EP0318544A1

EP0318544A1 - Method and apparatus for demodulating color television chrominance signals

Info

Publication number: EP0318544A1
Application number: EP88905004A
Authority: EP
Inventors: Carlo Basile; Alan P. Cavallerano; Mikhail Tsinberg
Original assignee: Philips Gloeilampenfabrieken NV; Koninklijke Philips Electronics NV
Current assignee: Koninklijke Philips NV
Priority date: 1987-06-02
Filing date: 1988-06-02
Publication date: 1989-06-07
Also published as: JPH01503591A; WO1988010053A1

Abstract

Un signal amélioré de sousporteuse couleur et de référence d'amplitude et un moyen d'utilisation dudit signal sont prévus pour des systèmes de télévision couleur, tels que notamment des systèmes de télévision à haute définition et avec un grand rapport hauteur-largeur d'image, dans lesquels le signal de haute définition est décomposé en deux signaux destinés à être transmis le long de deux canaux séparés et recombinés au niveau du récepteur, de façon à reproduire l'image d'origine à haute définition.An improved color subcarrier and amplitude reference signal and a means of using said signal are provided for color television systems, such as in particular high definition television systems and with a high aspect ratio. , in which the high definition signal is decomposed into two signals intended to be transmitted along two separate and recombined channels at the receiver, so as to reproduce the original image in high definition.

Description

METHOD AND APPARATUS FOR DEMODULATING COLOR TELEVISION CHROMINANCE SIGNALS

FIELD OF THE INVENTION

This invention relates to a method and apparatus for demodulating the chrominance signal of a composite color television signal, and in particular, television signals of the NTSC format. The present invention is applicable to standard single channel NTSC color television signals , wide bandwidth high definition wide aspect ratio color television signals, and perhaps of greatest importance, to NTSC- compatible two-channel high definition wide aspect ratio color television signals.

BACKGROUND OF THE INVENTION

In the latter system, a high definition wide aspect ratio (e.g. 16:9, also referred to as 5-1/3:3) single channel signal is decomposed into a center image signal and an enhancement signal. The center image signal is transmitted via a first channel in standard NTSC format and when received by a standard NTSC television receiver, produces the center image having a 4:3 aspect ratio. The enhancement signal is transmitted via a second channel. The enhancement signal contains information for side panel images which, together with the center image signal, reproduce the original wide aspect ratio image. The enhancement signal also carries additional information for the center image that cannot be accommodated by a standard NTSC signal. This additional or enhancement information, when properly combined with the NTSC center image signal, restores the high definition that was "lost" when the center image signal was converted to a standard NTSC signal. Additional video and audio enhancement information may be carried by the enhancement signal on the second channel. Such a two channel NTSC-compatible system is described in United States Patent US.P. 4,694,338 (PHA 21.323). In order to acceptably reproduce the full high definition wide aspect ratio picture transmitted by the two-channel system, it is necessary to have a receiver that receives both the first and second channel signals and properly stitches them back together so smoothly that the stitch is not observable by viewers. This requires that any phase difference between the first and second channel signals be compensated for and that the black level, white level and chrominance (hue and saturation) be reproduced equally for both channel signals so that no variation in brightness, contrast, hue or saturation is introduced by the process of decomposing the original high definition wide aspect ratio image into two signals, transmitting them via two separate channels, and recombining them to produce the display at the receiver.

SUMMARY OF THE INVENTION

The present invention solves some of the problems of properly recombining the two signals that are produced when the high definition signal is decomposed and transmitted via two separate channels by providing a training signal at the point of origination and using it at the receiver in a novel method of chrominance demodulation. The training signal is preferably transmitted during a line time, or portion thereof, within the vertical blanking interval. The training signal enables the receiver to establish and maintain a true phase reference for demodulating the chrominance signals of both the center image signal and the enhancement signal. The training signal is transmitted via both channels.

