How to use FEC feature for Opus codec

Question

I'm trying to use the opus Forward Error Correction (FEC) feature. I have a service which does the encoding with OPUS_SET_INBAND_FEC(1) and OPUS_SET_PACKET_LOSS_PERC(20) with 10ms packets and sends them over UDP.

I'm not clear on the decoding process though.

1. When a packet is lost, do I need to call decode with fec=1 ONLY or do I need to call decode with fec=0 after as well on the next packet?
2. How do I know up front the size of the pcm that I send to decode with fec enabled?

Answer 1

I managed to make it work.

The encoding part stated in the question was correct:

• Use the encoder OPUS_SET_INBAND_FEC(1) and OPUS_SET_PACKET_LOSS_PERC(X) where x>0 ans x<100
• Send packets with a duration of at least 10ms (for example: 480 samples at 48 kHz)

For the decoding part, when a packet is lost, call the decode function on the next packet first with fec=1 and again with fec=0.

When calling decode with fec=1, the pcm sent will be entirely filled. If you don't know the length that the pcm should be use on the decoder OPUS_GET_LAST_PACKET_DURATION(x) where x will get the the duration of the last packet.