GB2108804A

GB2108804A - Applying video component signals to a channel and receiving signals from the channel

Info

Publication number: GB2108804A
Application number: GB08229055A
Authority: GB
Inventors: Glenn Arthur Reitmeier
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1981-10-19
Filing date: 1982-10-12
Publication date: 1983-05-18
Also published as: FR2514978A1; DE3238506A1; JPS5880991A

Abstract

Component video signals such as the PAL color components (U,V) are applied to a vertical bandwidth limiter (10,12,14,16,18,20,22) and are line alternately applied by a switcher (24,26,28) to a channel such as a magnetic recording apparatus (30,32,34). The line alternated and vertical bandwidth limited signals are received from the channel and are applied to respective outputs either without further change or after further processing. <IMAGE>

Description

SPECIFICATION Applying video component signals to a channel and receiving signals from the channel The present invention relates to applying video component signals to a channel and receiving signals from the channel.

In certain recording or transmission channels, it may not be possibie to record or transmit a composite television signal, due to insufficient channel bandwidth, or due to nonlinearities in the response which interfere with the color subcarrier. In such a case, it may be desirable to separately record or transmit the luminance and chrominance information. For purposes of discussion, consider a recorder with a recording channel bandwidth of 2.5 MHz. While a single channel cannot record a composite NTSC signal having 4.2 MHz bandwidth, it would be possible to utilize two channels to separately record and replay luminance and chrominance.

The chrominance components (I and Q) of an NTSC color television signal could thus be recorded in a single recording channel by respectively frequency modulating a pair of carrier signals by said component signals, while the luminance signal is recorded by frequency modulating a single carrier in a second channel.

However, if it is desired to record the color components of a PAL or SECAM signal (U and V or R-Y, B-Y) in a similar manner, the channel bandwidth may not be sufficient since U and V (or R-Y and B-Y) both have bandwidths of 1.5 MHz, as compared to 1.5 MHz and 0.6 MHz for I and Q signals respectively. One way of partialiy overcoming this problem is to line sequentially record alternate color component signals from adjacent lines, each signal having its full bandwidth in the line scan direction, herein referred to as horizontal bandwidth. During playback, the missing components are reconstructed by repeating an adjacent line or by averaging adjacent lines.However, there is still a problem in that the bandwidth of the channel in the direction perpendicular to the line scan direction, (herein referred to as vertical bandwidth) is less than that of the original signal due to the line alternating transmission of a particular component. Thus aliasing in the vertical direction occurs. Thus, due to this aliasing a straight diagonal line will appear as a staircase, which is an objectionable artifact in television systems.

It is therefore desirable to have a transmission system that can transmit a video signal in a narrow bandwidth channel without such aliasing artifacts.

According to one aspect of the invention, there is provided a method or apparatus of applying component video signal to a channel, limiting the vertical bandwidth of at least one of the component video signals and line alternately applying the component signals to the channel.

According to another aspect there is provided a method or apparatus for receiving from a channel signals applied thereto by the method or apparatus of the said one aspect and at least applying the line alternated signals from the channel or signals derived therefrom to respective outputs.

For a better understanding of the invention, and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which: Figure 1 is a block diagram of an illustrative recording system in accordance with the invention; Figure 2 is a block diagram of an illustrative playback system for use with the recording system of Figure 1; Figure 3 is a block diagram of another illustrative recording system in accordance with the invention; and Figure 4 is a block diagram of an illustrative playback system for use with recording system of Figure 3.

Referring to Figure 1, input terminals 10 and 1 Oa respectively receive U and V color component signals from a camera or other video source, such as a PAL decoder which decodes the composite signal into its Y, U and V components.

The U signal is labelled U(n+1) (n being a line number starting with one at the top of the scanning raster) and is applied to a 1 H (one horizontal line period) delay line 1 2 and to a weighting resistor 14. The output signal from delay line 12 is therefore U(n), i.e. the line above line U(n+ 1). Line U(n) is referred to as the current reference line, and is applied to weighting resistor 1 6 and to 1 H delay line 18. The output signal from delay line 18 is therefore U(n-1) and is applied to weighting resistor 20. Resistors 14, 16, and 20 divide the amplitude of the applied signals by 1/4, 1/2, and 1/4 respectively and apply the respective signals to respective inputs of adder 22.Thus the output signal from adder 22 comprises 1/4U(n-1)+1/2 U(n)+1/4 U(n+1),and it is this weighted average which is recorded, rather than the signal U(n) as in conventional line sequential recording or transmission systems.

