[CM] Timing drift

[Heinrich Taube]

srry  i forgot havent tested 3.4.0 except on win32. please use chicken  
3.0.0 if you do this test.
On Apr 8, 2009, at 5:09 AM, Heinrich Taube wrote:

> it would be useful to know if you get the same drift using chicken
> 3.0.0 or 3.4.0
> (im soory  have very little time to work over the next week -- )
> if you can hack C you might try taking the time just before and after
> the process node calls into scheme and then subtracting the difference
> between these time stamps from the amount you increment the node by
> for its next run time. in other words to subtract out the time spent
> in scheme.
>
>
>
> On Apr 8, 2009, at 2:07 AM, Neil Baylis wrote:
>
>> I'm seeing timing drift between two processes running cm3 on OSX/
>> Leopard. I'm also seeing the timing drift on Linux that another user
>> reported, but I think this case is unrelated to that one.
>>
>> Here's the code:
>>
>> (define (note-cycle endtime keys rate)
>>  (process with pat = (make-cycle keys)
>> 	   while (< (elapsed) endtime)
>> 	   do
>> 	   (send "mp:midi" key: (next pat) dur: rate)
>> 	   (wait rate)))
>>
>> (let* ((k1 (key '(c4 g4 a4 bf4)))
>>       (k2 (key '(c2 e2 g2 a2 bf2 a2 g2 e2)))
>>       (stop 8000))
>>  (sprout
>>   (list (note-cycle stop k1 .4)
>> 	 (note-cycle stop k2 .2))))
>>
>>
>> If I define the two time delays to be equal, the processes seem to
>> stay in sync for at least a few minutes. But if I define them as
>> above, they drift apart fairly soon.
>>
>> I think the reason for this is that there is some overhead in the
>> process that gets added on to the wait time for each iteration. So
>> if the wait time is shorter, the loop executes more often, and the
>> overhead is incurred more often. (The overhead would be everything
>> in the loop apart from the wait time).
>>
>> What's the correct way to fix this? I can think of a way to do it
>> that involves sprouting only a single process, and sending multiple
>> notes each time through the loop, but it gets ugly fast if there are
>> more than a couple of parts/instruments.
>>
>> I guess what I want is to wait an amount of time that would bring me
>> to the start of the next beat, but I'm not sure how to express that.
>> I tried the following, but it didn't help:
>>
>> (define (note-cycle endtime keys rate)
>>  (process with pat = (make-cycle keys)
>> 	   for t = rate by rate
>> 	   while (< (elapsed) endtime)
>> 	   do
>> 	   (send "mp:midi" key: (next pat) dur: rate)
>> 	   (wait (- t (elapsed)))))
>>
>> (I was attempting to pre-compute the value of "elapsed" at the start
>> of the next iteration.)
>>
>> Basically I'm wanting to have one process run at an exact multiple
>> of the speed of another.
>>
>> Neil
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