Rasperry Pi Raw I²S I/O Virtual Codec

This project implements a simple suite of kernel modules and a device tree overlay to enable the Raspberry Pi I²S (PCM) audio interface to be used for both input and output with a virtual codec.

The mainline Raspberry Pi project provides integrations with specific boards and codecs such as the HiFiBerry DAC. The hifiberry-dac device tree overlay can be used as a substitude for raw I²S output, however input is not supported by this codec.

Other supported codecs support I²S input, output and duplex streaming, but these are more complex devices which have I²C control interfaces. Without the real codec being present, the drivers for these devices will not register them as available.

This project borrows heavily from the HiFiBerry DAC integration, but adds support for input.

The project will be useful to anyone who wishes to experiment with I²S signalling.

Installation

Install prerequisites:

 $ sudo apt install make gcc linux-headers device-tree-compiler

Build the modules and overlay:
```
 $ make
```
Install the modules and overlay:
```
 $ sudo make install
```
Enable device tree overlay.

Open the boot configuration file in a text editor:
```
 $ sudo editor /boot/config.txt
```
Find the line that has dtparam=audio=on. Disable it by putting a # in front.

Then add dtoverlay=rpi-raw-i2s-io on the next line.

If you have enabled any other audio-related overlays, remove them.
Reboot:
```
 $ sudo reboot
```

Check the virtual audio device is registered:

 $ aplay -l
**** List of PLAYBACK Hardware Devices ****
card 0: sndrpirawi2scod [snd_rpi_raw_i2s_codec], device 0: Raspberry Pi Raw I2S Codec HiFi rpi-raw-i2s-hifi-0 [Raspberry Pi Raw I2S Codec HiFi rpi-raw-i2s-hifi-0]
  Subdevices: 1/1
  Subdevice #0: subdevice #0

 $ arecord -l
**** List of CAPTURE Hardware Devices ****
card 0: sndrpirawi2scod [snd_rpi_raw_i2s_codec], device 0: Raspberry Pi Raw I2S Codec HiFi rpi-raw-i2s-hifi-0 [Raspberry Pi Raw I2S Codec HiFi rpi-raw-i2s-hifi-0]
  Subdevices: 1/1
  Subdevice #0: subdevice #0

Connections

By default the I²S connections on the Rasperry Pi pin-header are as follows:

Pin	Name	Description
12	`CLK`	Clock. Single pulse for each data bit.
35	`FS`	Frame Select. Indicates the channel of each data word.
38	`DIN`	Audio data in.
40	`DOUT`	Audio data out.

Test

To test the driver is working correctly, connect DIN and DOUT with a jumper lead. This will loop the output data back to the input.

Start recording in the background:

 $ arecord -r 48000 -t wav test.wav &

Now generate some output audio:

 $ speaker-test -r 48000 -c2 -twav -l1

Now kill recording session:

 $ killall arecord

Load up the audio file in an application such as Audacity. You should have a bit-for-bit perfect reproduction of the played audio.

jhol/rpi-raw-i2s-io

Rasperry Pi Raw I²S I/O Virtual Codec

Installation

Connections

Test