[FFmpeg-user] HTTP playlist stream copy to an IP QAM modulator

[Isaac Asimov]

> From: anacelia.sarlo at gmail.com
> Date: Wed, 20 Jul 2016 09:38:06 -0300
> To: ffmpeg-user at ffmpeg.org
> Subject: Re: [FFmpeg-user] HTTP playlist stream copy to an IP QAM modulator
> 
> Reuben, thanks to answer. I did 2 tests :
> 1. I received the stream with VLC and I have a similar problem like with
> the IP QAM.
> 2. Transcode the stream and the video its ok.
> 
> But my doubt is, if I receive the "stream copy" with ffplay, it runs
> perfect!, so
> it seems the response to my problem is, the way ffplay process/recover the
> time base.
> 
> If I enable a debug it looks like every time ffmpeg make an http request
> for
> the playlist, there is a delay until receive it, and seems at this point
> the video freeze, and then continue.
> Thanks for another suggestion.
> Ana
> 
> On Wed, Jul 20, 2016 at 12:56 AM, Reuben Martin <reuben.m at gmail.com> wrote:
> 
> > On Tuesday, July 19, 2016 10:16:03 PM CDT Anacelia Sarlo wrote:
> > > ffmpeg -re -y -i $stream  -c copy -f mpegts -map 0:0 -map 0:1
> > > -mpegts_pmt_start_pid 66 -streamid 1:71 -streamid 0:70 udp://
> > > 192.168.1.108:1234?pkt_size=1316
> > >
> > > When I watch the stream from the cable network,  the audio is OK but the
> > > video looks sometimes faltering and other times too fast, and out of
> > sync.
> > > But when I received this same output with ffplay it runs OK.
> > > It seems there is a problem with the time base, I did many testings with
> > > (genpts, copyts, etc)  with no good results.
> >
> > You might try and see what happens if you re-encode it rather than copying
> > the
> > media streams. Thing is with copying the stream you pass on any flaws that
> > might be able to be corrected by ffmpeg, but are not as gracefully dealt
> > with
> > by other implementations.
> >
> > At least that might help narrow the problem down a bit and determine if the
> > problem is with the origional source, or with ffmpeg.
> >
> > It would be nice to have a bit-stream filter to re-write time stamps when
> > doing a stream copy, but sadly no such filter exists yet.
> >
> > -Reuben

I don't know in cable networks but in DVB-T (terrestrial) there are "Null packets" in MPEG-TS mux, with PID 0x1FFF which target it's achieve a constant bit rate, will null data. AFAIK ffmpeg don't generate this kind of packets because it's unneeded in IPTV, that should be task of IP QAM firmware.

By the other side, MPEG-TS can use PTS and PCR info in each Packetized elementary stream (e.g. in each video frame) to achieve sync, it's easy play a video correctly without that info (and maybe that is the reason ffplay play ok), but most MPEG-TS players need PCR and PTS fields to avoid errors playing.