[sdiy] Consider this DAC

[cheater cheater]

I think that's a moment when topics get broken off. I guess answer
only the stuff relevant to the original topic in here, and answer the
more tangential stuff in a separate thread with the right subject, or
ignore it.

Cheers
D.

On Sun, Mar 14, 2010 at 12:32, Tom Wiltshire <tom at electricdruid.net> wrote:
> Cheater,
>
> Perhaps you should focus on fewer important points, rather than trying to
> answer everything.
> This message(below) is too much to reply to, without making the reply even
> longer.
>
> Thanks,
> Tom
>
>
> On 14 Mar 2010, at 10:50, cheater cheater wrote:
>
>> Piano tuning standards are inadequate for the discussion here for
>> several reasons:
>> 1. As Antti mentioned already, the notes are two- or three-fold unison
>> already and will beat no matter what;
>> 2. Piano timbre is acyclic which hides the phasing artefacts which are
>> easy to find in synth timbra
>> 3. The stretched tuning of a piano, which becomes extremely apparent
>> in the top octaves, hides exactly the effect of tuning deficiencies
>> which become clear on a synthesizer
>> 4. The way that piano is played. It is a different instrument from a
>> synthesizer and I hope everyone finally agrees that for all intents
>> and purposes the only thing it (sometimes) shares is the (unfortunate)
>> keyboard layout.
>> Sustained high-register notes are not used nor are they expected on
>> the piano; they are short, sudden stabs; sometimes they have some
>> sub-harmonic sustain after a glassy hit. Quite often the highest
>> octaves have such weird timbre (due to stretched tuning *and* tuning
>> difficulties) that they are only used as inharmonic stabs /
>> counterpoints / percussive noises. You'll notice that a lot in jazz.
>>
>> All this means that high tunning accuracy in the highest registers,
>> such as postulated in this thread, isn't necessary in the piano.
>>
>> What's more, I think there's no reason to look at industry standards,
>> better use your brain. Right now the industry standard in our craft is
>> the Juno Stage - want to follow industry standards? MIDI has been an
>> industry standard for 20 years and it kills music. Industry standards
>> in VCF modules are bad ripoffs of the EMS or Moog filter. Behringer
>> gear met and still meets all industry standards, such as
>> ISO-1234567-whatever and noise floor and THD.
>>
>> Magnus, if you just want to detune VCOs relatively, you can use a
>> second DAC (especially feeding a V/Hz input). But then you can ask
>> what happens if we use more than one instrument in a song (i.e.
>> always) and I will say that you're right with the absolute accuracy
>> having to meet relative accuracy standards.
>> I think that the answer could be that numerically controlled VCOs
>> should be controlled via a DAC in V/Hz mode. This coupled with an ADC
>> and all that put together into one big feedback mechanism for an
>> adaptive tuning algorithm could work quite well.
>>
>> The adaptive algorithm would only have to work when the pitch is
>> expected to be constant, i.e. when the CV from the DAC is not
>> changing, because that's where I think it really matters - if pitch is
>> changing rapidly, it's more difficult to notice lack of accuracy. So
>> for example, find the moment when the DAC output is constant, and feed
>> statistical data (i.e. frequency noticed vs frequency desired) back to
>> the brain.
>> Or, you could generalize the notion of 'constant', and take a
>> relativistic approach: if the desired pitch right now is x Hz, and
>> speed of change is xHz/s, then the statistical data should indicate n
>> cycles (+- several) during the testing time. That tests the first
>> derivative. You could go and check the second derivative fitting too.
>> This first-derivative approach can be expressed in this way: if you
>> look at the graph of your function of pitch (vertical axis) vs time
>> (horizontal axis), then the segment which you are looking at should
>> fall inside a box of specific height defined by the specific time you
