CA1331401C - Tv signal transmission system and method - Google Patents
Tv signal transmission system and methodInfo
- Publication number
- CA1331401C CA1331401C CA 595635 CA595635A CA1331401C CA 1331401 C CA1331401 C CA 1331401C CA 595635 CA595635 CA 595635 CA 595635 A CA595635 A CA 595635A CA 1331401 C CA1331401 C CA 1331401C
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- signal
- frequency components
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- analog
- television
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Abstract
ABSTRACT OF THE DISCLOSURE
The analog implementation of a television signal transmission system includes a line integrator for developing a first analog signal corresponding to low frequency components, that is, the average level of active video below 15 KHz during each horizontal line of signal. An A/D converter produces a digital value corresponding to the first analog signal and a D/A
converter develops a second analog signal corresponding to the digital value. The second analog signal is subtracted from the baseband video signal to develop the high frequency components of the video signal. The digital value is encoded in the horizontal blanking interval of the high frequency components which is modulated onto an RF carrier. A one horizontal line delay in the baseband video signal is introduced before subtraction. In the digital implementation, a digital average is taken of the active video below 15 KHz of each line and subtracted. In the receiver, the coded data is decoded to reconstruct the low frequency components signal, which is added to the high frequency.
components video signal to reconstruct the television signal.
The analog implementation of a television signal transmission system includes a line integrator for developing a first analog signal corresponding to low frequency components, that is, the average level of active video below 15 KHz during each horizontal line of signal. An A/D converter produces a digital value corresponding to the first analog signal and a D/A
converter develops a second analog signal corresponding to the digital value. The second analog signal is subtracted from the baseband video signal to develop the high frequency components of the video signal. The digital value is encoded in the horizontal blanking interval of the high frequency components which is modulated onto an RF carrier. A one horizontal line delay in the baseband video signal is introduced before subtraction. In the digital implementation, a digital average is taken of the active video below 15 KHz of each line and subtracted. In the receiver, the coded data is decoded to reconstruct the low frequency components signal, which is added to the high frequency.
components video signal to reconstruct the television signal.
Description
1331~ ~1 This invention relates generally to television signal transmission systems and methods an~ specifically concerns a system in which transmission power is substantially reduced without discernible degradation in signal fidelity.
The conventional NSTC type television signal occupies a 6 MHz bandwidth and imposes certain power demands on the transmitter. These power demands are directly related to the cost of operating the transmitter. In addition, many older cable television plants have restrictad power handling capabilities which limits the number of channel signals that may be transmitted through the plant. It would therefore be highly desirable to reduce the amount of power required to transmit a television signal, thereby reducing transmitter operating costs and also permitting a larger number of channel signals to be transmitted through a cable plant of given power handling capability. The system of the invention achieves a marked reduction in the power required to transmit a television signal without discernlble degradation of signal fidelity and is therefore a solution to the above-mentioned needs of the prior art. In this system, a portion of the power savings can be used as a trade-off to in~rease the signal to noise ratio of the transmitted signal.
The present invention seeks to provide a novel television signal transmission system and method that requires substantially less transmitting power.
The present invention therefore provides a hybrid analog-digital method of operating a television signal ;~
transmission system comprising: developing a first analog signal comprising predominantly high frequency components of a baseband video signal, developing a digitally coded representation of a second signal comprising predominantly low frequency components of said baseband video signal, and ~ "' ,~ ,, .
~331~1 transmitting said first analog signal and said digitally coded representation.
The present invention further provides a television signal transmisslon system comprising: means producing a baseband video signal, means developing a low frequency components signal indicate of the line averaged value of active video in said baseband video signal, means combining said low frequency components signal with said baseband video signal to develop a high frequency components signal, means developing a digitally coded representation of said low frequency components signal, and means for modulating a carrier with said high frequency componen~s signal and said digitally coded representation ~or transmission.
Additionally, the present invention provides a receiver for receiving a television signal including high frequency components of a video signal and a digitally coded representation of low frequency components of said video signal, aomprising: means for generating an additional signal from said digitally coded representation, and means for combining said high frequency somponents with said additional signal to reconstruct said video signal.
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These and other objects and advantages Or the invention will be apparent upon reading the following description in conjunction with the drawings, in which~
FIC. 1 is a block diagram of a television signal .
transmitter constructed in accordance with the invention; ~::
FIG. 2 is a series of waveforms useful in explaining the invention; -, - -FIGS. 3 and 4 are graphs of power distribution of a -- ~
. ;- ,:, : .
lo typical television signal;
FIG. 5 is a block diagram of a television reoeiver constructed in accordance with the invention and operating at RF ~
frequencies; ~ -FIG. 6 is a block diagram of a television receiver ~ ;
constructed in acoordance with the invention and operating at , baseband frequencies;
FIG. 7 is a digital implementation of a transmitter constructed ln accordance with the invention;
FIG. B is a series of waveforms explaining a novel : .
signal level compensator for use with the invention; and -.~
: . . ,. :. :
FIG. 9 illustrates a form of identification signal :
useful with the invention.
Description of the Preferred Embodiment I i The philasophy oP the system of the invention is to reduce the power required to transmit a television signal by ; extracting certain low frequency, high-power consumption !
components therefrom and transmitting such components in a coded, low-power-consumption form while transmitting the remaining high ~:
frequency components in a conventional manner~ As will be discussed, ,. ,~ , , this results in a substantial reduction in transmitter power, since the power demands are greatest for the low frequency components.
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, :
^-` 133~ D5779 More specifically1 at the transmitter the baseband composite video signal is subjected to "line averaging" to determine a line averaged value of the active video for each horizontal line. In an analog version of the transmitter, the video is subjected to a line integrator and the line averaged value is passed through an analog-to-digital tA/D) converter where it is converted to a digital value which is coded and transmitted with the high frequency components of the video signal. The high frequency components are obtained by subtracting an analog signal that corresponds to the line ~ -averaged digital value of the active video portion of each horizontal line of the baseband video signalO To assure that the subtracted signal corresponds to the appropriate portions of the -~
baseband video signal, the baseband composite video signal is ~ ~-,, 15 subjected to a one horizontal line delay. A digital version of ;-the transmitter is also disclosed in which the analog video signal is converted to a digital signal and a digital average of the active video portion of each horizontal line is obtained.
In the analog transmitter, the output of the line ~ ~
20 integrator may be substracted directly from the baseband video ~ -~ signal to obtain the hiBh frequency components. ~owever, this -`~ approach could introduce error since the coded representation of ~ :, that signal, which is used in the receiver to reconstitute the low frequency components9 may have resolution limitations.
Preferably, digital values, representing the line averaged video signals, are supplied to a digital-to-analog (D/A) converter for developing the ana]og signals (low frequency components), which are subtracted from the baseband video signal. This eliminates error due to resolution limitations. Accurate reconstruction of i the low frequency portions can now be accomplished in the receiver because each coded representation truly represents the subtracted low frequency portions for that video line. l~
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Referrin8 in greater detail to the drawings, in FIG. 1, a source 12 of baseband composite video signal supplies a video clamp circuit 14 in accordan~e with conventional techniques for establishing a base line reference, generally at blank level, ;
i.e., corresponding to the level of the sync signal back porch.
The output of video clamp 14 is supplied to a one horizontal line (lH) delay circuit 16, to a line inti~grator 24 and to a conventional sync separator circuit 28. The output of sync separator circuit 28 supplies sync pulses to a timing and control circuit 30. A data source 38 supplies information, in the form of data to be included in the transmitted television signal, to ~-timing and control circuit 30. The delayed baseband composite video signal output from delay line 16 is passed through a switch 18 that is operated by timing and control circuit 30. The lS output of ~witch 18 ls supplied to a summing network 20 which, in turn, supplies a multiplier circuit 22. Line integrator 24 is -also coupled to, and operated under the control of, timing and control circuit 30 for integrating only the active video portion -~
of each horizontal line of the baseband composite video slgnal.
lts output is supplied to an A/D converter 26 which is coupled over a bidirectional communication bus 27 to timing and control ~;
oircuit 39. Commanication bus 27 is also coupled to a digital~
to-analog (D/A) converter 32. A bidirectional control line 29 links A~D converter 26 and timing and control circuit 30. A
ROM 31 is coupled between timing and control circuit 30 and D/A
, converter 32. ROM 3t supplies certain fixed reference and i identification signals to D/A converter 32 as will be explained.
