W3C home > Mailing lists > Public > public-media-capture@w3.org > October 2012

Re: approaches to recording

From: Timothy B. Terriberry <tterriberry@mozilla.com>
Date: Tue, 16 Oct 2012 18:23:25 -0700
Message-ID: <507E088D.3040904@mozilla.com>
CC: public-media-capture@w3.org
Robert O'Callahan wrote:
> One option would be to have two APIs to initialize the recorder: one
> which takes a MediaStream, and one which takes a list of
> MediaStreamTracks. The former has the possibility of new tracks being
> added dynamically, and the latter does not. The UA could reject requests
> if the format doesn't support the specified tracks, or if the format
> doesn't support addition of tracks and the author passed in a MediaStream.

I think it would be helpful for the group to outline some of the 
capabilities of the formats that exist today.

The only container I'm aware of that _might_ be able to support 
dynamically adding tracks _and_ supports "streaming encoding" (by which 
I mean that the browser can hand you pieces of the data as it encodes 
them, and the application can reconstruct a valid file from them by 
concatenating them all afterwards) is MPEG Transport Streams. Support 
for MPEG TS, especially if you want it to do anything even slightly 
complicated, is universally bad (it is the only format that media 
developers can all agree is more difficult to deal with than Ogg).

Both Ogg and Matroska (WebM) can change the track list, but only by 
ending all currently active tracks and starting a new segment of the 
file with a completely new set of tracks. However, even though this is 
technically possible in the file format, almost no software actually 
supports it, except perhaps in the limited case where the number, types, 
and formats of the tracks in each segment do not actually change.

There are three other general-purpose containers I'm aware of that 
actually support streaming encoding: ASF, Matroska, and Ogg. ASF does 
not get used very much anymore, and I don't really know much about it, 
so I will ignore it. Although Matroska does support streaming encoding, 
it cannot include an index (because you can't store the index at the 
beginning and putting it at the end requires a pointer at the beginning 
to say where it is, which you also can't include without knowing how big 
the file will be in advance). I'm not aware of any software that can 
seek in Matroska files without an index except by linearly scanning 
through the entire file (this is, e.g., what Chrome does; Firefox 
refuses to seek in them entirely), with the sole exception of the Haali 
DirectShow filters available on Windows. Ogg files, by comparison, 
normally do not contain an index, so seeking in Ogg files produced this 
way is supported relatively well in comparison.

RIFF (WAV) can support streaming encoding, but only by lying about the 
size of the DATA chunk. Support for reading files of this type is 
hit-or-miss. There is no standard for it, so different applications lie 
in different ways, and even software we at Xiph have deployed and 
maintained for over a decade still occasionally runs into files which 
confound its attempt to handle this (as recently as a month or two ago). 
I am not aware of any attempt to support streaming encoding for RIFF in 
AVI (i.e., video), though some software can support playing (but not 
seeking in) AVI files without an index.

QuickTime (MP4) is completely non-streamable. The index is the _only_ 
thing in the file that tells the decoder where the packet boundaries 
are, and you can't write it out until you've seen all of the packets. 
HTTP live streaming is starting to use a "segmented MP4" format which 
chops data into separate MP4 files each 10 seconds long or so, but 
player support for files like that is extremely limited, the latency it 
introduces is quite significant, and using smaller chunks would add 
excessive overhead.
Received on Wednesday, 17 October 2012 01:23:54 GMT

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