Calculating the duration Options

[Sebastian Mares]

Hello!

I have a small question about calculating the duration of an MPEG Audio file.
At the moment, I use the following formula to calculate the play time in seconds:


MPEG 1.0 Layer I:
Seconds = Frame Number * 384 / Sampling Rate

MPEG 1.0 Layer II:
Seconds = Frame Number * 1152 / Sampling Rate

MPEG 1.0 Layer III:
Seconds = Frame Number * 1152 / Sampling Rate

MPEG 2.0 Layer I:
Seconds = Frame Number * 192 / Sampling Rate

MPEG 2.0 Layer II:
Seconds = Frame Number * 1152 / Sampling Rate

MPEG 2.0 Layer III:
Seconds = Frame Number * 576 / Sampling Rate

MPEG 2.5 Layer I:
Seconds = Frame Number * 192 / Sampling Rate

MPEG 2.5 Layer II:
Seconds = Frame Number * 1152 / Sampling Rate

MPEG 2.5 Layer III:
Seconds = Frame Number * 576 / Sampling Rate


Is the above method correct?

Regards,
Sebastian Mares

This post has been edited by Sebastian Mares: Nov 4 2003, 13:07

[Sebastian Mares]

Basicly:


Layer I will have 384 samples per frame in MPEG 1, and 192 samples per frame in MPEG 2(.5).

Layer II will always have 1152 samples per frame.

Layer III will have 1152 samples per frame in MPEG 1, and 576 samples per frame in MPEG 2(.5).


Is that correct?

This post has been edited by Sebastian Mares: Nov 3 2003, 20:23

[smack]

Yes, the samples-per-frame for MPEG-1 are correct.
As for MPEG-2 and 2.5: Layer III seems to be correct, but are Layer I and II actually specified? I thought only Layer III is used with with the LSF and VLSF extensions but I might be wrong.

Maybe you can find the answer in the source codes of a well-written decoder like amp11 by Niklas Beisert - I found it very helpful when I wrote my own decoder some time ago...

[Sebastian Mares]

I have just tested the above combinations and everything seems to be in order. For MPEG 2(.5) Layer I 192 samples per frame are used.

However, in an online document I have seen that only Layer III has different samples per frame in MPEG 2 (as only one granule is used instead of two) and that Layer II and Layer I are constant, but when I use MPEG 2 Layer I with 384 samples per frame, the duration is twice as large as original one.

This post has been edited by Sebastian Mares: Nov 4 2003, 12:06