The output of multiplier 22 is coupled to a low pass ~ I
, . . .
filter ~LPF) 23 to conform the data pulses to channel bandwidth ` 30 limitations. LPF 23 feeds a modulator 34 which, along with ~ -multiplier 22, is under control of timing and control circuit 30. ~
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1331~ D5779 Modulator 34 is also supplied with an R~ carrier and, in turn, supplies a summing network 36 ~hat combines an audio signal from a source of modulated sound 40 with the modulated video signal of the invention for transmission to suitable receivers.
In operation, timing and control circuit 30, under control of the sync signals from sync separator 28, sends appropriate timing signals to video clamp 14, line integrator 24, switch 18, multiplier 22, modulator 34, A/D converter 26, D/A
converter 32 and ROM 31. As mentioned, the video clamp 14 maintains the sync signal back porch of the composite baseband video signal at a predetermined level. The line integrator 24 is operated to independently integrate only the active video signal portion of each horiæontal line. Switch 18 is operated by timing and control circuit 30 to pass active video and color burst, but , lS not horizontal sync, to summing circuit 20. The line averaged value of video developed by integrator 24 for a particular video ~;
line is digitized by A/D converter 26 and coupled to both timing and control circuit 30 and D/A converter 32. D/A 3~ converts the output of A~D 26 to a corresponding analog signal which is . .
subtracted from active video in summing network 20. During the horizontal sync signal portion of the composite video signal, -ROM 31, in response to timing and control circuit 30, couples a i-`
digital pedestal signal to D/A 32, which is converted to a corresponding analog pedestal signal and inserted into the signal developed in summing network 20.
.` Timing and control circuit 30 develops a data signal comprising positive and negative voltage data pulses representing , `; the digitized line averaged signals from A/D 26 and applies these data pulses, during the horizontal blanking intervals, to multiplier 22. ~ -Multiplier 22 multiplies these data pulses with the analog pedestal signal previously inserted in the horizontal ùlanking 5~
1331~01 D5779 interval to develop positi~ve and negative data pulses during the horizontal blanking interval of the signal. As will be discussed below, the resultant coded representations of the line averaged signals are used to reconstitute the signal in the receiver.
While the number of data pulses that may be inserted in the horizontal blanking interval is dependent upon the data frequency, the inventive system envisions other coded data in the horizontal blanking interval as well, specifically data from data source 38. While the coded representation of the digital output of A/D converter 26 for each video line preferably comprises three or four bits (3/4), more bits can be used depending upon the resolution desired. To assure accuracy of the reconstituted signal in the receiver, this line varying 3/4 bit digital signal, :~
;~ which represents the high energy, low frequency components of the 15 video signal, is passed through D/A converter 32 to form an analog signal that is subtracted from the composite baseband video signal. Due to the resolution limitations of the 3/4 bit signal, a small residue of low frequency components may remain in the analog video signal that is passed by summing network 20 to `~
20 multiplier 22. However, as will be seen, since the low frequency ~i~
vidèo component added to the high frequency video component is j~-~
derived from the same 3/4 bit signal, it will precisely match that which was subtracted at the transmitter.
~` In FIG. 2, the series of not-to-scale waveforms 25 labelled A, B, C, D and E correspond to those appearing at correspondingly labelled portions of FIG. 1. Waveform A
represents the baseband composite video signal with a negative-going 15.75 KHz horizontal sync pulses 68, a 3.58 MHz color burst 70 and a horizontal blanking interval 72. Waveform B is 30 indicated as a dashed horizontal straight line and represents the output of line integrator 24 which corresponds to the average 13 ~ i 4 J3 ~ D5779 level of the active video signal between successive horizontal blanking in~ervals 72. Waveform C represents the result Or the subtraction of the line averaged video from the composite video signal and is centered at zero volts. It also includes a data s pedestal added in summing network 20. Waveform D illustrates the result Or multiplier 22 multiplying data with the data pedestal to develop positive and negative data pulses during the horizontal blanking period. While only two such pulses are shown for simplicity, a greater number of pulses is contemplated.
Modulator 34 modulates an RF televi.sion frequency carrier with ;
the bandwidth limited video signal, including the data pulses, for transmi~sion as illustrated by waveform E. The signal is centered about zero carrier and reverses phase each time the envelope passes through the zero carrier level. Thus, for example, each half cycle of the oolor burst, as well as each data j pulse, reverses phase. Portions 74 and 76 Or the waveform . ,. " . . .
represent RF carrier phase reversals in the video signal.
:~ Referring to FIG. 3, an idealized representative spectrum of power distribution of an NTSC type television signal o indicates the very significant power demand on the transmitter near the carrier frequency. FIG. 4 shows a very greatly expanded portion Or the curve of FIG. 3 near the video carrier frequency.
In particular the portion of the spectrum between the video ;- -carrier frequency and 15 KHz is illustrated. The shaded area ~; 25 bounded by the waveform and the dashed line illustrates pictorially the power saved with the invention because of the ! ;`
subtraction of the low frequency signal components, i.e., those below 15 KHz, from the transmitted signal. (They are instead transmitted as data in a coded low-energy utilization format as , explained above.) This shaded area is estimated to represent i`~
approximately 99% of the power of a typical television signal. A
reduction in transmitted power of 20dB (100:1~ is thus well ~ ~ -. 1 '~. .. '.. ,' ' '~
7 `~
1~31~1 within that contemplated by the invention. Some of this power reduction may, of course, he sacri:Eiced as a tradeoff to improve the signal-to-noise ratio of the transmitted signal. It should be borne in mind that since the power scales of the curves are logarithmic, they do not graphically convey the true magnitude of power reduction obtained with the invention.
In the receiver of FIG. 5, the transmitted signal is received by a tuner 41 and supplied to a buffer amplifier 42. The output of amplifier 42 supplies a sound carrier bandpass filter 44 and a video carrier bandpass filter 46. The receiver operates at RF frequencies, although operation at IF and baseband frequencies is also contemplated. The output of sound bandpass filter 44 is ;
supplied to one input of a summing network 66. The output of video bandpass filter 46 is supplied to a buffer amplifier 48. Buffer amplifier 48 feeds a multiplier 50 and a biphase stable phase locked loop (BPLL) circuit 52.
The output of multiplier ~0 is supplied through a switch ~ `
54 to a summing network 58. The data output of BPLL 52 is supplied to a controller 56 which, in turn, controls ~ ~
operation of multiplier 50 and switch 54. Controller 56 `
also supplies data, including the coded representation of the line averaged video, to a D/A converter 60. In the ~;-preferred embodiment, BPLL 52 is biphase stable and operates to provide recovered data, including the coded representation of line averaged video, to controller 56 and a fixed amplitude carrier Fo, that is either in phase or 180 out of phase with the received signal, to a ~ -multiplier 62. BPLL 52 may advantageously be constructed in accordance with the teachings of U.S. Patent No.
4,755,762 issued July 5, 1988 entitled COMBINED FPLL AND
PSK DATA DETECTOR, in the names of R. Citta and G.
Sgrignoli, and assigned to Zenith Electronics Corporation. That patent cites U.S. Patent Nos.
4,072,909, issued February 7, 1978, and 4,091,410, issued .
9 1331~1 May 23, 1978 both in the name of R~ Citta, as examples of biphase stable loops.
The received signal at the output of amplifier 48 is either in phase with, or 180 out of phase with Fo.
A special identification signal, ~to be discussed in further detail hereinafter) inserted at the encoder by ROM
31 (FIG. l) into the vertical int~erval of the television signal, is also recovered as part of the data and is interpreted by the controller to determine whether the phase of the received signal should be reversed to -establish the correct phase relationship. Multiplier 50, under control of controller 56, multiplies the signal at the output of amplifier 48 by either ~1 or -l to assure the correct phase relationship with Fo. Those skilled in the art will recognize that, alternatively, the phase of ;~
Fo may be controlled by appropriate multiplication rather than by controlling the phase of the received signal as described. In either case, after any necessary corrections, Fo and the received signal will have the same phase. It will of course be appreciated by those skilled in the art that any other well-known technique may be used in place of that discussed for determining the correct phase of Fo.
Controller 56 develops a number of clock or ~;
timing signals from the received color burst in a well-known manner. It will be recalled that the color ;~
burst of the encoded signal changes RF carrier phase every ;
half cycle thereby providing a conveniently detectable timing reference. These signals include a high frequency clock locked to the color burst and a horizontal rate clock derived by counting down therefrom. A low frequency ~`
clock is developed from an identification signal, to be described. Data is removed from the incoming signal by ~ `~
I opening D~779 ~33~
switch 54 during time peri~ods corresponding to the occurrence Or data. Sync information, i.e., a sync pulse and pedestal, is regenerated in the controller and applied via D/A 60 and multiplier 62 to summing network 58.
