Abstract: A transmission system in which a suitably modulated speech or data signal is combined with a low bit rate (100b/s) spread spectrum signal and transmitted simultaneously on the same channel with the modulated speech lying within the bandwidth of the spread spectrum signal.At a receiver the modulated speech or data signal is demodulated and the modulated speech or data signal together with the spread spectrum signal are applied to a multiplier in which the spread spectrum signal is despread.A code synchronizing signal may be included in the modulated signal prior to transmission and when recovered in the receiver, the synchronizing signal is applied to a pseudo-random code generating means. The transmission system may include a text transmission system in which case the low bit rate signal comprises pages of data. A number of pages may be transmitted simultaneously by spreading the data associated with each page by orthogonal psuedo-random codes.

Abstract: A direct conversion receiver for demodulating an FM signal converted frequency down converts the signal to quadrature related baseband signals (I,Q). These signals are applied by way of respective d.c. blocking capacitors (16, 18) and differentiating circuits (26, 28) to a differential arctan demodulator (20). The action of the differentiating circuits (26, 28) is to remove any d.c. component in the I and Q signals and to perform a frequency shaping which enables the demodulator (20) to recover correctly the modulating information. Measures are disclosed to overcome the effects of 180 degree phase jumps which will occur when the direction of rotation of the phasor reverses and of frequency dependent amplitude scaling introduced into the differentiated signals (I', Q').

Abstract: An SSB signal demodulator comprising an analog bandpass filter for band-limiting such signal, an analog-to-digital converter 20 for digitizing the band-limited signal, and a pair of quadrature related decimating filters (28, 29) for deriving quadrature related decimated signals therefrom which are applied to a Hilbert transform pair (30, 31). The upper or lower sideband signal is obtained by taking the sum or difference of the outputs of the Hilbert transform pair, and the resulting digital signal is reconverted to an analog signal. By using a decimation factor which is an odd numbered integer greater than 1, the periodic transfer function of the demodulator has alternate passbands and stopbands. This relaxes the performance requirements of the analog filter and of the Hilbert transform pair, and greatly enhances the adjacent channel selectivity of the demodulator.