[LAU] Multi-port MIDI interface for PC ?

Clemens Ladisch cladisch at fastmail.net
Thu Aug 14 03:26:55 EDT 2008

Andre Majorel wrote:
> On 2008-06-02 10:35 +0200, Clemens Ladisch wrote:
> > Andre Majorel wrote:
> > > - MOTU MIDI Timepiece   (around 600 EUR)
> > > - MOTU Express XT       (around 400 EUR)
> > > - MOTU MIDI Express 128 (around 200 EUR)
> > >
> > > I've heard complaints about the latency of the MOTU interfaces.
> > > How is it under Linux ?
> >
> > Under Linux, these devices have infinite latency, i.e., they do
> > not work at all.  If somebody were to write a driver, the
> > latency would be about as high as in other OSes.
> How do you know ? The latency could be in MOTU's drivers.

MOTU's devices use the "isochronous" transfer type which requires that
all transfers must be scheduled in advance, i.e., the driver has to tell
the host controller driver when a packet should be sent.  If the
scheduled time of a packet is not in the future, the host controller
driver drops the packet without informing the MIDI driver, so all MIDI
packets must be scheduled with a certain latency to ensure that they are
actually sent.

Isochronous transfers are intended for things like PCM audio where it
would not make sense to send packets later than at their scheduled time.
All other MIDI devices use bulk transfers, which do not guarantee a
specific delivery time but which are sent as fast as possible.

> The innards of the AMT8 are interesting. It contains an EPROM but
> no microprocessor that I can see, just a couple of FPGA.

There should be some USB interface chip, and most of those include some
8-bit microprocessor.


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