Abstract: A MODEM for a high frequency radio data link which substantially enhances the recovery of data transmitted via the link in the face of multipath distortion and fading. A serial digital input signal for transmission is split into odd bit and even bit data streams, having synchronous bit timing. Each pair of bits, referred to as a symbol, is phase shifted by .pi./4 which encodes its timing, for later clock recovery. The resulting signal is then twice differentially phase encoded which facilitates the recovery, when demodulated, of the original symbols overcoming the effect of Doppler shift and/or transmitted oscillator center frequency offset. The resulting signal then phase modulates a subcarrier signal having a frequency much higher than the symbol rate, and the phase modulated subcarrier is converted to a spread spectrum signal for transmission. This digital signal is provided to a high frequency analog transmitter for transmission with the signal spread over a particular HF channel, to a receive point.

Abstract: A transversal filter echo canceller includes two stores each for storing a set of coefficients for the transversal filter. The suitability of a set of coefficients stored in one of the stores is checked by supplying a pulse to the receive path and transversally filtering it, and comparing the level of the resultant transmit path signal, after subtraction of the transversally filtered pulse, with a threshold. At the same time the impulse response before the subtraction is stored in the other store. If the threshold is exceeded the functions of the two stores are interchanged and the check is repeated. The checking can be effected only once for a transmission line not subject to change, or, in the case of a transmission line which is subject to change, at the start of each telephone connection established via the line and optionally during conversation pauses during the connection.

Abstract: At a transmitter, an analog NTSC color video signal is converted into a digital video signal and all of the signal components in the chrominance spectral band in alternate television frames are inverted. The resultant signal is transmitted via a low-speed digital signal transmission channel using interframe difference coding. At a receiver, the received digital signal is converted into an analog video signal and all of the signal components in the chrominance spectral band in the other alternate television frames are inverted. The resultant signal, having inverted chrominance, is reproduced on a conventional NTSC color television receiver. Bandpass and comb filtering, preferably on the digital signals, are used at the transmitter and at the receiver to separate the luminance and chrominance signal components to enable inversion of the chrominance components.