FFMPEG Reduce RTSP Latency13 Nov 2015
You may be able to decrease initial “startup” latency by specifing that I-frames come “more frequently” (or basically always, in the case of x264’s zerolatency setting), though this can increase frame size and decrease quality, see here for some more background. Basically for typical x264 streams, it inserts an I-frame every 250 frames. This means that new clients that connect to the stream may have to wait up to 250 frames before they can start receiving the stream (or start with old data). So increasing I-frame frequency (makes the stream larger, but might decrease latency). For real time captures you can also decrease latency of audio in windows dshow by using the dshow audio_buffer_size setting. You can also decrease latency by tuning any broadcast server you are using to minimize latency, and finally by tuning the client that receives the stream to not “cache” any incoming data, which, if it does, increases latency.
Sometimes audio codecs also introduce some latency of their own. You may be able to get less latency by using speex, for example, or opus, in place of libmp3lame.
You will also want to try and decrease latency at the server side, for instance wowza hints.
Also setting -probesize and -analyzeduration to low values may help your stream start up more quickly (it uses these to scan for “streams” in certain muxers, like ts, where some can appears “later”, and also to estimate the duration, which, for live streams, the latter you don’t need anyway). This should be unneeded by dshow input.
Reducing cacheing at the client side can help, too, for instance mplayer has a “-nocache” option, other players may similarly has some type of pre-playback buffering that is occurring. (The reality is mplayers -benchmark option has much more effect).
Using an encoder that encodes more quickly (or possibly even raw format?) might reduce latency.
You might get less latency by using one of the “point to point” protocols described in this document, at well. You’d lose the benefit of having a server, of course.
NB that a client when it initially starts up may have to wait until the next i-frame to be able to start receiving the stream (ex: if receiving UDP), so the GOP setting (-g) i-frame interval will have an effect on how quickly they can start streaming (i.e. they must receive an i-frame before they start). Setting it to a lower number means it will use more bandwidth, but clients will be able to connect more quickly (the default for x264 is 250–so for 30 fps that means an i-frame only once every 10 seconds or so). So it’s a tradeoff if you adjust it. This does not affect actual latency (just connection time) since the client can still display frames very quickly after and once it has received its first i-frame. Also if you’re using a lossy transport, like UDP, then an i-frame represents “the next change it will have to repair the stream” if there are problems from packet loss.
You can also (if capturing from a live source) increase frame rate to decrease latency (which affects throughput and also i-frame frequency, of course). This obvious sends packets more frequently, so (with 5 fps, you introduce at least a 0.2s latency, with 10 fps 0.1s latency) but it also helps clients to fill their internal buffers, etc. more quickly.
Note also that using dshow’s “rtbufsize” has the unfortunate side effect of sometimes allowing frames to “buffer” while it is waiting on encoding of previous frames, or waiting for them to be sent over the wire. This means that if you use a higher value at all, it can cause/introduce added latency if it ever gets used (but if used, can be helpful for other aspects, like transmitting more frames overall consistently etc. so YMMV). Almost certainly if you set a very large value for this, and then see the “buffer XX% full! dropping!” message, you are introducing latency.
There is also apparently an option -fflags nobuffer which might possibly help, usually for receiving streams reduce latency.
By default, ffplay (as a receiver for testing latency) introduces significant latency of its own, so if you use it for testing (see troubleshooting section) it may not reflect latency accurately. FFplay introduces some video artifacts, also, see notes for it in “troubleshooting streaming” section Also some settings mentioned above like “probesize” might help it start more quickly.
Useful is mplayer with its -benchmark for testing latency (-noaudio and/or -nocache might be useful, though I haven’t found -nocache to provide any latency benefit it might work for you).
Using the SDL out option while using FFmpeg to receive the stream might also help to view frames with less client side latency: “ffmpeg … -f sdl “window title”” (this works especially well with -fflags nobuffer, though in my tests is still barely more latency than using mplayer -benchmark always). This doesn’t have a “drop frames if you run out of cpu” option so it can get quite far behind at times (introduce more latency variably).
Another possibly useful receiving client is “omxplayer -live”
See also “Point to point streaming” section esp. if you use UDP etc.
Here is a list of some other ideas to try (using VBR may help, etc.)