[sdiy] Bunching of MIDI clock messages

[Adam Inglis]

On 13/09/2013, at 3:28 AM, Colin f wrote:
>>
>> On 12/09/2013, at 10:20 PM, Richie Burnett wrote:

>> Well, that's exactly what machines like Roland's TB-303, TR-606,  
>> TR-808 and TR-909 actually do.  Their tempo clock is a free-running  
>> analogue oscillator that is polled every 2 milliseconds by the CPU,  
>> and the instrument trigger pulses are then all generated  
>> simultaneously for the appropriate instruments.


>>> And, before anyone says that it's the 2ms polling that's the  
>>> problem... It has been implemented this way because the real  
>>> instruments I'm modelling (TR-808 and TR-909) have a CPU that runs  
>>> on a 2ms interrupt clock.  All of their instrument triggers being  
>>> synchronised to this internal 2ms interrupt clock.  I'd like to  
>>> retain this as it is one of the things that gives these  
>>> instruments their much sought after "groove".
>>> -Richie,
>
>> Nah, that's a myth.
>> Slop and randomness do not make a groove.
>
> Randomness may not make a groove, but a cyclic relationship between  
> the
> master clock leading edges and a polling CPU responding to them at  
> different
> points in its 2ms period can.
>
> Cheers,
> Colin f


I appreciate you guys are talking at a technical level that is mostly  
beyond my ken, but I'm very interested in the practical result.

So a free-running tempo clock gets polled 500 times a second by the  
CPU, and this gives rise to a cyclic variation in an otherwise steady  
tempo?
Or, are you saying that the cyclic variation affects where exactly an  
intrument is triggered on a given beat subdivision? Either way, it  
would have the same effect.
OK, now if this cyclic variation was to have a constructive  
contribution to a particular machine's groove, I would have thought  
the cycle would have to affect some multiple of beat subdivisions that  
recurred in one, or maybe two, bars of 4/4 time (these machines aren't  
famous for their use in jazz time signatures!).
Years ago, before I adopted the Innerclock solution, I looked at tempo  
clock stability in various machines here including the 808 and 909,  
both in master and slave roles. I measured the number of samples  
between consecutive quarter notes. There was both random variation and  
cyclic variation, but the cyclic variation was odd-numbered, 9 or 7,  
something like that. That sort of cycle would actually have a  
destructive contribution to a 4/4 groove, worse than random, which is  
why I've always thought the claims made about these machines intrinsic  
magic/groove/feel/woteva were rubbish.

cheers
Adam.