linuxaudio.org

On Wed, Jul 14, 2010 at 08:09:29PM +0200, Ralf Mardorf wrote:

quoted text
> On Wed, 2010-07-14 at 19:56 +0200, Arnout Engelen wrote:

> > On Wed, Jul 14, 2010 at 03:23:03PM +0200, Ralf Mardorf wrote:

> > > I disconnected all audio connections for JACK and connected hw MIDI in

> > > to hw MIDI out. connected the DX7 MIDI out directly to the D4 MIDI in and

> > > then I reconnected to the PCI card.

> > >

> > > The difference is alarming :(.

> > >

> > > Yamaha DX7 --> Alesis D4 results in a 100% musical groove.

> > > Yamaha DX7 --> PC --> Alesis D4 results in extreme latency

> >

> > So here you're directly routing the MIDI IN to the MIDI OUT, and

> > experiencing latency. Are you using JACK here, or directly ALSA? In other

> > words, are you connecting 'in' to 'out' in the qjackctl 'MIDI' tab or in

> > the 'ALSA' tab?

>

> I'm connecting MIDI in the Qtractor (quasi QjackCtl) ALSA MIDI tab.

OK, if that's causing noticable latency, there's something odd going on - and

we should first see if we can fix this: layering more stuff (jack, a2j,

fluidsynth, qtractor etc) on top of this (apparently) weak foundation will

just confuse us.
Before finding out how to prevent 'noticable latency' for this use case, I'd

say it would be good to try and quantify this for a bit. 
I took a MIDI Keyboard (m-audio keystation) and a Synth (Yamaha VL70-m),

connected them to each other directly, put a mic close to the keyboard, and

hit a key repeatedly with my nail. 
The recording has a nice plastic 'tick' of me hitting the key with my nail,

and the softsynth sound starting a fraction of a second later. Looking with

audacity, the total nail-to-synthsound latency is about 20-26ms.
Then I plugged the synth into my USB audio/MIDI card (Edirol UA-25EX) and

connected the MIDI keyboard to my laptop (directly with USB). 
Did the same test again, recorded it, and the nail-to-synthsound latency now

seems to be rougly in the 23-26ms range.
To *me*, this doesn't really seem to be a very noticable/problematic latency -

but I'm not a keyboard player, I might not be so sensitive. I remember playing

a MIDI wind controller at different latencies (I'm a saxophone player) - I'm

not sure if I could *hear* it, but I could sure *feel* the difference.
The wavs are at http://arnout.engelen.eu/files/dev/linuxmusicians/latencytests/

for your enjoyment. Such beautiful music!
It might be interesting if you could make similar recordings - see what kind

of latency gets unacceptable, if the latency is mostly constant or very

jittery, if it's much bigger than here or that you're just more sensitive than

me, etc.
Arnout
(the laptop used in these tests is a 2ghz single-core debian machine without

much tuning - the kernel doesn't even have preemption enabled, let alone the

-rt patch)

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