[sdiy] AD/DA schematics? quantizers,etc

Thu May 14 18:47:11 CEST 2009

I am sure the staircase is excellent but what I am referring to is matching
the output steps to note voltages.

This stuff is only important if you are a perfectionist.

Take a theoretical VCO in perfect tune at 1volt per octave using -5v to +5v
as its input range. Each octave has 12 semitones or notes, this means that
each increase of .08333... volts will increase the frequency by exactly one
semitone (1volt/12semitones). If each step (100 cents) (or 2 steps 50 cents)
is at this voltage you are half way there. The drawback is that the
transitions are right on the note and bit jitter can cause you to be 50 to
100cents (a full semitone) off the note. If you center the desired voltage
( a multiple of .08333... volts ) in the "flat" part of the step instead of
the transition bit jitter in a "quite" system is no problem. An easy way to
do this is to add 1/2 of the .08333... volts on the input. You can match 2
"steps" to .08333... volts by setting up the converter(s) to divide evenly
by the number of semitones in the full 10 octave range. 10 Octaves with 12
Semitones each = 120. An 8 bit converter has 255 "steps" so you can use
2*120 or 240 steps and hit the semitones square on. To do this all you need
to do is set the converter(s) so that -5v = 0 steps and +5 = 240 steps. Of
course this requires that you use precision reference voltage(s) and limit
circuit noise as a variation of 8.333...e-4 volts on the output is One Cent
and people with "perfect pitch" can hear a variation of 3 Cents (0.0025
volts). The other consideration is using multiple voices with minor pitch
variation causes sub harmonics that even us mortals with far less that
"perfect pitch" can hear.

-----Original Message-----
From: synth-diy-bounces at dropmix.xs4all.nl
[mailto:synth-diy-bounces at dropmix.xs4all.nl]On Behalf Of Matthias
Herrmann
Sent: Thursday, May 14, 2009 10:50 AM
To: Synth-Diy
Subject: AW: [sdiy] AD/DA schematics? quantizers,etc

> Make perfect sense to me and sounds like a solid design.
> As long as you are not using the control voltage to run a VCO at
> 1v/o you do
> not have to be concerned about where the output voltages make transitions.

i do! and the staircase tracks nice over 10 octaves (at least for me it is
okay, i did not do exact measurements, though).

