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• How to improve web camera streaming latency to v4l2loopback device with ffmpeg ?

  11 mars, par Made by Moses

  I'm trying to stream my iPhone camera to my PC on LAN.

  


  What I've done :

  


  


  1. HTTP server with html page and streaming script :

     


     I use WebSockets here and maybe WebRTC is better choice but it seems like network latency is good enough

     


  


  


  async function beginCameraStream() {
  const mediaStream = await navigator.mediaDevices.getUserMedia({
    video: { facingMode: "user" },
  });

  websocket = new WebSocket(SERVER_URL);

  websocket.onopen = () => {
    console.log("WS connected");

    const options = { mimeType: "video/mp4", videoBitsPerSecond: 1_000_000 };
    mediaRecorder = new MediaRecorder(mediaStream, options);

    mediaRecorder.ondataavailable = async (event) => {
      // to measure latency I prepend timestamp to the actual video bytes chunk
      const timestamp = Date.now();
      const timestampBuffer = new ArrayBuffer(8);
      const dataView = new DataView(timestampBuffer);
      dataView.setBigUint64(0, BigInt(timestamp), true);
      const data = await event.data.bytes();

      const result = new Uint8Array(data.byteLength + 8);
      result.set(new Uint8Array(timestampBuffer), 0);
      result.set(data, 8);

      websocket.send(result);
    };

    mediaRecorder.start(100); // Collect 100ms chunks
  };
}


  


  


  2. Server to process video chunks

     


  


  


  import { serve } from "bun";
import { Readable } from "stream";

const V4L2LOOPBACK_DEVICE = "/dev/video10";

export const setupFFmpeg = (v4l2device) => {
  // prettier-ignore
  return spawn("ffmpeg", [
    '-i', 'pipe:0',           // Read from stdin
    '-pix_fmt', 'yuv420p',    // Pixel format
    '-r', '30',               // Target 30 fps
    '-f', 'v4l2',             // Output format
    v4l2device, // Output to v4l2loopback device
  ]);
};

export class FfmpegStream extends Readable {
  _read() {
    // This is called when the stream wants more data
    // We push data when we get chunks
  }
}

function main() {
  const ffmpeg = setupFFmpeg(V4L2LOOPBACK_DEVICE);
  serve({
    port: 8000,
    fetch(req, server) {
      if (server.upgrade(req)) {
        return; // Upgraded to WebSocket
      }
    },
    websocket: {
      open(ws) {
        console.log("Client connected");
        const stream = new FfmpegStream();
        stream.pipe(ffmpeg?.stdin);

        ws.data = {
          stream,
          received: 0,
        };
      },
      async message(ws, message) {
        const view = new DataView(message.buffer, 0, 8);
        const ts = Number(view.getBigUint64(0, true));
        ws.data.received += message.byteLength;
        const chunk = new Uint8Array(message.buffer, 8, message.byteLength - 8);

        ws.data.stream.push(chunk);

        console.log(
          [
            `latency: ${Date.now() - ts} ms`,
            `chunk: ${message.byteLength}`,
            `total: ${ws.data.received}`,
          ].join(" | "),
        );
      },
    },
  });
}

main();


  


  After I try to open the v4l2loopback device

  


  cvlc v4l2:///dev/video10


  


  picture is delayed for at least 1.5 sec which is unacceptable for my project.

  


  Thoughts :

  


  

  • Problem doesn't seems to be with network latency

  


  


  latency: 140 ms | chunk: 661 Bytes | total: 661 Bytes
latency: 206 ms | chunk: 16.76 KB | total: 17.41 KB
latency: 141 ms | chunk: 11.28 KB | total: 28.68 KB
latency: 141 ms | chunk: 13.05 KB | total: 41.74 KB
latency: 199 ms | chunk: 11.39 KB | total: 53.13 KB
latency: 141 ms | chunk: 16.94 KB | total: 70.07 KB
latency: 139 ms | chunk: 12.67 KB | total: 82.74 KB
latency: 142 ms | chunk: 13.14 KB | total: 95.88 KB


  


