Rick's b.log - entry 2013/02/24

RISC OS MIDI

My Yamaha keyboard has a USB socket that outputs some sort of MIDI.

My RaspberryPi has a USB socket.

The resultant "thought" ought to be obvious.

The Pi and the Keyboard.

When you are working with raw USB on RISC OS, you are dealing with DeviceFS and you will need to know two things - the name of the USB device to which you wish to receive data, and the correct endpoint. An endpoint is sort of like a pipe, either data goes in, or data falls out. But not at the same time. So a bi-di communication will have two endpoints. Actually, it is a lot more complex than that as some devices have numerous endpoints for different purposes.

I had originally determined my keyboard to be device "USB7" with endpoint 2.

Using !USBinfo to see what the keyboard identified itself as.

With the device and endpoint known, the first program was dead simple:

in% = OPENIN("devices#endpoint2:USB7")
REPEAT
  b% = BGET#in%
  PRINT b%
UNTIL 0

I will skip a lot of headscratching and say that there are some fundamental limitations in the RISC OS USB stack. The first of which is that if you use USB in interrupt mode, you need to request a certain amount of data to be received, or else the system will block awaiting it. This is not a problem with bulk storage as the important transfers will be a request to your actions so you'll know how much data will be coming in (say, a sector or somesuch). However erratic transfers such as MIDI - you cannot say how much data will be coming in, if any. My keyboard transmits a clock tick every quarter beat (every 2-10cs depending on tempo) and an Active Sense request (about every 3-4cs), but other keyboards don't do this. They may be silent until something happens.
The alternative is to just keep looking for bytes, but due to how the system is set up it will block if there is no byte to return. I don't know why, OS_BGet provides for the validity of data to be flagged with the Carry flag. In addition there is a call to return the size of the buffer and the number of bytes free...which doesn't seem to bear any semblance to reality.
So the accepted method is padding. Stuffing in zero bytes when there is nothing actually there to receive.

For the purposes of my MIDI receiver, I look for specific codes and ignore everything else. One thing that will certainly fail is USBMIDI events starting with a zero byte...thankfully it is "for expansion" so there aren't any. Yet.

My keyboard has an auto-off. Switching back on will cause the USB attachment to be reassigned, so I had to make the program look for the correct device.

REM Step one - enumerate USB devices looking for one with
REM an interface "class 1.3" (audio, midi streaming).
SYS "OS_ServiceCall", 1, &D2, 0 TO ,,blk%
devtmp$ = ""
devname$ = ""
devep% = 0
REPEAT
  REM Get pointer to next device
  next% = blk%!0

  REM Read device name
  blk%?13 = 13
  IF blk%?12 = 0 THEN blk%?12 = 13
  devtmp$ = $(blk%+8)

  REM Skip to description blocks
  blk% = blk% + (blk%!4 >> 16) + 4

  REPEAT
    blksize% = blk%?0
    blktype% = blk%?1

    REM Interface description block, looking for
    REM device with class 1 (audio), subclass 3
    REM (midi streaming).
    IF blktype% = 4 THEN
      IF ((blk%?5 = 1) AND (blk%?6 = 3)) THEN
        PRINT "Identified device: "+devtmp$
        devname$ = devtmp$
      ENDIF
    ENDIF

    REM Look at endpoints for the one to read
    REM in data from the device (addr > 128).
    IF devname$ <> "" THEN
      REM Only scan endpoints if device matched
      IF blktype% = 5 THEN
        IF (blk%?2 > 128) THEN
          devep% = (blk%?2 - 128)
          PRINT "Identified endpoint: "+STR$(devep%)
          blksize% = 0 : REM Force parsing to stop now...
        ENDIF
      ENDIF
    ENDIF

    blk% = blk% + blksize%
  UNTIL blksize% = 0

  blk% = next%
UNTIL next% = 0

Once the device and endpoint had been determined, we can open it with:

in% = OPENIN("devices#endpoint"+STR$(devep%)+":"+devname$)

And, then, it is as simple as just running BGET over and over.

USBMIDI, as opposed to plain MIDI, sends MIDI events with a prefix. The prefixes are provided in the low nibble (bits 0-3) of the first byte received:

0. Miscellaneous Function Codes (for future expansion)
1. Cable Events (for future expansion)
2. Two-byte System Common Messages
3. Three-byte System Common Messages
4. System Exclusive Messages Start/Continue
5. Single-byte System Common Messages
6. Two-byte End of System Exclusive Message
7. Three-byte End of System Exclusive Message
8. Note Off
9. Note On
10. Poly Keypress
11. Control Change (stuff like Sustain etc)
12. Program Change (active voice/sample/patch)
13. Channel Pressure
14. Pitch Bend Change
15. Single Byte Immediate Messages

The thing seems to be a little bit over-engineered, doesn't it? Why not just send System Exclusive messages in chunks with a start/continue, use the EOX byte (&F7) to mark the end, and pad it to fit with zero bytes. That would use one code, not the three provided.

That said, manufacturers are known for doing their own thing. My keyboard never sends a Note Off. Instead I receive a Note On with a velocity of zero.

Once this prefix code has been stripped, what remains is plain MIDI. Once all the quirks and headaches have been sorted, it seemed to be remarkably simple to turn a plugged in MIDI lead into a source of usable data.

MIDI events report.

Some things I encountered in my coding

Prerequisites: clk% and tempo% are initialised to TIME; tempotick% is initialised to zero.

  WHEN &F8 : PRINT "[Clock]        ";
             diff% = TIME - clk%
             PRINT "Interval = "+STR$(diff%)+" cs";
             clk% = TIME
             tempotick% = tempotick% + 1
             IF (tempotick% = 24) THEN
               diff% = TIME - tempo%
               PRINT ", tempo = "+STR$(INT(6000 / diff%))+"BPM"
               tempo% = TIME
               tempotick% = 0
             ELSE
               PRINT
             ENDIF

The biggest complication here is dealing with crappy Americanisms. The MIDI tick runs at a rate of 24ppqn - which means 24 pulses per quarter note. It is only when you see that there is a direct correlation between "quarter notes" and BPM (beats per minute) that you realise that they are talking about crotchets. This isn't, of course, universal as different time signatures may alter this. However it stands for 4:4 time.
As the RISC OS time is a centisecond ticker, we can work out the BPM by dividing 6000 (cs per minute) by the length of time it took to receive 24 MIDI ticks. The result is the BPM.

A note, specified by MIDI, is a value from zero to 127, with 60 being middle C (on a piano voice, at least). This can be fairly easily translated into octave and note by:

  oct% = INT(b% / 12)
  fra% = b% MOD 12

Converting this to a value to present to the RISC OS sound system is as follows:

    note% = (((oct% - 1) << 12) + (fra% * 341.3333))

The code

The program has a few bugs/quirks I have noticed:

• Looks for a device class 1 subclass 3. If your MIDI device presents something different, it won't be seen. Alter the code. This is neither a bug nor a quirk, AUDIO/MIDIStreaming is correct, but some devices identify themselves differently.
• If the program is aborted while a note is playing, the note won't be stopped. Add SOUND 1,0,0,0 into the second ON ERROR to fix this. - fixed
• Sound will not be turned off if your keyboard actually uses the correct Note Off commands; nor will Note Off be reported. Duh! Add some code into the note parser - if on% is FALSE then it was a Note Off command. - fixed
• The parsing of voices is missing, and of commands is incomplete. Got bored. I'll do it another time (along with the above fixes).

Written on a Pi! (explains the lower quality video grabs; CVBS != S-video)

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