CA2094381A1 - Decoding of composite television signals - Google Patents
Decoding of composite television signalsInfo
- Publication number
- CA2094381A1 CA2094381A1 CA002094381A CA2094381A CA2094381A1 CA 2094381 A1 CA2094381 A1 CA 2094381A1 CA 002094381 A CA002094381 A CA 002094381A CA 2094381 A CA2094381 A CA 2094381A CA 2094381 A1 CA2094381 A1 CA 2094381A1
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- Canada
- Prior art keywords
- signal
- test signal
- correction
- derived
- level
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Abandoned
Links
- 239000002131 composite material Substances 0.000 title claims abstract description 21
- 238000012937 correction Methods 0.000 claims abstract description 84
- 238000012360 testing method Methods 0.000 claims abstract description 59
- 238000005259 measurement Methods 0.000 claims abstract description 14
- 230000000694 effects Effects 0.000 claims abstract description 12
- 238000000034 method Methods 0.000 claims description 21
- 238000012545 processing Methods 0.000 claims description 5
- 230000015654 memory Effects 0.000 abstract description 9
- 230000005540 biological transmission Effects 0.000 description 3
- 238000010586 diagram Methods 0.000 description 3
- 238000012935 Averaging Methods 0.000 description 2
- 238000013461 design Methods 0.000 description 2
- 238000009795 derivation Methods 0.000 description 1
- 230000000737 periodic effect Effects 0.000 description 1
- 230000004044 response Effects 0.000 description 1
- 230000035945 sensitivity Effects 0.000 description 1
- 230000003068 static effect Effects 0.000 description 1
Classifications
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
- H04N9/00—Details of colour television systems
- H04N9/64—Circuits for processing colour signals
- H04N9/646—Circuits for processing colour signals for image enhancement, e.g. vertical detail restoration, cross-colour elimination, contour correction, chrominance trapping filters
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
- H04N17/00—Diagnosis, testing or measuring for television systems or their details
- H04N17/02—Diagnosis, testing or measuring for television systems or their details for colour television signals
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
- H04N5/00—Details of television systems
- H04N5/14—Picture signal circuitry for video frequency region
- H04N5/20—Circuitry for controlling amplitude response
- H04N5/205—Circuitry for controlling amplitude response for correcting amplitude versus frequency characteristic
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
- H04N5/00—Details of television systems
- H04N5/14—Picture signal circuitry for video frequency region
- H04N5/21—Circuitry for suppressing or minimising disturbance, e.g. moiré or halo
- H04N5/211—Ghost signal cancellation
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
- H04N9/00—Details of colour television systems
- H04N9/64—Circuits for processing colour signals
Landscapes
- Engineering & Computer Science (AREA)
- Multimedia (AREA)
- Signal Processing (AREA)
- Health & Medical Sciences (AREA)
- Biomedical Technology (AREA)
- General Health & Medical Sciences (AREA)
- Testing, Inspecting, Measuring Of Stereoscopic Televisions And Televisions (AREA)
- Television Systems (AREA)
- Color Television Systems (AREA)
Abstract
A broadcast system is provided in which the effects of differential distortions on composite television signals are reduced. At predetermined intervals, the broadcast signal includes a test signal having high frequency components at least three average video signal levels. In a suitably equipped receiver, following demodulation by the receivers vision demodulator (20), a test signal is identified by a test signal detector (34). Measurement apparatus (24) then measures the distortion of the test signal at a number of different average video signal levels, calculates the correction required to minimise the effect of distortion at each measured signal level, and stores the corrections in a memory (28) at address locations derived from the demodulated signal by a filter circuit (32). The stored corrections are applied to an interpolator (40) which derives further corrections for other average video signal levels, the derived corrections being applied to subsequently received television signals by a controllable equaliser (38).
Description
. ~vo 93/04563 2 0 9 ~ 3 81 PCT/CB92/01532 DECODING OF COMPOSITE TELEYISION SIGNALS
Technical Field:
The present invention relates to the field of composite television broadcasting systems and in particular to a method and apparatus for reducing the effect of differential distortions on demodulated broadcast video signals.
Back~round Art:
It is a known problem of PAL, NTSC and other similar composite tele~ision broadcasting systems that the broadcast signal is often subject to amplitude and phase distortions.
These distortions are usually introduced to the signal as a result of modulation of the signal onto a high frequency c~rrier signal prior to transmission and demodulation of the transmitted signal at the receiver. Several factors contributing to the distortion of the video signal are influenced by the mean level of the modulated signal carrier with the result that, when the video signal is demodulated, the distortions become differential distortions which vary with the mean level of the signal.