Multiplier 62 multiplies Fo with the output of D/A
converter 60 to produce a carrier signal, the amplitude Or which is determined by the ooded representation of the line averaged video, for addition to the received video signal supplied to summing network 58. The output of summing network 58 is therefore the reconstituted video portion of the television signal. This signal is supplied to a special AGC circuit ~1 and to summing network 66 where it is recombined ~ith the sound modulated carrier and passed to conventional television signal -~
processing circuitry (not shown). As will be explained, the output of AGC circuit 61 controls the gain of amplifier 42 to assure that the analog value of the digital representation of the line averaged video at the receiver matches that in the transmitter since the digital data is not altered by transmission attenuation as are the analog portions of the signal.
The special AGC circuit 61 includes an RF detector 64, a pair of sample and hold (S/H) circuits 63 and 65 and a ; comparator 67. As will be explained, a reference signal is transmitted and portions thereof are sampled in the receiver to ` ' determine attenuation effects on the analog portions of the signal and to compensate the receiver gain accordingly. ' As mentioned, the receiver of FIG. 5 operates at RF !~
frequencies. In many installations, it is desirable that the receiver operate at baseband frequencies and FIG. 6 illustrates such a receiver. A tuner/IF 41' receives the transmitted signal and applies an IF signal through an amplifier 42'to a video IF ;~
bandpass filter 46' and to a sound IF bandpass filter 44'.
~ -13 31~ ~1 D5779 Filter 46~, in turn, supplies the IF signal to a BPLL 52~ and to a multiplier 50'. Data is removed by BPLL 52' and applied to a controller 56'. BPLL 52' also recovers a pair of quadrature-related IF carriers Fo~ and Fo' 90, Fo' being applied to a -multiplier 58~ and Fo~ 90 to a multiplier 59'. Controller 56' determines on the basis of a receivecl reference signal whether the phases of the received signal and Fo' are the same and controls multiplier 50' to reverse the phase of the signal, if necessary, by multiplying by a ~1 or a -1. Multipliers 58' and 59' function as synchronous detectors for developing output baseband video and 4.5 MHz sound signals, respectively, in response to Fo' and Fo' 90. The 4.5 MHz sound signal is applied to a 4.5 MHz sound BPF 44" and the composite video signal is ;
. .
applied to a switch 54'. Switch 54' is operated by controller 56' to open during data and horizontal sync portions of the received signal. A D/A 60' is operated by controller 56' and supplies one input of a summing network 47', the other input ~ ~;
being sùpplied by switch 54'. D/A 60' supplies to summing ~ network 47'the sync and sync pedestal along with an analog `~ 20 signal corresponding to the coded representation of the low frequency components sent in the data recovered by BPLL 52', which are added to the baseband video signal developed at the output of switch 54'. A reconstituted baseband video signal ~;l therefore appears at the output of summing network 47' and is 25 applied directly to the S/H circuits 63' and 64', which are `
operated under control of controller 56' to sample the reference signal that is transmitted to determine the attenuation effects on the analog portions of the transmitted signals. Again a comparator 67' supplies any correction required to adjust the gain of amplifier 42' to match the analog signal portions with the digital representations. The reconstituted video signal is also combined with the 4.5 MHz audio signal in a summing 13314~ D~779 network 66' to provide an ~utput baseband television signal which may be applied to a television monitor or the like for viewing.
Referring to FIG. 7, a digital implementation of a transmitter constructed in accordance with the invention is shown. A baseband source of composite video signal 12 is coupled to video clamp 14, the output of which supplies an A/D
converter 78 and a sync separator circuit 28. A timing and control circuit 84 is intercoupled with A~D converter 78 and is supplied with the output of sync separator 28. Video clamp 14 is lo operated under control of timing and control cirouit 84 to clamp , :
the incoming video signal at the back porch levelO The source of ~ -data 38 is coupled to the timing and control circuit 84. The ~
output of A/D converter 78 is supplied to a digital averaging ~ ~ ;
circuit 79 and to a RAM memory 80. Digital averaging circuit 79 15 i9 operated under control of timing and control circuit 84 to sample the output of A/D converter 78 during the active video portions of the signal and to develop an average of the digital values for each individual horizontal line. This value is ~
supplied back to timing and control circuit 84 and to a summing - `
network 81 which is also supplied with the output of RAM
memory 80. RAM memory 80 comprises a two video line memory in which one video line is written in as the previous video line is read out. This arrangement introduces a one line delay to assure that the digital average signal is subtracted from the video 25 samples of the appropriate horizontal line. The output of ,~
summing network 81 is supplied to a multiplexing circuit 82 which is also coupled to the output of a ROM 31. ROM 31 supplies the reference and identification signals to the multiplexer 82 as will be described further below. Data from data source 38 and timing and control c:Lrcuit 84 is applied to a third input of multiplexer 82 during the horizontal blanking intervals of the t ~ , ''"~ ' 13 31~ ~1 D5779 signal. The data includes a coded representation of the line averaged values developed by digital averaging circuit 79. The output of multiplexer 82 is coupled to a D/A converter 86 whose output is supplied to a low pass filter 23 and thence to a modulator 34. Summing network 81, multiplexer 82, D/A
converter 86 and modulator 34 are al:L operated under control of timing and control circult 84. Modulator 34 is supplied with an RF signal and its output is further processed as indicated in FIG. 1.
Referring back to FIG. 2, it will be seen that the waveform C is obtained by subtracting waveform B from waveform A
during the active video portions of each horizontal line, except during the horizontal blanking interval 72. It will be appreciated by those skilled in the art that a similar result would be obtained by adding a waveform of magnitude B to the horizonta} blanking interval only and correcting for the resulting change in zero level. When considering the digital `~
. .~ .
implementation of the encoder, the latter technique involves considerable simplification and is the presently preferred method of implementation.
;;; In FIG. 7, the output of the A/D converter 78 ~ -preferably comprises approximately 910 samples per horizontal line with about 752 of those samples representing the active video portion of the line. Each sample is represented by either 8 or 10 bits depending upon the output resolution desired. For example, for ordinary commercial type television signals, an 8 bit resolution is suffioient, whereas for studio level quality `
and transmission applications, a 10 bit resolution is preferred. `
The number of bits selected for the active video portion is 30 prèferably divisible by 2 which greatly simplifies the hardwareO -. ~, . ..
As alluded to previously with respect to FIG. 2, it may be ~;
preferable to add the line averaged value (waveform B) to the ;
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~ 1331~1 D5779 horizontal blanking interv~l of the si~nal rather than subtract the line averaged value from the active video portion. This would entail approximately 60 additions as compared with approximately 752 subtractions and would again materially S simplify the operation and hardware. However, the result would be the same after correction for the zero level and the particular technique utiliæed should not be considered limiting of the invention. The digitally processed signal is then supplied from the summing network 81 to the multiplexer 82 along with data from timing and control circuit 84 and the fixed identification and reference signals from ROM 31. After passage through D/A converter 86, the signal is handled in the same manner as described with respect to the transmitter of FIG. 1.
; Because Or the nature of the transmitted television signal, that is, a hybrid of analog and coded digital information, a system for compensating for attenuation experienced by the analog signal (which does not alter the digital data) is provided. In order to properly reconstruct the received signal, the analog video signal may need to be adjusted to maintain the same relationship, between it and the digital -data, that existed at the transmitter. The invention essentially provides for sending a reference signal with a known relationship between the analog and digital data and detecting that signal in , the decoder and comparing the detected levels to determine the amount and polarity of adjustment required, if any.
