[sdiy] MIDI merging info requested

[Byron G. Jacquot]

On 8 May 2005 at 18:02, David Brown wrote:

> It seems like with running status, sys events, and such, that you
> really need to decode both MIDI streams to be able to merge complete
> messages.

That's correct...MIDI relies heavily on keeping individual messages 
intact.

> Is there a simple way to do this?  Perhaps it is just decoding the
> commands and deciding how many bytes to pass before looking at the
> other MIDI stream.  Seems like if the other stream doesn't have a
> complete command then you would hang.  Perhaps this is never a
> problem.

If you can afford to filter out stuff you don't care about, that 
simplifies it a bit...like dumping out sysex or realtime messages.

In general (and ignoring sysex and active sensing), it might look 
something like:
1> Pass all the single byte realtime stuff (clock, start/stop, etc) 
as it arrives.  It can interrupt other messages if necessary.
2> Buffer messages until they're complete...since they're usually 2 
or 3 bytes, this isn't hard.  Be careful to track running status, 
which will look like incomplete commands.
3> When a message buffer from either stream fills, send the complete 
message.  If you're clever, you can implement running status on that 
output, independently of the 2 input streams.  Hopefully, neither 
stream is so dense that you can't interleave the other one in a 
timely fashion.

Byron Jacquot