[sdiy] asm-1 connectivity

[Magnus Danielson]

From: "Oakley Sound" <oakley at techrepairs.freeserve.co.uk>
Subject: Re: [sdiy] asm-1 connectivity
Date: Sat, 8 Nov 2003 12:11:45 -0000

Hi Tony,

> 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.

I agree that this is a problem. With increased number of bits comes the
increased requirement on linearity, or rather, it remains about the same
(+/- 1/2 LSB) but that the amplitude is scaled down.

> 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.

I think we must think about not only notes themselfs but also in note-bending,
after-touch etc. If you want these to perform sufficiently smooth operation you
don't want them to jump 50 cent jumps or whatever, you want to perceive a fairly
smooth change. I think 14-16 bit is required for the range being in use in MIDI
gear. I.e. the full dynamics of control signals goes beyond just what note was
intended to play.

Cheers,
Magnus