Variable Bit Rate Explained (a side note) November 17, 2008Posted by ConnorSmith in : Chit Chat, File Compression, Project Studio , trackback
So, what is variable bit rate (or VBR)?
This is a tangent from the compression articles. I thought I should explain VBR.
Normally, when encoding a compressed audio files (let’s say, an MP3), you need to specify a bit rate in KBPS (kilobits per second). This tells the encoder how many bits it has to work with over the given time period. (See Everything About Compression: Take 2 for info on what the encoder is doing)
But, what if a REALLY complicated second of audio is followed by a really simple second (say a second of metal mathcore followed by a 1k sine wave). In this case, the encoder would struggle to fit all the metal into the allocated bits, but would probably have a lot of wiggle room with the sine wave encoding.
Well, a VBR encoder would realize this, and vary the bit rate for that section. It would allocate more bits to the second of complex signal (the mathcore), leaving less (but more than sufficient) bits available for the sine wave encoding.
To relate it to real life… You ate a sandwich (on wheat bread) that had 300 calories. Now, you have two things on your “to do” list:
1) Stand up
2) Do 9007 jumping jacks
If you were not a Variable Bit Rate encoded person, you would allocate 150 calories for standing up and 150 calories for doing 9007 jumping jacks. What’s going to happen? Standing up is no problem, but sometime after you start jumping jacks, you are going to fail. When the compression encoder “fails”, it does things like not giving enough bits to the audio signal to raise it above the (quantization-induced) noise floor, thus audible distortion.
If you were a VBR encoded person, you could allocate 1 calorie to standing up, and 299 to jumping jacks, thus giving yourself almost twice the chance of finishing them.
A rather ridiculous example… but hopefully it gets the point across.
The Studio Files