[LAD] [Fwd: Re: No nagging, a serious question]

[Pieter Palmers]

On 07/04/2010 11:16 PM, Dan Mills wrote:
> On Sun, 2010-07-04 at 22:56 +0200, Ralf Mardorf wrote:
>
>> Good to read about this issue. I always disable the on-board audio
>> devices, but I would add a second PCI card to my PC and sync it with
> the
>> already installed sound card, so I better don't do it. It at least
> would
>> be nice to have several MIDI IO by simply using some cheap Envy24
> cards.
>> Unfortunately those cheap cards seems to use just one of the two MPUs
>> supported by the Envy24.
>
> Note that I didn't say Linux does this (it would break separation
> between the midi and audio subsystems), just that you can do it if you
> know when developing the midi ISR exactly what hardware you have
> available on the audio side (Specifically what address the cards
> playback position register is).
>
> The thing is you really want that timestamp to be based on the soundcard
> clock that will ultimately be outputting the samples generated as a
> result of that midi, difficult with things like Pulse (and to an extent
> Jack) in the way and obviously unworkable with netjack.
>
> This sort of issue is what the ALSA midi sequencer is really intended to
> fix, by making midi timing a kernel problem rather then a user space
> one. It sort of solves HALF the problem, in that it makes getting midi
> messages out on time more or less possible (at least on paper), but it
> does nothing for reliably timestamping incoming midi using a clock that
> can be related simply to the audio sample clock and which would thus be
> useful when building software synths.

I cannot help but noting that the 'most bone-headed way of doing 
low-latency audio' called firewire provides timestamps  related to the 
sample clock for midi messages (even each MIDI byte). The timestamps are 
valid accross multiple devices.

The FFADO backend uses these to position the midi messages in the jack 
buffers with sample accuracy. Therefore all jitter you see when using 
the a firewire device with FFADO and jack is either due to MIDI itself 
or due to the hardware you use. But not due to the software stack 
(modulo bugs), the protocol nor the 1394 bus.

Regards,

Pieter