cheers,
matthias
___
www.modular.fonik.de

>
> It had slipped my mind that some control voltages are -5 to +5 and others
> are 0 to +10v.
>
>
> -----Original Message-----
> From: synth-diy-bounces at dropmix.xs4all.nl
> [mailto:synth-diy-bounces at dropmix.xs4all.nl]On Behalf Of
> Matthias.Herrmann at jeppesen.com
> Sent: Thursday, May 14, 2009 10:02 AM
> To: synth-diy at dropmix.xs4all.nl
> Subject: RE: [sdiy] AD/DA schematics? quantizers,etc
>
>
> i did not follow the whole thread, however i am currently working on a
> quantizer that would be usable for control votages in the range of -5V to
> +5V.
>
> i used a DAC0800, that is setup to operate in a bipolar mode (output from
> -5V to +5V using a REF102 10V reference). as we already learned the output
> is not exactly 10V p-p when using only 7 bits, therefor i added an
> invering mixer with offset and gain control to trim the DACs output to
> exactly -5V to +5V.
>
> hope that makes any sense in this context. as said above i did not follow
> the thread toroughly. sorry.
>
> cheers,
> matthias
> ___
> www.modular.fonik.de
>
>
>
>
> synth-diy-bounces at dropmix.xs4all.nl wrote on 05/14/2009 04:46:36 PM:
> > <<The Scaling is already in place on the BACK END D to A using the 10v
> vref
> > in
> > this schematic.
> > This does not take care of any Offset you may have.>>
> > ====
> > That's not quite right - with a 10v vref you still need to scale the
> output
> > but the factor is
> > (OUT 10v/9.412v = 1.06)
> > ====
> >
> > -----Original Message-----
> > From: synth-diy-bounces at dropmix.xs4all.nl
> > [mailto:synth-diy-bounces at dropmix.xs4all.nl]On Behalf Of Jerry
> > Gray-Eskue
> > Sent: Thursday, May 14, 2009 9:33 AM
> > To: Synth-diy at dropmix.xs4all.nl
> > Subject: RE: [sdiy] AD/DA schematics? quantizers,etc
> >
> >
> >
> > I should not try to do math at midnight when I am tired. I have a couple
> of
> > inaccuracies in the message below.
> >
> > 1 The "Step" value of an 8 bit converter with 5v ref should be 5v/255
> not
> > 5v/256 ~= 0.0196078 volts. This in turn modifies the Scale Factors (IN
> > 4.706v/10v = .4706) (OUT 10v/4.706v = 2.125) Note:"Step" 240 = 0.01960v
> *
> > 240 steps = 4.705872 ~= 4.706
> >
> > 2 Do not "SUBTRACT 0.01v" on the back end, this would null out Centering
> the
> > desired values on the steps and throw all the output "Steps" off 1/2
> "Step"
> > and off "Note"
> > ========
> > IF you setup your DAC0808 as in the Data Sheet- FIGURE 1. +10V Output
> > Digital to Analog Converter You will see that the DAC0808 Can take 10v
> for
> > Vref+ SO things get simpler You only need to do the FRONT END scaling
> into
> > the A to D
> >
> > The Scaling is already in place on the BACK END D to A using the 10v
> vref in
> > this schematic.
> > This does not take care of any Offset you may have.
> > ========
> > To clear up Offset and Scaling -
> >
> > Offset is a DC voltage in an audio signal that pushes the waveform away
> from
> > centering on 0 volts, Typical audio signals run balanced amounts of +
> and -
> > voltages. Mathematically this is ADDITION or SUBTRACTION of a Fixed
> Value to
> > ZERO out the offset, Decoupling Caps may also be used to remove offset.
> >
> > Scaling is MULTIPLYING or DIVIDING a value to change its overall peak,
> or
> > peak to peak voltage.
> > =======
> >
> >
> > -----Original Message-----
> > From: synth-diy-bounces at dropmix.xs4all.nl
> > [mailto:synth-diy-bounces at dropmix.xs4all.nl]On Behalf Of Jerry
> > Gray-Eskue
> > Sent: Thursday, May 14, 2009 12:03 AM
> > To: Synth-diy at dropmix.xs4all.nl
> > Subject: RE: [sdiy] AD/DA schematics? quantizers,etc
> >
> >
> >
> > Let me try to clear up the Vref thing for you.
> >
> > If you are just running audio signals through the converters all you
> need to
> > do is scale down the 0-10v to 0-5v run it into the A to D run the Data
> lines
> > into the D to A and scale the output back up to 0-10v, the drawback here
> is
> > that the audio signal is running with a 5v offset and needs to have that
> > removed either by subtracting the voltage or using capacitive coupling
> into
> > your amplifier.
> >
> > If your analog signal IN is centered on 0v i.e. -5v to +5v your scaling
> will
> > need to scale the input down to -2.5v to +2.5v and Add a 2.5v offset so
> that
> > the signal into the A to D is 0v to 5v. In this setup vref+ = 5v and
> Vref- =
> > 0v is good. Again the offset needs to be removed.
> >
> > Now If you are tiring to quantize a Control Voltage and want it to "Hit
> > Notes" you have other considerations. Take a control Voltage of 1 volt
> per
> > Octave from 0 - 10v. In this case Each Volt represents 12 "Notes" for 10
> > Octaves Or 12 * 10 = 120 distinct notes. An 8 bit converter has 0-255 as
> the
> > output or 256 "steps" 256/120 = 2.133 in other words it is a mismatch.
> you
> > need to match the 120 notes to "steps" So you can set it us to use 240
> of
> > the 256 steps with 16 "steps" unused. In other words Using a vref+ of 5v
> and
> > a vref- of 0v you want 10v in to result in the converter outputting
> "Step"
> > 240. Each Step of the A to D is 0.01953125 volts, we want 240 of these =
> > 4.6875 volts, So the input to the A to D should scale the 10volt input
> at
> > 4.6875/10 or 0.46875 .
> > If the D to A is also using vref+ 5v and vref- 0v we need to do the
> reverse
> > to scale back up to 10 volts 10/4.6875 or 2.1333...
> > I am just about to tired to think but I also believe that we have at
> this
> > point centered the "Notes" on the Step transitions, we really want them
> > centered between the transitions, so as part of the Front end (A to D)
> after
> > the scaling we need to ADD 1/2 of a Step to the Voltage into the
> Converter,
> > or 0.01953125/2, 0.01v is close enough, and coming out of the Back end
> (D to
> > A) we need to SUBTRACT 0.01v before scaling the voltage back up.
> >
> > I hope this helps clear things up and not create total confusion.
> >
> >
> >
> >
> > -----Original Message-----
> > From: synth-diy-bounces at dropmix.xs4all.nl
> > [mailto:synth-diy-bounces at dropmix.xs4all.nl]On Behalf Of Dan Snazelle
> > Sent: Wednesday, May 13, 2009 10:39 PM
> > To: sdiy
> > Subject: RE: [sdiy] AD/DA schematics? quantizers,etc
> >
> >
> >
> > ok
> >
> >
> >
> > i feel like a fool
> >
> > in all my reading and testing today i had somehow missed the fact that
> > on an ADCchip, the vref cant go below grnd (or not much below)
> >
> >
> >
> > so i am confused. usually when i see a - sign i think of putting the
> > negative rail. but in this case it seems you pick a top voltage and a
> > you pick a bottom but that has to be within 0-vref
> >
> >
> >
> > is this correct? and the vcc is always 5volts?
> >
> >
> >
> > how high can the vref be?
> >
> >
> >
> > a few people have mentioned that for cv, i want a 10v vref.
> >
> >
> >
> > would a high vref be useful for audio as well? or a lower vref useful
> even?
> >
> >
> >
> > thanks
> >
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