Re: random seeking with MAD

On Nov 27, 2003, at 8:57 AM, Jedediah Smith wrote:
> How would I implement fast, sample-resolution random seeking with MAD? 
> In other words, I need to be able to reposition the stream to a 
> specific sample and get resynced in constant time and without too much 
> overhead. The MP3 also needs to be streamed from disk rather than 
> loaded entirely in RAM. I imagine the main problem would be 
> reconstructing the bit pool at the seek point.
>
> From my understanding of the MP3 format, the furthest back bit pool 
> data can be stored is 9 frames. So here is my idea:
>
> 1. On initial load, create an index of all the frame offsets in the 
> mp3 and cache it in RAM
> 2. When seeking to sample N, calculate the containing frame number F 
> as F=N/1152
> 3. Use the cached frame index to find the file offset of frame F-9
> 4. Seek to that offset in the file and begin decoding
> 5. Decode 9 frames silently, ignoring any MAD_ERROR_BADDATAPTR
> 6. Begin playing at sample N which is within the current frame
>
> Will this work? Is there a faster way that avoids decoding the audio 
> data for the extra 9 frames?

This seems a reasonable approach. Keep in mind the number of samples 
per frame will only be 1152 for MPEG-1 Layer III or II. Likewise the 9 
frame reservoir limit only applies to MPEG-1. For MPEG-2 the 
theoretical maximum is 29 frames.

You don't have to fully decode each frame leading to your target frame; 
you can skip the synthesis step for all but the frame immediately prior 
to the target.

-- 
Rob Leslie
[email protected]