Referring to FIG. 8, waveforms A, B and C, depicting two horizontal lines of a transmitted signal are shown. Waveform A constitutes a reference signal which comprises of a white line (ir,dicated as digital level 255) that falls to zero or black level (indicated as 0) followed by a second line of no video or black level. Waveform B represents an encoded counterpart of the 1 3 3 ~ D~779 reference signal (A) in which the white line has been reduced to a digital level of 55 by subtraction of an assumed average level ~ Or 200. The black level portion Or the video line now occupies a `~ level of -200, reflecting the subtraction of the average level Or 200 therefrom. The second line, however, is unchanged in the ¦ active video portion since its average level is zero. Waverorm C
represents the decoded (reconstituted) signal and also indicates two sample areas identified as sample #1 and sample #2. Samples of the levels are taken at the indicated areas and stored in the sample and hold circuits of the receiver. Under conditions where the analog signal does not experience attenuation, sample #1 will reflect that the signal level has been returned precisely to zero level and will match sample #2. Should the decoded (reconstituted) signal be higher, as indicated by the dashed line portion H, sample #1 will be greater than sample #2 and the output of comparators (67 in FIG. 5 and 67' in FIG. 6) will generate a correction voltage for application to amplifier 42 or 42'. If, on the other hand, the decoded signal is at a lower - -level L, sample #1 will be less than sample #2 and an opposite o type correction will be supplied from the comparator to the ,~. .
amplifier. The provision of this reference signal, including one horizontal line with a significant analog video portion and a subsequent line with a zero analog video portion, provides a , built-in standard for determining what hlas happened to the analog ~;~
signal during transmission and processing.
In FIG. 9, one form of identification signal is shown ;~
. ~,::, that serves the dual purpose of providing a start signal for ~.
timing purposes and for identifying the proper phase relationship between the video carrier signal and Fo. A normal encoded line (shown not-to-scale) includes data horizontal pulses 90, a color ;~
burst 91 and an active video portion 92, which assures a certain ;
number of zero crossings. Detection is based upon no zero ;;
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crossings occurring during a line. An identification line is established without zero crossings by removing data pulses and color burst. The polarity of the video signal 93 may be used to indicate a particular phase relationship between the video 5 carrier and th~ recovered Fo signal. The next line, assumed to be in the vertical blanking interval, does not have color burst, but does have data pulses. Thus it too exhibits zero crossings.
While the invention has been described in terms Or transmitting a digital representation Or the low frequency components of the active video, it is contemplated that an analog signal level may be used as the coded representation. Thus the level of the data pedestal or of the horizontal blanking interval itself may represent the value of low frequency components that have been subtracted from the signal. Such an arrangement is believed to fall within the purview of the invention.
The invention thus permits a television transmission system requiring significantly less power without sacrificing signai quality. It is recognized that numerous changes in the described embodiment of the invention will be apparent to those skilled in the art without departing from its true spirit and scope. The invention is to be limited only as defined in the claims. ~
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The conventional NSTC type television signal occupies a 6 MHz bandwidth and imposes certain power demands on the transmitter. These power demands are directly related to the cost of operating the transmitter. In addition, many older cable television plants have restrictad power handling capabilities which limits the number of channel signals that may be transmitted through the plant. It would therefore be highly desirable to reduce the amount of power required to transmit a television signal, thereby reducing transmitter operating costs and also permitting a larger number of channel signals to be transmitted through a cable plant of given power handling capability. The system of the invention achieves a marked reduction in the power required to transmit a television signal without discernlble degradation of signal fidelity and is therefore a solution to the above-mentioned needs of the prior art. In this system, a portion of the power savings can be used as a trade-off to in~rease the signal to noise ratio of the transmitted signal.
The present invention seeks to provide a novel television signal transmission system and method that requires substantially less transmitting power.
The present invention therefore provides a hybrid analog-digital method of operating a television signal ;~
transmission system comprising: developing a first analog signal comprising predominantly high frequency components of a baseband video signal, developing a digitally coded representation of a second signal comprising predominantly low frequency components of said baseband video signal, and ~ "' ,~ ,, .
~331~1 transmitting said first analog signal and said digitally coded representation.
The present invention further provides a television signal transmisslon system comprising: means producing a baseband video signal, means developing a low frequency components signal indicate of the line averaged value of active video in said baseband video signal, means combining said low frequency components signal with said baseband video signal to develop a high frequency components signal, means developing a digitally coded representation of said low frequency components signal, and means for modulating a carrier with said high frequency componen~s signal and said digitally coded representation ~or transmission.
Additionally, the present invention provides a receiver for receiving a television signal including high frequency components of a video signal and a digitally coded representation of low frequency components of said video signal, aomprising: means for generating an additional signal from said digitally coded representation, and means for combining said high frequency somponents with said additional signal to reconstruct said video signal.
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Brief ~ 13 31~ (3 ~
These and other objects and advantages Or the invention will be apparent upon reading the following description in conjunction with the drawings, in which~
FIC. 1 is a block diagram of a television signal .
transmitter constructed in accordance with the invention; ~::
FIG. 2 is a series of waveforms useful in explaining the invention; -, - -FIGS. 3 and 4 are graphs of power distribution of a -- ~
. ;- ,:, : .
lo typical television signal;
FIG. 5 is a block diagram of a television reoeiver constructed in accordance with the invention and operating at RF ~
frequencies; ~ -FIG. 6 is a block diagram of a television receiver ~ ;
constructed in acoordance with the invention and operating at , baseband frequencies;
FIG. 7 is a digital implementation of a transmitter constructed ln accordance with the invention;
FIG. B is a series of waveforms explaining a novel : .
signal level compensator for use with the invention; and -.~
: . . ,. :. :
FIG. 9 illustrates a form of identification signal :
useful with the invention.
Description of the Preferred Embodiment I i The philasophy oP the system of the invention is to reduce the power required to transmit a television signal by ; extracting certain low frequency, high-power consumption !
components therefrom and transmitting such components in a coded, low-power-consumption form while transmitting the remaining high ~:
frequency components in a conventional manner~ As will be discussed, ,. ,~ , , this results in a substantial reduction in transmitter power, since the power demands are greatest for the low frequency components.
!
, :
^-` 133~ D5779 More specifically1 at the transmitter the baseband composite video signal is subjected to "line averaging" to determine a line averaged value of the active video for each horizontal line. In an analog version of the transmitter, the video is subjected to a line integrator and the line averaged value is passed through an analog-to-digital tA/D) converter where it is converted to a digital value which is coded and transmitted with the high frequency components of the video signal. The high frequency components are obtained by subtracting an analog signal that corresponds to the line ~ -averaged digital value of the active video portion of each horizontal line of the baseband video signalO To assure that the subtracted signal corresponds to the appropriate portions of the -~
baseband video signal, the baseband composite video signal is ~ ~-,, 15 subjected to a one horizontal line delay. A digital version of ;-the transmitter is also disclosed in which the analog video signal is converted to a digital signal and a digital average of the active video portion of each horizontal line is obtained.
In the analog transmitter, the output of the line ~ ~
20 integrator may be substracted directly from the baseband video ~ -~ signal to obtain the hiBh frequency components. ~owever, this -`~ approach could introduce error since the coded representation of ~ :, that signal, which is used in the receiver to reconstitute the low frequency components9 may have resolution limitations.
Preferably, digital values, representing the line averaged video signals, are supplied to a digital-to-analog (D/A) converter for developing the ana]og signals (low frequency components), which are subtracted from the baseband video signal. This eliminates error due to resolution limitations. Accurate reconstruction of i the low frequency portions can now be accomplished in the receiver because each coded representation truly represents the subtracted low frequency portions for that video line. l~
. -:: : ::: ~:
. ~
:~' ::-- ::
~, . ;: :,: .
~~' D$779 1331~
Referrin8 in greater detail to the drawings, in FIG. 1, a source 12 of baseband composite video signal supplies a video clamp circuit 14 in accordan~e with conventional techniques for establishing a base line reference, generally at blank level, ;
i.e., corresponding to the level of the sync signal back porch.
The output of video clamp 14 is supplied to a one horizontal line (lH) delay circuit 16, to a line inti~grator 24 and to a conventional sync separator circuit 28. The output of sync separator circuit 28 supplies sync pulses to a timing and control circuit 30. A data source 38 supplies information, in the form of data to be included in the transmitted television signal, to ~-timing and control circuit 30. The delayed baseband composite video signal output from delay line 16 is passed through a switch 18 that is operated by timing and control circuit 30. The lS output of ~witch 18 ls supplied to a summing network 20 which, in turn, supplies a multiplier circuit 22. Line integrator 24 is -also coupled to, and operated under the control of, timing and control circuit 30 for integrating only the active video portion -~
of each horizontal line of the baseband composite video slgnal.
lts output is supplied to an A/D converter 26 which is coupled over a bidirectional communication bus 27 to timing and control ~;
oircuit 39. Commanication bus 27 is also coupled to a digital~
to-analog (D/A) converter 32. A bidirectional control line 29 links A~D converter 26 and timing and control circuit 30. A
ROM 31 is coupled between timing and control circuit 30 and D/A
, converter 32. ROM 3t supplies certain fixed reference and i identification signals to D/A converter 32 as will be explained.