This weighted average is formed from signal samples displaced in time by multiples of H, and therefore the averages occur in a vertical direction relative to the position of the samples on a raster.

Since the signal is an average in the vertical direction, the maximum rate of change in the vertical direction and therefore the vertical bandwidth is reduced, and hence aliasing caused by having too much information in the vertical direction is eliminated. The output signal from adder 22 is applied to contact 24 of switch 26.

The circuitry for processing the V signal at input terminal 1 Oa is identical to that of the U signal described above, and therefore have been given corresponding reference numerals with an "a" suffix added. Thus 1 H delay lines 1 2a and 1 8a, resistors 1 4a, 1 6a, and 20a, and adder 22a produce at the output of adder 22a the signal 1/4 V(n-1)+1/2 V(n)+1/4 V(n+1), which signal is applied to contact 24a of switch 26.

Switching control generator 28, which generator, as well as the generators described below, can comprise a J-K flip-flop, receives horizontal and vertical synchronization signals and produces at its output a one-half horizontal line rate switching control signal that controls electronic switch 26 as indicated by a dotted line.

Said horizontal synchronization signal controls switch 26 so that during one horizontal line switch 26 provides to FM (frequency modulation) modulator 30 the averaged U output signal of adder 22, while during the next line switch 26 provides the averaged V output signal from adder 22a. Said vertical synchronization signal controls the initial position of switch 26 with respect to the field of frame rate, so that the switching sequence is fully defined with respect to the video signal. FM modulator 30 provides a carrier signal line alternately modulated by said output signals to recording head 32, which head sequentially records the signal on magnetic tape 34. Although shown as linear recording for the sake of simplicity, in a preferred embodiment the tape is wrapped about a drum with head 32 disposed and rotating therein for helical scan recording.

Also the luminance and sound signals are conventionally FM recorded on other tracks. The luminance signal is applied to recording head 1 52 by way of FM modulator 1 50.

Figure 2 shows a block diagram of a playback circuit which can be used with a magnetic tape recorded using the circuit of Figure 1. Recorded tape 34 moves past playback head 36, which provides an output signal to FM demodulator 38 in accordance with the signal recorded on the tape. The demodulated output signal from demodulator 38 will on one line be 1/4U(n-1)+1/2 U(n)+1/4 U(n+1) and is applied to l H delay line 40 and resistor 42. The output signal from delay line 40 comprises the information recorded during the previous line, which is 1/4V(n-2)+1/2 V(n-1)+1/4 (V(n) and is applied to i H delay line 44 and to contacts 50 and 56 of switches 48 and 54 respectively.The output signal from delay line 44 comprises the signal recorded two lines previous to that being presently reproduced by head 36, which is 1/4 U(n-3)+1/2 U(n-2)+1/4 U(n-1) and is applied to resistor 58. divide the amplitudes of the applies signal in half and apply the thus reduced amplitude signals to adder 60. The output signal from adder 60 is therefore 1/8 U(n-3)+1/4 U(n-2)+1/4 (U-1)+1/4 U(n) +1/8 U(n+1),which signal is applied to contacts 46 and 52 of switches 48 and 54 respectively.

Switching control generator 62 receives horizontal and vertical synchronization pulses, preferably from the luminance channel, since this channel has a wider bandwidth than the chrominance channel and therefore better preserves the edges of the pulse. Generator 62 provides the same one-half line rate switching control signal to switches 48 and 54 as was provided to switch 26 by switching control generator 28 in the record circuit of Fig. 1. The control signal is indicated by a dotted line.When switches 48 and 54 are in position shown in Figure 2 during one line, the signal at output terminal 64 comprises the output signal from adder 60, i.e. it is the U signal, which has been reconstituted by further averaging by delay lines 40 and 44, resistors 42 and 58, and adder 60, the two line averaged signals that were recorded, while the signal at output terminal 66 is the output signal from delay line 40, i.e. it is the line averaged V signal. During the next line, the positions of switches 48 and 54 are opposite to that shown in Figure 2. In addition, it will be appreciated that now that output signals from demodulator 38, delay line 44, and adder 60 are V signals, while the output signal from delay line 40 is a U signal.The arithmetic form of the signals are the same as shown in Figure 2 except of course for the fact that the line numbers in parenthesis are increased by one Thus output terminal 64 receives the now U signal from delay line 40 by way of contact 50, while output terminal 66 receives the now V signal from adder 60 by way of contact 52. During the next line the positions of switches 48 and 52 return to the positions shown in Figure 2. Therefore terminals 64 and 66 always provide the U and V signals respectively.

