Analog TV Transmitter

Analog TV Transmitter

Basic block diagram of a television transmitter showing how the composite video signal from the camera is combined with the frequency modulated sound signals and used to amplitude modulate the RF carrier frequency. The vestigial sideband filter is used to conserve the occupied channel space to satisfy the allocated channel bandwidth requirements.

Composite video from camera

Composite video contains all the information that a TV or video monitor needs to present an image on the screen. The colour or chroma burst signal is present on the back porch area of the horizontal sync block. Although this is a black and white image the chroma burst will still be present.

Audio Signal

Sound processing

The stereo sound signals from the program are processed here to be applied to the sound sub-carriers for modulation.

The main sound signal is frequency modulated onto an RF sub-carrier. This is a sine wave signal centred at 5.5 MHz and deviating by +/- 50 KHz on maximum sound levels. As part of the full video signal the main sound sub-carrier has a power level approximately 13 dB below the main vision carrier signal.

Full TV signal combiner

 The composite video and both sound sub-carriers form the full TV signal and are brought together in the combiner to be applied to the amplitude modulator as a complete TV signal .  Figure  shows the frequency spectrum of the full television signal being applied to the amplitude modulator of the RF carrier frequency. This television signal contains composite video, the main sound signal and the second sound signal sub-carriers.

RF carrier frequency generator

Each TV channel in the system has its own carrier frequency to be modulated with its TV program. Refer to the list of RF carrier frequencies allocated to the TV channels.

Carrier wave

The TV viewers around the transmission area are able to select the desired program by tuning their receivers to the carrier frequency of that TV station and rejecting all others. Once the channel frequency is selected the full TV signal is extracted from it as composite video and sound The carrier originates as a constant level sine wave signal and is passed through an amplitude modulator to have the TV signal added to it.

Amplitude modulator

The RF carrier signal is made to vary in amplitude following the full TV signal which is amplitude modulating it.

Figure shows that the output of the amplitude modulator for the TV signal is a full double sideband signal occupying some 11 to 12 MHz of spectrum space. A large portion of the lower sideband signal is removed by the vestigial sideband filter to limit the occupied bandwidth.

Vestigial sideband filter

The TV signal to display the grey scale step pattern has amplitude modulated the main RF carrier frequency signal and is applied to the transmitting antenna. This can be seen at below figure. Using amplitude modulation the instantaneous carrier level follows the modulating video signal. Here the sync tip corresponds to the peak carrier amplitude, and white level the minimum. This follows the principles of negative modulation.

The vestigial sideband filter removes a large portion of the lower sideband information, leaving only the lower frequencies up to 1.25 MHz. See TP5 F. This is to reduce the occupied bandwidth and remain within the 7 MHz allocated to each channel. TV signal transmissions use a vestigial sideband method which is somewhere between single sideband (SSB) and double sideband (DSB). The term vestigial comes from the word vestige meaning 'something that remains of a previous existence'. In this case only part of the lower sideband remains.