GB1487216A - Digital signal processor - Google Patents

Abstract

1487216 Multiplex communication systems TELECOMMUNICATIONS RADIOELECTRIQUES ET TELEPHONIQUES 26 Nov 1974 [29 Nov 1973] 51131/74 Heading H4M In arrangements for either converting a single side band frequency division multiplex signal into the corresponding base band channel signals or vice versa and in which the channel signals are separated or combined by digital techniques making use of digital filters and Fourier transform circuits, the digital filters are of recursive type. To convert a single side band frequency division multiplex signal applied at 1 into base band channel signals, binary coded samples of the multiplex signal are derived by A/D converter 5 (following frequency transposition at 2 and filtering at 4) and applied to a series parallel converter 6 which distributes the samples to signal paths S 0 -S 7 such that samples occur in each path at a frequency which is the same as the bandwidth of a channel signal. Each path incorporates a delay r 0 -r 7 to eliminate the mutual shifts between samples within a given cycle. The outputs of the delays are applied to recursive digital filters A 0 -A 7 , to which are also applied signals representing filter coefficients from a source 7, and the filter outputs pass to the inputs # 0 -# 7 of a Fourier transform circuit 8. This additionally receives signals representing complex coefficients corresponding with channel carrier signals from a source 9 and provides a set of output signals C 1 , C 2 , C 3 , corresponding to the required channel signals which are fed to demodulators d 1 , d 2 , d 3 . The filter coefficients characterize an amplitude frequency characteristic of a low pass filter which has a cut off frequency corresponding to the half band width of a channel signal and a phase frequency characteristic representing the difference between the phase frequency characteristic of the filter and that of the reference filter Ac associated with the reference delay ro. The phase frequency characteristic of the recursive digital filter is sawtooth shaped such that in combination with the linear characteristic of its associated delay, a stepped characteristic is established. The recursive filters, e.g. A 1 , each have a recursive section 12 which is the same for all filters A 0 -A 7 , all receiving the same coefficient signals and a non recursive section 10 which in each filter receives the same number of coefficient signals as does the recursive section. In a multiplexer (Fig. 10, not shown) similar but complementary techniques are used.