While these distortions are allowed for in the design of known composite television signal receivers, by the use of delay-line decoders and a saturation control in PAL system receivers and by the use of hue and saturation controls in ~TSC
receivers, these measures allow only an average correction to be applied. In order to improve the quality of signal that can be delivered by~e~isting-terrestrial television systems and by new television systeims which use the established.composite coded signal methods, improved receivers employing more accurate distortion correction methods are required.
... . . . . . . . . . . - . .
- - . .. - . ........ . .. . . . . ....... .
Disclosure of Invention~
; '' In accordance with the present-invention, there is provided a method~for reducing the effect of differential ; distortion on demodulated composite television signals, comprising the steps of:
. .-.... , . - . . ..
.. . - .
.. - . . . . - . . .
., . ~ . -~ -. : - - : . ~
~ .
broadcasting a test signal at predetermined intervals, the test signal including a component at at least one average video signal level;
receiving the test signal at a receiver, the receiver being adapted to carry out the steps of:
measuring the distortion of the video signal at the said at least one average video signal level;
deriving the required correction at that signal level; and applying the said derived required correction to subsequently received broadcast video signals at that signal level.
The present invention also provides a test signal for use in the above method, comprising one line of a composite television signal having an identifying portion to identify it as a test signal and having high frequency components at at least three average video signal levels. , The test signal may be transmitted once per frame or over r-a longer period with successive test signals being spaced by several seconds. , , The method of the present invention may include the step of measuring the amplitude and phase distortion for a range of average video signal levels of the test signal and deriving a correction to be applied to subsequently received broadcast signals for each such distortion at each of the measured average video signal levels. Thus, if the levels of demodulated ' chrominance are significantly reduced at high average signal levels (i.e. close to peak white) by distortion of the signal during modulation, transmission and demodulation, the effect of these distortions mayibe corrected by increasing the chrominance gain when the signal level is high. These corrections are ;
preferably applied.to the signal,in ,its com,posite form to allow , the level of high-frequency luminance (picture detail) to be corrected also. ,,The correction may be derived and applied at a number OI different frequencies to improve the accuracy of the luminance correction. ,~
. ,., ,The derived,corrections may be interpolated or extrapolated to derive the correction to,be applied at average ,,,~,video signal levels other than the signal levels of the test signal components. , , -' ' ' ' ' 2 0 9 4 3 8 1 ! .
W O 93/0~563 PCT/GB92/01532 Preferably, the corrections are applied to subsequentlyreceived broadcast signals at closely spaces signal levels to give a smoothly varying correction which adds no visible distortions to the broadcast signal.
Also in accordance with the present invention there is ' provided a receiver adapted to receive and demodulate a composite television signal, including: -' a test signal detector operable to detect a test signal broadcast at predetermined intervals;
measurement apparatus adapted to determine the level of differential amplitude and/or phase distortion of the received ;
test signal at at least one average video signal level and to derive the correction required to reduce the distortion at that signal level or levels;
.. . .
storage means connected to receive the derived required -correction or corrections; and ,L~
- signal processing apparatus adapted to apply the required correction or corrections to subsequently received television signals at the average video signal level or levels for which the correction or corrections was derived.
The receiver operates to derive and apply corrections in the way described above. The apparatus may be implemented in analogue or digital'circuitry or, where the receiver includes a high speed computer, certain of the apparatus operations may be ' implemented in software.
' ' '' ~ The present invention provides a broadcast'system, including a transmitter configured to modulate and transmit a ' composite'television'signal'including t~e test signal, and at least one receiver'of the type described above. In such a :
' ' broadcast system differential distortion of the broadcast signals ' ., . - . -, . - . . . ~ . . . . .
' may be corrected more accurately than by the average correction ' methods currently known and used. ' ~, ' ~rief DescriPtion of Drawinqs: ' .. . , .................... .... . , .................... .
One particular preferred embodiment of the present ' invention will now be described, by way of e~ample only, with reference to the accompanying drawings in which:
Figure 1 is a general schematic diagram of the correction : ~ . - . . . - . .:
.
W 0 93/04563 43 8 ~ PCT/G892/OlS32 derivation and application stages of the receiver of the present invention;
Figure 2 shows two suitable forms of the test signal of the present invention; and Figure 3 is a schematic diagram showing the receiver stages of Figure 1 implemented in digital circuitry.
Modes for CarrYinq Out the Invention:
Referring initially to Figure 2, two suitable forms of test signals according to the preser.t invention are shown. Both signals, employed to correct chrominance errors, are in the form of a broadcast line signal including high frequency components ; which may be measured at at least three different average video signal levels. The first signal lOA shown in Figure 2A, is in the form of a television line ramp and the second lOB, in Figure 2B, is in the form of a television line staircase. Both signals shown are ausmented by a subcarrier.
i; Signals of this type are easy to detect, being identified by a preceding chrominance burst 12 of at least 20 microseconds duration.
