[PD] Debian, Pd and Midi

[Nicola Pandini]

Il 13/02/2012 01:07, Martin Peach ha scritto:
> On 2012-02-12 17:32, Nicola Pandini wrote:
>> Hi, I'd like to use Pd for midi processing, and I'm trying to find the
>> flags to have the best "Pd for midi" possible.
>> For me midi latency is an extremely important factor, so all my efforts
>> were in the way to optimize it.
>> I did some experiments, playing with flags and seeing what happens (I
>> use Pd 0.43.1 on Debian Wheezy, with jack 44.100 - 128 frames):
>>
>> With
>> -rt -jack -audiobuf 1 -alsamidi
>> I get 2.9 ms of midi latency
>>
>> Adding
>> -noaudio
>> I get two possible values: 1.45 or 2.9 ms
>>
>> Adding
>> -r 192000
>> I get 1.33 ms
>>
>> I don't know why I have to modify the sample rate (-r) to affects the
>> midi latency.
>
> Pd sends non-audio messages in between audio blocks, usually every 64 
> sample frames. MIDI is sent via a Pd message. You may not get latency 
> so much as jitter on the millisecond level, as all messages are 
> quantized to the audio block period.
> At 44100Hz, 64 sample frames take up 1.4512 ms, so Pd messages are 
> spaced at multiples of that period.
> So you get either one or two frames latency depending on when you send 
> the message relative to the audio block.
> Running at a higher sample rate gives you lower message latency.
> Add to that the latency of the MIDI driver and hardware, which is 
> probably less than 1ms. As well a full 3-byte MIDI message takes 
> 0.96ms to send at 31250Hz (30 bits * 32us/bit).
> Probably you could get lower latency by encoding the MIDI directly as 
> an audio waveform but you'd need to make a hardware circuit to convert 
> the audio levels to MIDI levels.
> Since sound travels about 1 meter in 3ms, it's not all that much latency.
>
> Martin

Thank you Martin, now it's all more clear.

-- 
Nicola