Forward error correction adds redundant information to the stream so the receiver can rebuild lost packets without asking for a retransmission — which real-time audio has no time to wait for. Opus's in-band FEC carries a low-bitrate copy of the previous frame inside the current packet, so a single lost frame can be recovered from the next one to arrive; the RED scheme (RFC 2198) instead bundles whole previous payloads alongside the new one. The cost is bandwidth: redundancy only helps if you spend extra bits, and it only protects against the loss pattern it was sized for. FEC and packet loss concealment are complementary — FEC recovers data, PLC hides what FEC couldn't.
The book · Volume 5 of 7
Audio for Video: A Complete Guide to Sound in Video Products: From Jitter Buffers and Lip Sync to Atmos, Quality Metrics, and Neural Codecs
FEC lets real-time audio survive packet loss without waiting for retransmission, at the price of extra bitrate on every call. Volume 5 covers it in detail with Opus in-band FEC, RED redundancy, packet loss concealment, jitter buffers and NetEq, so you can decide how much redundancy your calls need on the networks your users have. Written by Nikolay Sapunov, CEO at Fora Soft.
Calls breaking up under packet loss, and wondering whether FEC or RED will fix it?
Send us network stats from your calls and we will recommend FEC and RED settings with the bitrate overhead and expected quality gain. 250+ video products shipped since 2005.
