Sysex messages containing bytes greater than 127

[dhjdhj]

Has anyone ever come across a device that allows sysex messages with bytes > 127 in them? I was under the impression that such a byte immediately terminates sysex but I’m being told that apparently some control surfaces use such bytes for displaying text characters that are represented by bytes > 127?

[danielrudrich]

A System Exclusive code set begins with 11110000 (240 decimal or F0 hex), followed by the manufacturer ID#, then by an unspecified number of data bytes of any ranges from 0-127) and ends with 11110111 (decimal 247 or F7 hex), meaning End of SysEx message.

Source: http://www.indiana.edu/~emusic/etext/MIDI/chapter3_MIDI9.shtml

So it might work as long the value isn’t 247

[asimilon]

I think it would be flaky at best; it wouldn’t be beyond the realms of possibility that a particular synth might not even really check for F7 as the end marker, but assume that the MSB being 1 is the end of the SYSEX data.

Found this in the “File Dump Data Packet” section of this document

Encoding of data :

The 8-bit file data needs to be converted to 7-bit form, with the result that every 7 bytes of file data translates to 8 bytes in the MIDI stream.

For each group of 7 bytes (of file data) the top bit from each is used to construct an eigth byte, which is sent first. So :

AAAAaaaa BBBBbbbb CCCCcccc DDDDdddd EEEEeeee FFFFffff GGGGgggg

becomes :

0ABCDEFG 0AAAaaaa 0BBBbbbb 0CCCcccc 0DDDdddd 0EEEeeee 0FFFffff 0GGGgggg

The final group may have less than 7 bytes, and is coded as follows (e.g. with 3 bytes in the final group) :

0ABC0000 0AAAaaaa 0BBBbbbb 0CCCcccc

so to be sure you should just encode any 7 bytes of 8-bit data into 8 bytes.

[dhjdhj]

unspecified number of data bytes of any ranges from 0-127

So it might work as long the value isn’t 247

No, as they note – the values have to be between 0 and 127

[dhjdhj]

I agree – it’s not me trying to do this — one of our users was claiming to have a device that requires sysex messages with bytes > 127 and I wasn’t convinced, but wanted to verify.

[bgporter]

Remember that many control surfaces don’t interface over MIDI but instead (or additionally? Nothing would surprise me in this arena) use the Mackie Control interface or some other USB HID protocol, so it’s entirely possible that they’re using 8-bit data, just not inside SysEx messages, or they’re using something like the 8->7 bit tunneling technique that @richie pointed at above.