   150ms is actually too much for 15KB on LAN but there can some issue with my router

  


  

  • As far as I can tell it neither ties to ffmpeg throughput :

  


  


  Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'pipe:0':
  Metadata:
    major_brand     : iso5
    minor_version   : 1
    compatible_brands: isomiso5hlsf
    creation_time   : 2025-03-09T17:16:49.000000Z
  Duration: 00:00:01.38, start:
0.000000, bitrate: N/A
    Stream #0:0(und): Video: h264 (Baseline) (avc1 / 0x31637661), yuvj420p(pc), 1280x720, 4012 kb/s, 57.14 fps, 29.83 tbr, 600 tbn, 1200 tbc (default)
    Metadata:
      rotate          : 90
      creation_time   : 2025-03-09T17:16:49.000000Z
      handler_name    : Core Media Video
    Side data:
      displaymatrix: rotation of -90.00 degrees

Stream mapping:
  Stream #0:0 -> #0:0 (h264 (native) -> rawvideo (native))

[swscaler @ 0x55d8d0b83100] deprecated pixel format used, make sure you did set range correctly

Output #0, video4linux2,v4l2, to '/dev/video10':
  Metadata:
    major_brand     : iso5
    minor_version   : 1
    compatible_brands: isomiso5hlsf
    encoder         : Lavf58.45.100

Stream #0:0(und): Video: rawvideo (I420 / 0x30323449), yuv420p, 720x1280, q=2-31, 663552 kb/s, 60 fps, 60 tbn, 60 tbc (default)
    Metadata:
      encoder         : Lavc58.91.100 rawvideo
      creation_time   : 2025-03-09T17:16:49.000000Z
      handler_name    : Core Media Video
    Side data:
      displaymatrix: rotation of -0.00 degrees

frame=   99 fps=0.0 q=-0.0 size=N/A time=00:00:01.65 bitrate=N/A dup=50 drop=0 speed=2.77x
frame=  137 fps=114 q=-0.0 size=N/A time=00:00:02.28 bitrate=N/A dup=69 drop=0 speed=1.89x
frame=  173 fps= 98 q=-0.0 size=N/A time=00:00:02.88 bitrate=N/A dup=87 drop=0 speed=1.63x
frame=  210 fps= 86 q=-0.0 size=N/A time=00:00:03.50 bitrate=N/A dup=105 drop=0 speed=1.44x
frame=  249 fps= 81 q=-0.0 size=N/A time=00:00:04.15 bitrate=N/A dup=125 drop=0 speed=1.36
frame=  279 fps= 78 q=-0.0 size=N/A time=00:00:04.65 bitrate=N/A dup=139 drop=0 speed=1.31x


  


  


  • I also tried to write the video stream directly to video.mp4 file and immediately open it with vlc but it only can be successfully opened after 1.5 sec.

    


  


  • I've tried to use OBS v4l2 input source instead of vlc but the latency is the same

    


  


  


  Update №1

  


  When i try to stream actual .mp4 file to ffmpeg it works almost immediately with 0.2sec delay to spin up the ffmpeg itself :

  


  cat video.mp4 | ffmpeg -re -i pipe:0 -pix_fmt yuv420p -f v4l2 /dev/video10 & ; sleep 0.2 && cvlc v4l2:///dev/video10


  


  So the problem is apparently with streaming process

  


• How to quote a file name with a single quote in ffmpeg movie= filter notation ? [closed]

  26 mai, par PieterV

  I am trying to run ffmpeg using a file that contains a single quote ' in the filename.

  


  I tried to follow the docs that say I should replace a ' with '\''.
  
And a ticket that says I should replace a ' with \\\\\'.

  


  I've tried both, and can't get get it working.

