[sdiy] Analogue Drift (was Re: HF VCOs and tracking problems)

[Tom Wiltshire]

Richie,

As Ian said, thanks for such a full report. Fascinating. And well done for getting the results I failed to get.

As I'd come to suspect, the *actual* sources of drift in an analog synth don't look much like people expect - no nice slow wobbly drift, instead, a bit of noise, some hum, and some spikes and jumps!

Your comment about sliders being firmly down is also significant. Designers of digital synths get used to zero being zero, but if the aim is for "virtual analog" authenticity, it'd be better if zero on the control was in fact just a few units above zero in reality! Of course, this setting is available already on every digital synth, so perhaps the secret to realistic analog-style sounds would be to leave everything turned on just a tiny bit. Personally, I'm a fan of the almost-imperceptible modulation, so I'm in favour.

It certainly isn't difficult to add a smattering of 50Hz modulation and a dash of noise to VCO CVs, along with just a hint of sub-osc and some subtle soft-clipping to the output. This probably gets you most of the way there. Pep it up with a splash of tabasco and a grind of black pepper. If you've got more destinations and more sources, go for your life, being sure to keep everything just below the level at which it becomes obvious what you're up to!!

Thanks again,
Tom


On 14 Nov 2012, at 10:15, rburnett at richieburnett.co.uk wrote:

> Hi Tom,
> 
>> Have you managed to get any decent data for this, Richie?
> 
> I did quite a bit of this VCO drift characterisation in my lunch hour at work a few years ago now.  Sadly don't have the actual results files now as my lab PC at work died, or I would have sent them to you.
> 
> The synths I tested were SH-09 (old single oscillator Roland monosynth), TB-303, JX-3P (DCO), alpha-juno2 (DCO), Casio VZ-1 (digital).  I set each synth to generate a raw oscillator waveform in the 3 kHz region, tuned my rock-solid GPS locked oscillator to the same frequency and then let the drift test commence.  I'd summarise the results as follows.  The SH-09 had the most noticeable drift, then 303, JX, alpha-juno and finally VZ-1 in that order.  As for the causes of the pitch drift...  The SH-09 had significant frequency-modulation of the VCO by 50Hz, 150Hz, 350Hz, 450Hz components.  I tried beefing up the power supply smoothing, but it turns out that this is due to capacitive coupling between the mains wiring to the transformer and the sensitive traces of the expo-converter and VCO that are only 6" or so away.  (Proved this by temporary screening of mains cable with grounded foil shield.) I remember noting that the shape of the "mains modulation" waveform was also highly distorted, presumably due to the capacitive coupling favouring the higher harmonics of the nasty looking mains voltage waveform.
> 
> The second biggest contributor was LFO modulation, even though the VCO MOD slider was firmly at zero.  Again this waveform was also distorted, and its influence on the VCO frequency looked like a severely high-pass filtered square wave.  My proposed explanation for this is that as the LFO cycles between charging and discharging, it's current demands from the +ve and -ve rails switch abrupty.  It also flashes a LED on the panel, all of which cause momentary supply voltage fluctuations before the regulators servo the supply voltage back to the nominal values a millisecond or two later.  This affects the oscillator pitch slightly.  I also noticed that notes started ever so slightly flat each time a key is pressed and then quickly slid into tune!  Presumably also due to transient loading on the supply by the envelope circuitry, or voltage drop created along common ground traces on the PCB?  Then finally there was an underlying long-term drift.  This tended to always be in one direction, presumably as the synth warmed up, or the room temperature changed, but it would often go in fits and starts.  Sometimes the drift would *almost* stop, and other times it would suddenly jump a few fractions of a cent.  This could easily have been due to people opening the door to the lab, and wafting air currents etc.  What I didn't see was any interesting filtered noise type fluctuations in pitch that I had theorised might be due to shot-noise or something in the expo-converter. I also didn't see anything that I could put down to supply voltage fluctuations, but the building has its own dedicated distribution transformer right near where I was working that so I wouldn't have expected the supply to vary much at my location.
> 
> I think the tests for the TB-303 were done at a lower frequency than 3kHz, which limited the bandwidth of the drift I could measure.  The majority of the drift on the TB-303 was from four sources.  Firstly, long-term thermal, secondly the tempo clock seems to modulate the VCO pitch ever so slightly, thirdly the flashing of panel LEDs shifts the VCO frequency by almost 0.5 cent, and lastly the pitch bends quite noticeably at the onset of notes again!  I guess that's to be expected though on a cheaply made battery powered device.  (It was run from batts during the testing, and you can even see the battery LED flicker slightly as notes start and stop!)
> 
> The digital VZ-1 and alpha-juno were pretty much rock solid relative to the GPS clock, with only ppm level drift immediately after switching on. The JX-3P was the strange beast where I got some conflicting results, depending on the pitch offset between the oscillators!  I think it plays tricks with detuning one high-frequency RF oscillator relative to another inside to get small oscillator detuning amounts and suspect this is susceptible to one oscillator injection locking the other when they are close to integer ratios in frequency.  I never spent any time looking into this, but suspect the designer knew about this issue because there's a little metal plated mounted on the PCB to screen each oscillator from the other!
> 
> Unfortunately didn't have a minimoog available to test, but would have been interested to see how it's oscillators behaved as they are obviously held in high regard for their sought-after analogue sound.
> 
> So there you have it, if you want your digital oscillators to sound more "analogue" then modulate the pitch with a bit of mains hum, and a bit of differentiated LFO waveform, and make it occasionally lurch up or down in pitch when someone opens a nearby door or window!
> 
>> I had a go using a friend's 24-bit 192-KHz audio interface to record
>> files which I then analysed post-recording with the computer. I was
>> hoping to get enough data to allow me to generate a realistic "drift
>> waveform" for a given synth or oscillator, but I never got very far
>> with it. I wasn't convinced that the results I got weren't just
>> artefacts of the process rather than actual data.
> 
> You can do it this way as well.  The trick is to up-sample (interpolate) your recorded waveform so that you can determine the VCO period to a resolution better than whole samples.  Otherwise you'll get severe period quantisation in your results.  You only need to interpolate the waveform around the zero crossings though.  So for instance you could record the VCO waveform at 48kHz, but up sample by 1024 using sinc interpolation just around where the waveform went though zero and changes sign.  This would then give you an effective sample rate of 49MHz which is enough to see tiny variations in the period of oscillation from one cycle to the next.  (A lot of people think that detailed timing information like this is lost when you sample a signal at a finite rate.  The information is still there embedded in the sampled data, but you need to sinc-interpolate the sampled data to see where events happened in between discrete sample points.)  Bare in mind that this technique is also subject to the clock jitter in your sound-card too, and any less-than-ideal sample-rate conversions etc. that Windows might decide to apply to its output data, so beware!  That's why it's handy to have a rock steady pure tone available as a control to check your measurement process is sound.
> 
> -Richie,
> 
> 
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