21 Mar
2015
21 Mar
'15
3:56 a.m.
jack MIDI bridges delay all MIDI events by 1 process cycle, after
timestamping them with the offset into the current process cycle when they
are received.
this increases latency by 1 cycle, but eliminates jitter, which is the most
desirable tradeoff.
so a MIDI event arriving T samples into process cycle N will be delivered
to clients in process cycle N+1 with a timestamp of T.
On Fri, Mar 20, 2015 at 7:52 PM, Len Ovens <len(a)ovenwerks.net> wrote:
On Fri, 20 Mar 2015, Joakim Hernberg wrote:
with jitter. Further, at least on windows the midi isn't handled at all
by the ASIO driver, so I doubt that the ASIO
buffer size would have any
impact at all on midi timing. I think this is different on linux using
jack midi, but i suspect that the alsa sequencer won't be tied at all
to any buffer size that JACK uses. I might be wrong about this though?
The alsa sequencer gets it's timing from the OS, It seems to say from the
wall clock, but is in fractions of a second from when the alsa sequencer is
started. Raw MIDI is just as soon as it comes in... but I am not sure what
buffering there is or not. I would think that the buffer should be cleared
each and every time any byte comes in. That would be the only way single
byte midi rt commands could be real time. I do not see irq that belongs to
my midi port (mpu401 clone), but it may be called from the ensoniq driver.
I do not know how jack gets it's date stamp for a midi event ( or if that
time is at the first or last byte of an event... jack deals with midi
events) I would assume if it is using alsa-seq, it uses the time stamp from
there. With raw I don't know. From Paul's comment, I would guess the sample
at the end of the last buffer input plus time to calc current sample time
stamp.
jack1 uses ajmidid internally, and ajmidid is recommended forjack2 as
well. I don't have a clue how time stamp is created with this.
I think with firewire, the time stamp is a part of the transport... but it
has been a while since I looked at it.
--
Len Ovens
www.ovenwerks.net
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