The output of multiplier 22 is coupled to a low pass ~ I
, . . .
filter ~LPF) 23 to conform the data pulses to channel bandwidth ` 30 limitations. LPF 23 feeds a modulator 34 which, along with ~ -multiplier 22, is under control of timing and control circuit 30. ~
, . , ." ~ .
,' .~ .
1331~ D5779 Modulator 34 is also supplied with an R~ carrier and, in turn, supplies a summing network 36 ~hat combines an audio signal from a source of modulated sound 40 with the modulated video signal of the invention for transmission to suitable receivers.
In operation, timing and control circuit 30, under control of the sync signals from sync separator 28, sends appropriate timing signals to video clamp 14, line integrator 24, switch 18, multiplier 22, modulator 34, A/D converter 26, D/A
converter 32 and ROM 31. As mentioned, the video clamp 14 maintains the sync signal back porch of the composite baseband video signal at a predetermined level. The line integrator 24 is operated to independently integrate only the active video signal portion of each horiæontal line. Switch 18 is operated by timing and control circuit 30 to pass active video and color burst, but , lS not horizontal sync, to summing circuit 20. The line averaged value of video developed by integrator 24 for a particular video ~;
line is digitized by A/D converter 26 and coupled to both timing and control circuit 30 and D/A converter 32. D/A 3~ converts the output of A~D 26 to a corresponding analog signal which is . .
subtracted from active video in summing network 20. During the horizontal sync signal portion of the composite video signal, -ROM 31, in response to timing and control circuit 30, couples a i-`
digital pedestal signal to D/A 32, which is converted to a corresponding analog pedestal signal and inserted into the signal developed in summing network 20.
.` Timing and control circuit 30 develops a data signal comprising positive and negative voltage data pulses representing , `; the digitized line averaged signals from A/D 26 and applies these data pulses, during the horizontal blanking intervals, to multiplier 22. ~ -Multiplier 22 multiplies these data pulses with the analog pedestal signal previously inserted in the horizontal ùlanking 5~
1331~01 D5779 interval to develop positi~ve and negative data pulses during the horizontal blanking interval of the signal. As will be discussed below, the resultant coded representations of the line averaged signals are used to reconstitute the signal in the receiver.
While the number of data pulses that may be inserted in the horizontal blanking interval is dependent upon the data frequency, the inventive system envisions other coded data in the horizontal blanking interval as well, specifically data from data source 38. While the coded representation of the digital output of A/D converter 26 for each video line preferably comprises three or four bits (3/4), more bits can be used depending upon the resolution desired. To assure accuracy of the reconstituted signal in the receiver, this line varying 3/4 bit digital signal, :~
;~ which represents the high energy, low frequency components of the 15 video signal, is passed through D/A converter 32 to form an analog signal that is subtracted from the composite baseband video signal. Due to the resolution limitations of the 3/4 bit signal, a small residue of low frequency components may remain in the analog video signal that is passed by summing network 20 to `~
20 multiplier 22. However, as will be seen, since the low frequency ~i~
vidèo component added to the high frequency video component is j~-~
derived from the same 3/4 bit signal, it will precisely match that which was subtracted at the transmitter.
~` In FIG. 2, the series of not-to-scale waveforms 25 labelled A, B, C, D and E correspond to those appearing at correspondingly labelled portions of FIG. 1. Waveform A
represents the baseband composite video signal with a negative-going 15.75 KHz horizontal sync pulses 68, a 3.58 MHz color burst 70 and a horizontal blanking interval 72. Waveform B is 30 indicated as a dashed horizontal straight line and represents the output of line integrator 24 which corresponds to the average 13 ~ i 4 J3 ~ D5779 level of the active video signal between successive horizontal blanking in~ervals 72. Waveform C represents the result Or the subtraction of the line averaged video from the composite video signal and is centered at zero volts. It also includes a data s pedestal added in summing network 20. Waveform D illustrates the result Or multiplier 22 multiplying data with the data pedestal to develop positive and negative data pulses during the horizontal blanking period. While only two such pulses are shown for simplicity, a greater number of pulses is contemplated.
Modulator 34 modulates an RF televi.sion frequency carrier with ;
the bandwidth limited video signal, including the data pulses, for transmi~sion as illustrated by waveform E. The signal is centered about zero carrier and reverses phase each time the envelope passes through the zero carrier level. Thus, for example, each half cycle of the oolor burst, as well as each data j pulse, reverses phase. Portions 74 and 76 Or the waveform . ,. " . . .
represent RF carrier phase reversals in the video signal.
:~ Referring to FIG. 3, an idealized representative spectrum of power distribution of an NTSC type television signal o indicates the very significant power demand on the transmitter near the carrier frequency. FIG. 4 shows a very greatly expanded portion Or the curve of FIG. 3 near the video carrier frequency.
In particular the portion of the spectrum between the video ;- -carrier frequency and 15 KHz is illustrated. The shaded area ~; 25 bounded by the waveform and the dashed line illustrates pictorially the power saved with the invention because of the ! ;`
subtraction of the low frequency signal components, i.e., those below 15 KHz, from the transmitted signal. (They are instead transmitted as data in a coded low-energy utilization format as , explained above.) This shaded area is estimated to represent i`~
approximately 99% of the power of a typical television signal. A
reduction in transmitted power of 20dB (100:1~ is thus well ~ ~ -. 1 '~. .. '.. ,' ' '~
7 `~
1~31~1 within that contemplated by the invention. Some of this power reduction may, of course, he sacri:Eiced as a tradeoff to improve the signal-to-noise ratio of the transmitted signal. It should be borne in mind that since the power scales of the curves are logarithmic, they do not graphically convey the true magnitude of power reduction obtained with the invention.
In the receiver of FIG. 5, the transmitted signal is received by a tuner 41 and supplied to a buffer amplifier 42. The output of amplifier 42 supplies a sound carrier bandpass filter 44 and a video carrier bandpass filter 46. The receiver operates at RF frequencies, although operation at IF and baseband frequencies is also contemplated. The output of sound bandpass filter 44 is ;
supplied to one input of a summing network 66. The output of video bandpass filter 46 is supplied to a buffer amplifier 48. Buffer amplifier 48 feeds a multiplier 50 and a biphase stable phase locked loop (BPLL) circuit 52.
The output of multiplier ~0 is supplied through a switch ~ `
54 to a summing network 58. The data output of BPLL 52 is supplied to a controller 56 which, in turn, controls ~ ~
operation of multiplier 50 and switch 54. Controller 56 `
also supplies data, including the coded representation of the line averaged video, to a D/A converter 60. In the ~;-preferred embodiment, BPLL 52 is biphase stable and operates to provide recovered data, including the coded representation of line averaged video, to controller 56 and a fixed amplitude carrier Fo, that is either in phase or 180 out of phase with the received signal, to a ~ -multiplier 62. BPLL 52 may advantageously be constructed in accordance with the teachings of U.S. Patent No.
4,755,762 issued July 5, 1988 entitled COMBINED FPLL AND
PSK DATA DETECTOR, in the names of R. Citta and G.
Sgrignoli, and assigned to Zenith Electronics Corporation. That patent cites U.S. Patent Nos.
4,072,909, issued February 7, 1978, and 4,091,410, issued .
9 1331~1 May 23, 1978 both in the name of R~ Citta, as examples of biphase stable loops.
The received signal at the output of amplifier 48 is either in phase with, or 180 out of phase with Fo.
A special identification signal, ~to be discussed in further detail hereinafter) inserted at the encoder by ROM
31 (FIG. l) into the vertical int~erval of the television signal, is also recovered as part of the data and is interpreted by the controller to determine whether the phase of the received signal should be reversed to -establish the correct phase relationship. Multiplier 50, under control of controller 56, multiplies the signal at the output of amplifier 48 by either ~1 or -l to assure the correct phase relationship with Fo. Those skilled in the art will recognize that, alternatively, the phase of ;~
Fo may be controlled by appropriate multiplication rather than by controlling the phase of the received signal as described. In either case, after any necessary corrections, Fo and the received signal will have the same phase. It will of course be appreciated by those skilled in the art that any other well-known technique may be used in place of that discussed for determining the correct phase of Fo.
Controller 56 develops a number of clock or ~;
timing signals from the received color burst in a well-known manner. It will be recalled that the color ;~
burst of the encoded signal changes RF carrier phase every ;
half cycle thereby providing a conveniently detectable timing reference. These signals include a high frequency clock locked to the color burst and a horizontal rate clock derived by counting down therefrom. A low frequency ~`
clock is developed from an identification signal, to be described. Data is removed from the incoming signal by ~ `~
I opening D~779 ~33~
switch 54 during time peri~ods corresponding to the occurrence Or data. Sync information, i.e., a sync pulse and pedestal, is regenerated in the controller and applied via D/A 60 and multiplier 62 to summing network 58.
