>>> MPEG-1 Layer I is 384 samples. MPEG-1 Layer II and III is 1152 samples.
>>> Values are halved for LSF.
>>
>> The number of samples is only halved for Layer III LSF (576). Layer I and
>> Layer II are always 384 and 1152 samples, respectively.
>
> What is Layer III "LSF" ?
LSF refers to the MPEG 2 extension to Lower Sampling Frequencies (24000,
22050, and 16000 Hz.) The "ID" bit in the frame header is 0 for LSF.
In LSF, Layer III has only one granule rather than two, so there are half as
many output samples per frame.
> I had a problem with some files to compute the length in second of a
> frame. I was using various Layer III format and is some cases the formula
> "frame_time = samples / frame_per_second" wouldn't give me the right
> length. I assume it might be comming from this LSF.
How do you calculate frame_per_second?
A better formula would be
frame_time = frame_samples / sampling_frequency
to get frame_time in seconds.
LSF can affect both frame_samples and sampling_frequency:
MPEG 1 MPEG 2 LSF
Sampling Frequency 48000 24000
44100 22050
32000 16000
Samples/Frame
Layer I 384 384
Layer II 1152 1152
Layer III 1152 576
> By the way, the formula never work with 'free format' files (encoded with
> LAME 3.86beta), but I've found another one which works fine. (can't
> remember, but I can give it if you want).
The above formula will work regardless of bitrate, free format or otherwise.
--
Rob Leslie
[EMAIL PROTECTED]
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