This is a simple Surround Processor with the degital delay method. This audio processor is not using any special function ICs that difficult to get parsonaly, and desined in only general purpose ICs.
Of course the Digital Delay Block of A-D, D-A and delay logic is also using only general purpose ICs. This is the key feature that easy to build it.
The type of this Surround Processor is generating the surround effect with processing two channels of stereo source. Most of those are generates the surround effect with separates reverberations from source signal and applies any processes, and then mix it to front channels or output as rear channel.
Normally, the sounds (vocal or instruments) are contained in both channels with same amplitude and phase. But phase of the reverberations is disordered between both channels, and it appears as difference between both channels. When erase the same phase sounds between both channels in any way, it will able to separete only reverberations.
This is easy to do depend on subract R-channel from L-channel with a differential amplifier. This method separates reverberations, and difference of other than those (panned part, phase error, etc) as well. It is not much influence on the actual use. And these effects vary by confditions of the source signal.
The sound created in digigtal processing is separated better than plain sound as live recording. When build an experiment circuti temporary, you will be able to make sure the effect that supressing the center voice and separating the reverberations.
Separated reverberations are applied to delay as sounds refrected in the hall. This is important process on the every surround processors. Delay time is tipicaly several ten miliseconds, and this time affects the tendency of the surround effect.
Reverberations are amplified with power amplifire, and output to the rear speakers. In actual case, the reverberations reach to the listener from all directions. However main speakers are plased in front normaly so that reverberations channels place in the rear of the listener to create surround effect.
Please open the Circuit Diagram, and read below with looking it.
The difference between both channels is separated with difference amplifire of the OPAMP. And higher frequencies in the difference signal are cut with the LPF.
These are composed with general purpose ICs. The A-D converter is simply delta modurator that using a comparator and a D type flip-flop. And it converts analog signal into digital data of one bit, 2 Msa/sec. After passed digital delay, the bit stream is directly conversion into analog signal with the integrater.
It is not good that signal to noise ratio and distortion as this A-D, D-A converter.
For the digital delay process, a DRAM chip is used as a FIFO memory. This circuit design requires a 64K bit DRAM, but I used a 256K bit DRAM found in the junk box. Shift register generates Read-Modify-Write cycle, read out old data and store new data in a single cycle.
And lower byte of the address counter is assined as row address of the DRAM to increment row address every cycle, so that refresh cycle can be omitted. The FIFO length is 65536 bits and 2 Msa/sec makes 33 msec of delay time.