[sdiy] Choosing a DAC

Jerry Gray-Eskue jerryge at cableone.net
Mon Jun 29 17:31:00 CEST 2009 

<<My plan is to have 64 note values (a bit more than 5 octaves)>>

Well if you want to have some fun and dink around with this you could build
your own DAC pair.

I would suggest setting up two R2R DACS (one 4 bit and one 3 bit )using hand
matched resistors, This gives you control inputs of 0-15 for 4 bits, so if
you use 1.25 volts as the Vref of the first DAC it will have the magic
0.0833... volts per step.

The second 3 bit DAC ranges 0 - 7 so if you use 7.00 volts as its Vref you
have 1 volt per bit, or one Octave per bit.

Now that you have the two hand made DACs you feed the outputs into a voltage
summing setup using an opamp.

Here is the fun part, The music scale is basically base 12 so if you use
base 12 to drive the DACs it all works out, that is to say your input to the
DACs runs like this:

Octave DAC  Note DAC
   0            0
   0            1
   0            2
   0            3
   0            4
   0            5
   0            6
   0            7
   0            8
   0            9
   0            10
   0            11
   1            0
   1            1
   1            2
   1            3
   1            4
   1            5
   1            6
   1            7
   1            8
   1            9
   1            10
   1            11
   2            0
....

- Jerry

-----Original Message-----
From: synth-diy-bounces at dropmix.xs4all.nl
[mailto:synth-diy-bounces at dropmix.xs4all.nl]On Behalf Of stew
Sent: Monday, July 06, 2009 9:46 PM
To: Synth-diy at dropmix.xs4all.nl
Subject: Re: RE: [sdiy] Choosing a DAC


Thanks for all the replies.

I was more concerned about accurracy than resolution. Some 12 bit DAC's have
2 LSB intial error, whereas others have 0.5LSB.

My plan is to have 64 note values (a bit more than 5 octaves). I could
possibly change it to have more but I don't think it would be very useful
(others might) and the pot (user input) resolution starts to become a
problem. For example, using a 12 bit DAC I would multiply each note value by
64, so there would be 64 steps between each value that is output from the
DAC.

I guess I'll have to use an adjustable reference. I was planning on scaling
it afterwards but it would make sense to have the reference scale it.

I'll have to think more about this when I get home from work...

Regards,
Stewart


> Jerry Gray-Eskue <jerryge at cableone.net> wrote:
>
>
> 5 octaves * 12 semitones = 60 steps.
>
> The 1 volt per octave is a liner input at 0.0833... Volts per semitone
> (1Volt/12).
> If you setup your LSB at this voltage an 8 bit DAC can range over 21
> octaves
> (255/12) however
> that requires over a 21 volt range.
>
> Assuming you are thinking of 0-5v for your output and using 4 steps per
> semitone you can cover 5.3125 octaves using 5.3125 as the Vref. each
> step
> would be 25cents or 0.02083... Volts.
>
> If you wish to use a 5.00 Volt Vref you will need a higher resolution
> DAC,
> for Good results you want to always be +- 3 Cents or less, preferably
> less
> than +- 1 Cent.
>
> roughly at 5 Volts Vref it runs
> Bits
> 		  Steps/Octave	S/Semi	C/Step
> 8	255	      51	      4.25	      23.52941176
> 9	511	      102.2	      8.516666667	11.74168297
> 10	1023	      204.6	      17.05	      5.865102639
> 11	2047	      409.4	      34.11666667	2.93111871
> 12	4095	       819	      68.25	      1.465201465
> 13	8191	      1638.2	136.5166667	0.732511293
> 14	16383	      3276.6	273.05	0.366233291
> 15	32767	      6553.4	546.1166667	0.183111057
> 16	65535	      13107	      1092.25	0.091554131
> 17	131071	26214.2	2184.516667	0.045776716
> 18	262143	52428.6	4369.05	0.022888271
> 19	524287	104857.4	8738.116667	0.011444114
> 20	1048575	209715	17476.25	0.005722051
> 21	2097151	419430.2	34952.51667	0.002861024
> 22	4194303	838860.6	69905.05	0.001430512
> 23	8388607	1677721.4	139810.1167	0.000715256
> 24	16777215	3355443	279620.25	0.000357628
>
> As you can see an 12 bit DAC looks very good at 1.46 Cents per step.
> And a 16 Bit DAC is well into the overkill range at 9 Hundredths of a
> Cent
> per step.
>
> - Jerry
>
>
>
> -----Original Message-----
> From: synth-diy-bounces at dropmix.xs4all.nl
> [mailto:synth-diy-bounces at dropmix.xs4all.nl]On Behalf Of Stewart Pye
> Sent: Monday, July 06, 2009 4:57 PM
> To: Synth DIY
> Subject: [sdiy] Choosing a DAC
>
>
> Hi,
>
> I'm designing an "analogue" sequencer around an AVR micro and I need to
> choose a DAC for the CV output. It will on only span 5 octaves (0-5V)
> and output only the equally tempered scale. (there will be a separate
> circuit for glide).
>
> I guess my question really is - how many millivolts can you be off by in
> a 1V/octave system before it becomes noticeable?
>
> Regards,
> Stewart.
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