lemanschik/pw.md

## pw.md

      
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    Achieving synchronized audio and video playback is a complex but crucial task in multimedia programming. With a framework like PipeWire, which is designed to handle all types of media streams, synchronization is a core concept. The key to this is understanding the role of Presentation Timestamps (PTS) and a shared clock.
Here’s a breakdown of the concepts and a step-by-step guide on how to approach A/V sync when creating a C++ player with PipeWire.
The Core Concept: A Common Clock and Timestamps

Imagine you have two separate players: one for video frames and one for audio samples. To keep them in sync, you can't just play them as fast as possible. Instead, you need a shared "wall clock" that both players can look at.


The Clock: PipeWire provides a global clock for the entire media graph. This clock is typically driven by an audio device (like your sound card) because audio playback is very sensitive to timing errors. If audio samples aren't delivered at a precise, steady rate, you get pops, clicks, and distorted sound (an "underrun" or "overrun"). Video is more forgiving; dropping or displaying a frame a few milliseconds late is often unnoticeable.


Presentation Timestamps (PTS): Every single audio buffer and video frame that you decode from a media file (like an MP4) has a timestamp attached to it. This PTS value says, "According to the timeline of the media file, this piece of data should be presented (heard or seen) at exactly this moment."


The synchronization logic is then straightforward:

The application gives PipeWire an audio buffer with a PTS.
The application gives PipeWire a video frame with a PTS.
PipeWire's internal clock advances.
When PipeWire's clock time matches the PTS of a buffer or frame, it releases that data to the hardware (the sound card or the display server/GPU).

How to Implement A/V Sync with PipeWire in C++

Let's expand on the previous audio-only example. A full A/V player would require a demuxing and decoding library (like FFmpeg), but we can outline the logic for handling the PipeWire side.
You would need to create two separate PipeWire streams:

One pw_stream for audio playback.
One pw_stream for video playback.

Here are the essential steps:
Step 1: Initialize and Set Up the Media Source (e.g., FFmpeg)

Before touching PipeWire, you need to read the media file. A library like FFmpeg is standard for this.

Open the Media File: Use FFmpeg to open the video file. This will give you access to its various streams (audio, video, subtitles).
Find Streams and Codecs: Identify the audio and video streams and initialize the appropriate decoders.
Get Time Base: Crucially, get the time_base for each stream. This is a rational number (like 1/90000) that tells you the unit of the PTS values in the stream. You will need this to convert the stream's PTS into nanoseconds, which is what PipeWire's clock uses.

Step 2: Create Two PipeWire Streams (Audio and Video)

You will create two streams, much like the audio example, but with different properties.
Audio Stream Creation:
// (Inside your main function)
pw_stream *audio_stream = pw_stream_new_simple(
    loop,
    "my-player-audio",
    pw_properties_new(
        PW_KEY_MEDIA_TYPE, "Audio",
        PW_KEY_MEDIA_CATEGORY, "Playback",
        // ... other properties
        nullptr),
    &audio_stream_events, // A struct with your audio callbacks
    &app_data);
Video Stream Creation:
The key difference is the PW_KEY_MEDIA_TYPE.
pw_stream *video_stream = pw_stream_new_simple(
    loop,
    "my-player-video",
    pw_properties_new(
        PW_KEY_MEDIA_TYPE, "Video", // This is the important part
        PW_KEY_MEDIA_CATEGORY, "Playback",
        // ... other properties
        nullptr),
    &video_stream_events, // A separate struct for video callbacks
    &app_data);
Step 3: Connect Streams with Correct Formats

When connecting each stream, you must provide the format decoded from the media file.

For Audio: This would be SPA_AUDIO_FORMAT_S16, SPA_AUDIO_FORMAT_F32P (planar float), etc., along with the sample rate and channels.
For Video: This would be the pixel format, like SPA_VIDEO_FORMAT_RGB or SPA_VIDEO_FORMAT_YV12, along with the video's width and height.

Step 4: The on_process Callbacks and PTS

This is where synchronization happens. You'll have two on_process functions: one for audio and one for video.


Read and Decode a Packet: In your main application loop (outside the callbacks), continuously read packets from the media file using FFmpeg. A packet can be either audio or video.


Store Decoded Data: When you decode a packet, you get raw audio samples or a raw video frame, each with its PTS. Store these in thread-safe queues.


Inside on_audio_process:

Dequeue a buffer from the audio stream: pw_stream_dequeue_buffer(audio_stream).
Pop decoded audio data from your audio queue.
Set the PTS on the PipeWire buffer: This is the most critical step. Convert the frame's PTS from its time_base to nanoseconds.
struct pw_buffer *pw_buf = pw_stream_dequeue_buffer(audio_stream);
struct spa_buffer *spa_buf = pw_buf->buffer;

// FFMpegFrame *frame = your_audio_queue.pop();
// int64_t pts_ns = av_rescale_q(frame->pts, ffmpeg_stream->time_base, {1, 1000000000});

// The time for this buffer is now set
spa_buf->datas[0].chunk->offset = 0;
spa_buf->datas[0].chunk->size = /* size of audio data */;
// Copy your audio samples into spa_buf->datas[0].data

// Associate the timestamp with this buffer
pw_buf->time = pts_ns;

pw_stream_queue_buffer(audio_stream, pw_buf);


Inside on_video_process:

Do the exact same thing for video: dequeue a video buffer, get the decoded video frame from your video queue, convert its PTS to nanoseconds, set pw_buf->time, copy the pixel data, and queue the buffer.


Step 5: Let PipeWire Do the Work

Once you are feeding both streams with correctly timestamped buffers, PipeWire handles the rest.

PipeWire's scheduler looks at the PTS of the buffers you've queued.
It monitors its internal master clock (driven by the audio sink).
It will only release a video buffer to be rendered when the clock time is greater than or equal to that buffer's time (PTS).
If the video rendering is falling behind, PipeWire's rate-matching mechanism might cause it to automatically drop video frames to catch up to the audio clock. If audio is behind, you might hear a glitch as it tries to catch up.

Summary of the Workflow


Setup: Use a library like FFmpeg to open a file and prepare decoders.
PipeWire Init: Create two streams: Audio and Video.
Main Loop:

Continuously read A/V packets from the file.
Decode them into raw frames/samples, keeping their PTS.
Push the decoded data into separate, thread-safe audio and video queues.


PipeWire Audio Callback (on_process):

When PipeWire needs audio, pull from your audio queue.
Copy the audio samples into the dequeued pw_buffer.
Set the pw_buffer->time field to the frame's PTS (converted to nanoseconds).
Queue the buffer.


PipeWire Video Callback (on_process):

When PipeWire needs a video frame, pull from your video queue.
Copy the pixel data into the dequeued pw_buffer.
Set the pw_buffer->time field to the frame's PTS (converted to nanoseconds).
Queue the buffer.


By providing accurate timestamps, you are no longer just pushing data blindly; you are telling PipeWire when each piece of data should be presented, allowing its internal clock and scheduling mechanisms to ensure perfect A/V sync.
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