How to Use FFmpeg asetpts Filter to Adjust Audio Timestamps

Adjusting audio timestamps in FFmpeg is crucial for fixing audio-video synchronization issues, altering playback speed, and resetting timelines for seamless concatenation. This article explains how to use the asetpts (audio set presentation timestamp) filter, covering its basic syntax and demonstrating how to apply practical expressions to shift, scale, and reset audio timestamps.

Understanding the asetpts Filter

The asetpts filter works by modifying the Presentation Timestamp (PTS) of each audio frame. The PTS determines exactly when a specific frame of audio should be played relative to the start of the stream.

The basic syntax for the filter is:

-af "asetpts=EXPRESSION"

FFmpeg provides several built-in variables for the expression, including: * PTS: The original presentation timestamp of the current frame. * STARTPTS: The presentation timestamp of the first frame. * TB: The time base of the audio stream. * SAMPLE_RATE (or SR): The audio sample rate.

Common Use Cases

1. Resetting Audio Timestamps to Zero

When cutting or merging audio files, the resulting stream may inherit non-zero starting timestamps, causing playback issues in some media players. You can reset the timeline so the audio starts precisely at zero using PTS-STARTPTS:

ffmpeg -i input.mp3 -af "asetpts=PTS-STARTPTS" output.mp3

2. Shifting Audio to Fix Desynchronization

If your audio is out of sync with your video, you can shift the audio timeline forward or backward.

• Delay audio by 1.5 seconds:

  ffmpeg -i input.mp4 -af "asetpts=PTS+1.5/TB" output.mp4

• Advance audio by 1.5 seconds (make it play earlier):

  ffmpeg -i input.mp4 -af "asetpts=PTS-1.5/TB" output.mp4

(Note: TB represents the time base, which translates the real-world seconds into the stream’s internal timestamp units.)

3. Changing Playback Speed

While the atempo filter is typically used to change audio speed without changing pitch, the asetpts filter can be used to adjust the container-level playback timestamps to match video speed changes (like those done with the video setpts filter).

• Double the playback speed (half the presentation time):

  ffmpeg -i input.wav -af "asetpts=0.5*PTS" output.wav

• Halve the playback speed (double the presentation time):

  ffmpeg -i input.wav -af "asetpts=2.0*PTS" output.wav

Combining Video and Audio Timestamp Adjustments

When adjusting timestamps for a video file, you must apply setpts to the video stream and asetpts to the audio stream simultaneously to keep them in sync.

For example, to fast-forward a video to 2x speed while keeping the audio synchronized:

ffmpeg -i input.mp4 -filter_complex "[0:v]setpts=0.5*PTS[v];[0:a]asetpts=0.5*PTS[a]" -map "[v]" -map "[a]" output.mp4