Convert AC4 to AU

Why Convert AC4 to AU?

Converting AC4 to AU transforms Dolby's next-generation immersive audio into a lossless format, creating an uncompressed or losslessly-compressed working copy suitable for editing, archiving, or further processing. While the conversion cannot restore audio data removed during AC-4 encoding, it provides a format that avoids additional quality degradation during subsequent editing and re-encoding operations.

Dolby AC-4 was designed for next-generation broadcasting (ATSC 3.0) and streaming platforms, delivering immersive Dolby Atmos audio at remarkably efficient bitrates. However, professional audio workflows often require uncompressed or lossless formats for editing, mixing, and mastering. Converting to AU provides a stable working format compatible with standard audio production tools and DAWs.

The AU format offers lossless audio storage, ensuring that once converted from AC-4, no further quality loss occurs during editing, processing, or format conversion. This makes AU an excellent intermediate format when you need to work with AC-4 content outside of Dolby's broadcast ecosystem.

Note that the resulting AU file will reflect the quality of the decoded AC-4 stream rather than the original pre-encoding source. The file size will increase compared to the highly efficient AC-4 encoding, but you gain broad compatibility and editability across professional audio tools.

Key Benefits of Converting AC4 to AU:

• Broad Compatibility: AU is supported on far more devices than AC-4
• Editing Ready: Convert AC-4 broadcast content for standard DAW workflows
• Platform Flexibility: Distribute on platforms that do not support AC-4
• Simplified Playback: No specialized Dolby decoder required for AU
• Archival Option: Create AU copies alongside original AC-4 masters
• Workflow Integration: Seamlessly incorporate AC-4 content into existing audio pipelines
• Future-Proof: Maintain access to content as AC-4 support evolves

Practical Examples

Example 1: Broadcast Content Repurposing

Scenario: A broadcasting engineer needs to convert ATSC 3.0 content encoded in AC-4 to AU for distribution on platforms that do not yet support Dolby AC-4.

Source: atsc3_broadcast_segment.ac4 (5.1 channels, 192 kbps)
Conversion: AC4 → AU
Result: atsc3_broadcast_segment.au

Workflow:
1. Extract AC-4 audio from ATSC 3.0 transport stream
2. Convert AC-4 → AU for platform compatibility
3. Verify channel layout and audio levels
4. Deliver to distribution platform
5. Archive original AC-4 for future use

Example 2: Post-Production Audio Conversion

Scenario: A sound engineer receives Dolby Atmos content in AC-4 format and needs to create a AU version for editing in a standard DAW that does not support AC-4 input.

Source: dolby_atmos_mix.ac4 (7.1.4 channels, 512 kbps)
Conversion: AC4 → AU (downmixed to stereo/5.1)
Result: dolby_atmos_mix.au

Benefits:
✓ Compatible with standard audio editing software
✓ Preserves core audio channels from Atmos mix
✓ Editable without AC-4 decoder dependency
✓ Ready for integration into post-production workflow
✓ Can be re-encoded to distribution format

Example 3: Device Compatibility Conversion

Scenario: A content distributor has AC-4 encoded audio files from a next-gen broadcast workflow and needs AU versions for playback on devices without AC-4 support.

Source: live_event_audio.ac4 (stereo, 128 kbps, 45 min)
Conversion: AC4 → AU
Result: live_event_audio.au

Device compatibility achieved:
✓ Playable on all AU-compatible devices
✓ No specialized Dolby decoder required
✓ Suitable for web embedding and app integration
✓ Standard format recognized by all media players
✓ Maintains acceptable audio quality for distribution

Frequently Asked Questions (FAQ)

Q: What is Dolby AC-4 and why would I need to convert from it?

A: Dolby AC-4 is the newest audio codec from Dolby Laboratories, designed for ATSC 3.0 next-generation TV broadcasting and streaming platforms. It supports immersive Dolby Atmos audio with up to 7.1.4 channels. You may need to convert from AC-4 when your playback device, editing software, or distribution platform does not yet support this relatively new codec.

Q: Does converting AC4 to AU preserve Dolby Atmos spatial audio?

A: The conversion preserves the core audio channels but Dolby Atmos object-based metadata is specific to Dolby's ecosystem. When converting to AU, the immersive audio is downmixed to the channel layout supported by AU. For stereo output, a spatial downmix is applied; for multichannel AU, the bed channels are preserved.

Q: Will there be quality loss when converting AC4 to AU?

A: Since AU is a lossless format, the decoded AC-4 audio is preserved perfectly in the output. However, the quality reflects the AC-4 encoding — data discarded during AC-4 compression cannot be recovered.

Q: Is AC-4 widely supported on consumer devices?

A: AC-4 support is still limited compared to established formats. It is primarily found in ATSC 3.0 compatible TVs, some streaming devices, and Dolby-enabled mobile phones. This limited support is a primary reason for converting AC-4 to more widely supported formats like AU for broader playback compatibility.

Q: How does AC-4 compare to E-AC-3 (Dolby Digital Plus)?

A: AC-4 achieves approximately 50% better coding efficiency than E-AC-3, delivering equivalent audio quality at half the bitrate. Both support Dolby Atmos, but AC-4 also adds features like dialogue enhancement, personalized audio mixing, and broadcast-optimized loudness management that E-AC-3 lacks.

Q: What channel layouts does AC-4 support?

A: AC-4 supports channel layouts from mono up to 7.1.4 (seven surround channels, one LFE, and four height channels). It also supports Dolby Atmos object-based audio, where individual sound elements can be positioned in 3D space. When converting to AU, the output channel layout depends on what AU supports.

Q: Can I convert AC4 to AU using FFmpeg?

A: FFmpeg has experimental AC-4 decoding support in recent builds. The basic command is: ffmpeg -i input.ac4 output.au. However, full AC-4 support may require specific FFmpeg builds with Dolby codec libraries. Our online converter handles this automatically without any software installation.

Q: How long does AC4 to AU conversion take?

A: AC-4 to AU conversion is typically fast, completing in seconds for most files. The exact time depends on file duration, channel count (7.1.4 Atmos files take longer than stereo), and the target AU encoding complexity. Our online converter processes most audio files within a few seconds.