RTP Streaming to Raspberry Pi with CamillaDSP from a desktop computer

[kbasaran]

Hello all,
I just finished my setup where I can digitally stream my audio from the desktop PC to a CamillaDSP server connected to the speakers. This took me two days to get running properly, so I wanted to post here a breakdown of it. Hopefully it will help some of you.

• Desktop PC as source
  • Ubuntu with Pulseaudio
  • Pulseaudio resamples all to 96kHz with speex-float-5
    • PA uses 0.3% CPU still after this, on a Ryzen 5700g
  • PA transmits to network using rtp module (Only 16 bit integer was possible.)
    • load-module module-null-sink sink_name=rtp_2_camilla channels=2 format=S16BE rate=96000 sink_properties="device.description='RTP to CamillaDSP'" load-module module-rtp-send source=rtp_2_camilla.monitor destination_ip=192.168.0.22 port=46908
  • Running pulseaudio with -vvvv, I find all the RTP settings as SDP data. I put these in attached sdp file.
• Raspberry Pi with Raspbian OS 64 - Server for DSP
  • Uninstalled Pulseaudio, only using Alsa
  • Installed ffmpeg, camilladsp, all other dependencies
  • Running attached Python script to check for incoming streams
    • Pings my desktop PC to see if it is on. It shuts down the amplifier if there is no ping response.
    • If my desktop PC pings back, script will turn on the amplifier and run ffmpeg to check for RTP stream.
      • Uses the sdp file to recognize the incoming stream
  • ffmpeg outputs the stream to snd-aloop loopback device at hw:0
    • This creates a virtual device for Alsa and makes it possible to capture audio from an application, in my case ffmpeg.
    • Channel count needed to be corrected for snd-aloop, to match the RTP that has 2 channels. By default it has 8 channels.
      • $ cat /etc/modprobe.d/sound.conf alias alsa-loopback snd-aloop options snd-aloop pcm_substreams=2,2 index=0,0 enabled=1,1
    • It is tricky to minimize delay of this RTP stream but I believe I found good settings. See inside the Python script.
  • CamillaDSP captures from loopback and does its DSP magic
    • CamillaDSP is running as service using camilladsp.service file from camilldsp-config repository
    • Rate adjust has to be on - otherwise stream falls out of sync with sound card
    • For the capture input I had to select not just the card but the device under the card. It had to be hw:0,1 and not just hw:0 apparently (hw:0,0 is playback, hw:0,1 is capture). This took me HOURS to figure out.. oh God
  • CamillaDSP plays output to USB sound card
    • I had to disable hdmi audio and onboard audio by editing the /boot/config.txt. Otherwise sound card number in alsa was changing sometimes.
      • dtoverlay=vc4-kms-v3d,noaudio # dtparam=audio=on
      • While editing the /boot/config.txt, I also overclocked the Raspberry Pi to 2147MHz
    • Used the 'Alsa instructions' within the documentation of camilladsp to find what formats are supported (192kHz and S32LE with my Behringer UMC204HD)

The setup has very little buffering overall. If I try to summarize latency sources:

• 2ms in local PC Pulseaudio RTP stream output (checked with $pactl list sinks)
• About 20ms I believe for ffmpeg to receive package from network and send to Alsa
• No latency expected from loopback device in Alsa
• Latency inside CamillaDSP - Buffer level is dancing around 1000 samples. My settings are: 192kHz sampling, 1024 chunk size, max queue of 4. Not sure what my latency is, I didn't understand if queue length is set to 4 or another value currently.
• Latency from USB port and sound card - not sure how much

I checked until now only with Youtube video/audio sync test videos and it feels very good. Still adding my loudspeaker filters to CamillaDSP though. I hope cpu load will not go so high that I have to increase buffers.

Hereby I want to thank Henrik of CamillaDSP for the great tool and its just as great documentation. Without the documentation and all support files, I wouldn't have been able to set this up. Great job from him really. So nice now to just use the browser to setup my DSP and also to know that all I need is a sound card and a low cost PC for such precision DSP work.