[sdiy] Analysis of frequency variation in analogue synths

[Tom Wiltshire]

Hello All,



>> I realize its been debated a lot, and I don't think Tom or I was
>> trying to heat the debate.

Sorry if anyone feels like I took the lid off the wormcan when we'd  
only just managed to force it back on!

>> What we both seem to be after is finding
>> the conclusive evidence you mention.

Absolutely.

> Conclusive evidence is hard. Not only do you need to measure  
> various sources
> for a number of suspected effects (AM, PM, Moon phase or whatever),  
> but you
> need to measure them well enought from a large number of sources  
> and then have
> them blind-tested on a large enought scale of people that know  
> whatever
> psycho-acoustical sensation(s) you are after and is able to rate  
> that sensation
> for each of the test objects. This part of the test also needs  
> about the same
> signal or else the test subjects have too easy to spot a Moog from  
> an ARP or
> whatever. Thus the blindness needs to be fairly complete. Only  
> after analyzing
> this and seeing correlations between the test-persons judgement and  
> some of
> the measures you made on the source, you are approaching conclusive  
> evidence.
> You can naturally convince yourself by reducing the test-person to  
> yourself,
> but you must recall that you don't have conclusive evidence that  
> will count
> with everyone. The experiment I presented above would also have to  
> be repeated
> by independent groups and come to similar enought conclusions for  
> it to attempt
> to be conclusive. It also needs to be published in a good enought  
> journal with
> sufficient review to start to count as evidence. This is how the  
> scientific
> community works. Everything else we do is interesting exercises.  
> Those can be
> made more or less skillfull.

For myself, before I can think about the next step of whether a  
certain type of error or drift is associated reliably with 'the  
analogue sound' or the 'moog sound' or 'arp sound' or whatever, I  
need to see some measurable effect, which I haven't managed to do  
reliably.
Although I've heard much heated debate about VCOs being better than  
DCOs (or not) and many presentations of the reasons that this might  
(or might not) be the case, there isn't a great deal (notice I'm not  
saying none) of actual experimental data showing measurable  
differences. What does the frequency/phase drift on a VCO look like?  
Show me a trace! How much comparator jitter is there on a Moog VCO?  
Give a percentage! Unless I can measure and identify these effects, I  
don't stand a chance of trying to emulate them in a digital  
oscillator, which is my ultimate motivation for all this.

Given that a bench frequency generator, a Moog modular, an ARP  
odyssey, and a Prophet 5 can all generate a ramp wave of a given  
frequency (say, concert A 440Hz for the sake of argument) and some  
aficionados would claim that all of these sources have a different  
character (and I'm quite prepared to believe that they do) then what  
are differences between them? What aspects of the waveshape? What  
variations of time? Which parameters? Once we know that, we can start  
trying to work out which of those factors are significant to the  
individual sounds and do the sort of "Is it a Moog or a fake?" double- 
blind trials that would turn it into a serious scientific paper (if  
anyone felt the need!).

Magnus clearly has much more experience and equipment for measuring  
these things that I do, and it may be that the CD quality sampling  
that I was using to get data is insufficiently accurate to find any  
useful effects. However, even if I were able to show that this were  
the case, that in itself would be an interesting result for a digital  
oscillator design.

If none of that works - do Rentokill deal with escaped worms?!

Tom