It is well known that color burst and sync signals are often changed, intentionally or unintentionally, between the signal source and the ultimate receiver. This can occur when the source signal is recorded and played back for later broadcast, and also during its passage through the signal delivery system, which may include coaxial cable, satellite up and down links, and cable television head end equipment. Distortion will also be introduced if the signal delivery system does not have a flat frequency response and thereby introduces a variation in gain (usually a loss) that differs at low frequencies from that at the color subcarrier frequency. This will create an error in color satuation at the receiver. Furthermore, due to variations in modulation index, demodulator tolerances, analog component tolerances, and other factors, the center and panel signals will not have the proper gain or correct black level. When viewing images transmitted via standard NTSC signals, these variations are usually not objectionable bacause the errors are uniform across the entire displayed image. However, when considering the two-channel system described above, there will be chroma (phase and amplitude), gain, and black level errors associated with each the center image signal and the enhancement signal. An image displayed without properly correcting for these errors would be unacceptable. Variations in image properties between the center image and the panels would be particularly noticeable and annoying. In order to provide for correct color reproduction at the ultimate receiver, notwithstanding that the burst phase has been shifted or that the signals were transmitted through channels that do not have flat frequency responses, the vertical interval reference (VIR) signal was introduced. The training signal of the present invention has certain features in common with the VIR, but is structured to provide more useful information to the receiver. A system using the methods and apparatus of the present invention compensates for the errors mentioned above, and furthermore, may be operated without reference to the NTSC color burst, thereby permitting additional information to be transmitted during the time otherwise occupied by the color bust.

Demodulation according to the present invention is accomplished by sampling and digitizing each signal, i.e. the center image signal and the enhancement signal. Each digitized signal is then processed to correct for channel attenuation and to separate the chrominance and luminance signals. The chrominance signal is demodulated by algebraically adding the values of successive samples to produce demodulated outputs.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1a is an illustration of a portion of the waveform of an NTSC television signal showing how the training signal of the present invention may be accommodated.

Figure 1b is an illustration of a signal that switches state every other horizontal sync pulse and is denoted as H/2. Figure 1c is a more detailed illustration of the training signal of the present invention.

Figure 2a is a detailed illustration of a portion of the training signal of the present invention showing its relationship to a sample clock signal, a counter state, and a training gate signal.

Figure 2b is a sample clock signal.

Figure 2c is a sequence of numbers illustrating the states of a counter.

Figure 2d illustrates the training gate signal of the present invention.

Figure 3 is a block diagram of the functional components of a two-channel receiver in accordance with the present invention. Figure 4 is a block diagram showing an embodiment of the chroma demodulator of Figure 3 in more detail.

Figure 5 is a graph showing the gain vs. frequency characteristic of a transmitted signal and that of the same signal at a receiver. Figure 6 is a vector diagram showing the relationships between a random value of the chrominance signal and the sampling axes and the I and Q axes.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in the environment of a two-channel high definition wide aspect ratio television system that is compatible with a standard NTSC television signal. That is, one of the two channels (the center image signal) may be viewed on a standard NTSC television receiver while the original high definition wide aspect ratio image may be viewed on a high definition wide aspect ratio two-channel receiver receiving both the center image signal and the enhancement signal.

In the described system, the originating signal is a high definition (e.g. 500 line horizontal, 500 line vertical) signal having a wide aspect ratio (e.g 16:9) compared to the standard NTSC aspect ratio (4:3). In accordance with the method described in United States Patent US.P. 4,694,338 (PHA 21.323), the originating signal is decomposed into two signals, a center image signal and and enhancement signal. These two signals are then transmitted via two standard broadcast or cable channels to the ultimate receiver, which may be either a standard single channel NTSC receiver or a high definition wide aspect ratio receiver. This disclosure will address primarily the latter. Referring now to Figure la, there is shown a waveform representative of a portion of a standard NTSC television signal. A detailed presentation of NTSC and other standards for television broadcast signals is presented in Benson, Television Engineering Handbook, McGraw-Hill, Inc., 1986. During the vertical blanking interval 21 there are transmitted a number of equalizing pulses, followed by the vertical sync signal, followed by post equalizing pulses. There follows several scan lines that do not carry picture information. Several of the scan lines during the vertical blanking interval have been designated or reserved for teletext data, closed captioning data, VIR, etc. See, for example, United States Patent No. 4,092,674, in particular, Figure 2 thereof. There are still several lines that are available for new uses. The present invention uses one of these lines, for example line 18, for the training signal. The training signal of the present invention comprises several cylces (e.g. 50) of the color subcarrier signal, f_sc, indicated by the reference numeral 22, preceded by a black reference level 23 and a white reference level 24. The training signal is shown in more detail in Figure 1c, and in still more detail in Figure 2a, wherein the black reference level 23, the white reference level 24, and several cycles of the color subcarrier frequency, f_sc, are illustrated. The phase of f _sc in the training signal is selected for convenience to equal 0 degrees with respect to the Q or I axis. This is not a necessary condition, however, the mathematical explanation of the method of demodulation used by the present invention is simplified for this condition, and therefore, this condition will be assumed to exist for purposes of explanation.

