GB1310538A - Data transmission systems - Google Patents

Abstract

1310538 Multiplex transmission INTERNATIONAL BUSINESS MACHINES CORP 30 April 1970 [30 April 1969] 20895/70 Heading H4R Data signals are transmitted simultaneously with speech signals in a gap formed in the middle of the speech frequency band. At the receiver a portion of speech is synthesized, using vocoder techniques, to fill the gap in which the data was transmitted. As shown in Fig. 2a, a speech signal from a subscriber's line circuit LC is passed through parallel high and low pass filters HPF1 and LPF1 to remove the band from 1300 to 2300 Hz and replaced by data signals from a transmitter DT. At the receiver the data signals are recovered by a band-pass filter BPF7 and passed to a data receiver DR while the speech bands are recovered using high and low pass filters HPF2 and LPF3 similar to those used at the transmitter. The speech signals are fed direct to a summing unit SA4 and to a non-linear circuit NLC which derives an excitation function signal which is separated out into two synthesis channels by BPF1 and BPF3, passing respectively 1200 to 1900 Hz and 1700 to 2400 Hz, to be modulated by signals derived from an analysis and interpolation of the energy in the bands of the received speech signal adjacent the edges of the gap. BPF5 and BPF6 pass signals in the ranges 900-1300 Hz and 2300 to 2700 Hz, respectively, and the energy in these bands is detected by rectifiers R1 and R2 and low pass filters LPF3 and LPF4. The signals corresponding to the energy levels in these bands are fed to combining networks SA2 and SA3 where they are combined in appropriate proportions to provide interpolations of energy level at frequencies corresponding to the locations of the synthesis channels. The outputs of the networks SA2 and SA3 are fed to the modulators in the synthesis channels to provide signals which fill in the gap in the speech spectrum. In an alternative system, Fig. 3a (not shown), the energy level in the untransmitted band in the original speech is detected at the transmitter and the resulting signal modulates the data signal amplitude. At the receiver the energy level signal is recovered and used to control a synthesis channel in the centre of the untransmitted band and to provide a signal for interpolation with further analysis channels as previously described to provide further synthesis channels at the outer edges of the untransmitted band.