ffmpeg/doc/HOWTO-STREAM

The FFserver streaming HOWTO
----------------------------

Philip Gladstone <philip-ffserver@gladstonefamily.net>
Last updated: July 26, 2002

0. What is this HOWTO about?

This covers only the streaming aspects of ffserver / ffmpeg. All questions about
parameters for ffmpeg, codec questions, etc. are not covered here.

You should also read the ffserver.txt file in this directory. It contains
roughly the same information.

1. What can this do?

When properly configured and running, you can capture video and audio in real
time from a suitable capture card, and stream it out over the Internet to
either Windows Media Player or RealAudio player (with some restrictions).

It can also stream from files, though that is currently broken. Very often, a
web server can be used to serve up the files just as well.

2. What do I need?

I use Linux on a 900MHz Duron with a cheapo Bt848 based TV capture card. I'm
using stock linux 2.4.17 with the stock drivers. [Actually that isn't true,
I needed some special drivers from my motherboard based sound card.]

I understand that FreeBSD systems work just fine as well.

3. How do I make it work?

First, build the kit. It *really* helps to have installed LAME first. Then when
you run the ffserver ./configure, make sure that you have the --enable-mp3lame
flag turned on.

LAME is important as it allows streaming of audio to Windows Media Player. Don't
ask why the other audio types do not work.

As a simple test, just run the following two command lines:

./ffserver -f doc/ffserver.conf &
./ffmpeg http://localhost:8090/feed1.ffm

At this point you should be able to go to your windows machine and fire up
Windows Media Player (WMP). Go to Open URL and enter 

    http://<linuxbox>:8090/test.asf

You should see (after a short delay) video and hear audio. 

WARNING: trying to stream test1.mpg doesn't work with WMP as it tries to
transfer the entire file before starting to play.

4. What happens next?

You should edit the ffserver.conf file to suit your needs (in terms of 
frame rates etc). Then install ffserver and ffmpeg, write a script to start
them up, and off you go.

5. Troubleshooting

* I don't hear any audio, but video is fine

Maybe you didn't install LAME, or get your ./configure statement right. Check
the ffmpeg output to see if a line referring to mp3 is present. If not, then
your configuration was incorrect. If it is, then maybe your wiring is not
setup correctly. Maybe the sound card is not getting data from the right 
input source. Maybe you have a really awful audio interface (like I do)
that only captures in stereo and also requires that one channel be flipped. 
If you are one of these people, then export 'AUDIO_FLIP_LEFT=1' before 
starting ffmpeg.

* The audio and video loose sync after a while.

Yes, they do.

* After a long while, the video update rate goes way down in WMP.

Yes, it does. Who knows why?

* WMP 6.4 behaves differently to WMP 7.

Yes, it does. Any thoughts on this would be gratefully received. These
differences extend to embedding WMP into a web page. [There are two
different object ids that you can use, one of them -- the old one -- cannot
play very well, and the new one works well (both on the same system). However, 
I suspect that the new one is not available unless you have installed WMP 7].

6. What else can it do?

There seems to be a bunch of code that allows you to replay previous
video. I've never tried it, so it probably doesn't work properly. YMMV.
In fact, in order to get some level of stability, ffserver now deletes
all the previously sent video whenever it restarts.

You can fiddle with many of the codec choices and encoding parameters, and
there are a bunch more parameters that you cannot control. Post a message
to the mailing list if there are some 'must have' parameters. Look in the
ffserver.conf for a list of the currently available controls.

It will automatically generate the .ASX or .RAM files that are often used
in browsers. These files are actually redirections to the underlying .ASF
or .RM file. The reason for this is that the browser often fetches the
entire file before starting up the external viewer. The redirection files
are very small and can be transferred quickly. [The stream itself is
often 'infinite' and thus the browser tries to download it and never 
finishes.]

7. Tips

* When you connect to a live stream, most players (WMP, RA etc) want to
buffer a certain number of seconds of material so that they can display the
signal continuously. However, ffserver (by default) starts sending data
in real time. This means that there is a pause of a few seconds while the
buffering is being done by the player. The good news is that this can be
cured by adding a '?buffer=5' to the end of the URL. This says that the
stream should start 5 seconds in the past -- and so the first 5 seconds 
of the stream is sent as fast as the network will allow. It will then
slow down to real time. This noticeably improves the startup experience.

You can also add a 'Preroll 15' statement into the ffserver.conf that will
add the 15 second prebuffering on all requests that do not otherwise
specify a time. In addition, ffserver will skip frames until a key_frame
is found. This further reduces the startup delay by not transferring data
that will be discarded.

* You may want to adjust the MaxBandwidth in the ffserver.conf to limit
the amount of bandwidth consumed by live streams.

8. Why does the ?buffer / Preroll stop working after a time?

It turns out that (on my machine at least) the number of frames successfully
grabbed is marginally less than the number that ought to be grabbed. This
means that the timestamp in the encoded data stream gets behind real time.
This means that if you say 'preroll 10', then when the stream gets 10
or more seconds behind, there is no preroll left.

Fixing this requires a require in the internals in how timestampts are 
handled.

9. Does the ?date= stuff work.

Yes (subject to the caution above). Also note that whenever you start
ffserver, it deletes the ffm file, thus wiping out what you had recorded
before. This behaviour is a temporary fix to various crashes. The aim is
to fix it so that the old data is saved if possible.

The format of the ?date=xxxxxx is fairly flexible. You should use one
of the following formats (the 'T' is literal):

* YYYY-MM-DDTHH:MM:SS     (localtime)
* YYYY-MM-DDTHH:MM:SSZ    (UTC)

You can omit the YYYY-MM-DD, and then it refers to the current day. However 
note that ?date=16:00:00  refers to 4PM on the current day -- this may be
in the future and so unlikely to useful.

You use this by adding the ?date= to the end of the URL for the stream.
For example:   http://localhost:8080/test.asf?date=2002-07-26T23:05:00