The circuitry measuring the test signal distortion and deriving the corrections required to minimise its effect includes ~ -storage for the corrections to allow them to be applied to successive received television broadcast signals.~ It is therefore the case that the test signal, which sets up the stored corrections, need ~e transmitted only sufficiently often to ensure that there is no appreciable delay in obtaining a good picture after the viewer has changed channel or after a change in received signal quality as a result of changes to the signal path for distribution, transmission or reception.~ Accordingly, whilst the test signal may be transmitted once per frame, lower repetition frequencies such as once per second or once per ten seconds may be used.
The receiver circuitry which derives and applies the ' required correction of amplitude and phase distortion affecting one signal frequency (colour subcarrier) at different average video signal levels is shown in Figure 1.
The television broadcast signal, including at periodic . - : .
, ~VO 93/0~563 2 0 9 ~ 3 81 PCT/G892/01532 intervals the test signal, is received and demodulated by the receivers vision demodulator 20. The demodulated signal 22 is applied to measurement apparatus 24 which measures the difference between the chrominance level of the received (distorted) signal and an expected chrominance level for the transmitted test signal and derives a correction which will minimise the effect of the distortion. The derived corrections 26 are then stored in a memory 28 at address locations 30 determined by the average luminance level of the demodulated signal 22. The average luminance level, and hence the memory address locations 30, are derived by passing the demodulated signal 22 through a suitable low-pass filter arrangement 32. -To ensure that only corrections derived from a receivedtest signal are stored in the memory 28, a test signal detector 34 is provided. ~ When a test signal is received, identified as- - -such by the preceding 20 us chrominance burst 12 (Figure 2), the detector ~4 generates a writ~F~-enable signal 36 which allows derived corrections to be written into the memory 28 for the duration of the test signal.
The stored corrections are applied to subse~uently received television broadcast signals by a controllable equaliser 38. Control of the equaliser 38 may be achieved by a number of established methods.
The stored corrections are passed through an interpolator 40 prior to application by the ëqualiser 38. The interpolator ' -40 derives further required corrections to be applied at average video signal levels other than those fo,r which the measurement apparatus 24 measured the distortion'of the test signal. Use of ¦
the interpolator 40 allows the corrections to be applied at closely spaced average video signal levels to provide a smoothly varying correction which àdds no'visible additional distortions to the video signal. -- ~''`" A more'detaiied-implementation of'the receiver circuitry is shown in Figure 3. In the circuitry shown, the distortion is measured and the corrections derived from the test signal of Figure 2A (the television line ramp). The output from the receivers vision demodulator 20 is digitised by an analogue to digital converter 42: this allows the subsequent measurement and 93/04563 20~38~ PCl`tGB92/01532.
correction stages to be defined in terms of known digital operations although it will be appreciated that some of these stages could be implemented in analogue circuitry.
The digitised output from the analogue to digital converter g2 is applied to a chrominance demodulator 44. The chrominance demodulator 44, which could be the normal receiver demodulator, is shown in Figure 3 as a separate demodulator for the sake of clarity.
The demodulated chrominance components U,V are applied to the measurement apparatus 24. The amplitude of the demodulated signal is calculated, in a PROM (programmable read-only memory) 24A, as the root of the sum of the squares of the two demodulated colour components U,V. The phase of the demodulated signal is also calculated in a PROM 24P as the inverse tan ratio of the two demodulated colour components U,V. These two measurements 26A, 26P which define the corrections to be applied to a subsequently received broadcast signal are then stored in amplitude and phase correction memories 28A, 28P respectively.
As described previously, the corrections are stored at memory address locations 30 derived from the average luminance level of the demodulated signal by a low pass filter 32. As also described, these corrections may only be stored when a write-enable signal 36 from a test signal detector 34 indicates the presence of a test signal.
Following interpolation by respective amplitude and phase interpolators 40A, 40P the corrections are applied to a ~ , .. . .. . . . . .
controllable egualiser 38. The equaliser 38 is an adjustable transversal equaliser having a signal input 50 for the demodulated broadcasL video signal and separate symmetric 52 and - - - .. ..
anti-symmetric 54 control inputs. The amplitude correction signal is applied to the symmetric control input 52: amplitude correction is achieved by varying the size of the signal I -coefficients symmetrically about the centre of the filter. The phase correction signal i5 applied to the anti-sy~metric control input 54: phase correction is achieved by varying the size of the -~: signal coefficients anti-symmetrically about the centre of the ¦
filter.