  


  E.g. docs format :

  


  ./ffprobe -loglevel error -read_intervals %00:30 -select_streams s:0 -f lavfi -i "movie='D\:\\Test\\Interlaced - Dragons'\'' Den - S14E02 - Episode 2.mkv'[out0+subcc]" -show_packets -print_format json

{
[Parsed_movie_0 @ 00000222a2f82200] Failed to avformat_open_input 'D:\Test\Interlaced - Dragons Den - S14E02 - Episode 2.mkv'
[AVFilterGraph @ 00000222a2f76ec0] Error processing filtergraph: No such file or directory
movie='D\:\\Test\\Interlaced - Dragons'\'' Den - S14E02 - Episode 2.mkv'[out0+subcc]: No such file or directory


  


  E.g. ticket format :

  


  ./ffprobe -loglevel error -read_intervals %00:30 -select_streams s:0 -f lavfi -i "movie='D\:\\Test\\Interlaced - Dragons\\\\\' Den - S14E02 - Episode 2.mkv'[out0+subcc]" -show_packets -print_format json

{
[Parsed_movie_0 @ 00000158613d2080] Failed to avformat_open_input 'D:\Test\Interlaced - Dragons\\ Den - S14E02 - Episode 2.mkv[out0+subcc]'
[AVFilterGraph @ 00000158613c6ec0] Error processing filtergraph: No such file or directory
movie='D\:\\Test\\Interlaced - Dragons\\\\\' Den - S14E02 - Episode 2.mkv'[out0+subcc]: No such file or directory


  


  > dir "D:\Test\Interlaced - Dragons' Den - S14E02 - Episode 2.mkv"

    Directory: D:\Test

Mode                 LastWriteTime         Length Name
----                 -------------         ------ ----
-a---           4/20/2025 11:38 AM       18059051 Interlaced - Dragons' Den - S14E02 - Episode 2.mkv


  


  This is on Win11 using FFmpeg7.
  
Any ideas ?

  


  [Update]
  
I found a doc on escape filtergraph strings, did not help, I tried 0 to 7 \.

  


  I also found and tried the ffescape utility, the output it produces just uses a single \' and does not work.

  


  > echo "D:\Test\Interlaced - Dragons' Den - S14E02 - Episode 2.mkv" | ./ffescape.exe
=> D:\\Test\\Interlaced - Dragons\' Den - S14E02 - Episode 2.mkv\

> ./ffprobe -loglevel error -read_intervals %00:30 -select_streams s:0 -f lavfi -i "movie='D:\\Test\\Interlaced - Dragons\' Den - S14E02 - Episode 2.mkv\'[out0+subcc]" -show_packets -print_format json
{
[Parsed_movie_0 @ 0000021348f12200] Failed to avformat_open_input 'D'
[AVFilterGraph @ 0000021348f06ec0] Error processing filtergraph: No such file or directory
movie='D:\\Test\\Interlaced - Dragons\' Den - S14E02 - Episode 2.mkv\'[out0+subcc]: No such file or directory


  


  [Update]
  
I found docs for ffmpeg filter script where I can place commands in a file.

  


  I tried ./ffprobe -loglevel error -read_intervals %00:01 -select_streams s:0 -f lavfi -/i "d:\filtergraph.txt" -show_packets -print_format json, and it load the script.

  


  Works : movie=test.mkv[out0+subcc]\ if test.mkv is in ffprobe dir.
Works : movie=test\'.mkv[out0+subcc]\ if test'.mkv is in ffprobe dir.
  
Not : movie=D:\test.mkv[out0+subcc]
  
Not : movie=D\:\\test.mkv[out0+subcc]
  
Not : movie=test space.mkv[out0+subcc]
  
Not : movie='test space.mkv[out0+subcc]'
  
Not : movie="test space.mkv[out0+subcc]"
  
Not : 'movie=test space.mkv[out0+subcc]'
  
Not : "movie=test space.mkv[out0+subcc]"

  


   :(

  


  Update with working answer

  


• FFMPEG send RTP audio at 8k bytes/sec [closed]

  10 mai, par Muzza

  I'm trying to use FFMPEG to mimick a device that transmits G711U audio over UDP/RTP at 8k bytes per second.
The device im mimicking sends rtp packets every 20ms with 160byte payload.

  


  I've had limited success using the following command

  


  ffmpeg -f dshow -i audio="Microphone (Realtek(R) Audio)" -ac 1 -ar 8000 -ab 8 -acodec pcm_mulaw -f rtp rtp://127.0.0.1:12345?pkt_size=160


  


  This sends G711U encoded audio, in 160byte chunks, but streams at 64kB/s, not the 8kB/s that my device is expected, so the device errors out ?

