What's algorithm WinAmp use to caculate its time positions?

yxdacdemic · 8th July 2002, 22:09

Hi,

I tested WinAmp time positions for VBR mp3 files. It seems there is a problem, the output time positions (in milliseconds) by using WM_USER Messages 105 are not accurate after a fast forward or rewind operation. I guess WinAmp may just simply get a current bit rate and use it to calculate a file length to seek. But I also realized that WinAmp can dynamically update its track length. It is the sign that WinAmp may average the bit rates for time estimations.

It is highly appreciated if somebody can give me some detailed guidelines about how WinAmp calculate its time position during fast forward or rewind operations? Thank you very much.

David

grv575 · 22nd July 2002, 17:57

If you load a 100mb mp3 you'll notice that it loads right away. This seems to indicate that winamp doesn't get exact times be running through all the frame headers and calculating the total play time. It most likely gets an estimate from either a small sample of frames or perhaps from the filesize. In any case this time is accurate enough to the second. Don't see why you would need or even want ms accuracy (since large files would then take too long to load).

Gourou · 23rd July 2002, 00:28

Vbr files have a certain frame rate already set, to wit:
a 128k vbr, can go down to 112k, or up to 160k, but it only does this momentarily, they can even go to greater extremes, but they still have a certain bitrate that they conform to, it is this bitrate that winamp calculates by. There is a chance I'm wrong, but I have written 2 decoders, and this is the method I used, when I calculate out the time, I was RARELY more than a 10th of a second off, and I could usually correct the display for this, before the user even heard one output sample

8th July 2002, 22:09	#1
yxdacdemic Junior Member Join Date: Jan 2002 Location: Chicago Posts: 5	What's algorithm WinAmp use to caculate its time positions? Hi, I tested WinAmp time positions for VBR mp3 files. It seems there is a problem, the output time positions (in milliseconds) by using WM_USER Messages 105 are not accurate after a fast forward or rewind operation. I guess WinAmp may just simply get a current bit rate and use it to calculate a file length to seek. But I also realized that WinAmp can dynamically update its track length. It is the sign that WinAmp may average the bit rates for time estimations. It is highly appreciated if somebody can give me some detailed guidelines about how WinAmp calculate its time position during fast forward or rewind operations? Thank you very much. David

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22nd July 2002, 17:57	#2
grv575 Junior Member Join Date: Oct 2001 Posts: 7	If you load a 100mb mp3 you'll notice that it loads right away. This seems to indicate that winamp doesn't get exact times be running through all the frame headers and calculating the total play time. It most likely gets an estimate from either a small sample of frames or perhaps from the filesize. In any case this time is accurate enough to the second. Don't see why you would need or even want ms accuracy (since large files would then take too long to load).

23rd July 2002, 00:28	#3
Gourou Senior Member Join Date: Feb 2002 Location: The backside of the universe on the trailing edge of eternity Posts: 238	Vbr files have a certain frame rate already set, to wit: a 128k vbr, can go down to 112k, or up to 160k, but it only does this momentarily, they can even go to greater extremes, but they still have a certain bitrate that they conform to, it is this bitrate that winamp calculates by. There is a chance I'm wrong, but I have written 2 decoders, and this is the method I used, when I calculate out the time, I was RARELY more than a 10th of a second off, and I could usually correct the display for this, before the user even heard one output sample