In order to reduce the sensitivity of the circuitry to ., . I
. .
.: : '' ; , . ~ ~
W O 93/045~3 2 0 9 ~ ~ ~ 1 PCT/C~/0153~ ~
disturbances which may affect the received test signal, averaging circuits (not shown) may be provided around the correction memories 28A, 28P. Such averaging circuits may average successive test signals prior to measurement of distortion or may average successively derived corrections for a given average video signal level.
It is envisaged that new designs of broadcast receiver, which include means for improving the quality of the received television signal, will include a wide range of sophisticated circuits. The present invention provides such means for improving the quality without requiring a significant increase in receiver circuitry.
It will be appreciated that, where the receiver apparatus includes high-speed computing capability, the method of the present invention may be impiemented in software. In such a case, the schematic diagrams of figures l and 3 may be regarded as flow charts representing the steps of a software decoding and correction process.
The decoding and correction circuitry of the present invention may also include means for correcting static losses within the receiver to compensate for multipath effects, receiver ; and/or transmitter responses'and the''low'frequency effects of mistuning. ' ' ' .. .- . . . ,~ . .,. :. ... ..
~'' ' ' , . . .
....... . .
.i .
,,, ~.. . , .' ' ' - . - - . .
.. j;..
.
,. ~, . . . . , . . . .. ' .. ... .. .. _ _ .. _ .. ,_ ~,, .. . .. .
.' ' '"'",.'", . ~
Technical Field:
The present invention relates to the field of composite television broadcasting systems and in particular to a method and apparatus for reducing the effect of differential distortions on demodulated broadcast video signals.
Back~round Art:
It is a known problem of PAL, NTSC and other similar composite tele~ision broadcasting systems that the broadcast signal is often subject to amplitude and phase distortions.
These distortions are usually introduced to the signal as a result of modulation of the signal onto a high frequency c~rrier signal prior to transmission and demodulation of the transmitted signal at the receiver. Several factors contributing to the distortion of the video signal are influenced by the mean level of the modulated signal carrier with the result that, when the video signal is demodulated, the distortions become differential distortions which vary with the mean level of the signal.
While these distortions are allowed for in the design of known composite television signal receivers, by the use of delay-line decoders and a saturation control in PAL system receivers and by the use of hue and saturation controls in ~TSC
receivers, these measures allow only an average correction to be applied. In order to improve the quality of signal that can be delivered by~e~isting-terrestrial television systems and by new television systeims which use the established.composite coded signal methods, improved receivers employing more accurate distortion correction methods are required.
... . . . . . . . . . . - . .
- - . .. - . ........ . .. . . . . ....... .
Disclosure of Invention~
; '' In accordance with the present-invention, there is provided a method~for reducing the effect of differential ; distortion on demodulated composite television signals, comprising the steps of:
. .-.... , . - . . ..
.. . - .
.. - . . . . - . . .
., . ~ . -~ -. : - - : . ~
~ .
broadcasting a test signal at predetermined intervals, the test signal including a component at at least one average video signal level;
receiving the test signal at a receiver, the receiver being adapted to carry out the steps of:
measuring the distortion of the video signal at the said at least one average video signal level;
deriving the required correction at that signal level; and applying the said derived required correction to subsequently received broadcast video signals at that signal level.
The present invention also provides a test signal for use in the above method, comprising one line of a composite television signal having an identifying portion to identify it as a test signal and having high frequency components at at least three average video signal levels. , The test signal may be transmitted once per frame or over r-a longer period with successive test signals being spaced by several seconds. , , The method of the present invention may include the step of measuring the amplitude and phase distortion for a range of average video signal levels of the test signal and deriving a correction to be applied to subsequently received broadcast signals for each such distortion at each of the measured average video signal levels. Thus, if the levels of demodulated ' chrominance are significantly reduced at high average signal levels (i.e. close to peak white) by distortion of the signal during modulation, transmission and demodulation, the effect of these distortions mayibe corrected by increasing the chrominance gain when the signal level is high. These corrections are ;
preferably applied.to the signal,in ,its com,posite form to allow , the level of high-frequency luminance (picture detail) to be corrected also. ,,The correction may be derived and applied at a number OI different frequencies to improve the accuracy of the luminance correction. ,~
. ,., ,The derived,corrections may be interpolated or extrapolated to derive the correction to,be applied at average ,,,~,video signal levels other than the signal levels of the test signal components. , , -' ' ' ' ' 2 0 9 4 3 8 1 ! .