Aside from the number of cycles of color subcarrier, the training signal differs from the standard VIR signal in that the subcarrier amplitude and phase and specific white level are different. These differences will be explained later. Assuming that the encoder which splits the wide aspect ratio source image into the center and panel video signals uses digital processing it is important to note that best results will be obtained if this training signal is inserted digitally within the encoder. This will eliminate any alignment and drift problems normally associated with standard VIR and VIT signal insertion techniques. At the receiver, both signals are sampled at 910f_h = 4f_sc. Figure 2b represents the signal generated by a sample clock running at this frequency. The frequency of the sample clock may be derived from the horizontal sync pulse in one of the received channels. The sample clock signal is used as the input to a four-state counter which repetitively counts 0, 1, 2, 3. Counter states for the counter are shown in Figure 2c.

Turning now to Figure 3, there is shown a block diagram of a portion of a two channel receiver for receiving the center image signal and the enhancement signal and recombining them to display the original high definition wide aspect ratio image.

After being converted to baseband by tuners 31 and 32, the two channel signals are fed via automatic gain control (AGO circuits

33 and 34 to A/D converters 35 and 36, respectively. Quantization is 9- bit linear, providing 512 possible digital values for each sample.

Ordinarily, eight bit quantization is deemed satisfactory for digitizing color television signals, providing 256 possible values. Because AGC performance may permit as much as 6 db variation, however, it was decided to increase quantization to nine bits. Theoretically, the AGC function could be performed in processors 43, 44 to be described below, providing that the level of quantization is adequate. We have chosen for the present to continue the use of AGC circuits as shown.

As noted earlier, there will be some transmission delay time for each channel. It must be assumed that the two delay times will not be the same. Accordingly, the two channel signals, after being digitized by A/D converters 35 and 36, are then delay equalized or time base corrected (the terms may be used interchangeably) by time base corrector (TBC) 37, which receives both channel signals and delays the earlier-to-arrive signal by a time equal to the delay difference so that its outputs both signals, the center image signal on channel 1 and the enhancement signal on channel 2, have the same time relationship as the components of the original high definition wide aspect ratio signal that was decomposed to form them. Time base correction is the only parameter which is corrected independently of microprocessor 55. All other parameters are controlled in conjunction with microprocessor 55.

Both digitized signals from A/D converters 35 and 36 are routed to a one line buffer memory 58. The only lines written into the buffer memory 58 are the lines containing the training signal of each channel. As indicated, the microprocessor is integral in controlling hue, saturation, gain and black level of both video signals since it is completely responsible for providing coefficients necessary for the operation of these correction circuits. The training signals contain sufficient information such that with the proper analysis these parameters can be normalized. In the present configuration the training signal is placed on line 18 of each field which is indicated by address counter 57 and timing generator 48 creates at 59 a training gate signal as shown in Figure 2d which enables the write cycles of buffer memory 58 during this line. Simultaneously, microprocessor 55 releases control of the bus so as not to inhibit this write cycle. Immediately following the end of this cycle microprocessor 55 takes control of buffer memory 58 in order to execute its analysis and provide the proper coefficients. Microprocessor 55 will have one field time (16.33 milliseconds) to analyze the samples before a new line must be written.

We have used an 8086 processor running at or approximately 9.54 MHz with a program execution time of approximately 9 msec.

To the output of AGC 33, there is connected a sync stripper 47, which detects horizontal sync signals and outputs them to a phase lock loop and timing circuit 48 that runs at 910 times the horizontal sync pulse repetition rate of f_h. The frequency 910f_n is equal to four times the color subcarrier frequency f_sc . The sync stripper 47 and phase lock loop (PLL) and timing circuit 48 operate as a sample clock 49 to provide sampling pulses to A/D converters 35 and 36 and other circuits that will be described below. As noted above, phase lock loop and timing circuit 48 also produces the training gate signal shown in Figure 2d. It should be noted that sync stripper 47 may operate from either the channel 1 signal or the channel 2 signal, so long as the operative channel contains the horizontal sync signal. In the two channel NTSC-compatible system, it is necessary that the center image channel signal include sync signals so that the signal will be compatible with existing NTSC receivers. Sync may or may not be included in the enhancement signal. It is contemplated that horizontal sync and color burst may be omitted from the enhancement signal. In fact, all of the standard NTSC information contained in the horizontal blanking interval may be removed from the enhancement signal, making those time slots available for the transmission of other information.

