GB1462203A - Speech-compressor - Google Patents

Abstract

1462203 Signal compression system CAMBRIDGE RESEARCH & DEVELOPMENT GROUP DT LIQUIDATING PARTNERSHIP S D GREENBERG and M M SCHIFFMAN 31 Jan 1974 [12 Feb 1973 (2) 4 Jan 1974] 04482/74 Heading H4R Sound signals reproduced from a sound record played back at a speed different from the recording speed by a factor C are time compressed or expanded to their original frequency by conversion by a factor I/C with a repetitive and varying time delay which is a function of C. Gramophone record or tape motor speed is adjusted by control 17, Fig. 2, and a function of C, e.g. K(C-1)/(C+1) or lnC, is established in 45 to produce signal V EC which determines by comparison with a reference E the sign and the slope of a ramp voltage V C generated at 42. In the compression mode (C> 1) the ramp starts at 7�8 v. and falls during which time generator 54 produces by charge and discharge of a capacitor (Fig. 2A, not shown) a series of pulses of increasing duration on line 55. The pulses are split, 65, to produce antiphase drive pulses to regulate the transfer of sound from the record through analogue shift registers 68, 69 feeding output amplifier 74 and operating in antiphase to reduce distortion products. When the ramp V C falls to 3 v., an enabling pulse is sent out by unit 43 on line 49 to enable flip-flop 76 such that on the next zero crossing of the sound detected at 74 on line 75 a blanking signal is provided on line 77 to reduce the sound output to zero. The blanking signal also activates generator 101 to cause reset 52 to provide resetting outputs on lines 53 and 83 to commence generation of a new ramp and to reset a counter 81. If there should be no zero-crossing before the ramp falls to 2 v., a signal on line 51 activates the reset 52 to initiate a blanking signal. Further drive pulses 55 are thus generated to empty the contents of the registers and read in the next sound interval during the blanking period. At the end of this time the 256 pulses counted in counter 81 produce a reset output on line 78 so that the next zero-crossing of the sound input to amplifier 74 removes the blanking signal to restore sound output. For C = 1, the "ramp" has no slope and the driver pulses 55 are of constant repetition rate thereby producing no frequency change of the sound. For expansion (0<C<1) the ramp is of positive slope and the drive pulses are initially long but reducing. The level of reset value in the ramp generator is then V EC rather than the fixed value E = 7�8 v. Otherwise the operation is similar. Gaps in the stretched sound output in the compression mode, and possibly also in the expansion mode where C is near unity, may be filled by portions of the sound delayed in a shift register (168, Fig. 5, not shown) controlled by a square wave generator (152) and inserted into the blanked portions of the output by a flip-flop (176) and amplifier (174) in the same configuration as Fig. 2. Gap-filling in the expansion mode operates primarily by altering the ramp reset characteristic such that for small values of C reset value is linear, falling with increasing C, but is constant at 2 v. for 1>C#0�92.