W O 93/0~563 PCT/GB92/01532 Preferably, the corrections are applied to subsequentlyreceived broadcast signals at closely spaces signal levels to give a smoothly varying correction which adds no visible distortions to the broadcast signal.
Also in accordance with the present invention there is ' provided a receiver adapted to receive and demodulate a composite television signal, including: -' a test signal detector operable to detect a test signal broadcast at predetermined intervals;
measurement apparatus adapted to determine the level of differential amplitude and/or phase distortion of the received ;
test signal at at least one average video signal level and to derive the correction required to reduce the distortion at that signal level or levels;
.. . .
storage means connected to receive the derived required -correction or corrections; and ,L~
- signal processing apparatus adapted to apply the required correction or corrections to subsequently received television signals at the average video signal level or levels for which the correction or corrections was derived.
The receiver operates to derive and apply corrections in the way described above. The apparatus may be implemented in analogue or digital'circuitry or, where the receiver includes a high speed computer, certain of the apparatus operations may be ' implemented in software.
' ' '' ~ The present invention provides a broadcast'system, including a transmitter configured to modulate and transmit a ' composite'television'signal'including t~e test signal, and at least one receiver'of the type described above. In such a :
' ' broadcast system differential distortion of the broadcast signals ' ., . - . -, . - . . . ~ . . . . .
' may be corrected more accurately than by the average correction ' methods currently known and used. ' ~, ' ~rief DescriPtion of Drawinqs: ' .. . , .................... .... . , .................... .
One particular preferred embodiment of the present ' invention will now be described, by way of e~ample only, with reference to the accompanying drawings in which:
Figure 1 is a general schematic diagram of the correction : ~ . - . . . - . .:
.
W 0 93/04563 43 8 ~ PCT/G892/OlS32 derivation and application stages of the receiver of the present invention;
Figure 2 shows two suitable forms of the test signal of the present invention; and Figure 3 is a schematic diagram showing the receiver stages of Figure 1 implemented in digital circuitry.
Modes for CarrYinq Out the Invention:
Referring initially to Figure 2, two suitable forms of test signals according to the preser.t invention are shown. Both signals, employed to correct chrominance errors, are in the form of a broadcast line signal including high frequency components ; which may be measured at at least three different average video signal levels. The first signal lOA shown in Figure 2A, is in the form of a television line ramp and the second lOB, in Figure 2B, is in the form of a television line staircase. Both signals shown are ausmented by a subcarrier.
i; Signals of this type are easy to detect, being identified by a preceding chrominance burst 12 of at least 20 microseconds duration.
The circuitry measuring the test signal distortion and deriving the corrections required to minimise its effect includes ~ -storage for the corrections to allow them to be applied to successive received television broadcast signals.~ It is therefore the case that the test signal, which sets up the stored corrections, need ~e transmitted only sufficiently often to ensure that there is no appreciable delay in obtaining a good picture after the viewer has changed channel or after a change in received signal quality as a result of changes to the signal path for distribution, transmission or reception.~ Accordingly, whilst the test signal may be transmitted once per frame, lower repetition frequencies such as once per second or once per ten seconds may be used.
The receiver circuitry which derives and applies the ' required correction of amplitude and phase distortion affecting one signal frequency (colour subcarrier) at different average video signal levels is shown in Figure 1.
The television broadcast signal, including at periodic . - : .
, ~VO 93/0~563 2 0 9 ~ 3 81 PCT/G892/01532 intervals the test signal, is received and demodulated by the receivers vision demodulator 20. The demodulated signal 22 is applied to measurement apparatus 24 which measures the difference between the chrominance level of the received (distorted) signal and an expected chrominance level for the transmitted test signal and derives a correction which will minimise the effect of the distortion. The derived corrections 26 are then stored in a memory 28 at address locations 30 determined by the average luminance level of the demodulated signal 22. The average luminance level, and hence the memory address locations 30, are derived by passing the demodulated signal 22 through a suitable low-pass filter arrangement 32. -To ensure that only corrections derived from a receivedtest signal are stored in the memory 28, a test signal detector 34 is provided. ~ When a test signal is received, identified as- - -such by the preceding 20 us chrominance burst 12 (Figure 2), the detector ~4 generates a writ~F~-enable signal 36 which allows derived corrections to be written into the memory 28 for the duration of the test signal.
The stored corrections are applied to subse~uently received television broadcast signals by a controllable equaliser 38. Control of the equaliser 38 may be achieved by a number of established methods.