It should be noted that there is an average delay of 1 H during both record and playback for a total delay of 2H. Thus ideally the conventional luminance channel comprising playback head 1 54 and FM demodulator 156 should also have a 2H delay line 1 58 in order to compensate for the delay in chrominance channels. However, in an inexpensive system, this may not be needed.

Since only one chrominance component is recorded during a given line, only a single adder and a single set of three amplitude weighting resistors are needed in the recording circuit of Figure 1. The inputs of the resistors are then switched between two sets of delay lines by three single pole double throw switches controlled by generator 28.

Figures 3 and 4 show a simpler system than that of Figures 1 and 2, which uses for recording only 1 H of delay in each channel, as well as the modified switching arrangement discussed above.

It will be noted that the performance will not be as good as the previously discussed system incorporating 2H of delay, since the filters of Figures 1 and 2 are symmetrical about the reference line, and thus provide a linear phase response. This means that a vertical transition will be averaged (smeared) in both upward and downward directions, as opposed to only a single direction as in the case of the 1 H system shown in Figures 3 and 4. Further, the additional weighting coefficients of Figs. 1 and 2 provide a better, i.e., more nearly ideal, magnitude response.However, it will be appreciated that the system of Figures 3 and 4 still provides a great improvement over a conventional line sequential color recording or transmission system by comb filtering before the line sequential recording transmission in order to reduce vertical aliasing and does so while incorporating the simplified possible circuitry, which is of the utmost importance in portable equipment.

Figure 3 shows input terminals 70 and 70a that respectively receive the U(n) and V(n) signals which are respectively applied to 1 H delay lines 72 and 72a as well as being respectively applied to switch contacts 74 and 76 of switch 82. The output signals from delay lines 72 and 72a respectively comprise U(n-1) and V(n-1), which signals are respectively applied to contacts 78 and 80 of switch 84. Switching control generator 28 receives vertical and horizontal synchronization pulses and provides a one-half line rate switching control signal that controls switches 82 and 84 as indicated by a dotted line.

The output signal from switch 82 thus line alternately comprises U(n) and V(n), while the output signal from switch 84 line alternately comprises U(n--l)and V(n-1). These output signals are amplitude reduced by a factor of onehalf by resistors 88 and 90 respectively and then applied to adder 92. The output signal from adder 92 comprises on one line 1/2 U(n)+ 1/2 U(n-1) and on the next line 1/2 V(n+1 )+ 1/2 V(n), (the numbers in parenthesis for the V signal having in the meantime advanced by one), which signal is applied to FM modulator 30. The output signal from modulator 30 is applied to recording head 32, which head records on moving tape 34.

Figure 4 shows a playback circuit for use with the record circuit of Figure 3. Tape 34 moves past playback head 36, which head applies its output signal to FM demodulator 38. The demodulated output signal from demodulator 38 which on one line comprises 1/2 U(n)+1/2 U(n-1), is applied to contacts 96 and 100 of switches 104 and 106 respectively as well as to 1 H delay line 94. The output signal from delay line 94 comprises the information recorded during the previous line, which is 1/2 V(n-1 )+1/2 V(n-2) and is applied to contacts 98 and 102. Switching control generator 62 provides a one-half line rate signal for control of switches 104 and 106 as indicated by a dotted line. Thus with the switches in the positions shown, output terminal 108 provides the U signal, while output terminal 110 provides the V signal.

During the next line demodulator 38 provides a V signal, while delay line 94 provides a U signal.

Since the position of switches 104 and 106 are now opposite to that shown in Figure 4, output terminals 108 and 110 continue to provide the line alternating U and V signals respectively.

It should be understood that any combination of the pre- and post-filtering circuits that have been discussed may be used. For example, the recording circuit of Figure 3 may be desirable in portable equipment, while the playback circuit of Figure 2 is preferred for the studio playback machine. Further, any arrangement for synchronizing the switch positions with field or frame rates is acceptable, although a frame alternation may be desirable, this providing a 4:1 -- vertical interlace in the chrominance channels.

Such interlacing would allow for future quality improvements using frame stores and temporal averaging on replay of the recorded information.

Further, the system can be used to transmit video signals in channels other than recording circuits.

Still further, throughout the description and claims by "vertical" is meant "perpendicular to the line scan direction".

Claims

1. A method of applying component video signal to channel, comprising limiting the vertical bandwidth of at least one (U or V) of the component video signals and line alternately applying the component signals to the channel.