The digitized and time base equalized signals are then processed by processors 43 and 44 which compensate for attenuation of the composite video signal at both low frequencies and at the color subcarrier frequency. This compensation is performed in order to approximately restore, each composite video signal to its original gain versus frequency characteristic across the relevant bandpass. It is well known that finite bandwidth channels usually attenuate the transmitted signal more at higher frequencies than at lower frequencies and that the attenuation is not uniform with frequency. This is illustrated in Figure 5 which, shows at T a source signal that has a substantially constant gain G across the relevant bandwidth, say from near zero to a frequency F above f_sc. At R is shown the gain G versus frequency F characteristic of the same signal as received at the receiver. This general characteristic exists for both channels, however, it will not likely be exactly the same for both channels. Therefore, the gain versus frequency compensation must be carried out for each of the two channels. The low frequency attenuation is represented at B and the attenuation at f_sc is indicated at A. Determination of the required gain and offset correction coefficients will be described below.

Processing of the two digitized signals is performed at the sample rate of 910f_h = 4f_sc. This is accomplished by using the strobe signal from the same sample clock (sync stripper 47 and PLL and timing circuit 48) that is used by the A/D converters 35 and 36. In addition to compensating for attenuation in the two transmission channels, processors 43 and 44 also separate chrominance and luminance. Inasmuch as we are not here concerned with how the chrominance and luminance signals are separated or with further processing of the luminance signal for either channel, it is merely noted that luminance is separated from chrominance in processors 43 and 44. This is symbolized by the outputs from each processor labeled Y.

The digitized chrominance signals C(n) from processors 43 and 44 are input respectively to chroma demodulators 53 and 54 along with additional signals that are used to cary out the novel method of chrominance demodulation. These additional signals comprise, for each demodulator 53 and 54, a coefficient calculated by microprocessor 55 and the state of a four-state counter ([4) 56. The counter 56 is advanced through its four states, 0, 1, 2, 3, at the rate of the sample clock, 4f_sc. The microprocessor 55 reads and writes to buffer memory 58 that state, as noted above, and also samples several cycles of f_sc in each training signal under control of the training gate 59 from PLL and timing circuit 48. The microprocessor 55 utilizes these samples in conjunction with data from a ROM 60 to calculate the coefficients that are supplied to the chroma demodulators. The stored data in ROM 60 is representative of the original form of the training signal as inserted at the point of origination.

Gain and offset correction by processors 43 and 44 utilizes coefficients calculated for each of the two received signals The training signal allows for at least 16 samples of white level, 16 samples of black level, and 32 cycles of subcarrier to be processed on the selected line (which occurs once per field). In order to suppress the noise component present on each sample, all samples representing the same information are averaged with each other and the averaged result is then taken to represent the value of that parameter. Analysis of the received white and black levels will yield results representing the low frequency gain and DC offset of the signals which can then be used to correct for contrast and black level. Determination of these two parameters is performed by solving a set of simultaneous equations. We define the received white level, S_w1, and black level S_b1, as: S_w1 = B x WL_ref + DC

S_b1 = B x BL_ref + DC where B represents the low frequency gain (or attenuation) as shown in Figure 5, DC represents the DC offset and WL_ref and BL_ref are white level and black level reference values stored in ROM 60 at the receiver. The present configuration of the training signal contains white level at 75 IRE and black level at 7.5 IRE and the reference values stored in ROM 60 represent these values normalized to the 8-bit video bus. These values were chosen over the standard 50 IRE and 7.5 IRE levels used in the VIR signal in order to gain a signal-to-noise advantage. Therefore the gain at low frequencies is:

and the DC offset is:

DC = S_w1-B x WL_ref These results are calculated each field and averaged with the results from previous fields. The filtered results are then output to processors 43 and 44 which contain real time signal processing circuits where corrections to the video samples take place. Up to the present we have used equal weighted averaging, however, it would seem that IIR filtering might yield better results.

Conventional methods to separate luminance and chrominance, or more sophisticated techniques, can be employed. In either case the outputs after separation are center and panel luma and chroma signals. The luma signals, which have been normalized, can now be recombined (stitched). Due to the effects of transients, stitching techniques are of reasonable complexity and will not be covered here. Reference is made, however, to U.S. Patent Application, Serial No. 057,849, filed June 2, 1987, (PHA 21.379). Before the chroma signals are combined they first must be demodulated. The demodulator used here consists of a two tap digital filter whose coefficients are dynamic and are provided by the microprocessor. Once the chroma signals have been demodulated they can be recombined in a similar manner as the luma signals are.

