A signal carried in the field interval of a television signal as it passes through the distribution network (and invisible to normal viewers), which can readily be used to make an objective assessment of the quality of any part of the distribution network.
 Less than half of the 140Mbit/s signal because of the presence of some signalling information which is not split between the two ‘packages’.
 The terminology follows a water-borne analogy with a number of individual tributaries feeding the final multiplexed stream!
 Today, by contrast, synchronous multiplexing has become the preferred technique. The availability of very accurate and universal timing information via geo-stationary satellites (e.g. the Global Positioning System) has made this more efficient technique feasible.
 In a well-known reference article, Nyquist defined the minimum distortion free sampling frequency of a signal to be twice the highest frequency contained in the signal. Sub-Nyquist sampling indicates the use of a sampling frequency lower than this ideal.
 Some might be tempted to claim that some TV programmes are so predictable that it shouldn’t be necessary to send any information at all!
 This is essentially the same system as is used today to transmit the stereo sound component of analogue TV, though this uses a higher bit-rate of 728kbit/s.
In those days, the optimism about the benefits of digital transmission was such that there was a feeling that fibre-optic transmission would be perfect and that, on this type of link, the error-protection might be a waste of time!
 On many transmission links, bursts of errors are more often encountered than randomly distributed single errors.
 We discovered one of the unknown properties of the common soldering iron, which proved to have an unmatched ability to create large spikes of impulsive interference.