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View Full Version : WMA codec bitrate peculiarity..



GSV3MiaC
07-04-2005, 07:05 PM
Encoding a track to 2-pass 64kbps VBR WMA 9.1, I get a filesize of 2362kBytes,
for a file which is 4 minutes 56 seconds (per the tags). Now when I do the sums that is just about exactly 64kbps .. so why does the 'bitrate' tag lie to me and say 73 .. is that the maximum or something??

LtData
07-04-2005, 07:38 PM
Bitrate tag in the Popup tag, the Explorer properties, or in dAP?
I have seen VBR mp3s have "bitrates" in the Explorer properties of 400+kbps. This is obviously incorrect.

ChristinaS
07-04-2005, 10:26 PM
VBR - Variable Bit Rate. What you ask for is just an indication, the actual bitrate will vary from below to above that number, depending on contents of the audio source. All along the wave pattern it will not stay a constant bitrate. Probably the bitrate for the starting point which is reported is 73. In other places it may be 50, and so on.

GSV3MiaC
07-05-2005, 05:38 AM
Bitrate tag in the Popup tag, the Explorer properties, or in dAP?
I have seen VBR mp3s have "bitrates" in the Explorer properties of 400+kbps. This is obviously incorrect.


They are all the same. I'm loking at a 48kbps .WMA 2-pass VBNR rip right now, and they all say 59kbps (so at least they are consistent), however file size divided by track length gives 48, so they are all consistently wrong.

GSV3MiaC
07-05-2005, 05:41 AM
VBR - Variable Bit Rate. What you ask for is just an indication, the actual bitrate will vary from below to above that number, depending on contents of the audio source. All along the wave pattern it will not stay a constant bitrate. Probably the bitrate for the starting point which is reported is 73. In other places it may be 50, and so on.

Possibly Christina, however for all examples I have looked at so far the reported number is higher than the actual number (remember this is two pass VBR, so the actual average should come out close to what you asked for).

I would expect the start of each track to have very low bitrate (mostly being silence). I'm beginning to suspect that the reported number (presumably plonked into the tags by the WMA9.1 codec) is actually the maximum used on the track.

neilthecellist
07-05-2005, 03:47 PM
VBR has always given me incorrect bitrates using any program that can identify the bitrate of the file. No worries.

ChristinaS
07-05-2005, 04:42 PM
I'm not sure an average is what is reported by VBR. Because in that case you'd be using ABR - average bitrate.
I doubt it's a maximum also. In truth I don't know what the heck it is :D

Defintiion:


VBR - A type of encoding of compressed audio that allows for different bitrates in the same file. VBR can reduce file size and increase quality. The file size can be reduced by keeping bit rates low in non-complex parts of audio. Increased quality comes from allowing high bitrates in more complex segments of audio.


In another broader context:


Short for variable bit rate, or Class B quality of service, an ATM bandwidth-allocation service that allows users to specify a throughput capacity (i.e., a peak rate) and a sustained rate but data is not sent evenly. VBR is often used when transmitting compressed packetized voice and video data, such as videoconferencing.