The stored corrections are passed through an interpolator 40 prior to application by the ëqualiser 38. The interpolator ' -40 derives further required corrections to be applied at average video signal levels other than those fo,r which the measurement apparatus 24 measured the distortion'of the test signal. Use of ¦
the interpolator 40 allows the corrections to be applied at closely spaced average video signal levels to provide a smoothly varying correction which àdds no'visible additional distortions to the video signal. -- ~''`" A more'detaiied-implementation of'the receiver circuitry is shown in Figure 3. In the circuitry shown, the distortion is measured and the corrections derived from the test signal of Figure 2A (the television line ramp). The output from the receivers vision demodulator 20 is digitised by an analogue to digital converter 42: this allows the subsequent measurement and 93/04563 20~38~ PCl`tGB92/01532.
correction stages to be defined in terms of known digital operations although it will be appreciated that some of these stages could be implemented in analogue circuitry.
The digitised output from the analogue to digital converter g2 is applied to a chrominance demodulator 44. The chrominance demodulator 44, which could be the normal receiver demodulator, is shown in Figure 3 as a separate demodulator for the sake of clarity.
The demodulated chrominance components U,V are applied to the measurement apparatus 24. The amplitude of the demodulated signal is calculated, in a PROM (programmable read-only memory) 24A, as the root of the sum of the squares of the two demodulated colour components U,V. The phase of the demodulated signal is also calculated in a PROM 24P as the inverse tan ratio of the two demodulated colour components U,V. These two measurements 26A, 26P which define the corrections to be applied to a subsequently received broadcast signal are then stored in amplitude and phase correction memories 28A, 28P respectively.
As described previously, the corrections are stored at memory address locations 30 derived from the average luminance level of the demodulated signal by a low pass filter 32. As also described, these corrections may only be stored when a write-enable signal 36 from a test signal detector 34 indicates the presence of a test signal.
Following interpolation by respective amplitude and phase interpolators 40A, 40P the corrections are applied to a ~ , .. . .. . . . . .
controllable egualiser 38. The equaliser 38 is an adjustable transversal equaliser having a signal input 50 for the demodulated broadcasL video signal and separate symmetric 52 and - - - .. ..
anti-symmetric 54 control inputs. The amplitude correction signal is applied to the symmetric control input 52: amplitude correction is achieved by varying the size of the signal I -coefficients symmetrically about the centre of the filter. The phase correction signal i5 applied to the anti-sy~metric control input 54: phase correction is achieved by varying the size of the -~: signal coefficients anti-symmetrically about the centre of the ¦
filter.
In order to reduce the sensitivity of the circuitry to ., . I
. .
.: : '' ; , . ~ ~
W O 93/045~3 2 0 9 ~ ~ ~ 1 PCT/C~/0153~ ~
disturbances which may affect the received test signal, averaging circuits (not shown) may be provided around the correction memories 28A, 28P. Such averaging circuits may average successive test signals prior to measurement of distortion or may average successively derived corrections for a given average video signal level.
It is envisaged that new designs of broadcast receiver, which include means for improving the quality of the received television signal, will include a wide range of sophisticated circuits. The present invention provides such means for improving the quality without requiring a significant increase in receiver circuitry.
It will be appreciated that, where the receiver apparatus includes high-speed computing capability, the method of the present invention may be impiemented in software. In such a case, the schematic diagrams of figures l and 3 may be regarded as flow charts representing the steps of a software decoding and correction process.
The decoding and correction circuitry of the present invention may also include means for correcting static losses within the receiver to compensate for multipath effects, receiver ; and/or transmitter responses'and the''low'frequency effects of mistuning. ' ' ' .. .- . . . ,~ . .,. :. ... ..
~'' ' ' , . . .
....... . .
.i .
,,, ~.. . , .' ' ' - . - - . .
.. j;..
.
,. ~, . . . . , . . . .. ' .. ... .. .. _ _ .. _ .. ,_ ~,, .. . .. .
.' ' '"'",.'", . ~
Claims (25)
1. A method for reducing the effect of differential distortion on demodulated composite television signals, comprising the steps of:
broadcasting a test signal at predetermined intervals, the test signal including a component at at least one average video signal level;
receiving the test signal at a receiver, the receiver being adapted to carry out the steps of:
measuring the distortion of the video signal at the said at least one average video signal level;
deriving the required correction at that signal level; and applying the said derived required correction to subsequently received broadcast video signals at that signal level.
broadcasting a test signal at predetermined intervals, the test signal including a component at at least one average video signal level;
receiving the test signal at a receiver, the receiver being adapted to carry out the steps of:
measuring the distortion of the video signal at the said at least one average video signal level;
deriving the required correction at that signal level; and applying the said derived required correction to subsequently received broadcast video signals at that signal level.
