https://github.com/mill1000/raspdif
Raspberry Pi S/PDIF digital audio without a HAT.
https://github.com/mill1000/raspdif
audio raspberry-pi
Last synced: 7 days ago
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Raspberry Pi S/PDIF digital audio without a HAT.
- Host: GitHub
- URL: https://github.com/mill1000/raspdif
- Owner: mill1000
- License: mit
- Created: 2020-04-25T20:31:27.000Z (almost 6 years ago)
- Default Branch: main
- Last Pushed: 2025-12-20T20:29:19.000Z (about 1 month ago)
- Last Synced: 2025-12-22T20:13:41.357Z (about 1 month ago)
- Topics: audio, raspberry-pi
- Language: C
- Homepage:
- Size: 118 KB
- Stars: 76
- Watchers: 4
- Forks: 8
- Open Issues: 5
-
Metadata Files:
- Readme: README.md
- License: LICENSE.md
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README
# 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
* 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.
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.