Television receivers and colour processing:Colour burst processing

Television receivers: colour processing

You will recall from Chapter 2 that the chrominance information is contained in a 4.43-MHz modulated sub-carrier which forms part of the incoming composite video signal. Colour difference signals B’- Y’ and R’- Y’ are used to modulate the sub-carrier, which is then suppressed to leave two quadrature components, U and V. At the receiver, the chrominance infor- mation is separated from the luminance signal by a comb filter, decoded and applied to a matrix network, which performs the operation necessary to reproduce the original RGB colour signals that can be applied to a colour tube. Figure 14.1 shows a block diagram of the major functional units required for TV video processing. It consists of two major processing sec- tions: luminance and chrominance. Chrominance processing itself consist of four distinct parts: a colour burst section, a colour decoding section, a matrix and a colour drive amplifier.

Colour burst processing

Colour burst processing separates the sub-carrier burst signal from the chrominance information so it may be used to recreate the sub-carrier which has been suppressed at the transmitter. The sub-carrier has to be restored in both its frequency and phase to ensure correct colour repro- duction. Two sub-carriers at 90° to each other have to be produced, one for each colour difference, B’- Y’ and R’- Y’. Furthermore, in the PAL sys- tem, the sub-carrier for the R’- Y’ demodulator has to be phase reversed on alternate lines. The burst processing section is also used to provide automatic chrominance control (a.c.c.) as well as the colour killer signal for monochrome-only transmissions.

The colour burst, which consists of about 10 cycles of the original sub- carrier, is mounted on the back porch of the line sync pulse. A burst gate amplifier triggered by the line sync is used to separate the burst from the composite video. The burst gate amplifier is turned on by a delayed line fly- back pulse. The delay in the flyback pulse is necessary because, having been placed on the back porch arrives immediately after the line sync. The delay ensures that the amplifier begins to conduct on the arrival of the burst. The burst gate amplifier allows the burst to go through a phase-locked loop (PLL) network known as the automatic phase control (a.p.c.) consisting of the phase discriminator, filter and the voltage-controlled oscillator (VCO).

The phase discriminator compares the phase of the burst with the 4.43 MHz output of the voltage-controlled crystal oscillator. If there is an error, a correction voltage is produced, which after going through a low-pass filter is fed to the VCO to bring the frequency and phase of the oscillator into step with the burst. For a more precise and stable sub-carrier, the VCO natural resonant frequency is doubled to 2 X 4.43 = 8.86 MHz. The 4.43 MHz sub- carrier is obtained by dividing the output of the oscillator by 2. For PAL, the burst signal is not of constant phase but swings to convey information of the phase reversal of the V component at the transmitter. For this reason, the phase discriminator must compare the phase of the oscillator with the average phase of the burst signal.

Finally, the sub-carrier for the B’- Y’ demodulator is obtained by the insertion of a simple 90° phase shift at the output of the oscillator. The sub- carrier for the R’- Y’ demodulator must suffer a phase reversal on alternate lines. To achieve this, a square wave at half-line frequency is needed. A component of this frequency is present at the incoming signal because of the 90° phase change of the colour burst on alternate lines. Since one complete swing of the burst phase takes place every two lines, the frequency of the ‘swing’ is half the line frequency, i.e. 15.625/2 = 7.8125 kHz, which is normally quoted as 7.8 kHz and referred to as the identification or ident signal. After amplification, the ident signal is fed into a PAL switch that reverses the phase of the 4.43 MHz oscillator signal on alternate lines.

Two other functions are derived from the burst processing section: colour killing and a.c.c. The purpose of the colour killer circuit is to close down the chrominance amplifier path on monochrome-only transmis- sions to prevent random colour noise appearing on the screen. The ident signal is therefore used to provide a normal bias for the chrominance amplifier, which will be turned off if ident is absent.

The a.c.c. prevents varying propagation conditions from changing the amplitude of the chrominance signal in relation to the luminance. To realise this, the gain of the chrominance amplifier is made variable by a control voltage in a similar way to the control of the gain of the i.f. stage by an a.g.c. signal. The a.c.c. control voltage must be proportional to the amplitude of the chrominance signal. This voltage cannot be derived from the actual chrominance signal during the active picture scan as this varies in ampli- tude as the colour information changes. It is derived instead by monitoring the amplitude of the colour burst. A fall in the amplitude of the burst sig- nifies an attenuated chrominance. This is corrected when the a.c.c. control voltage into the chrominance amplifier increases its gain, and vice versa.

NICAM STEREO SOUND:SERVICING NICAM DECODERS.

VIDEO DISC TECHNOLOGY:TILT SERVO AND SYSCON, FIELD AND CHAPTER INDEXING.

Multimedia convergence:Transmission modes

MPEG encoding:Quantising the DCT block

Flat panel television receivers:DC power failure

TV AND VIDEO WAVEFORMS AND STANDARDS:CHANNEL FIVE BROADCASTING AND WORLDWIDE TV STANDARDS.

Multimedia convergence:Enhanced FEC and Enhanced signalling

DVD:Dolby digital (AC-3) encoding.

Flat panel television receivers:Fault finding in an LCD television receiver

The digital TV reception:Memory faults and their symptoms

VIDEO DISC TECHNOLOGY:DVD PLAYERS

CONTROL SYSTEMS:EXTERNAL CONTROL SYSTEMS.