FFmpeg - Encode AC3
AC-3 (Dolby Digital) is a widely supported lossy multichannel audio codec used in DVD, broadcast, set-top boxes, and legacy home-cinema systems. This article explains how to encode AC-3 audio using FFmpeg with safe, standards-compliant settings.
Summary
AC-3 audio supports mono, stereo, and multichannel output at sample rates of 32 kHz, 44.1 kHz, and 48 kHz. For stereo sources, the usable quality range begins at 160 kbps, with 192–224 kbps being typical for transparent or near‑transparent output.
This article provides a recommended FFmpeg command, bit‑rate guidance, and reference links to official documentation.
Recommended FFmpeg Command
To encode a stereo AC‑3 stream at 160 kbps:
ffmpeg -i <input.file> -c:a ac3 -ac 2 -b:a 160k <output.file>
Explanation of parameters
- -c:a ac3 — Use the native FFmpeg AC‑3 encoder
- -ac 2 — Downmix or encode as 2‑channel stereo
- -b:a 160k — Audio bitrate (minimum advisable for acceptable stereo AC‑3)
Bit‑Rate Considerations
Stereo (2.0)
| Bitrate | Notes |
|---|---|
| 160 kbps | Minimum usable quality for music or mixed content |
| 192 kbps | Common DVD‑quality stereo bitrate |
| 224–256 kbps | Higher clarity, useful for rich stereo music |
| >256 kbps | Rarely necessary for stereo; diminishing returns |
Supported Sample Rates
AC‑3 allows:
- 32000 Hz
- 44100 Hz
- 48000 Hz (most common for film, video, and broadcast)
FFmpeg will preserve the input sample rate unless explicitly changed.
Channel Layouts
Although this article focuses on stereo, AC‑3 supports:
- 1.0 (Mono)
- 2.0 (Stereo)
- 5.1 Surround
Examples:
5.1 at 448 kbps:
ffmpeg -i input.wav -c:a ac3 -b:a 448k output.ac3
5.1 at 640 kbps (maximum AC‑3 bitrate):
ffmpeg -i input.wav -c:a ac3 -b:a 640k output.ac3
When to Use AC‑3
AC‑3 remains useful when:
- Targeting DVD‑Video or broadcast specifications
- Playback device requires Dolby Digital compatibility
- You need a widely-supported legacy format
- Building media for set-top boxes, old TVs, or home-cinema receivers
Troubleshooting
"Bitrate not allowed" Errors
AC‑3 only accepts specific bitrates depending on channel count. Try using: 160k, 192k, 224k, 256k, 320k for stereo.
"Invalid samplerate"
Ensure the sample rate is one of:
- 32000
- 44100
- 48000
Audio seems quieter
AC‑3 includes Dialog Normalization (dialnorm) metadata. FFmpeg sets default values; receivers may adjust playback gain accordingly.
