[LAD] MIDI granularity

[Greg]

What Will said. Numbers:

For a hypothetical controller and MIDI device that speaks USB 2...
USB 2 has a minimum latency of ~.125ms and a bandwidth of 480 Mbps.
Note on/off = 3 bytes; 24/480M = .00005ms, which we can round to 0.
Even on USB 1.1, that's only 24/12M = .002ms.
(I ignored any other bits that may have to be transmitted.)

It seems like <1ms for a note message is pretty reasonable expectation.
In fact, it sure seems like <1ms total for 100 messages might be reasonable. :)

I figure most of the meaningful delays will come from the driver and OS
scheduling, but I think the hardware is more than capable.

cheers,
Greg

On Thu, Jun 16, 2022 at 10:38:11PM +0200, Fons Adriaensen wrote:
>On Thu, Jun 16, 2022 at 08:37:49PM +0100, Will Godfrey wrote:
>
>> Over a hardware DIN port this is of course approx. 1mS, but does anyone know
>> if it's the same over a USB link?
>>
>> Presumably, it would have to be if the source was also sending to the DIN
>> route.
>
>Not really, it could be much faster. When the source starts transmitting the
>first byte on the standard MIDI port, it probably has the complete message
>(3 bytes for a note on/off) ready. There is no reason why the USB message
>should be derived from the serial MIDI data.
>
>> Following on from that, what about the multiport adaptors that have 4 hardware
>> ports going down one USB cable. I would guess that these could be interleaved
>> so that (assuming the 1mS granularity holds) the overall rate is still 1mS with
>> the ports spaced up to 250uS apart.
>
>AFAIK, USB transfers are not interleaved at byte level, doing that  would
>create a lot of overhead. So even if you have a note event on all four ports
>at the same time, that would probably be 4 separate USB transfers, one after
>the other. Still this could take less than 1 ms, it just depends on the USB
>data rate.
>
>Ciao,
>
>-- 
>FA
>
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