Patent GB-1406831-A (GB)
Title: Controlled delay line signal processor for sound reproduction
IPC: G10L21/04
Filed: 1972-08-11
Abstract: 1406831 Time compression or expansion systems CAMBRIDGE RESEARCH & DEVELOPMENT GROUP DT LIQUIDATING PARTNERSHIP S D GREENBERG and M M SCHIFFMAN 11 Aug 1972 [13 Aug 1971 7 Feb 1972] 37553/72 Heading H4R Signals from a tape recorder which is playing back a recording at the wrong speed are fed to a delay line which is arranged to provide an an output having a linearly variable delay relative the input so that the signals have their frequency components restored to the original range. Discontinuities between samples after passage through the delay line are eliminated by blanking and gap filling, smoothing, or arranging that the sample ends at a zero crossing and a gap filling signal commences at a zero crossing in the same direction. The variable delay may be provided either by an analogue delay line whose overall delay can be continuously charged, e.g. by having the line comprise R-C sections in series with the R's varied to vary the delay, using an analogue or digital shift register whose shift rate is varied or using an analogue or digital shaft register or memory matrix which is filled at a different rate to its reading rate. As described in Fig. 7, a playback device 51 has its speed controlled by a device 52 which also controls, in 58, the slope of a sawtooth voltage, produced by generator 64, which controls the delay of a delay line 56 through which the output from the recorder 51 passes. Sections of the output from the recorder 51 are therefore expanded or compressed in time at the output of the delay line 56 depending on the polarity of the sawtooth voltage. When the delay of line 56 has been varied from one extreme to the other a discontinuity will be generated during a fast change back to the original delay and during this time the output of the line is blocked by the blanking circuit 71, and a signal can be interposed in the resulting gap by the gap filler 75. Fig. 9 (not shown) shows a system for derivation of the gap filling signal from another variable delay line fed with a similar but phase shifted sawtooth voltage to the first variable delay line. The second line may be of similar length to the first, or, for economy, a short line just producing a signal long enough to extend over the gap. Another alternative, Fig. 12 (not shown), uses a fixed delay line connected to the output of the variable delay line to provide the gap filling signal. To produce a binaural output each channel contains a variable delay line controlled by respective relatively phase shifted sawtooth voltages with gap filling signals for one channel provided by the signal in the other channel and vice versa, Fig. 15 (not shown). A shift register may be used to provide the delay with the delay controlled either by varying the shift rate, Figs. 17, 19, 20 (not shown), or by feeding signals in at a different rate to that at which they are read-out, Fig. 26 (not shown). Alternatively addressable memory matrices may be used of either analogue form, Fig. 21 (not shown), or digital form, Fig. 22 (not shown). In order to reduce discontinuities at the beginning and end of the gap filling signal the blanking signal can be arranged to start at a zero crossing of the variably delayed signal and the gap filling signal to start at a similarly directed zero crossing, with a similar arrangement at the end of the gap filling signal.