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    SubjectRe: Bad MIDI performance : 10ms latency instead of the expected 1-1.5ms
    On Thu, 26 Aug 1999, Jaroslav Kysela wrote:

    > > I wrote this to measure the MIDI output-to-input delay.
    > > I tried both with blocking I/O and non-blocking I/O.
    > > Unfortunately I get bad values in both modes:
    > > about 11.5ms , and this is exactly the MIDI transfer time (1.3ms) plus
    > > 10ms = 1 jiffie.
    > > If I run the test on a HZ=1000 kernel I get about 2ms.
    > MPU401 is very bad hardware for transmit, because it doesn't use interrupt
    > to determine that Tx FIFO is free. Some vendors offers large FIFOs
    > (8,12,16 bytes), but unfortunately guys at Creative designed SB AWE with
    > 2 byte FIFO.

    Hmm .. so you are saying that this is a HARDWARE limitiation ?
    How does Windoze manage this ?

    > This doesn't explain why your latencies are so big for OSS/Free, because
    > OSS/Free code uses polling mode (busy loop for transmit).

    What is the main drawback of this ?
    What happens when I send a block of 3000 bytes to the midi device under
    OSS/Free ?
    A 100% CPU usage for 1sec ?
    I can't believe this.

    > ALSA uses system timer to avoid busy loop, but the performance depends on
    > your HZ value:
    > 100Hz, 2 byte FIFO = 200bytes/sec
    > 100Hz, 8 byte FIFO = 1600bytes/sec
    > 100Hz, 12 byte FIFO = 2400bytes/sec
    > 100Hz, 16 byte FIFO = 3200bytes/sec

    200bytes/sec is just rudiculous , how do you plan to drive an external synth
    with that little MIDI bandwidth ?
    Many songs uses up much of the 3000 bytes/sec bandwidth, especially
    when there are many controller/pitchbend events present.

    Do you know if the RX FIFO of the MPU401 has the same problem (no interrupt) or
    is there an interrupt present ?

    Is there no other way to make the MIDI response better, without increasing HZ
    to 1000 ?

    Does this mean that sequencers running under Windows have the same problems
    as we in Linux on the AWE64 ?
    Or does Windows a "sane" busywaiting all the time ? :-)
    With these big midi delays you can't simply use your PC to act as a midi-tru
    box, playing an external synth via masterkeyboard connected to the midi-in.

    > Not affected hardware:
    > GUS soundcards
    > AudioPCI chips (ES1370/1371/1373)

    If this is true, then I will throw away my AWE64 and buy an AudioPCI


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