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.

Table Of Contents

• Building
• Arguments
• ALSA Configuration
• Manual Usage
• Signal Levels
• Alternate GPIO
• 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 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 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 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.

  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.