[LAD] No nagging, a serious question

[Ralf Mardorf]

On Mon, 2010-07-05 at 00:52 +0200, fons at kokkinizita.net wrote:
> On Sun, Jul 04, 2010 at 11:05:02PM +0100, Dan Mills wrote:
> 
> > On Sun, 2010-07-04 at 23:35 +0200, fons at kokkinizita.net wrote:
> > 
> > > Well, at least those sound cards that provide both audio and midi
> > > *could* do this - timestamp incoming midi and provide that info
> > > in some way, and requiring similar time stamps for outgoing midi.
> > 
> > Could do it even with separate midi and audio cards, as long as the midi
> > RX ISR can somehow grab a timestamp from the audio card driver (in an
> > interrupt context), and as long as the midi RX ISR is getting serviced
> > with negligible latency (Otherwise you need to timestamp in hardware). 
> > 
> > In reality is is probably only practical in an embedded context where
> > you have total control over the hardware. 
> > 
> > Note that even with this you will have some jitter due to the way a UART
> > typically over samples to get the centre location of each bit (But at
> > midi rates this should only be measured in tens of microseconds). 
> 
> It shouldn't be, and you don't even need an UART. Just handle it
> as an analog signal, i.e. one more audio channel.
> 
> Even 44.1 kHz is enough to preserve all information. The ratio
> of bit frequency vs. sample frequency is a bit odd, but that is
> in practice not a real problem. On input just use some DSP to
> recover the bits (peanuts on a modern CPU). On output all you
> need to ensure is that the zero-crossings of the generated analog
> waveform (after the antialias filter) are correct, then let the
> HW turn it into the required square wave.
> 
> Result: sample-accurate MIDI.
> 
> Ciao,

Cool idea :). Lets say just MIDI out is wanted, right, no UART is
needed, but there's still the need to get the correct level.

Btw. http://www.maxmidi.com/diy/serial/index.html