Meter Level Sysex Decoding for QU-SB

[AlexandernautParticipant]

Hey all, I’m trying to parse meter level data from the QU via a TCP connection, but getting confusing results. I’ve successfully made the connection and receive the meter data, but am unable to parse it properly.

I follow the specs outlined in the Get Meter Data section of this document: https://www.allen-heath.com/media/Qu_MIDI_Protocol_V1.9.pdf.

As a test, I’m first trying to get the data from CH 1, Post Preamp. If my understanding is correct, this should be first 3 response bytes after the response header.

So to get these 3 bytes, I look at the response buffer. The first 10 bytes are the response header, so I grab the following 3 bytes:

uint8_t extraByte = _meterBuffer[10];
uint8_t msb = _meterBuffer[11];
uint8_t lsb = _meterBuffer[12];

I then perform the logic from the spec using those bytes. Here’s the log for one pass:

extraByte: 0101 0101
msb: 0001 0011
lsb: 0101 1001
msb (parsed): 1001 0011            // after prepending with the appropriate bit from extraByte
lsb (parsed): 0101 1001            // after prepending with the appropriate bit from extraByte
combined: 1001 0011 0101 1001      // after combining them all to make a 16 bit int
offsetedValue: 0001 0011 0101 1001 // after subtraction of 0x8000
dbValue: 19.347656                 // after dividing by 256.0f

All of the logic makes sense to me and follows the midi spec, and the values do change as the mixer receives input, but… over time, the values are simply not what I expect. They don’t follow the meter ui in the Qu-pad app, and seem to jump all over the place.

Here are some dbValues following a short impulse response from a synthesizer. All values of complete silence are -126.929688 (as you can see these values bookend the response):

dbValue: -126.929688
dbValue: -126.929688
dbValue: 19.414062
dbValue: 57.402344
dbValue: 93.390625
dbValue: -124.617188
dbValue: -86.628906
dbValue: -71.640625
dbValue: -12.652344
dbValue: 27.335938
dbValue: 67.324219
dbValue: 105.312500
dbValue: -112.695312
dbValue: -74.707031
dbValue: -37.718750
dbValue: 1.269531
dbValue: 39.257812
dbValue: 73.246094
dbValue: 114.234375
dbValue: -103.773438
dbValue: -66.785156
dbValue: -26.796875
dbValue: 8.191406
dbValue: 1.179688
dbValue: 86.167969
dbValue: 125.156250
dbValue: -93.851562
dbValue: -55.863281
dbValue: -17.875000
dbValue: 21.113281
dbValue: 58.101562
dbValue: 96.089844
dbValue: -122.917969
dbValue: -84.929688
dbValue: -126.929688
dbValue: -126.929688

As you can see, the response data does not show a simple decaying impulse. Anyone have any idea what’s going on?

[Alex A&HKeymaster]

Hi Alexander,

I’ve had a chat with Keith about this and he’s suggested that when you combine the MSB and LSB you may need to swap them round:

So instead of combined: 1001 0011 0101 1001

Try instead combined: 0101 1001 1001 0011

This gives a value of -38.43dB. Is that closer?

Also, the minimum value the meters should display is -110dB, so hopefully swapping the MSB and LSB solves that too.

Thanks!
Alex

[AlexandernautParticipant]

That did it! Thanks so much :]

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