2. A method according to claim 1, in which the test signal includes components at at least three average video signal levels and in which a required correction is derived for each such level.
3. A method according to claim 2, in which measured values for distortion of the test signal are interpolated or extrapolated to derive the required correction at signal levels other than the average video signal levels of the measured test signal components.
4. A method according to claim 1, in which the required correction derived and applied includes correction of phase and amplitude distortions present in the demodulated television signal.
5. A method according to claim 1, in which the required correction is derived and applied at a number of different frequencies.
6. A method according to claim 1, in which successive test signals are stored and averaged with the preceding test signal and the required correction is derived from the measured distortion of the averaged test signal.
7. A method according to claim 1, in which successive derived corrections for a given average video signal level are averaged and the correction subsequently applied to broadcast video signals is the averaged correction for that average video signal level.
8. A method according to claim 1, in which the or each required correction is applied to the video signal in its composite form.
9. A method according to claim 2, in which the average video signal levels at which required corrections are applied are closely spaced such that no visible additional distortions are added to the signal.
10. A test signal for use in a method according to claim 1, comprising one line of a composite television signal having an identifying portion to identify it as a test signal and having high frequency components at at least three average video signal levels.
11. A test signal according to claim 10, including a chrominance burst preceding the said high frequency components thereof.
12. A test signal according to claim 11, in which the chrominance burst has a duration of at least 20 microseconds.
13. A composite television signal including, at predetermined intervals, a test signal according to claim 10.
14. A receiver adapted to receive and demodulate a composite television signal, including:
a test signal detector operable to detect a test signal broadcast at predetermined intervals;
measurement apparatus adapted to determine the level of differential amplitude and/or phase distortion of the received test signal at at least one average video signal level and to derive the correction required to reduce the distortion at that signal level or levels;
storage means connected to receive the derived required correction or corrections; and signal processing apparatus adapted to apply the required correction or corrections to subsequently received television signals at the average video signal level or levels for which the correction or corrections was derived.
a test signal detector operable to detect a test signal broadcast at predetermined intervals;
measurement apparatus adapted to determine the level of differential amplitude and/or phase distortion of the received test signal at at least one average video signal level and to derive the correction required to reduce the distortion at that signal level or levels;
storage means connected to receive the derived required correction or corrections; and signal processing apparatus adapted to apply the required correction or corrections to subsequently received television signals at the average video signal level or levels for which the correction or corrections was derived.
15. Apparatus according to claim 14, in which the signal processing apparatus includes an interpolator configured to derive the required correction for average video signal levels other than the at least one average video signal level at which the distortion is determined by the measurement apparatus, the derived required corrections being applied to subsequently received television signals by the signal processing apparatus.
16. Apparatus according to claim 14, in which the measurement apparatus is adapted to determine the level of distortion of the received test signal at at least three average video signal levels and to determine a required correction for each such signal level.
17. Apparatus according to claim 14, including an analogue to digital converter through which the received test signal is passed, in which the measurement apparatus and storage means are implemented in digital circuitry.
18. Apparatus according to claim 17, in which the measurement apparatus comprises a programmable read-only memory device.
19. Apparatus according to claim 17, in which the storage means comprises a memory device in which the derived required correction or corrections are stored at address locations determined by the average luminance level of the received test signal.
20. Apparatus according to claim 19, including at least one filter through which the analogue test signal is passed to derive the luminance level thereof.
21. Apparatus according to claim 14, including further storage means adapted to store a received test signal, to compare a subsequently received test signal with the stored test signal and to derive an average test signal therefrom, the measurement apparatus being adapted to determine the level of distortion of the average test signal.
22. Apparatus according to claim 14, including additional storage means adapted to compare a subsequently derived correction for a given average video signal level with a stored derived correction for that average video signal level, the signal processing apparatus being adapted to apply the averaged derived correction to subsequently received television signals.
23. A receiver adapted to receive and demodulate a composite television signal, including high speed computer circuitry programmed to effect correction of the effects of differential distortion on a demodulated composite television signal in accordance with the method of claim 1.
24. A composite television broadcast transmitter including means for modulating and transmitting a composite television signal according to claim 13.
25. A composite television broadcasting system comprising a transmitter according to claim 24 and at least one receiver according to claim 14.
