[Icecast] Icecast WebM Support Patch Second Edition
dennis at heerema.net
Thu Feb 9 02:37:06 PST 2012
Streaming live WebM would also give a boost to this open format.
Whitch Client do you use to stream the webm format to icecast with?
From: Krad Radio <kradradio at gmail.com>
To: icecast-dev at xiph.org, icecast at xiph.org
Date: Wed, 8 Feb 2012 23:53:05 -0500
Subject: [Icecast] Icecast WebM Support Patch Second Edition
tl;dr: Nothing new or interesting to non-developers
Attached is a newer Icecast WebM support patch for Icecast SVN, there is
simply aesthetic changes. Whitespace has been altered to match Icecast
some functions have been renamed and moved around.
The format_ebml.c file lines 0-296 operates much the same as the
format_ogg.c file does. It uses a parsing library api to feed in and pull
out bytes via
the _get_buffer function. In the case of Ogg this is of course libogg, in
the case of ebml, the "parsing library" consists of the functions below line
This mini-included library comes from some debugging code that I wrote
during my work on a much more robust EBML muxing library, originally I had
icecast would need to do some manipulation of the EBML segment header in all
cases, but it turns out in most cases this is better done by the source
This mini-included library implements the same function calls that Icecast
would need to use if it was using this EBML muxing library, but "parse" is a
generous descriptor of whats actually happening. If you are familiar with
the internals of Ogg streaming, you know that the stream starts out with a
few header pages
that are then followed by pages containing the acutual video and audio
packets. EBML doctype WebM and MKV (The only currently known) work in a
There is a "Segment Header" followed by "Clusters" (Its actually more
nuanced than this, but this oversimplification will suffice for the moment).
Clusters are comparable to
Ogg pages. The connecting client needs to be sent the header, and then it
can start on any cluster as long as it starts exactly on a cluster. (This is
actually not a format requirement per se but all media players I am aware of
lack the ability to re-sync themselves if started on a random byte,
something I intend not to be a limitation of my own work). So, in a live
WebM stream, everything before the first cluster is the "Header" and then
the rest is the clusters, of which the boundary between them is marked with
4 specific bytes. So at every cluster the refbuf is marked as a sync point.
The size of clusters could vary significantly during a stream or from stream
to stream, but on a properly constructed one would indicate a keyframe in
the case of a stream with video. The 'mini-library' doesn't actually 'parse'
the stream at all, all it does is look for the four byte sequence indicating
a cluster boundary and informs the format functions appropriately. I am a
fraudulent mathematician at best, but I calculate that there is a 1 in 4.2
billion chance of this happening for any given 4 bytes, and 1 in 4294 per
megabyte, and likely once per 4.2 gigabytes. This is not a problem once the
stream has started for the client however, it only matters when the source
connects and the header is stored, and when a client connects and needs a
proper starting point. I suppose that makes it very unlikely to cause a
problem, even though its technically wrong. It also means that each byte is
being compared, whilst when properly parsing most would be skipped, but
computers are so damn fast that its moot. At any rate proper parsing could
be added this this mini-library or provided by the external library as it
-------------- next part --------------
An HTML attachment was scrubbed...
More information about the Icecast