14 Nov
2003
14 Nov
'03
11:05 a.m.
On Fri, Nov 14, 2003 at 09:20:47AM -0500, Fred Gleason wrote:
FYI, from the vorbis docs:
Vorbis provides none of its own framing, synchronization or protection
against errors; it is solely a method of accepting input audio,
dividing it into individual frames and compressing these frames into
raw, unformatted 'packets'. The decoder then accepts these raw packets
in sequence, decodes them, synthesizes audio frames from them, and
reassembles the frames into a facsimile of the original audio stream.
Vorbis is a free-form variable bit rate (VBR) codec and packets have
no minimum size, maximum size, or fixed/expected size. Packets are
designed that they may be truncated (or padded) and remain decodable;
this is not to be considered an error condition and is used
extensively in bitrate management in peeling. Both the transport
mechanism and decoder must allow that a packet may be any size, or end
before or after packet decode expects.
Vorbis packets are thus intended to be used with a transport mechanism
that provides free-form framing, sync, positioning and error
correction in accordance with these design assumptions, such as Ogg
(for file transport) or RTP (for network multicast).
So, if I'm reading this correctly, the vorbis framesize is not fixed
within the general spec, so you'd have to be prepared for all sorts of
frame sizes.
784 - Michael C. Piantedosi - drclaw(a)dogsolitude.org
On Thursday 13 November 2003 16:48, J_Zar wrote:
> I' ve done some tests on a bunch of songs in different compressed formats
- snip -
different
values for Ogg and Mp3? Ogg will be affected by bitrate?
An MPEG frame always contains 1152 PCM frames, as per the standard. This is
true of Layers One, Two and Three. Thus, the time length of an MPEG frame
would be:
l=1152/fs
where
l = Length of frame in mS
fs = Sample rate in kHz
Thus, for your example case, the calculated length would be:
1152/44.1 = 26.1 mS
thus yielding good agreement with your measured results.
I don't know how it is with Vorbis, but I'd suspect something similar. See:
http://www.xiph.org/