>> are testing for.
>> The first-derivative approach (note that I am not sure this is the
>> right term, I'm sure someone has named this already and it's probably
>> got a smarter name) also answers the question: 'how to make sure that
>> my notes are in tune between eachother when I am performing a
>> glissando?'
>>
>> Joachim, I heard the Q+ has something like 'first order tuning error
>> correction', is that what it is? I never figured out what that meant.
>>
>> With regards to the recurring noise floor question: the biggest
>> problems right now are bad layout; in physically bigger (than 0.5 m^2)
>> systems, the use of single-wire signaling; the parts themselves give
>> us loads of accuracy.
>>
>> With regards to midi tuning: the fact that it specifies an
>> evenly-distributed tuning table only means that no note will be on the
>> pitch that it should be at; this sort of bare-data approach was nice
>> in the 80s when processing one byte could take minutes, but nowadays
>> there's no reason to not specify the exact tuning in a literal way,
>> i.e. send a string with the content "325.14555".
>>
>>
>>
>> On Sun, Mar 14, 2010 at 09:59, ASSI <Stromeko at nexgo.de> wrote:
>>>
>>> On Saturday 13 March 2010, Antti Huovilainen wrote:
>>>>
>>>> Please correct me if I'm wrong, but a TL072 for example has 18
>>>> nV/sqrt(Hz) voltage noise. For 20 kHz bandwidth (RC lowpass at
>>>> 10kHz meaning settling time of less than 0.2 msec) the resulting
>>>> RMS noise is 2.5 uV. That's quite a lot less than 1mV.
>>>
>>> Look up the graph of noise vs. frequency to see that you've missed to
>>> integrate a substantial amount of noise.  The flicker noise corner is
>>> at around 400Hz for the TL072 and the noise figure on the first page
>>> is for 1kHz and a source impedance of 50Ohm (guess what?).  There's
>>> also this pesky little thing called offset and offset drift (with both
>>> temperature and common mode voltage) that will need attention.  But
>>> maybe I'm still missing your point.
>>>
>>> My definition of 1mV _precision_ is that there is at most +-0.5mV
>>> deviation from the ideal CV across all operating points.  For a 10Vpp
>>> signal range that results in about 14bit precision across the whole
>>> system (in voltmeter speak: 4½ digits) - which is doable, but not as
>>> easily or cheaply as you seem to believe.
>>>
>>> Getting back to the CV->VCO example: "merely 1mV precision" across the
>>> whole system (just having this for the CV is not going to cut it)
>>> would allow you to set up two VCO five octaves apart (say 220Hz /
>>> 7040Hz, after calibrating the beats out with both VCO at the same
>>> frequency) and have them beating at less than 2.5Hz.  Not only that,
>>> if you reverse which VCO plays the lower and the higher frequency, you
>>> still get less than 2.5Hz beats.  Modulating both VCO with the same
>>> voltage never produces any faster beats than said 2.5Hz and
>>> transposing the patch four octaves lower (13.75Hz / 440Hz) produces
>>> less than 0.2Hz beat frequency of the 32nd harmonic of VCO1 vs. the
>>> fundamental of VCO2.  Lastly, if you switch that hypothetical synth
>>> off, let it sit for a year and switch it back on, you would get the
>>> same results (not exactly the same beat frequencies, mind you, but in
>>> the same range, perhaps after warm up).  Ian Fritz might have such
>>> wonderfully precisely calibrated VCO, but there surely are a lot of
>>> systems that would fail this test.  In other words their precision is
>>> worse than 14bit/1mV and yet they are still perfectly useable.
>>>
>>>
>>> Achim.
>>> --
>>> +<[Q+ Matrix-12 WAVE#46+305 Neuron microQkb Andromeda XTk Blofeld]>+
>>>
>>> Waldorf MIDI Implementation & additional documentation:
>>> http://Synth.Stromeko.net/Downloads.html#WaldorfDocs
>>>
>>> _______________________________________________
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>>>
>>
>> Cheers,
>> D.
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>
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