My evil modular plans -- patch cords

[Mikko Helin]

   DSP's are not that expensive now, but you may like to implement
   more than one module with one DSP. So you need some patching
   inside a module, and when you are going to expand your system
   you may want patch one function on DSP 1 to another on DSP 2,
   so you'll come to decision to use a common shared bus. I think
   it's best done with physically shared RAM and with an operating
   system that treats all modules inside a DSP as separate processes
   which get their input from OS scheduler - a process which looks
   for messages sent to special process (like VCF) containing data
   to be processed. OS patching system then decides where that data
   is then sent next. I'm not sure if OS is needed at all, if
   every process has it's own dedicated data input and data output buffers,
   propably ping-pong style dual-port RAMs (or conventional RAM
   with flags). Then it's only question of knowing which address
   to use for input and which one for output, and some timing
   information is needed, at least a flag which tells when it's
   ok to read or write data. D/A is then the last module process
   which then simply sends it's input data through serial port
   to DAC, which may be CS433x series or better one. Between
   DSPs real dual-port RAMs are needed anyway (visit
   www.qualitysemi.com for more information). If data format
   is fixed between and inside DSPs, whether it's integer, fixed
   or floating point, and (dual-port RAM) interface is defined
   I guess different DSPs can be mixed.

   But if you limit one DSP for one function at a time, it's
   easiest to use digital signals without A/D & D/A between,
   simply connect (buffered!) sample clock, sample data and word clock
   signals like in analog system, and derive all clocks from a
   common master clock to avoid EMI. There may be problems with
   jitter if clocks are chained too long.

   Just some thoughts,

   -Mikko

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>
>gstopp at fibermux.com wrote:
>> 
>>      Okay if you make a modular synthesizer out of modules that are digital
>>      inside, do you bring out all the patchpoints to panel jacks so that
>>      you can use patchcords to connect them to other modules, analog or
>>      digital? If you do, then this means A/D and D/A and the subsequent
>>      potentially cumulative conversion errors.
>> 
>	Why not bring out both digital and analog to a single patch connector ?
>For example, if you are using 1/4" phone plugs run the analog to the tip
>and digital to the ring. If you use a standard mono patch cord you would
>route analog to another module or external device. If you use a patch
>cord (stereo for example) that contains the ring, you would instead
>route the digital. You could even use a Y cord and split off analog and
>digital. There could be some issues of possible interference due to the
>close proximity of the digital and analog but I think this could be
>taken care of with some careful design.
>
>Dan Slater (dslater at ix.netcom.com)