Turning now to Figure 4, there is shown a block diagram of each of the chroma demodulators 53 and 54 of Figure 3. Samples C(n) are applied to a first input 401 of a first multiplier 403, and via a one horizontal line time delay 405 to a first input 407 of a second multiplier 409. Additional input coefficients and , provided by the microprocessor 55 of Figure 3, are applied to second inputs 411 and 413, respectively, of the first multiplier 403 and the second multiplier 409, respectively. Outputs 415 and 417 of the first multiplier 403 and the second multiplier 409, respectively, are connected to inputs 419 and 421, respectively, of an adder 423. An output 425 of the adder 423 is connected to a first input 427 of a 2:1 multiplexer 429, and via an inverter 431 to a second input 433 of the 2:1 multiplexer 429. An output 435 of the 2:1 multiplexer 429 is connected to an input 437 of a 1:2 demultiplexer 439. Outputs 441 and 443 of the 1:2 demultiplexer 439 supply I and Q signals, respectively. A counter state signal from the four state counter 56 of Figure 3 is applied to inputs 445 and 447 of the 2:1 multiplexer 429 and the 1:2 demultiplexer 439, respectively. The chroma demodulator consists of a two tap filter and a demultiplexer 439. Ordinarily with NTSC signals digitized at 4f_sc, demodulation is accomplished with only a demultiplexer, however this technique requires that there is a precise phase relationship (zero degrees for R-Y, B-Y demodulation, 33 degrees for I, Q demodulation) between the sampling clock and the color burst and that the color burst is a true phase reference for the active color video. In this system, in order to compensate for the possibility that the color burst is not a precise phase reference and to allow flexibility in the sampling clock phase, an alternate method was employed. As shown in the vector diagram Figure 6 each chrominance sample contains an I component and a Q component. Decomposition of the I and Q components from a single sample is not possible unless the angle, θ, between the sampling clock and the I and Q axis is 0 degrees. However, if θ is known, combining adjacent samples S₁ and S₂ with the appropriate functions of θ will decompose the vector into its I and Q components. By processing information contained in the training signal we are able to calculate, and therefore know, θ. Unlike known techniques, this technique is precise if the color vector is not varyingwith time. This demodulation scheme also implements saturation adjustment since the multiplier coefficients can be normalized to provide gain compensation. Derivation of the coefficients necessary for this technique will now be described.

The chrominance signal C can be represented by:

with the sampling frequency of 910f_h = 4f -sc, t takes on values of

where T_α represents the arbitrary static phase of the sampling clock with respect to subcarrier. Therefore, the sampled chrominance signal at the input to the demodulator is:

At this point it will be shown that I and Q can be recovered from the sampled chrominance signal, C(n), with hardware configured as shown in

Figure 4 and additional input coefficients and A provided by the microprocessor 55. Using trigonometric identities it can be shown that:

C(-1)=A[+I(-1)sinθ-Q(-1)cosθ] n = -1

C(+0)=A[+I(0)cosθ+Q(0)sinθ] n = 0

C(+1)=A[-I(1)sinθ+Q(1)cosθ] n = 1 C(+2)=A[-I(2)cosθ-Q(2)sinθ] n = 2

C(+3)=A[+I(3)sinθ-Q(3)cosθ] n = 3

C(+4)=A|+I(4)cosθ+Q(4)sinθ] n = 4

Applying this signal, C(n), and the coefficients and to the chroma demodulator circuitry (see Figure 4), the output of the adder 423, C'(n), will be: Taking advantage of the bandlimited nature of the I and Q signals we can make the approximation that: Kn-1) - I(n) = I and Q(n-1) - Q(n) = Q And substituting:

C (0) =[+I(O)cos²θ+Q(0)sinθcos0)+[I(-1)sin²0-Q(-1)sinθcosθ]=+I C' (l)=[-I(1)cosθsinθ+Q(1)cos²Q|+f +I(0)cosθsinθ+Q(0)sin²θ]=+Q C(2)=[-I(2)cbs²θ-Q(2)sinθcosθ]+[-I(1)sin²θ+Q(l)sinθcosθ]=-I C(3)=[+I(3)cosθsinθ-Q(3)cos θ]+[-I(2)cosθsinθ-Q(2)sin²θ]=-Q C(4)=[+I(4)cos²θ+Q(4)sinθcosθ]+[+I(3)sin²θ-Q(3)sinθcosθ]=+I C(5)=[-I(5)cosθsinθ+Q(5)cos²θ]+[+I(4)cosθsinθ+Q(4)sin²G]=+Q

