From: Jens Arnold <arnold-j_at_t-online.de>
Date: Sat, 19 Nov 2005 09:31:21 +0100

On 18.11.2005, Bluechip wrote:

> Is there any reason why the Audio frame which contains the
> Xing header should be at the same sample rate (32K, 44.1K,
> 48K) as the main Audio data?

Yes.

First, there may be players which don't support xing headers, so
they treat them as a standard audio frame. That's the reason why
the xing header mimics a standard frame in the first place.

Second, the xing header should provide as much information about
the audio stream as possible. It contains the frame count, but
in order to compute the playtime, you also need the frametime
which depends on the sample rate.

> That is, (by example) what if my Xing header were in an Audio
> frame which claimed to be at 48K ...but the first _real_ Audio
> frame (and all subsequent Audio frames) are actually 44.1K
> ...would any-body or more to the point any-software complain?

Many mp3 frame analysers complain if the sample rate or mpeg
version changes mid-stream. I found no written standards for
this, but I think only the following fields in the frame headers
are allowed to change within one stream: Bitrate, Padding bit,
Private bit, Mode extension. One more field may change between
xing header and the rest of the stream: the Protection bit.

> BC

Regards, Jens
Received on 2005-11-19