Post by thread:[PIC] acceptable MIDI data rate deviation

I need to communicate via MIDI both with a PIC and with other MIDI devices from an embedded Linux world. The Linux device's UARTs are cabable of: 14.7456M/16*29 = 31779 14.7456M/16*30 = 30720 MIDI spec says 31250 +-1%. My error is 1.7%. Does anyone know if this is this close enough? I figure this problem can't be new, but I'm finding very little info about it on the net. Some sites hint that an error of up to 3% is OK with certain MIDI devices. I wonder if anyone has real-world experience with this. Any advice appreciated, darren

>I need to communicate via MIDI both with a PIC and with other MIDI >devices > Some sites hint that an error of up to 3% is OK I know nothing about this :-) BUT if it's an asynchronous serial protocol then the MAXIMUM allowable error is close enough to 50/(start bits + data bits [+ stop bits]) % (Stop bits may not be necessary depending on system design) ie the error required to get half a bit error so that sampling wanders off the proper bit onto the adjacent one. eg the standard 1+ 8 + 1 serial data protocol gives 50/(1+8) =~ 5.6% With stop bit that's 50/(1+8+1) = 5% That's the error at BOTH ends. If the error is in the same direction at both ends you can tolerate higher error rates. If it goes opposite ways then the above applies. Murphy says it goes the same way during design and opposite ways in real world must-work situations. Resonator gives you 1% odd. Crystal gives maybe 0.1% or better over life/temperature/phase of moon/bank holiday/ .... If it's not an async serial protocol then the above doesn't apply :-) RM

Russell McMahon wrote: >>I need to communicate via MIDI both with a PIC and with other MIDI >>devices >> Some sites hint that an error of up to 3% is OK > With stop bit that's 50/(1+8+1) = 5% > That's the error at BOTH ends. > If it's not an async serial protocol then the above doesn't apply :-) It is an async serial protocol, and it applies. All MIDI devices that I've seen from the inside use standard UARTs to communicate. The only thing a bit odd about it is the bit rate. So you are probably quite safe with 1.7%, and the sources that say up to 3% are fine are correct, too. (No hard experience data, though.) Gerhard

>> With stop bit that's 50/(1+8+1) = 5% >> That's the error at BOTH ends. > So you are probably quite safe with 1.7%, and the sources that say > up to 3% > are fine are correct, too. (No hard experience data, though.) That's presumably 3% at one end and hopefully almost 0% at the other. (ie Crystal or resonator at one end). Two ends with 3% each going in the opposite directions could be bad. RM

Thanks for the info! It's good to know I'm in the ballpark at least... On Feb 3, 2005, at 4:28 AM, Russell McMahon wrote: {Quote hidden}>>> With stop bit that's 50/(1+8+1) = 5% >>> That's the error at BOTH ends. > >> So you are probably quite safe with 1.7%, and the sources that say up >> to 3% >> are fine are correct, too. (No hard experience data, though.) > > That's presumably 3% at one end and hopefully almost 0% at the other. > (ie Crystal or resonator at one end). > > Two ends with 3% each going in the opposite directions could be bad.