Rockbox.org home
release
dev builds
extras
themes manual
wiki
device status forums
mailing lists
IRC bugs
patches
dev guide



Rockbox mail archive

Subject: Re: MPEG Audio format

Re: MPEG Audio format

From: Bluechip <csbluechip_at_gmail.com>
Date: 2006-01-15

> > >>Am I ever likely to encounter an MPEG Audio file which will contain
> AUDIO
> > >>frames which differ in VERSION, LAYER, SAMPLERATE and/or CHANNELS? Eg.
> > >>Some frames are 48KHz and other frames are 44.1KHz ...or some frames in
> > >>Stereo, some in Joint-Stereo?
>
>a change of LAYER or SAMPLERATE requires a restart of the decoder.

Excellent information.

I notice you did not mention VERSION or CHANNELS ...Given a LAYER change
will trigger a restart it seems logical to presume a VERSION change would
also trigger a restart even if the change was Ver1/Lyr1 -> Ver2/Lyr1

Would you care to comment on CHANNEL changes ...even if your answer is
"don't know" I would consider that to be helpful feedback.

> > Thanks, I too use razorlame, and also have no idea what short or long
> > blocks are - and google isn't helping much either. LOL
>
>IIRC, the audio data that belongs to each MPEG-1 layer III audio header
>(in fact, the frame header can be located just in the middle of the
>audio data it belongs to)

:O
A frame header can exist in the middle of an [a pair of] audio frame[s]??

Could you explain a little more on that - it seems very important to my util.

If my file has only TWO [a pair of] audio frames (unlikely, but valid)
...the file can/will look like "AUDIO_DATA -- HEADER -- AUDIO_DATA"
...surely lack of a header at the start of the file will mean the first
block of audio data cannot be found by the decoder?

Or do you mean that there would be two [a pair of] audio frames which each
contained half of the data required to recreate the original sound?

Do you have a test file which I can throw at my util for testing please?
(or instructions for making one)

> is parted into 2 halves AKA granules. Those in
>turn are parted into n (frequency) subbands, which can consist of either
>1 long or 3 short blocks (windows) of samples (it's even possible to have long
>blocks in the lower and short blocks in some upper subbands). The former
>have a better frequency resolution, the latter a better time resolution
>(eg to reduce pre-echoes). Try a percussive sound like from a
>glockenspiel or triangle to hear the difference when short blocks are
>disabled.

Perfect, thank you.

>Jacob

BC
Received on Sun Jan 15 04:05:40 2006


Page was last modified "Jan 10 2012" The Rockbox Crew
aaa