Applications Claiming Priority (3)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB9118057.0 | 1991-08-21 | ||
| GB9118057A GB2258969A (en) | 1991-08-21 | 1991-08-21 | Decoding of composite television signals |
| PCT/GB1992/001532 WO1993004563A1 (en) | 1991-08-21 | 1992-08-20 | Decoding of composite television signals |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2094381A1 true CA2094381A1 (en) | 1993-02-22 |
Family
ID=10700317
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA002094381A Abandoned CA2094381A1 (en) | 1991-08-21 | 1992-08-20 | Decoding of composite television signals |
Country Status (14)
| Country | Link |
|---|---|
| EP (1) | EP0558704A1 (en) |
| JP (1) | JPH06501832A (en) |
| CN (1) | CN1074073A (en) |
| AU (1) | AU2440292A (en) |
| BG (1) | BG97648A (en) |
| BR (1) | BR9205328A (en) |
| CA (1) | CA2094381A1 (en) |
| CZ (1) | CZ61693A3 (en) |
| FI (1) | FI931798A (en) |
| GB (1) | GB2258969A (en) |
| HU (1) | HUT64668A (en) |
| NO (1) | NO931440L (en) |
| SK (1) | SK34293A3 (en) |
| WO (1) | WO1993004563A1 (en) |
Families Citing this family (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| GB2429868B (en) * | 2005-09-03 | 2010-11-10 | Amulet Electronics Ltd | Video display system |
| JP4968146B2 (en) * | 2008-03-31 | 2012-07-04 | ソニー株式会社 | Broadcast signal receiver, reception control method thereof, and IC |
Family Cites Families (7)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| IT1050574B (en) * | 1975-07-01 | 1981-03-20 | Indesit | IMPROVEMENTS OF COLOR TELEVISION SYSTEMS |
| US4237486A (en) * | 1978-11-09 | 1980-12-02 | Comsonics, Inc. | Compatible transmission of an encoded signal with a television |
| SU1184108A1 (en) * | 1981-11-11 | 1985-10-07 | Nikolaj M Eremin | Meter of parameters of television signal |
| GB2176669A (en) * | 1985-06-05 | 1986-12-31 | Philips Electronic Associated | Digitising television signals |
| US4825379A (en) * | 1986-08-29 | 1989-04-25 | Tektronix, Inc. | Method and apparatus for processing waveform records for jitter elimination prior to averaging in determining signal to noise ratio |
| EP0308776B1 (en) * | 1987-09-25 | 1993-09-01 | Nippon Hoso Kyokai | Decoding equalizer |
| US5136368A (en) * | 1991-01-24 | 1992-08-04 | The Grass Valley Group, Inc. | Television signal decoder with improved architecture |
-
1991
- 1991-08-21 GB GB9118057A patent/GB2258969A/en not_active Withdrawn
-
1992
- 1992-08-20 BR BR9205328A patent/BR9205328A/en not_active Application Discontinuation
- 1992-08-20 WO PCT/GB1992/001532 patent/WO1993004563A1/en not_active Application Discontinuation
- 1992-08-20 HU HU9300945A patent/HUT64668A/en unknown
- 1992-08-20 CN CN92110458.8A patent/CN1074073A/en active Pending
- 1992-08-20 EP EP92917600A patent/EP0558704A1/en not_active Withdrawn
- 1992-08-20 JP JP5504200A patent/JPH06501832A/en active Pending
- 1992-08-20 AU AU24402/92A patent/AU2440292A/en not_active Abandoned
- 1992-08-20 CA CA002094381A patent/CA2094381A1/en not_active Abandoned
- 1992-08-20 SK SK34293A patent/SK34293A3/en unknown
-
1993
- 1993-04-09 CZ CZ93616A patent/CZ61693A3/en unknown
- 1993-04-20 BG BG97648A patent/BG97648A/en unknown
- 1993-04-20 NO NO93931440A patent/NO931440L/en unknown
- 1993-04-21 FI FI931798A patent/FI931798A/en not_active Application Discontinuation
Also Published As
| Publication number | Publication date |
|---|---|
| BR9205328A (en) | 1994-06-21 |
| GB2258969A (en) | 1993-02-24 |
| HUT64668A (en) | 1994-01-28 |
| FI931798A0 (en) | 1993-04-21 |
| AU2440292A (en) | 1993-03-16 |
| EP0558704A1 (en) | 1993-09-08 |
| HU9300945D0 (en) | 1993-06-28 |
| CZ61693A3 (en) | 1993-11-17 |
| NO931440D0 (en) | 1993-04-20 |
| BG97648A (en) | 1994-03-31 |
| CN1074073A (en) | 1993-07-07 |
| WO1993004563A1 (en) | 1993-03-04 |
| SK34293A3 (en) | 1993-07-07 |
| GB9118057D0 (en) | 1991-10-09 |
| NO931440L (en) | 1993-04-20 |
| FI931798A (en) | 1993-04-21 |
| JPH06501832A (en) | 1994-02-24 |
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| FZDE | Discontinued |