[sdiy] DAC uses (was Re: Appearance of many SSM2300 chips)

[dan snazelle]

great explanation Tom

I for one have always wanted to understand better the logic required to make Multiplexing a reality.




On May 29, 2011, at 6:28 AM, Tom Wiltshire wrote:

> 
> On 29 May 2011, at 02:42, Dylan Distasio wrote:
> 
>> Hi all-
>> 
>> I'm new to the list and new to diy synth design, and am trying to soak
>> up as much as possible.  I realize this is a very beginner question
>> for most of you on the list, but if someone has a few minutes, could
>> you please explain the typical applications of Digital to Analog
>> Converters / sample and hold multiplexers in the synth world.  I saw
>> in the datasheet of the original that these can be used as compressors
>> or expanders (I assume of dynamic range), but was hoping to get some
>> real world examples of how these are used in a design.
>> 
>> Hope it's ok to ask questions this basic on the list.  Thanks!
>> 
> 
> Hi Dylan and welcome,
> 
> A typical 1980's-era analogue polysynth uses DACs to provide control voltages (CVs) to the analogue voice circuitry. There's a microprocessor that reads the position of front panel controls (with an ADC) and deals with keys pressed and MIDI input if its present. It can store patches into memory or alter settings using the panel controls.
> 
> The voices themselves could be entirely analog (Prophet 5 or Jupiter, say) and the microprocessor controls them by sending each voice several control voltages - typically at least VCO pitch, Filter cutoff, and filter resonance, plus voltages to control envelope generators (voltages for A, D, S and R, possibly twice if you've got two envelopes) and any modulations that might be present. Multiply this by 5, 6 or 8 voices and you need a lot of CVs.
> Since DACs were expensive back then (especially good ones) the usual way of doing it was to use one DAC followed by a lot of multiplexers with sample-and-hold cells. The multiplexers are often 4051 analog switches, and the sample-and-holds are often a simple cap to ground followed by a high impedance op-amp buffer. You can find many schematics online that follow this general pattern or some variation of it. The Prophet 5 is one of the first and a classic example. In fact, you can argue that many of the others simply copied the design.
> 
> As time passed, more parts of this system became digital, with envelope generator and modulation voltages being generated by the microprocessor (or by a dedicated processor in some instruments) rather than using CVs to control analog env gen hardware (like SSM2056 or CEM3310/CEM3312 chips). This evolution is visible in other later Sequential gear - it's interesting to compare the Prophet 5 with things like the Multitrak or the Prophet T8, which use a lot less analog circuitry.
> 
> HTH,
> Tom
> 
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