In those transparent tone-in-band systems which employ a single oscillator in the transmitter to generate a frequency notch between upper and lower portions of a band which is to be transmitted, there is not complete freedom to choose the width of the notch. In the present invention which also uses a single transmitter oscillator, one portion of the band transmitted is selected using a filter. A mixer is used to change the frequency of the input band in a way which does not allow the lower sideband to "fold" back at zero frequency and interfere with the lower transmitted portion. A further filter selects the other portion for transmission from the output of the mixer.
In deriving a control signal for recombining the parts of the spectrum separated by a frequency notch in transparent tone-in-band signals, cross-correlation products give rise to undesirable frequency and phase jitter. In a receiver which overcomes this problem an input signal E containing the notch is passed to two first mixers and then portions corresponding to the lower and upper portions of the input spectrum are selected by two mirror filters before being translated in frequency by two second mixers to a final output band in which the notch is eliminated. Two selection filters select parts of the spectra at the outputs of the first mixers which correspond to an overlap region of the said lower and upper portions when derived in a transmitter and apply them to a control-signal mixer. A control signal for the second mixers is derived from the output of the control-signal mixer by noise filtering and frequency division by two. The above mentioned problem is overcome by giving the mirror filters transfer functions H.sub.1- (f) and H.sub.1+ (f) which are the same as the transfer functions of mirror filters in the transmitter which produced the notch and by giving the selection filters transfer functions H.sub.2- (f) and H.sub.2+ (f) which satisfy the relationships H.sub.2- (f)=.vertline.H.sub.1- (f).vertline..sup.2 and H.sub.2+ (f)=.vertline.H.sub.1+ (f).vertline..sup.2.
A system for voice distribution amongst a plurality of telephone subscriber locations operating on a broadband cable network wherein RF transmitting and RF receiving channels for carrying voice and signalling information are established at the subscriber locations and coupled with the broad band network and a central switch is also coupled with the broadband network and enables each RF transmitting channel to be selectively coupled to any of the RF receiving channels. Phase correction pilot tones having a known phase relationship are generated at the telephone subscriber locations and transmitted in one or more phase correction pilot tone channels separate from the RF transmitting channels to the central switch. At the switch, the phase correction pilot tones are recovered and the phases of the pilot tones are used to correct the phases of the transmitting channels.
