Philip-Dylan Gleonec: >> Could you elaborate? >> I would have expected that the normal use case is not have a >> lossy input and that the new feature is always useful if data >> was lost. > > The use-case for FEC is typically RTP stream where audio is compressed > with opus. In that case, depending on the network conditions, packets > can be lost. From the decoder side, this can be observed as a packet > with a pts that is incremented more than we would expect. > > With this patch, if no discontinuity is detected, the packet is decoded > as usual. If a discontinuity is detected, its duration is checked to > deduct how many samples should be reconstructed. One limitation of opus > FEC is that packets can be reconstructed with a granularity of 2.5ms > (120 samples), so libopus can not jsut reconstruct an arbitrary number > of samples. This patch manages this by "rounding" the number of lost > samples to the closest number of samples that libopsu can reconstruct. > >> Or is there no way to distinguish between a stream with >> unusual timestamps and a stream with packet loss? > > From my understanding, the only information at the decoder side are the > metadata associated with the packet to decode, notably the pts, dts and > packet duration. It does not have information about how the packet has > been received (RTP, SDP, other...). Packet loss can typically be > detected at higher layers (RTCP packets for RTP) but not at the decoder > layer. > > From observation it seemed that a pts different from the last pts + > packet duration would indicate that some samples were lost. The decoder > than tries to restore as much sample as it can. > > Note that packet duration seem to change units depending on how the file > is encapsulated: the same file had a packet->duration of 20 (ms) in a > .mkv container but 960 (samples) in a .opus container. Since the decoder > is not aware of which container is used, it has to "guess" which unit is > used. As FEC is ony used with SILK frames, which have a 10ms (480 > samples) minimal duration, and an opus packet has a max duration of > 120ms, a threshold can find how the pts is expressed.
There is AVCodecContext.pkt_timebase. > > An alternative could be to simply provide a new function to decode FEC > data and let the higher layers manage if FEC should be decoded, but this > implies more modification to the stack to make FEC work, while here only > an AVOption has to be set. _______________________________________________ ffmpeg-devel mailing list ffmpeg-devel@ffmpeg.org https://ffmpeg.org/mailman/listinfo/ffmpeg-devel To unsubscribe, visit link above, or email ffmpeg-devel-requ...@ffmpeg.org with subject "unsubscribe".
