[PD] about latency

[vanDongen/Gilcher]

Hi,

A soundcard has an input and an output buffer. Both are divided into chunks 
called fragments or periods. Audio is read and written in these chunks.
The input latency is usually one chunk, and the output latecy is the entire 
output buffer.
I am not sure how portaudio handles all this, but the principle is the same 
for all systems.

To sync recorded material with an input stream you need to add a delay to the 
recorded material of a chunk's duration.

But there is also latency jitter. It can be caused by processor load. An 
(almost) inaudible buffer overrun (xrun) can cause a jump in the input 
latency, because the systems misses a deadline, so it skips a chunk.
It will probably also result in output buffer underruns,but that doesn't 
affect sync, it just caused an unsteadiness in tempo I think (?). The overall 
result is that the latency changes. Because your chunk size is probably 
pretty large, it is very noticable. 

 You could try increasing the buffer so that the processor has more "breathing 
space", or you could try to decrease your fragment/chunk size, so that the 
jitter is smaller. But that requires more processing power, so you will have 
more smaller but more frequent variations.
There are some command line arguments to set these. 

The processorload  might be caused by the loading of the patch. So you should 
check if it also happens if you turn on the audio processing after you have 
loaded all the patches. 

hope this helps
Gerard



On Wednesday 16 July 2003 00:03, Alex wrote:
> Hi,
>
> When I start up pd My version of portaudio tells me the latency I have:
> 46ms
> I have built a patch that records sound input on a tempo and I have
> noticed that my sound takes some times to get into pd. I have always
> thought that latency was the time the sound would put to get in then
> out but I thought that the sound would put something like 0.005ms to
> get in and a file played from the HD would put the same. So as I did
> not care about direct monitoring stuff, I did not mind.
> Recently I have recorded samples on a tempo generated in pd and when
> playing the samples back I have noticed that my samples were not
> synchronized with the tempo. Can somebody confirm me that the signal
> get some significant time before getting into pd ? Or else ?
> Also, I have then tried to play my samples a bit earlier than the tempo
> something like 89ms and then my samples where back on tempo. But 2 days
> later that 89ms became 130ms. (I used different patches) Can the
> in/output latency be variable according to the CPU I use ? Humm...
>
> Alex
>
>
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