Re: obtaining stream length from mad decoder

On Jul 28, 2005, at 3:48 PM, Langley, Adam wrote:
> I have written a Managed C++ wrapper for the libMad decoder, and I  
> would like to know…. How can I get EITHER, the _decoded_ length in  
> bytes, or the duration in seconds of the whole file, without  
> decoding the whole file?
>
> I need to just evaluate a set of mp3 files, and obtain their  
> lengths (preferably in bytes, then I can calculate the length in  
> seconds by the bytes-per-second)

There is no official decoded length in bytes, as the number of output  
bits per sample is unspecified. Rather, you can determine the length  
in samples -- although it is often simpler to determine the length  
directly in units of time.

There are four possible methods for determining the length of an  
entire file:

1. Read the length from a 'TLEN' frame of an ID3v2 tag. If present,  
this is supposed to contain the length of the audio file in  
milliseconds (as a numeric string).

2. Read a Xing tag from the ancillary data of the first decoded  
frame. Multiply the playing time of one frame (given by frame- 
 >header.duration) by the number of frames in the file as given by  
the tag.

3. Assuming the file has a constant bitrate (not VBR), divide the  
file size in bytes (less any ID3 tags) by (125 * kbps) to obtain the  
playing time in seconds.

4. Scan every frame header of the file and accumulate the playing  
time of each to arrive at a total. This does not require a full  
decode of each frame, so it can be done relatively quickly.

-- 
Rob Leslie
[email protected]