  


  Any idea's would be massively appreciated !

  


  Thank you

  


  Log from FFMPEG

  


  >ffmpeg -f dshow -i audio="Microphone (Realtek(R) Audio)" -ac 1 -ar 8000 -ab 8 -acodec pcm_mulaw -f rtp rtp://127.0.0.1:12345?pkt_size=160
ffmpeg version 2025-04-23-git-25b0a8e295-essentials_build-www.gyan.dev Copyright (c) 2000-2025 the FFmpeg developers
  built with gcc 14.2.0 (Rev3, Built by MSYS2 project)
  configuration: --enable-gpl --enable-version3 --enable-static --disable-w32threads --disable-autodetect --enable-fontconfig --enable-iconv --enable-gnutls --enable-libxml2 --enable-gmp --enable-bzlib --enable-lzma --enable-zlib --enable-libsrt --enable-libssh --enable-libzmq --enable-avisynth --enable-sdl2 --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxvid --enable-libaom --enable-libopenjpeg --enable-libvpx --enable-mediafoundation --enable-libass --enable-libfreetype --enable-libfribidi --enable-libharfbuzz --enable-libvidstab --enable-libvmaf --enable-libzimg --enable-amf --enable-cuda-llvm --enable-cuvid --enable-dxva2 --enable-d3d11va --enable-d3d12va --enable-ffnvcodec --enable-libvpl --enable-nvdec --enable-nvenc --enable-vaapi --enable-libgme --enable-libopenmpt --enable-libopencore-amrwb --enable-libmp3lame --enable-libtheora --enable-libvo-amrwbenc --enable-libgsm --enable-libopencore-amrnb --enable-libopus --enable-libspeex --enable-libvorbis --enable-librubberband
  libavutil      60.  2.100 / 60.  2.100
  libavcodec     62.  0.101 / 62.  0.101
  libavformat    62.  0.100 / 62.  0.100
  libavdevice    62.  0.100 / 62.  0.100
  libavfilter    11.  0.100 / 11.  0.100
  libswscale      9.  0.100 /  9.  0.100
  libswresample   6.  0.100 /  6.  0.100
  libpostproc    59.  1.100 / 59.  1.100
[aist#0:0/pcm_s16le @ 00000198256b73c0] Guessed Channel Layout: stereo
Input #0, dshow, from 'audio=Microphone (Realtek(R) Audio)':
  Duration: N/A, start: 135470.702000, bitrate: 1411 kb/s
  Stream #0:0: Audio: pcm_s16le, 44100 Hz, stereo, s16, 1411 kb/s, Start-Time 135470.702s
Stream mapping:
  Stream #0:0 -> #0:0 (pcm_s16le (native) -> pcm_mulaw (native))
Press [q] to stop, [?] for help
[pcm_mulaw @ 00000198256cf240] Bitrate 8 is extremely low, maybe you mean 8k
Output #0, rtp, to 'rtp://127.0.0.1:12345?pkt_size=160':
  Metadata:
    encoder         : Lavf62.0.100
  Stream #0:0: Audio: pcm_mulaw, 8000 Hz, mono, s16 (8 bit), 64 kb/s
    Metadata:
      encoder         : Lavc62.0.101 pcm_mulaw
SDP:
v=0
o=- 0 0 IN IP4 127.0.0.1
s=No Name
c=IN IP4 127.0.0.1
t=0 0
a=tool:libavformat 62.0.100
m=audio 12345 RTP/AVP 0
b=AS:64

[out#0/rtp @ 00000198256cdd00] video:0KiB audio:973KiB subtitle:0KiB other streams:0KiB global headers:0KiB muxing overhead: 8.467470%
size=    1055KiB time=00:02:04.51 bitrate=  69.4kbits/s speed=   1x
Exiting normally, received signal 2.


  


  Wireshark :
Wireshark Log

  


  Shows packets being sent every 0.20ms