USB audio device support

[mikeoliphant]

Any thoughts on the possibility of USB audio interface support?

It would be great to have a bare-metal solution on Raspberry Pi that could use a USB-connected audio interface instead of integrated I2S.

My application of choice is guitar effects processing, and I would love to be able to get Linux out of the way and go bare-metal to get the best performance at the lowest latency.

[rsta2]

Unfortunately this is currently not planned. The problem is, that the USB host controller drivers do not support isochronous USB transfers. I thought about dealing with this, when I have much time in the future, but this is far away.

[mikeoliphant]

Yes, it is a non-trivial problem to solve and there aren't a lot of public implementations floating about.

I did find this, though:

https://github.com/Themaister/libmaru

And, of course, the Linux kernel...

[sebastienNEC]

Hello Mike, I have actually worked on something similar, over I2S though. I am using circle for low latency I2S audio processing, and I have statically linked-in a lot of the usual suspects in audio fx (guitarix, rakarrack, etc).
I am currently working on streaming low latency audio via WiFi to have a distributed audio fx system...

[sebastienNEC]

Maybe I should open another topic but @rsta2 : I have hacked CI2SSoundBaseDevice to provide I2S audio input, but this is really quick and dirty. Is I2S input on your list possibly ?

[rsta2]

@mikeoliphant I think, this library is Linux based, not an USB driver itself. Yes, there is the dwc_otg Linux kernel driver, probably the only reference for this purpose. I analyzed it eight years ago to implement the current Circle driver, but I let the isochronous mode out, to reduce complexity.

@sebastienNEC I have I2S input on my TODO list. How did you extend the class interface to provide the received sound data? There was no sound input support so far in Circle. It would be great, if you could send me your modifications as a patch file per email. I would give you the credits for being the author, of course. A question is, how I can test the I2S input. Can I use a second RPi as I2S transmitter?

[mikeoliphant]

@sebastienNEC - your project sounds very interesting. What hardware are you using for audio in/out?

@rsta2 I understand better now. I was just thinking of the code that talks to the USB driver to implement audio, but you are saying that there is also work to be done to actually implement isochronous mode for the RPI USB controller.

[sebastienNEC]

@rsta2 I've sent you an email :)

@mikeoliphant: I've used the (not available anymore) fe-pi audio z v2, and a Pisound interface, which has instrument level inputs. I've sent you an email if you wish to discuss in more details :)

[rsta2]

I2S input support has been implemented on the develop branch. There is a test program i2s-input in the new test/ subdirectory, which receives I2S data (24-bit, 2 channels) and sends it to the headphone jack (PWM device). The sample/34-sounddevices can be used to generate the I2S data on a second Raspberry Pi.

@sebastienNEC Thanks again for sending the I2S input patch! It helped me very much, to get it working. I will also use your WLAN AP patch, this will come later.

[sebastienNEC]

You're welcome, I'm glad I could help :)

[probonopd]

It looks like @BenZonneveld has implemented i2s over USB UAC2 audio:
https://github.com//MiniDexed_Midi_Master/pull/2

I don't know more about it but perhaps it could be useful for someone and a candidate for being backported into Circle?

[rsta2]

Thanks for info. I will have a look on it.

[rsta2]

As I see it, the referenced project contains an I2S to USB audio converter, which runs on the Raspberry Pi Pico. It cannot be ported to Circle.

[probonopd]

Ah now I understand. Thanks for the explanation and sorry for the noise.

[rsta2]

There is some initial support for USB Audio class streaming devices on branch usb-audio. Currently only USB Audio class 1.0 devices (int1-2-0) are supported and only in output direction. These devices normally are USB full speed devices.

Using these devices on the Raspberry Pi 1-3 and Zero requires the system option USE_USB_SOF_INTR defined (default setting) and should work even better with USE_USB_FIQ defined. With the latter option defined, the FIQ is used by the USB host controller driver and cannot be used for other purposes (e.g. serial device) at the same time.

The sample programs 29-miniorgan and 34-sounddevices are prepared to work with USB Audio class devices. You have to define USE_USB in the file miniorgan.h for the first sample program, or add the option sounddev=sndusb to the file cmdline.txt on the SD card for the second sample.

You may also have to add the option usbpowerdelay=1000 (1000ms) to the file cmdline.txt, because some USB audio devices requires some additional time to come up after powering on. The default setting of 510ms may be not enough there.

Feedback is welcome.

[probonopd]

USE_USB_FIQ defined. With the latter option defined, the FIQ is used by the USB host controller driver and cannot be used for other purposes (e.g. serial device) at the same time.

Just to clarify, does this mean that when it is defined, then RX/TX (e.g., for MIDI) cannot be used at the same time?