FTDI latency on Linux (w/ patch)

[Angus Gratton]

By default, it seems like the FTDI drivers on Linux can introduce perceptible amounts of latency. Running the example sketch on an Duemilanove, I found I could hear the 3 notes of the chord arpeggiated (briefly) each time the 3 note_on messages were sent.

I think this is because the FTDI accumulates individual bytes from the serial link, in order to send them in a single USB packet. It has an adjustable timer for this "Nagling".

The patch I've attached sets the ASYNC_LOW_LATENCY flag, which the FTDI driver interprets as a request to drop the timer to 1ms (ie send all accumulated bytes after 1ms.)

It seems you can also set the timer via the sysfs interface. I didn't realise that until after I wrote the patch! It still seems like it might be helpful if ttymidi does it automatically, though.

I haven't put #ifdef __linux__ around the linux-specific bits, because AFAIK ttymidi is Linux-only. This does add an additional dependency on the Linux userspace headers, though.

Hope the patch is useful. Ttymidi seems like a great package, by the way (we are planning to use it to run a "make your own MIDI controller" workshop at the Electrofringe electronic arts festival in Newcastle, Australia!)

Cheers

- Angus.

[Thiago Teixeira (tvst)]

Hi Angus, thanks for the bug report and for the patch! I love patches :)

At a glance your code looks good. I will give it a quick test and update trunk as soon as possible.

Cheers,

- T

[Jari Suominen (jari-tasankokaiku)]

Howdy Angus! I tested your patch and everything seems to work fine! I also updated the trunk! Thanks!

.j

[Thiago Teixeira (tvst)]

Thanks Jari! I will update the site in a bit.