Multiplier 62 multiplies Fo with the output of D/A
converter 60 to produce a carrier signal, the amplitude Or which is determined by the ooded representation of the line averaged video, for addition to the received video signal supplied to summing network 58. The output of summing network 58 is therefore the reconstituted video portion of the television signal. This signal is supplied to a special AGC circuit ~1 and to summing network 66 where it is recombined ~ith the sound modulated carrier and passed to conventional television signal -~
processing circuitry (not shown). As will be explained, the output of AGC circuit 61 controls the gain of amplifier 42 to assure that the analog value of the digital representation of the line averaged video at the receiver matches that in the transmitter since the digital data is not altered by transmission attenuation as are the analog portions of the signal.
The special AGC circuit 61 includes an RF detector 64, a pair of sample and hold (S/H) circuits 63 and 65 and a ; comparator 67. As will be explained, a reference signal is transmitted and portions thereof are sampled in the receiver to ` ' determine attenuation effects on the analog portions of the signal and to compensate the receiver gain accordingly. ' As mentioned, the receiver of FIG. 5 operates at RF !~
frequencies. In many installations, it is desirable that the receiver operate at baseband frequencies and FIG. 6 illustrates such a receiver. A tuner/IF 41' receives the transmitted signal and applies an IF signal through an amplifier 42'to a video IF ;~
bandpass filter 46' and to a sound IF bandpass filter 44'.
~ -13 31~ ~1 D5779 Filter 46~, in turn, supplies the IF signal to a BPLL 52~ and to a multiplier 50'. Data is removed by BPLL 52' and applied to a controller 56'. BPLL 52' also recovers a pair of quadrature-related IF carriers Fo~ and Fo' 90, Fo' being applied to a -multiplier 58~ and Fo~ 90 to a multiplier 59'. Controller 56' determines on the basis of a receivecl reference signal whether the phases of the received signal and Fo' are the same and controls multiplier 50' to reverse the phase of the signal, if necessary, by multiplying by a ~1 or a -1. Multipliers 58' and 59' function as synchronous detectors for developing output baseband video and 4.5 MHz sound signals, respectively, in response to Fo' and Fo' 90. The 4.5 MHz sound signal is applied to a 4.5 MHz sound BPF 44" and the composite video signal is ;
. .
applied to a switch 54'. Switch 54' is operated by controller 56' to open during data and horizontal sync portions of the received signal. A D/A 60' is operated by controller 56' and supplies one input of a summing network 47', the other input ~ ~;
being sùpplied by switch 54'. D/A 60' supplies to summing ~ network 47'the sync and sync pedestal along with an analog `~ 20 signal corresponding to the coded representation of the low frequency components sent in the data recovered by BPLL 52', which are added to the baseband video signal developed at the output of switch 54'. A reconstituted baseband video signal ~;l therefore appears at the output of summing network 47' and is 25 applied directly to the S/H circuits 63' and 64', which are `
operated under control of controller 56' to sample the reference signal that is transmitted to determine the attenuation effects on the analog portions of the transmitted signals. Again a comparator 67' supplies any correction required to adjust the gain of amplifier 42' to match the analog signal portions with the digital representations. The reconstituted video signal is also combined with the 4.5 MHz audio signal in a summing 13314~ D~779 network 66' to provide an ~utput baseband television signal which may be applied to a television monitor or the like for viewing.
Referring to FIG. 7, a digital implementation of a transmitter constructed in accordance with the invention is shown. A baseband source of composite video signal 12 is coupled to video clamp 14, the output of which supplies an A/D
converter 78 and a sync separator circuit 28. A timing and control circuit 84 is intercoupled with A~D converter 78 and is supplied with the output of sync separator 28. Video clamp 14 is lo operated under control of timing and control cirouit 84 to clamp , :
the incoming video signal at the back porch levelO The source of ~ -data 38 is coupled to the timing and control circuit 84. The ~
output of A/D converter 78 is supplied to a digital averaging ~ ~ ;
circuit 79 and to a RAM memory 80. Digital averaging circuit 79 15 i9 operated under control of timing and control circuit 84 to sample the output of A/D converter 78 during the active video portions of the signal and to develop an average of the digital values for each individual horizontal line. This value is ~
supplied back to timing and control circuit 84 and to a summing - `
network 81 which is also supplied with the output of RAM
memory 80. RAM memory 80 comprises a two video line memory in which one video line is written in as the previous video line is read out. This arrangement introduces a one line delay to assure that the digital average signal is subtracted from the video 25 samples of the appropriate horizontal line. The output of ,~
summing network 81 is supplied to a multiplexing circuit 82 which is also coupled to the output of a ROM 31. ROM 31 supplies the reference and identification signals to the multiplexer 82 as will be described further below. Data from data source 38 and timing and control c:Lrcuit 84 is applied to a third input of multiplexer 82 during the horizontal blanking intervals of the t ~ , ''"~ ' 13 31~ ~1 D5779 signal. The data includes a coded representation of the line averaged values developed by digital averaging circuit 79. The output of multiplexer 82 is coupled to a D/A converter 86 whose output is supplied to a low pass filter 23 and thence to a modulator 34. Summing network 81, multiplexer 82, D/A
converter 86 and modulator 34 are al:L operated under control of timing and control circult 84. Modulator 34 is supplied with an RF signal and its output is further processed as indicated in FIG. 1.
Referring back to FIG. 2, it will be seen that the waveform C is obtained by subtracting waveform B from waveform A
during the active video portions of each horizontal line, except during the horizontal blanking interval 72. It will be appreciated by those skilled in the art that a similar result would be obtained by adding a waveform of magnitude B to the horizonta} blanking interval only and correcting for the resulting change in zero level. When considering the digital `~
. .~ .
implementation of the encoder, the latter technique involves considerable simplification and is the presently preferred method of implementation.
;;; In FIG. 7, the output of the A/D converter 78 ~ -preferably comprises approximately 910 samples per horizontal line with about 752 of those samples representing the active video portion of the line. Each sample is represented by either 8 or 10 bits depending upon the output resolution desired. For example, for ordinary commercial type television signals, an 8 bit resolution is suffioient, whereas for studio level quality `
and transmission applications, a 10 bit resolution is preferred. `
The number of bits selected for the active video portion is 30 prèferably divisible by 2 which greatly simplifies the hardwareO -. ~, . ..
As alluded to previously with respect to FIG. 2, it may be ~;
preferable to add the line averaged value (waveform B) to the ;
,.'`~' '''' '' ... . ~, , . , ,.~ .:
13 ~
~ 1331~1 D5779 horizontal blanking interv~l of the si~nal rather than subtract the line averaged value from the active video portion. This would entail approximately 60 additions as compared with approximately 752 subtractions and would again materially S simplify the operation and hardware. However, the result would be the same after correction for the zero level and the particular technique utiliæed should not be considered limiting of the invention. The digitally processed signal is then supplied from the summing network 81 to the multiplexer 82 along with data from timing and control circuit 84 and the fixed identification and reference signals from ROM 31. After passage through D/A converter 86, the signal is handled in the same manner as described with respect to the transmitter of FIG. 1.
; Because Or the nature of the transmitted television signal, that is, a hybrid of analog and coded digital information, a system for compensating for attenuation experienced by the analog signal (which does not alter the digital data) is provided. In order to properly reconstruct the received signal, the analog video signal may need to be adjusted to maintain the same relationship, between it and the digital -data, that existed at the transmitter. The invention essentially provides for sending a reference signal with a known relationship between the analog and digital data and detecting that signal in , the decoder and comparing the detected levels to determine the amount and polarity of adjustment required, if any.
