The standard for the Australian colour television system is known as Phase Alternate Line (PAL). The main features of the PAL system can be described as follows. Images are divided into a series of 625 line blocks (or frames) which are updated 25 times a second. Each line is transmitted together with synchronising information at the rate of one line per 64 microseconds. Figure 18 shows a line of a PAL "colourbar" waveform. The voltages on the active part of this line correspond to the brightness of the scene as the camera scans it from left to right.
Figure 18: Diagram of the waveform of a line of a PAL colour-bar test signal.
To understand how the colour information is carried in the PAL signal it is best to look at the signal in the frequency domain. For reasons of transmission efficiency and compatibility a colour camera tube captures information as RGB colour signals but converts them into black and white (luminance) and two colour difference (chrominance) signals. The PAL system then modulates the chrominance information onto a frequency known as the colour subcarrier so that it sits in the upper part of the video baseband.
Sound signals associated with the television signal are located at frequencies just above the video baseband.
This "frequency division multiplexing" in the PAL signal allows all the elements of the television signal to be carried in a single channel (or on a single cable) without causing interference to each other. Also the method of chrominance modulation was chosen because it allowed compatible monochrome and colour transmissions.
Figure 19: Frequency domain representation of PAL colour bar signal [Communications Laboratory measurement].
While the PAL system has worked well for the terrestrial amplitude modulated environment for which it was designed, it is less suited to frequency modulation transmission. The presence of important parts of the signal in the upper parts of the baseband frequency spectrum makes them more prone to noise due to the triangular noise spectrum of FM signals. While this can be reduced somewhat for single component signals by the use of pre-emphasis it is less effective where the luminance and chrominance signals have been combined in the baseband spectrum. Similar problems arise for the sound carrier(s) however in this case emphasis can be more easily applied.
The presence of multiple carriers, ie vision carrier, colour subcarrier and sound carrier(s), will generate a broad spectrum of intermodulation products which look like an increase in background noise level, and will reduce the power in the vision carrier.
A further disadvantage of conventional PAL for satellite transmission is that line synchronisation pulses uses 30% of the available FM deviation but does not transmit much in the way of useful information. However a system known as E-PAL has attempted to overcome this disadvantage by inverting the PAL line sync pulse.
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