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Raspberry Pi S/PDIF digital audio without a HAT.

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# raspdif - Raspberry Pi S/PDIF
S/PDIF audio output on Raspberry Pi without a HAT.

raspdif accepts 16 or 24 bit PCM samples from stdin, a file, or a FIFO (named pipe). Samples are encoded and transmitted as S/PDIF data on GPIO21 (Pin 40 on J8). For older boards without a 40-pin header, see [here](#Alternate-GPIO).

**Table Of Contents**
- [Building](#building)
- [Arguments](#arguments)
- [ALSA Configuration](#alsa-configuration)
- [Manual Usage](#manual-usage)
- [Signal Levels](#signal-levels)
- [Alternate GPIO](#alternate-gpio)
- [TOSLINK Optical Output](#toslink-optical-output)

## Building
### Prerequisites
* Raspberry Pi 1, 2, 3 or 4. The Raspberry Pi 5 has a different SOC architecture and is not currently supported.
* clang - Tested against `clang version 3.8.1-24+rpi1` and `clang version 11.0.1-2+rpi1`.
* 32-bit Raspberry Pi OS (64-bit support is experimental).

Install necessary packages
```
sudo apt-get install build-essential clang git
```

Just run make. No configuration right now.
```
make
```
Install to default location (`/usr/local/bin`)
```
sudo make install
```

## ALSA Configuration
ALSA can be configured to use raspdif in a seamless manner. Any application that supports ALSA will output via raspdif. It also allows access to other ALSA plugins like `softvol`.

First, a PCM device is defined that outputs raw samples to a FIFO. This [example configuration](asound.conf) can be used in either `/etc/asound.conf` for system wide configuration or `~/.asoundrc` for user configuration.
```
pcm.!default {
  type plug
  slave.pcm "raspdif"
}

pcm.raspdif {
  type plug
  slave {
    pcm {
      type file
      file "/tmp/spdif_fifo"
      format "raw"
      slave.pcm null
    }
    rate 44100
    format S16_LE
    channels 2
  }
  hint {
    description "S/PDIF output via raspdif FIFO"
  }
}
```

A raspdif device should now be visible with `aplay -L`.
```
$ aplay -L
null
    Discard all samples (playback) or generate zero samples (capture)
default
raspdif
    S/PDIF output via raspdif FIFO
```

Next, create a raspdif service that monitors the FIFO. This minimal [systemd service](raspdif.service) does a decent job.
```
[Unit]
Description=raspdif
After=syslog.service

[Service]
ExecStartPre=/usr/bin/rm -f /tmp/spdif_fifo
ExecStartPre=/usr/bin/mkfifo /tmp/spdif_fifo -m 777
ExecStart=/usr/local/bin/raspdif --input /tmp/spdif_fifo
StandardOutput=journal
StandardError=journal
SyslogIdentifier=raspdif

[Install]
WantedBy=multi-user.target
```

Copy `raspdif.service` to `/etc/systemd/system` and run the following.
```
sudo systemctl daemon-reload
sudo systemctl enable raspdif.service
sudo systemctl start raspdif.service
```

Playback is now as simple as `aplay some_wav_file.wav` or `gst-play-1.0 some_media_file.flac`.

## Optional Arguments
Check `raspdif --help` for additional optional arguments to tweak behavior.
```
Usage: raspdif [OPTION...]

-d, --disable-pcm-on-idle  Disable PCM during underrun.
  -f, --format=FORMAT        Set audio sample format to s16le or s24le.
                             Default: s16le
  -i, --input=INPUT_FILE     Read data from file instead of stdin.
  -k, --no-keep-alive        Don't send silent noise during underrun.
  -r, --rate=RATE            Set audio sample rate. Default: 44.1 kHz
  -v, --verbose              Enable debug messages.
  -?, --help                 Give this help list
      --usage                Give a short usage message
  -V, --version              Print program version

Mandatory or optional arguments to long options are also mandatory or optional
for any corresponding short options.

Report bugs to https://github.com/mill1000/raspdif/issues.
```

## Manual Usage
### Monitor a FIFO
raspdif can monitor a FIFO and transmit samples written to it. Silence is output if there is no data in the FIFO.

```
mkfifo /tmp/spdif_fifo
sudo raspdif --input /tmp/spdif_fifo
```

Now provide PCM data to to the FIFO in some matter.

#### Examples
Configure [gmrender-resurrect](https://github.com/hzeller/gmrender-resurrect) to output to the FIFO.
```
gmediarender --gstout-audiopipe 'audioresample ! audio/x-raw, rate=44100,format=S16LE ! filesink location=/tmp/spdif_fifo'
```

or construct a gstreamer pipeline manually that outputs to the FIFO.
```
gst-launch-1.0 uridecodebin uri="file://some_audio_file.flac" ! audioconvert ! audioresample ! audio/x-raw, rate=441
00,format=S16LE ! filesink location=/tmp/spdif_fifo
```

FFMPEG can also write directly to the FIFO.
```
ffmpeg -i some_audio_file.flac -f s16le -acodec pcm_s16le -ar 44100 /tmp/spdif_fifo
```

### Play a file directly
raspdif can play a file directly. Files must be raw PCM in S16LE or S24LE format.
```
sudo raspdif --input ~/some_pcm_file.pcm
```

### Pipe data via stdin
```
ffmpeg -i some_audio_file.flac -f s16le -acodec pcm_s16le -ar 44100 - | sudo raspdif
```

### Set the sample rate
raspdif defaults to a sample rate of 44.1 kHz. Alternate sample rates can be specified on the command line with the `--rate` option. Rates up to 192 kHz have been tested successfully.

### Set the sample format
raspdif supports 16 or 24 bit PCM samples. Use the `--format` option to select between `s16le` and `s24le`.

## Signal Levels
S/PDIF specification calls for .5 V Vpp when 75 Ohm is connected across the output. To achieve these level from the Raspberry Pi's nominal 3.3 V signaling a simple resistive divider can be build with a 390 Ohm resister is series with the output.

![Resistive Divider](https://raw.githubusercontent.com/mill1000/raspdif/main/raspdif_divider.png)

However, my DAC and receiver have not had an issue receiving the full 3.3 V signal. Use at your own risk.

## Alternate GPIO
raspdif can be reconfigured to output on GPIO31 instead of the default GPIO21. For Rev 2 boards, GPIO31 is located on the P5 header.

To reconfigure for GPIO31 the following changes need to be made.
```diff
  gpio_configuration_t gpioConfig;
  gpioConfig.eventDetect = gpio_event_detect_none;
-  gpioConfig.function = gpio_function_af0;
+  gpioConfig.function = gpio_function_af2;
  gpioConfig.pull = gpio_pull_no_change;
-  gpioConfigureMask(1 << 21 , &gpioConfig);
+  gpioConfigureMask(1 << 31 , &gpioConfig);
```

Thanks to @marcoalexcampos0 for digging into this and testing.

## TOSLINK Optical Output
Thanks to @tlalexander.
> I have discovered that if you hook up a red LED and a resistor (I used about 600 ohms) to the output pin and ground, this works for optical audio output.

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