audio data stream will now be described with reference to fig7 which shows the coder in diagrammatic block form the audio data stream in the form of serial 32 bit words as shown in fig1 with gaps in the last three bit positions where the user bit the channel status information bit and the parity bit respectively are to be inserted is supplied by way of an input terminal 1 to the data input of a multiplexer 2 a clock pulse signal of frequency 1 536 mhz is supplied by way of an input terminal 3 to the clock input of the multiplexer 2 and also to the input of a divider 4 which divides by 32 and can be reset by a frame reset pulse supplied to a terminal 5 the divider 4 supplies a 48 khz clock pulse signal to a divider 6 which divides by 64 with an optional reset and to the clock input of a channel status multiplexer and coder 7 the divider 4 also supplies a channel status enable pulse and a user enable pulse to the multiplexer 2 aligned with the 30th and 31st bit positions in the 32 bit audio data words and a frame sync enable to a bi phase mark encoder 8 the coder also includes a user block multiplexer and coder 9 which also receives the 48 khz clock pulse signal from the divider 4 the divider 6 supplies a word start signal to both the channel status multiplexer and coder 7 and to the user block multiplexer and coder 9 channel status data as required is supplied by way of a data input 10 to the channel status multiplexer and coder 7 which may comprise a microcomputer likewise.