5.1 surround codec with lossy compression — DVD, cinema, and streaming standard. Transparent enough for delivery, unsuitable for mastering archival.
We work with AC3 daily—whether for DVD mastering, cinema DCPs, or streaming delivery. The codec lossily compresses multichannel audio to one-third of its uncompressed file size, but makes compromises in bit depth and frequency resolution. In practice, this works significantly better than theory suggests. At 448 kbps or 640 kbps—the common mastering rates in cinema—the human ear can barely discern the difference from PCM, as long as the mix is properly done.
The typical AC3 configuration is 5.1: three front channels (Left, Center, Right), two surround channels (Ls, Rs), and the LFE channel for subwoofer effects below 120 Hz. On average, one calibrates to −27 dBFS for the center channel—this is the reference level for clear dialogue. When mixing, one should test AC3 early in the process, not just at the final delivery. Compression artifacts quickly reveal themselves with orchestral sounds, atmospheric layers, or very quiet passages, where the codec sometimes falters minimally. A pre-decoded AC3 reference file gives you immediate feedback on whether your mix is robust enough.
Practical Limits: AC3 should not be used for archival purposes or for the highest quality cinema mastering (DCI standard)—for that, you need TrueHD or PCM. But for television, home cinema, and mainstream streaming, AC3 has been the proven standard solution for twenty years. HDR video with AC3 audio is completely normal; format mismatch is not an issue here. Ensure your encoder settings are correct: Dialnorm should be set to −27 dBFS so that volume variations in the receiver do not lead to uncontrolled ducking. Many streaming platforms now reject AC3 and demand AAC or, even more so, Atmos—but for DVD replication and for legacy hardware, AC3 remains indispensable.
Compared to DTS (the other major surround codec), AC3 is compressed slightly more and sometimes sounds a touch more sterile; however, its compatibility is greater. Do not confuse AC3 with E-AC3 (Enhanced AC3)—this is a newer, more efficient variant with better bitrate efficiency, but it is decoded less universally.