Referring to FIG. 8, waveforms A, B and C, depicting two horizontal lines of a transmitted signal are shown. Waveform A constitutes a reference signal which comprises of a white line (ir,dicated as digital level 255) that falls to zero or black level (indicated as 0) followed by a second line of no video or black level. Waveform B represents an encoded counterpart of the 1 3 3 ~ D~779 reference signal (A) in which the white line has been reduced to a digital level of 55 by subtraction of an assumed average level ~ Or 200. The black level portion Or the video line now occupies a `~ level of -200, reflecting the subtraction of the average level Or 200 therefrom. The second line, however, is unchanged in the ¦ active video portion since its average level is zero. Waverorm C
represents the decoded (reconstituted) signal and also indicates two sample areas identified as sample #1 and sample #2. Samples of the levels are taken at the indicated areas and stored in the sample and hold circuits of the receiver. Under conditions where the analog signal does not experience attenuation, sample #1 will reflect that the signal level has been returned precisely to zero level and will match sample #2. Should the decoded (reconstituted) signal be higher, as indicated by the dashed line portion H, sample #1 will be greater than sample #2 and the output of comparators (67 in FIG. 5 and 67' in FIG. 6) will generate a correction voltage for application to amplifier 42 or 42'. If, on the other hand, the decoded signal is at a lower - -level L, sample #1 will be less than sample #2 and an opposite o type correction will be supplied from the comparator to the ,~. .
amplifier. The provision of this reference signal, including one horizontal line with a significant analog video portion and a subsequent line with a zero analog video portion, provides a , built-in standard for determining what hlas happened to the analog ~;~
signal during transmission and processing.
In FIG. 9, one form of identification signal is shown ;~
. ~,::, that serves the dual purpose of providing a start signal for ~.
timing purposes and for identifying the proper phase relationship between the video carrier signal and Fo. A normal encoded line (shown not-to-scale) includes data horizontal pulses 90, a color ;~
burst 91 and an active video portion 92, which assures a certain ;
number of zero crossings. Detection is based upon no zero ;;
" - - :~
.. :,, : "....
~ 13 3 3 ~ ~1 D577~
crossings occurring during a line. An identification line is established without zero crossings by removing data pulses and color burst. The polarity of the video signal 93 may be used to indicate a particular phase relationship between the video 5 carrier and th~ recovered Fo signal. The next line, assumed to be in the vertical blanking interval, does not have color burst, but does have data pulses. Thus it too exhibits zero crossings.
While the invention has been described in terms Or transmitting a digital representation Or the low frequency components of the active video, it is contemplated that an analog signal level may be used as the coded representation. Thus the level of the data pedestal or of the horizontal blanking interval itself may represent the value of low frequency components that have been subtracted from the signal. Such an arrangement is believed to fall within the purview of the invention.
The invention thus permits a television transmission system requiring significantly less power without sacrificing signai quality. It is recognized that numerous changes in the described embodiment of the invention will be apparent to those skilled in the art without departing from its true spirit and scope. The invention is to be limited only as defined in the claims. ~
','"' ' ' '~ ' - ',~'.'' li'.' ' ,;''~
Claims (51)
1. A hybrid analog-digital method of operating a television signal transmission system comprising: developing a first analog signal comprising predominantly high frequency components of a baseband video signal, developing a digitally coded representation of a second signal comprising predominantly low frequency components of said baseband video signal, and transmitting said first analog signal and said digitally coded representation.
2. The method of claim 1, further including the step of incorporating said digitally coded representation as data in said first analog signal.
3. The method of claim 2 where said second signal is representative of the line averaged value of the active video portion of each horizontal line in said baseband video signal and wherein said first analog signal is the difference between said second signal and said baseband video signal.
4. The method of claim 3, wherein said baseband video signal includes non-active video portions and wherein said data is included in said non-active video portions.
5. The method of claim 4, further including the steps of generating digital values corresponding to the line averaged values of each of said horizontal lines to form said data, and developing said first analog signal by converting said digital values into analog signals that are algebraically combined with said baseband video signal.
6. The method of claim 5, wherein said digital values have a predetermined resolution that leaves a relatively small residue of said low frequency components in said first analog signal.
7. A method of operating a television signal transmission system comprising the steps of: developing a predominately high frequency components video signal from a composite baseband video signal by subtracting a signal indicative of the low frequency components of said composite baseband video signal, developing a digitally coded representation of said low frequency components indicative signal, and transmitting said high frequency components video signal and said digitally coded representation.
8. The method of claim 7 wherein said low frequency components indicative signal comprises the average value of the active video portion of each horizontal line of said composite baseband video signal and wherein said digitally coded representation is in the form of digital data, and further including the step of: incorporating said digital data in non-active video portions of said high frequency components video signal.
9. The method of claim 8 wherein said composite baseband video signal includes horizontal blanking intervals and wherein said digital data i inserted in said horizontal blanking intervals.
10. The method of claim 9, further including the step of delaying said composite baseband video signal by one horizontal line prior to subtracting said low frequency components indicative signal.
11. The method of claim 10 wherein said digital data comprises the digital value of said low frequency components indicative signal, and further including the step of:
generating an analog signal from said digital value and subtracting it from said composite baseband video signal to form said high frequency components video signal.
generating an analog signal from said digital value and subtracting it from said composite baseband video signal to form said high frequency components video signal.
12. A television signal transmission system comprising: means producing a baseband video signal, means developing a low frequency components signal indicate of the line averaged value of active video in said baseband video signal, means combining said low frequency components signal with said baseband video signal to develop a high frequency components signal, means developing a digitally coded representation of said low frequency components signal, and means for modulating a carrier with said high frequency components signal and said digitally coded representation for transmission.
13. The system of claim 12, further including: line integrating means for developing said line averaged value of active video in said baseband video signal, and analog-to-digital converter means coupled to said integrating means for developing said digitally coded representation.
14. The system of claim 13, further including: delay means for delaying said baseband video signal by one horizontal line, and digital-to-analog converter means coupled to said analog-to-digital converter means for developing said low frequency components signal for combining with said baseband video signal.
5. The system of claim 12, wherein said baseband video signal is in digital form, and further including: means for determining the average digital value of the active video portion of each horizontal line of said digital baseband video signal, means for combining said average digital values with said digital baseband video signal to develop a digital high frequency components signal, and means for incorporating said average digital values as data in said digital high frequency components signal.
16. The system of claim 15, further including a digital-to-analog converter for converting said high frequency components digital signal and digital data into an analog signal for transmission.
17. A receiver for receiving a television signal including high frequency components of a video signal and a digitally coded representation of low frequency components of said video signal, comprising: means for generating an additional signal from said digitally coded representation, and means for combining said high frequency components with said additional signal to reconstruct said video signal.
18. The receiver of claim 17, wherein said digitally coded representation is of limited resolution and wherein said additional signal has the same limited resolution.
19. The receiver of claim 18 further including digital-to-analog converter means for converting said digitally coded representation into said additional signal.
20. The receiver of claim 19 wherein said television signal includes a reference signal, and further including means for detecting said reference signal and controlling the gain of said receiver as a function thereof.
21. A television signal transmission method comprising: developing a first signal representing the frequency components of the television signal below the horizontal scanning frequency thereof, developing a second signal representing the remaining frequency components of the television signal, and transmitting the first and second signals in different formats, the format for the first signal comprising a coded relatively low power utilization format including a digitally coded representation of said low frequency components signal.
22. A television signal transmission method comprising: developing first and second signals, respectively comprising frequency components of the television signal utilizing relatively low and high transmission power levels, digitally encoding the second signal for transmission in a relatively low power utilization format, and transmitting the first and digitally coded second signals.
23. A television signal transmission method comprising: developing a first signal representing frequency components of the television signal having a relatively low picture detail content and a relatively high transmission power demand, developing a second signal representing frequency components of the television signal having a relatively high picture detail content and a relatively low transmission power demand, digitally coding the first signal for transmission in a relatively low power utilization format, and transmitting the second and digitally coded first signals.
24. A television transmission and reception system comprising: at a transmitter: means for developing a first signal comprising predominantly high frequency components of a baseband video signal, means for developing from a second signal comprising predominantly low frequency components of said baseband video signal a limited resolution digitally coded representation thereof and means for transmitting said first signal and said digitally coded representation, at a receiver:
means for receiving said first signal and said limited resolution digitally coded representation, means for regenerating said second signal from said limited resolution digitally coded representation, and means for combining said first signal and said second signal to reconstruct said baseband video signal.
means for receiving said first signal and said limited resolution digitally coded representation, means for regenerating said second signal from said limited resolution digitally coded representation, and means for combining said first signal and said second signal to reconstruct said baseband video signal.
25. The system of claim 24, wherein said limited resolution digitally coded representation is a digital value of a given number of bits.
26. The system of claim 25, wherein said second signal represents the line averaged value of the active video of each horizontal line in said baseband video signal and wherein said first signal is the difference between said baseband video signal and said second signal.
