On Sat, 07 Feb 2009 16:52:57 +0000 (GMT), Jim Lesurf
wrote:

On 07 Feb, wrote:
In article 498f921e.236483187@localhost, Don Pearce (Don Pearce) wrote:
On Sat, 07 Feb 2009 13:19:45 +0000 (GMT), Jim Lesurf
wrote:


http://www.mpgedit.org/mpgedit/mpeg_...MP3Format.html

I'll have a look. But as I say, since I know how the files I've snipped
by editing are created I can be sure that those other than the first
won't have any header that only appears at the start of the source data
file. Yet they play OK.

Just had a quick look at the above page. It seems to confirm what I thought
was the case. This is that the MPEG relies on *frame* headers, *not* a file
header. This means that with fixed bitrates you can snip the files OK
provided you know the correct bitrate for that file and each snip is at a
frame boundary.

So if I snip something like a 10min file into two 5min sections I don't
need to add or change the 'headers' as each frame will still have the
header for that frame.

Thanks for the URL. I'll need to look at it in more detail as it has info
I've wanted. But IIUC it confirms what I was saying. Just as well, given
that devices like my players seem happy with the results.

I'll write a utility that examines the frame headers. I'll also check that
against FFmpeg results if I can. That might allow me to produce a better
snipper that deals with VBR and adjusts the rate according to header info.
At present I tell the editor what fixed bitrate to assume.

Slainte,

Jim

Interesting. Presumably the player just ignores the first data chunks
until it finds a frame header, then uses the info from that to read
the succeeding stuff.

d
--
Pearce Consulting
http://www.pearce.uk.com