1. Telecommunications apparatus for transmitting analogue signals, comprising an analogue signal input, pulse-coding means connected to said analogue signal input, for generating a stream of digital word signals, wherein each digital word signal will represent a sampled instantaneous value of an analogue signal applied to said analogue signal input and will comprise one polarity-bit signal representing the sign of the difference between the sampled value and a predetermined value and at least one amplitude-bit signal representing the magnitude of the modulus of the said difference, and reference-level deriving means, connected to outputs of the said pulse-coding means, for deriving a variable referencelevel signal dependent on the magnitudes of a plurality of the moduli represented in a sequence comprising a plurality of the said digital word signals which represent successively sampled values of the said analogue signal, the said pulse-coding means including means for generating the amplitude-bit signals, connected to receive the said referencelevel signal from the said reference-level deriving means and controlled thereby, for generating the amplitude-bit signals to represent the magnitudes of the sampled values of the analogue signal quantized in terms of units whose size will vary according to the value of The said reference-level signal.

8. Telecommunications apparatus, for receiving a continuous stream of pulse code signals formed of word signals wherein each word signal comprises one polarity-bit signal and at least one amplitude-bit signal sequentially transmitted, and represents one sampled instantaneous value of an analogue signal; the said apparatus comprising receiver means for receiving pulse code signals from a telecommunications channel; switching means, having a signal input connected to receive said pulse code signals from the said receiver means, and having a control input and at least two outputS, for distributing successive ones of said pulse code signals sequentially to the said at least two outputs of said switching means in a predetermined cyclic order, and causing a single modification to the distribution whenever a signal is applied to the said control input; threshold detector means, having an input connected to one of the said outputs of the said switching means, and having an output connected to the said control input of the said switching means, for applying a signal to the said control input whenever an average of a sequence of signals successively developed on the said one of the said outputs of said switching means occurs outside a predetermined range of values; and decoder means, having a polarity-bit signal input and at least one amplitude-bit signal input connected to separate outputs of the said switching means, for reproducing the said analogue signal by generating a sequence of signals having their polarities determined by the signals applied to its polarity-bit signal input and their amplitudes individually dependent on the bit signals applied to the said at least one amplitude-bit signal input and also proportional to an average of the values of a sequence of the signals applied to at least one amplitude-bit signal input of the said decoder means.

15. Telecommunications apparatus, for receiving a continuous stream of pulse code signals formed of word signals wherein each word signal comprises one polarity-bit signal and at least one amplitude-bit signal sequentially transmitted, and represents one sampled instantaneous value of an analogue signal; the said apparatus comprising receiver means for receiving pulse code signals from a telecommunications channel; switching means, having a signal input connected to receive said pulse code signals from the said receiver means, and having at least two outputs, for distributing successive ones of said pulse code signals sequentially to the said at least two outputs in a predetermined cyclic order; a plurality of threshold detector means, wherein each threshold detector means is connected to a separate one of the outputs of the said switching means, for generating a binary-signal output of a predetermined kind whenever an average of the pulse code signals applied to the one of the outputs of the switching means to which it is connected occurs within a predetermined range; gate circuit means, comprising a plurality of gate circuits and having pulse code signal inputs separately connected to the said outputs of the said switching means, control inputs separately connected to receive the binary-signal outputs of the said plurality of threshold detector means, a polarity-bit signal output channel and at least one amplitude-bit signal output channel, for passing the pulse code signals developed on the separate outputs of the said switching means to separate output channels in cyclic order so that the pulse code signals applied to the one of the said threshold detector means which has most recently produced a binary signal output of the said predetermined kind will also be applied to a predetermined one of the output channels of the gate-circuit means; and decoder means, having a polarity-bit signal input connected to the said polarity-bit signal output channel and at least one amplitude-bit signal input connected to the said at least one amplitude-bit signal output channel of the said gate-circuit means, for reproducing the said analogue signal by generating a sequence of signals having their polarities determined by the signals applied to the said polarity-bit signal input and their amplitudes individually dependent on the bit signals applied to the said at least one amplitude-bit signal channel and also proportional to an average of the values of a sequence of the signals applied to at least one amplitude-bit signal channel.