[sdiy] asm-1 connectivity

[Oakley Sound]

> I remember a discussion on this list about 2 years ago about the
accuracy of the Paia midi/cv and its quite surprisingly accurate,
surpassing some 12bit designs.

Folks may want to check the archives on this. I had quite a lot to say
on the Paia design, and even wrote to John Simonton about this after I
bought mine.

To summarise, there are quite a few deficiences with the design. The
main one being that it uses only an 8-bit DAC. OK, midi note CV is only
a 7-bit [or even 6-bit with reduced note cover] application, but we need
better accuracy than the +/-0.5LSB at eight bits to give you the correct
pitch CV. The DAC-08 used in the midi2CV8 is simply not accurate enough
to hit the right note all the time. In other words, you may set your
octave perfect to C0 to C1, but it won't give you a perfect octave from
C1 to C2. In other words for every incremental increase in midi note
number, the output CV does not jump exactly 83.33mV. It could be 70mV,
but it could also be 90mV.

You can improve the accuracy of the Paia unit by fitting the DAC08HP
from Analog Devices. Its pin for pin compatible but features +/-0.25LSB
accuracy. This will improve CV accuracy noticeably, so that you will get
worst case accuracy of around +/- 5mV.

We have had many a discussion on this list about what the required
accuracy should be, but I feel that 12 bits (+/-0.5LSB) is the minimum
that I would now accept. The proviso with any multiplying 12bit DAC is
that you must chose 10.66V for the reference, so that every incremental
step is an exact fraction of 1/12V (within the bounds of +/-0.5 LSB).
Voltage output DACs can also be used but scaling should be applied after
the DAC to get the appropriate 1V/octave. In these cases, the DAC's
input data's lowest 5 bits would be tied to ground, or held low at the
appropriate time for a serial stream.

16 bit DACs (or higher) can probably be used with any reasonable
reference, and the scaling be done digitally via all 16 input bits using
look up tables or whatever. I guess the newer midiCV convertors from
Kenton and others do their scaling in this manner.

Regards,

Tony Allgood                         www.oakleysound.co.uk

Oakley Modular Synthesisers      Penrith, Cumbria, England