27. A method of operating a television signal transmission system comprising the steps of: developing a first signal comprising predominantly high frequency components of a baseband video signal, developing a digitally coded representation of a second signal comprising predominantly low frequency components of said baseband video signal such that the power required to transmit the coded low frequency components of said baseband video signal is very substantially less than the power required to transmit the same frequencies in uncoded form and, transmitting said first signal and said digitally coded representation.
28. A method of operating a television signal transmission system comprising the steps of: developing a first signal comprising predominantly high frequency components of a baseband video signal, developing a digitally coded representation of a second signal comprising predominantly low frequency components of said baseband video signal, and transmitting said first signal and said digitally coded representation such that said digitally coded representation is transmitted with a small fraction of the total transmitted signal power.
29. A hybrid analog-digital television signal transmission and reception method comprising: developing a first analog signal comprising a first band of frequency components of a baseband video signal, developing a digitally coded representation of a second signal comprising a second band of frequency components of said baseband video signal, transmitting said first analog signal and said digitally coded representation of said second signal, receiving said first analog signal and said digitally coded representation of said second signal, and regenerating said second signal and combining the resulting signal with said first analog signal.
30. The method of claim 29, further including the step of incorporating said digitally coded representation as data in said first analog signal.
31. The method of claim 30, wherein said second signal is representative of the line averaged value of the active video portion of each horizontal line in said baseband video signal and wherein said first analog signal is the difference between said second signal and said baseband video signal.
32. The method of claim 31, wherein said baseband video signal includes non-active video portions and wherein said data is included in said non-active video portions.
33. The method of claim 29, further including generating digital values corresponding to the line averaged values of each of said horizontal lines to form said data, and developing said first analog signal by converting said digital values into analog signals that are algebraically combined with said baseband video signal.
34. The method of claim 33, wherein said digital values have a predetermined resolution that leaves a relatively small residue of said low frequency components in said first analog signal.
35. A hybrid analog-digital receiver for receiving a television signal including analog high frequency components of a video signal and a digitally coded representation of low frequency components of said video signal, comprising: means for generating an additional signal from said digitally coded representation, and means for combining said analog high frequency components with said additional signal to reconstruct said video signal.
36. The receiver of claim 35 wherein said digitally coded representation is of limited resolution and wherein said additional signal has the same limited resolution.
37. The receiver of claim 35, further including digital-to-analog converter means for converting said digitally coded representation into said additional signal.
38. The receiver of claim 37, wherein said television signal includes a reference signal, and further including means for detecting said reference signal and controlling the gain of said receiver as a function thereof.
39. A television signal transmission and reception method comprising: developing a first signal representing higher frequency components of a television signal, developing a second signal representing low frequency components of the television signal below the horizontal scanning frequency thereof, transmitting the first and second signals in different formats the format for the second signal comprising a coded relatively low power utilization format including a digitally coded representation of said low frequency components, receiving said first and second signals, and decoding said digitally coded representation and combining the result with said first signal.
40. A television signal transmission and reception method comprising: developing analog first and digital second signals, respectively comprising frequency components of the television signal utilizing the relatively low and high transmission power levels, digitally coding the second signal for transmission in a relatively low power utilization format, transmitting the first and digitally coded second signals, receiving said first and second signals, and decoding said digitally coded representation and combining the result with said first signal.
41. A hybrid analog-digital television signal transmission and reception method comprising: developing a first signal representing frequency components of the television signal having a relatively low picture detail content and a relatively high transmission power demand, developing a second analog signal representing frequency components of the television signal having a relatively high picture detail content and a relatively low transmission power demand, digitally coding the first signal for transmission in a relatively low power utilization format, transmitting the analog second and digitally coded first signals, receiving said first and second signals, and decoding said first signal and combining the result with said second signal.
42. A method of processing a television signal for transmission at drastically reduced power levels, comprising:
removing from the television signal at least a portion of the energy in selected frequency components of the signal which represent low picture detail, said selected frequency components constituting a small part of the overall television signal bandwidth, but containing a majority of the overall television signal energy, developing a digital signal representing a digital encoding of the selected frequency components, and developing an analog second signal representing other frequency components of the television signal which represent high picture detail.
removing from the television signal at least a portion of the energy in selected frequency components of the signal which represent low picture detail, said selected frequency components constituting a small part of the overall television signal bandwidth, but containing a majority of the overall television signal energy, developing a digital signal representing a digital encoding of the selected frequency components, and developing an analog second signal representing other frequency components of the television signal which represent high picture detail.
43. A hybrid digital-analog method of transmitting and receiving an information signal, comprising: removing from the information signal at least a portion of the energy in selected frequency components of the signal requiring relatively high transmission power, developing a digital signal representing a digital encoding of the selected frequency components, transmitting the information signal in analog form, transmitting the digital signal characterizing the selected frequency components, receiving the analog information signal and the digital signal, and decoding the digital signal and combining it with the analog information signal.
44. A method of transmitting and receiving a television signal at significantly reduced average power levels, comprising: removing from the television signal at least a portion of the energy in selected frequency components of the signal which represent low picture detail, said selected frequency components constituting a small part of the overall television signal bandwidth, but containing a majority of the overall television signal energy and requiring relatively high transmission power, developing a digital signal representing a digital encoding of the selected frequency components, transmitting a second signal representing other frequency components of the television signal which represent high picture detail, transmitting the digital signal characterizing the selected frequency components, receiving the second signal and the digital signal and combining the digital signal with the second signal.
45. The method defined by claim 44, wherein said digital signal is transmitted during inactive portions of the television signal.
46. A method of transmitting and receiving a television signal at drastically reduced average power levels, comprising: removing from the television signal at least a portion of the energy in selected frequency components of the signal which represent low picture detail, said selected frequency component constituting a small part of the overall television signal bandwidth, but containing a majority of the overall television signal energy and normally requiring relatively high transmission power, developing a digital signal representing a digital encoding of the selected frequency components, transmitting a second signal representing other frequency components of the television signal which represent high picture detail, transmitting the digital signal characterizing the selected frequency components at very low average power levels compared to the average power required to transmit said selected frequency components in analog form and to the overall transmitted signal power, receiving the second signal and the digital signal, and combining the digital signal with the second signal.
47. The method described by claim 46, wherein said digital signal is transmitted during inactive portions of the television signal.
48. A hybrid digital-analog method of transmitting and receiving an analog television signal at drastically reduced average power levels, comprising: removing from the television signal at least a portion of the energy in selected frequency components of the signal which represent low picture detail, said selected frequency components constituting a small part of the overall television signal bandwidth, but containing a majority of the overall television signal energy and normally required relatively high transmission power, developing a digital signal representing a digital encoding of the selected frequency components, transmitting in an analog form a second signal representing other frequency components of the television signal which represent high picture detail, transmitting the digital signal characterizing the selected frequency components at very low average power levels compared to the average power required to transmit said selected frequency components in analog form and to the overall transmitted signal power, receiving the analog second signal and the digital signal, and converting the digital signal to analog form and combining it with the analog second signal.
49. A method of processing an analog information signal from which has been removed at least a portion of the energy in selected frequency components requiring relatively high transmission power, the removed selected frequency components being included in the analog information signal as a digitally encoded representation, said method comprising:
receiving the analog information signal and the digitally encoded representation, decoding the digitally encoded representation, and combining the decoded representation with the analog information signal.
receiving the analog information signal and the digitally encoded representation, decoding the digitally encoded representation, and combining the decoded representation with the analog information signal.
50. A method of processing a television signal from which has been removed at least a portion of the energy in selected frequency components representing low picture detail to provide an analog television signal, the selected frequency components constituting a small part of the overall television signal bandwidth, but containing a majority of the overall television signal energy, the removed selected frequency components being included in the analog television signal as a digitally encoded representation, said method comprising:
receiving the analog television signal and the digitally encoded representation of the selected frequency components removed from the television signal, decoding the digitally encoded representation, and combining the representation with the analog television signal.
receiving the analog television signal and the digitally encoded representation of the selected frequency components removed from the television signal, decoding the digitally encoded representation, and combining the representation with the analog television signal.
51. The method defined by claim 50, wherein said digitally encoded representation occupies inactive portions of the television signal.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US17689388A | 1988-04-04 | 1988-04-04 | |
| US176,893 | 1988-04-04 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1331401C true CA1331401C (en) | 1994-08-09 |
Family
ID=22646318
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA 595635 Expired - Fee Related CA1331401C (en) | 1988-04-04 | 1989-04-04 | Tv signal transmission system and method |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1331401C (en) |
-
1989
- 1989-04-04 CA CA 595635 patent/CA1331401C/en not_active Expired - Fee Related
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