The resulting data stream is then appropriately inverted by inverter 431 and demultiplexed by 1:2 demultiplexer 439 in agreement with the counter states as shown in Table 1 and Figure 2c resulting in an I data stream at 441 and a Q data stream at 443 both at a 2f_sc rate. The relationship of the clock frequency to the horizontal rate is such that this sequence will repeat itself every other line. In order to calculate the proper coefficients, the processor 55 must have knowledge of the phase of this sequence with respect to the samples stored in the buffer. This is accomplished by the generation of an H/2 signal as shown in Figure 1b. The phase of this signal (either "1" or "0") as it occurs on line 18 of the television signal is also stored in buffer memory 58. The H/2 signal is also used to reset divide-by-four counter 56 and address counter 57 of buffer memory 58. This process ensures complete phasing of the demodulator and accounts for all arbitrary initial conditions.

Table 1

Counter State Action

0 No Inversion, Connect to I output

1 No Inversion, Connect to Q output

2 Inversion, Connect to I output 3 Inversion, Connect to Q output

Determination of the coefficients for chroma demodulation, cos sin performed by analysis of the reference burst in the training signal and the associated phase of the H/2 signal. The sampled reference burst present at the receiver and written into memory 58 can be expressed by:

S(n) = L + + N(n)

where A is the gain (or attenuation) of the channel at the subcarrier frequency, ω_sc, L is an arbitrary DC component, K is a constant representing the original transmitted amplitude, and N(n) is the noise. In order to maximize signal-to-noise, the value of K was chosen to be 50 IRE as opposed to the 20 IRE used in the known VIR signal. After the training signal is written into memory 58, microprocessor 55 reads the samples and the H/2 phase associated with that line. Since the sampling clock frequency is 4f_sc, every fourth sample (i.e all samples occurring with the same counter state) will have the same value except for the noise component. Therefore, all data values with the same counter state of counter 56 are averaged to suppress the noise component, resulting in a set of four values:

S₀=L+AKcosθ = L+AKcosθ counter state = 0

S₁=L+AKcos( = L-AKsinθ counter state = 1 S₂=L+AKcos = L-AKcosθ counter state = 2

S₃=L+AKcos = L+AKsinθ counter state = 3

The value of K is a constant and is stored in ROM at the receiver. The output coefficients defined as C₁ and C₂ c _A can be obtained by solving the following equations:

To further increase the performance of this system, the results for C₁ and C₂, which are computed every field, can also be filtered with results from previous fields. These filtered results can then be output to the multipliers.

Claims

1. A television transmission system of the kind wherein a high definition television signal is decomposed into two signals for transmission via two separate channels, at least one of which signals includes first and second color component signals modulated onto a color subcarrier to produce a chrominance signal, and wherein said two signals are recombined at a receiver to produce a high definition display in accordance with said high definition television signal, characterized in that at a transmitter a training signal is inserted into and transmitted as a part of each of said two signals and said receiver comprising means for utilizing said training signal to demodulate and recombine said two signals.

2. The television transmission system according to Claim 1, characterized in that said training signal comprises a color subcarrier reference portion having a plurality of cycles at said color subcarrier frequency and having a predetermined phase relationship to said color subcarrier.

3. The television transmission system according to Claim 2, characterized in that said training signal further comprises a black level reference portion having a predetermined black level amplitude and a white level reference portion having a predetermined white level amplitude.

4. The television transmission system according to Claim 3, characterized in that the predetermined black level amplitude is substantially 7.5 IRE and the predetermined white level amplitude is substantially 75 IRE.

5. The television transmission system according to Claim 1, characterized in that means at said receiver for utilizing said training signal to demodulate and recombine said two signals comprises a time base corrector for time base correcting said two signals and a microprocessor for analyzing said training signal in each of said two channels and for controlling hue, saturation, gain and black level of said two signals.

6. The television transmission system according to Claim 5, characterized in that the receiver further comprises memory means for storing data representative of the original form of said training signal as inserted into said two signals.

7. The television transmission system according to Claim 6, characterized in that said microprocessor uses said data representative of the original form of said training signal to correct each of said two signals for contrast and black level.

8. A transmitter suitable for the television transmission system as claimed in Claim 1, 2, 3 or 4.

9. A receiver suitable for the television transmission system as claimed in any one of Claims 1, 2, 3, 4, 5, 6 and 7.