Maximum Channels of Audio

[h5n1xp]

Super happy with this fork of the of the USB library, works great, I can get 4 channels of 16bit/48kHz audio just fine.

But what is the maximum number of channels possible? I edited the Adafruit_audio_config.h file define AUDIO_USB_MAX_CHANNELS 8 and the project compiles, and the device is recognised as having 8 channels by the computer... but my USB data receive callback is never called... so I can't get that lovely 8 channel data!

[pschatzmann]

I am not aware of any size limits, but I guess you are overwhelming the system with some many channels and such a high sample rate.
Try to decrease the sample rate or try to overclock the processor...

[h5n1xp]

I will experiment some more and see if I can identify what the issue may be.

[h5n1xp]

After much playing with different Audio Format configurations. The sample rate doesn't affect the maximum number of channels but the bit depth does seem to!

16bit depth maxes out at 4 channels even down a 8000Hz.
If I drop down to 8bit then I can get up to 8 channels of audio at 48000Hz

Based on the assumption that each USB packet is 1ms of audio we can workout the size of the payload:

16bit * 8 channels * 48000Hz = 768 bytes per 1ms USB packet (doesn't work)
16bit * 8 channels * 8000Hz = 128 bytes per 1ms USB packet (doesn't work)
16bit * 5 channels * 48000Hz = 480 bytes per 1ms USB packet (doesn't work)
16bit * 5 channels * 44100Hz = 441 bytes per 1ms USB packet (doesn't work)
16bit * 5 channels * 42000Hz = 420 bytes per 1ms USB packet (does work)
16bit * 5 channels * 8000Hz = 80 bytes per 1ms USB packet (does work)
16bit * 6 channels * 8000Hz = 96 bytes per 1ms USB packet (doesn't work)
16bit * 7 channels * 8000Hz = 112 bytes per 1ms USB packet (doesn't work)
16bit * 4 channels * 48000Hz = 384 bytes per 1ms USB packet (does work)

8bit * 9 channels * 8000Hz = 72 bytes per 1ms USB packet (doesn't work)
8bit * 8 channels * 48000Hz = 384 bytes per 1ms USB packet (does work)
8bit * 7 channels * 8000Hz = 56 bytes per 1ms USB packet (does work)

The data rate doesn't seem to be the issue, so I'm wondering if there is an alignment issue with the payload of USB data packet?

-Edit- Also I have read the maximum payload size on an isochronous packet is 1023bytes, so we are far below that limit. I'm running my Audio from an Apple MacStudio Ultra so the machine has perfectly capable of driving this...

[h5n1xp]

Reading the USB Full Speed spec, it seem the payload size is defined at configuration time by the setting of the endpoint size! Now to dig into the code and see if there is a bug in the endpoint configuration code!

[h5n1xp]

In Adafruit_USBD_Audio.h the methods getMaxPEPSize() and getIOSize() both use the macro TUD_AUDIO_EP_SIZE which appears to be doing a far more funky calculation than my basic assumption above...

#define TUD_AUDIO_EP_SIZE(_maxFrequency, _nBytesPerSample, _nChannels)
((((_maxFrequency + (TUD_OPT_HIGH_SPEED ? 7999 : 999)) / (TUD_OPT_HIGH_SPEED ? 8000 : 1000)) + 1) * _nBytesPerSample * _nChannels)

virtual uint16_t getMaxEPSize() {
return TUD_AUDIO_EP_SIZE(CFG_TUD_AUDIO_FUNC_1_SAMPLE_RATE,
_bits_per_sample / 8, _channels);
}

virtual uint16_t getIOSize() {
return TUD_AUDIO_EP_SIZE(_sample_rate, _bits_per_sample / 8, _channels);
}

Since we aren't using High Speed, the ternary returns 999 and 1000 respectively... CFG_TUD_AUDIO_FUNC_1_SAMPLE_RATE is (AUDIO_FREQ_MAX * 2), which means that 8 channels of 16bit 48000hz audio would give an endpoint size of 1552bytes, well over the Full Speed limit... The questions why 48000 * 2?

[h5n1xp]

I almost gave up too soon:

found Feature Unit Descriptor(4.7.2.8) in src/device/usbd.h where only 4 channels are defined, but a comment that says:
// For more channels, add definitions here

I will try later and see if I can increase the channel count.

[h5n1xp]

Nope false alarm, need to focus on the multiple EndPoint idea.

[pschatzmann]

Initially I planned to allow multiple Instances of Adafruit_USBD_Audio. Unfortunately the TinyUSB API does not allow any differenciation for some callbacks: so I am afraid that this can't be supported either.

[h5n1xp]

Are you able to be more specific? Is this something I should raise with the TinyUSB devs directly?