2. A method according to Claim 1, comprising limiting the vertical bandwidth of all the component video signals.

3. A method according to Claim 1 or 2, wherein the or each component signal is bandwidth limited before the line alternate application of the signals to the channel.

4. A method according to any preceding claim, wherein the vertical bandwidth limiting comprises forming the weighted combination of one line of a component video signal with at least one adjacent line.

5. A method according to Claim 4 comprising weighting one line of a component video signal and weighting the preceding line and the succeeding line and forming a combination of the weighted lines.

6. A method according to any preceding claims wherein the component video signals are PAL component color signals.

7. A method according to any one of claims 1 to 7 wherein the component video signals are SECAM component color signals.

8. A method of receiving from a channel signals applied to the channel by the method of anyone of claims 1 to 7, comprising at least applying the line alternated signals from the channel or signals derived therefrom to respective outputs.

9. A method according to Claim 8, comprising forming weighted combinations of the line alternated signals from the channel before applying the weighted combinations to the respective outputs.

10. A method according to Claim 9, wherein a component signal from the channel and associated with one line is applied to one of the outputs and a weighted combination of the component signals from the channel and associated with the preceding line and the succeeding line is applied to another of the outputs.

11. Apparatus for applying component video signals to a channel, comprising means for limiting the vertical bandwidth of at least one of the component video signals and for line alternately applying the signals to the channel.

12. Apparatus according to Claim 12, comprising means for limiting the vertical bandwidth of all the component video signals.

1 3. Apparatus according to Claim 11 or 12, wherein the limiting means comprises means for forming a weighted combination of one line of a component video signal with at least one adjacent line.

14. Apparatus according to Claim 13, wherein the limiting means comprises means for the forming a weighted combination of one line of a component video signal with the preceding line and with the succeeding line thereof.

1 5. Apparatus according to Claim 13, wherein the limiting means comprises a delay line having an input and an output and a delay of one video line period therebetween combining means and weighting resistors coupling the input and output of the delay line to the combining means to form a weighted combination of one line of a component video signal with an adjacent line.

1 6. Apparatus according to Claim 14, wherein the limiting means comprises a series arrangement of two delay line the series arrangement having an input, and output, and a junction of the two delay lines each delay line having a delay of one video line period, a combining means and weighting resistors coupling the input the output and the junction to the combining means to form the said weighted combination of the said one line with the said succeeding and preceding lines.

1 7. Apparatus according to Claim 13, wherein the limiting means comprises, for the component signals, respective series arrangements of two delay lines having an input for receiving the component signal an output and a junction of the lines, each line having a delay of one video line period, and comprises, for all the series arrangements, a common combining means having an output for connection to the channel and three weighting resistors common to all the series arrangements for coupling the input, output and junction of each series arrangement to the combining means, and the applying means line alternately applies the component signals from the series arrangement to the resistors.

1 8. Apparatus according to any one claims 11 to 1 7 wherein the applying means comprises at least one double throw single pole switch and switching control means for synchronizing the operation of the switch with the video line repetition period.

1 9. Apparatus for receiving from a channel signals applied to the channel by apparatus according to anyone of the Claim 11 to 1 8 and comprising at least means for applying the line alternated signals on the channel, or signals derived therefrom to respective outputs.

20. Receiving apparatus according to Claim 1 9 wherein it further comprises means for forming weighted combinations of the line alternated signals on the channel and in that the applying means applies the combinations to the outputs.

21. Receiving apparatus according to Claim 20, wherein applying means applies to one output a signal on the channel and associated with one line and the formal means forms a weighted combination of signals on the channel associated with the preceding and succeeding lines.

22. Receiving apparatus according to Claim 21 wherein the forming means comprises a series arrangement of two delay lines each having a delay of one video line period, a combining means and weighting resistors coupling the input and the output of the series arrangement to the combining means.

23. Apparatus according to any one of claims 11 to 22, wherein the component video signals are PAL component color signals.

24. Apparatus according to anyone of claims 11 to 22 wherein the component video signals are SECAM component color signals.

25. Apparatus according to anyone of Claim 11 to 24, coupled to the channel and wherein the channel comprises a frequency modulator for receiving the line alternated and vertical bandwidth reduced component signals, and a magnetic recording head coupled to the modulator.

26. A method or apparatus for applying component video signals to a channel substantially as hereinbefore described with reference to Fig. 1 or Fig. 3.

27. A method or apparatus for receiving component signals from a channel substantially as herein before described with reference to Fig. 2 or Fig. 4.