[sdiy] as3340 PW/tuning issue

[Richie Burnett]

Deficiencies of the as3340 aside, if you apply pwm to a pulse train generated from a sawtooth source using a comparator it *DOES* intrinsically modulate the pitch.

This is because generating a pulse train from a sawtooth source only modulates one of the edges of the pulse. For example, the rising edge is coincident with the saw reset event, but the falling edge moves back and forth as the LFO output varies over time. This means that the centroid of each pulse moves earlier or later as the LFO waveform is either rising or falling. The faster the rate of change of the LFO waveform the greater the pitch modulation. Try whacking LFO speed up and listen to the seasick pitch warble on most synths with pwm!

However, pwm generated from a triangle waveform source results in both edges of the pulse waveform being modulated equally and in opposite directions. Therefore the centroid of the pulses doesn't move and there is no resulting pitch modulation. That's one of the advantages of using a triangle wave instead of sawtooth to generate PWM waveforms. 

-Richie,

Ps. Happy new year! 



---- Brian Willoughby wrote ----

>I didn’t pay close enough attention the first time through. I assumed that people were listening to the Square a.k.a. Pulse waveform output when adjusting Pulse Width, since that’s what it’s intended for.
>
>Tom, you said you were listening to the Ramp wave alone on Oscillator A, with LFO routed to PW on Oscillator A. That shouldn’t have any audible effect at all.
>
>Are folks saying that the pitch of non-Pulse non-Square waveforms drift when Pulse Width modulation is added on the appropriate input pin? That would be quite unexpected.
>
>I recreated the quick test on my Pro-One, and I hear what sounds like a sort of frequency modulation when the Triangle LFO is directed through the Wheel to Oscillator A PW and the shape selector for Square engaged. But there are psychoacoustic effects in play, because even amplitude modulation can be perceived as pitch modulation.
>
>In any event, some clarification would be educational … or at least a link to those Muff’s threads.
>
>
>On Dec 19, 2019, at 6:57 AM, Tom Wiltshire <tom at electricdruid.net> wrote:
>> A quick test on my Pro-One doesn’t reveal any tuning problem. I routed the LFO to PW on oscillator A, then listened to Oscillator A’s ramp wave alone (so no beating to confuse things) and couldn’t hear any difference, even with maximum mod depth and maximum LFO rate. Checking the output on the oscilloscope showed no pitch variation either.
>> 
>> That said, there’s some evidence Sequential *did* select 3340s. The ones regarded as “not good enough” were used for LFO duties, and if you check the PCBs you’ll see paint blobs on those ones. I’ve never seen anything documenting what the tests were though. Maybe they just built the things and swapped out any that caused a problem, put a blob on them, and then used them later where it wouldn’t matter.
>> 
>> A more interesting test which I might get to later would be an A/B/C comparison on my 3340 test board between CEM3340 G, AS3340, and V3340. Despite having all three chips on the shelf, I’ve never actually done this.
>> 
>> Tom
>> 
>> On 19 Dec 2019, at 12:06, Mattias Rickardsson <mr at analogue.org> wrote:
>>> I remember talking to Dieter Doepfer about this back in the day... Ah, here it is:
>>> 
>>> /mr
>>> 
>>> At 17:52 2003-02-02, Dieter Doepfer wrote:
>>> > > -----Ursprüngliche Nachricht-----
>>> > > Von: Mattias Rickardsson 
>>> > > Gesendet am: Mittwoch, 29. Januar 2003 11:10
>>> > > An: hardware at doepfer.de; technik at doepfer.de
>>> > > Betreff: A-111 problem? (PW affects Frequency)
>>> > >
>>> > > Hi,
>>> > > I have a problem with several of my A-111 High-End VCOs:
>>> > >
>>> > > The pulsewidth (both PW knob and PWM input+knob) affects the frequency
>>> > > of the oscillator. I.e., when turning the PW knob or PWM modulating with
>>> > > a slow LFO, the frequency makes a clearly audible change.
>>> > >...
>>> >
>>> >Dear Mattias,
>>> >I forward your inquiry to our A-111 specialist Matthias Marass
>>> >(mailto:keyboards at doepfer.de). He is responsible
>>> >for the final tests and repairs
>>> >of the A-111. He will answer you directly.
>>> 
>>> At 15:36 2003-06-11, Döpfer Musikelektronik - Keyboardservice wrote:
>>> Dear Mr. Rickardsson,
>>> 
>>> the influence of the external and internal PW
>>> control voltage to the oscillator frequency is a
>>> problem of nearly 80..90% of the CEM3340 we ever
>>> used. Unfortunately there is no chance to repair
>>> this problem. The only way would be a strict
>>> selection of the CEM3340 circuits.
>>> ...
>>> Best regards,
>>> Matthias Marass
>>> 
>>> 
>>> Den tors 19 dec. 2019 10:42Oakley Sound via Synth-diy <synth-diy at synth-diy.org> skrev:
>>>  > ... "So as a conclusion the goal is to have on pin3 as close as 
>>> possible to -5V and the PWM to tune error will not manifest itself."
>>> 
>>> I did not find this to be the case since I always run any 3340 circuit 
>>> from a precision 5V reference. The solution for me was to get the 
>>> CEM3340 rev G. The CEM3340 has the detune too, it's just that it is 
>>> quite small. I can't remember exactly, but I think it's in the region of 
>>> +/-1 cent. It would almost certainly be there on the SH-101.
>>> 
>>> Alfa's solution is to use the newer AS3340-HYB. The other thing to do is 
>>> make your own triangle or saw to pulse wave convertor from an op-amp or 
>>> comparator external to the chip.
>>> 
>>> When repairing an OB-Sx I found that the AS3340 did not work well as a 
>>> sub for the original CEM3340. Even with the various resistor changes 
>>> required to get the AS3340 to work properly, the VCOs in last three 
>>> voice card positions on the motherboard displayed a very audible pitch 
>>> instability at pulse widths at less than 45%. Clearly, there was some 
>>> layout issue but although I tried all sorts of things (including bus bar 
>>> 0V and liberal dousing of capacitance) it wouldn't solve the problem. 
>>> Again, the solution was to replace all the VCOs with CEM3340 G.
>>> 
>>> Tony
>>> 
>
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