Patent GB-1365127-A (GB)
Title: Simulating reverberation
IPC: G10K15/12
Filed: 1972-08-17
Abstract: 1365127 Artificial reverberation systems BRITISH BROADCASTING CORP 17 Aug 1972 [17 Aug 1971] 38582/71 Heading H4R Artificial reverberation is added to an input signal by storing an information signal consisting of or derived from the input signal, modifying the stored information signal by an at least approximately random signal and controlling the envelope of the resultant signal to provide a predetermined decay characteristic. An input 10 (Fig. 1) is connected to an analogue to digital converter 12. The digital output of converter 12 is applied successively through storage units 1, 2 ... N consisting of clocked shift registers. The signal at input 10 is sampled in converter 12 at a frequency f greater than twice the highest frequency of interest. Each shift register has a capacity of n samples and is clocked at a rate greater than nf. The effective reverberation time is adjusted by varying f. Each sample is read into shift register 1 through switch SW1a. Switches SW1a ... SWNb are ganged together and positioned so as to move the train of samples in sequence through the shift registers. The switches are then returned to the position shown, to complete recirculation loops for each shift register, and in the time intervals between samples the entire contents of each shift register are clocked once around the recirculation loop. Whilst the samples are being recirculated they are fed to multipliers 14, the other inputs of which receive an output of a pseudo-random sequence generator 16. Each multiplier receives a different output from generator 16. Generator 16 is clocked at the same rate as the storage assembly 1, 2 ... N. Integrators 18 sum the n products generated during each input sampling period. At the end of the summation process the totals obtained are converted from digital to analogue form in converters 20. The outputs of successive converters 20 are modified signals with progressively increasing delay. The outputs are summed in amplifier 24 with a weighting determined by resistors 22. The output of amplifier 24 is added to the original input signal in amplifier 26. The overall envelope of the output is a staircase function having N steps (Fig. 2, not shown). Resistors 22 can be adjusted so that the staircase envelope has a desired shape. The apparatus may be adapted to operate entirely with analogue or digital signals or with any other convenient combination of the two. The described apparatus may have a shift register delay connected between ADC 12 and shift register 1. Additional recirculation loops may be provided around some or all shift registers 1, 2 ... N. The apparatus may be adapted to work with signals in pulse form other than binary pulse code modulation, e.g. differential pulse code modulation and pulse amplitude modulation. Artificial reverberation for stereo signals may be obtained by processing the equivalent monophonic signal (a + b)/2 using the apparatus described and then dividing the reverberant signal between the two output channels. Alternatively such apparatus may be provided for each of the two input signals with appropriate cross-connections between them to give the required degree of correlation between the output signals. Similar extensions may be provided for polyphonic signals.