Whenever we deal with uncompressed audio formats such as WAV, AIFF or Red Book Audio, we talk about sample rates and bit depths. But when we talk about compressed formats such as MP3, AAC, or FLAC its bit rates. So, what is meant by bit rate and what is its relationship to sample rate and bit depth. As far as audio goes this can be explained quite simply. Bit rate is the number of bits transferred or processed per some unit of time (in our case a second).
Here’s the equation for converting sample rate and bit depth to bit rate:
Bit Rate = Sample Rate x Bit Depth x Number of Channels
For Example a standard Red Book Audio CD has a sample rate of 44.1 kHz (that’s 44,100 samples per second), a bit depth of 16 bits (that’s 16 bits per sample), and is stereo (2 channels). Using the equation above, we get this:
44,100 x 16 x 2 = 1,411,200 (1,411,200 bits per second = 1411.2 kbit/s)
So a CD has a bit rate of about 1411 kbit/s and a decent quality MP3 starts at about 160 kbit/s. That’s nearly a 9:1 ratio of compression. This demonstrates two things. One, that although my MP3 albums cost a few bucks less than their CD equivalents, I’m getting 1,251,200 bits less a second for my money. Two, the fact that losing all those bits allows me to fit a hundred plus albums in my pocket and the quality is still listenable at all is pretty impressive.
Many audiophiles may proclaim the MP3 as some ugly bastard child of the recording industry and there is some truth to that. But the MP3 and its various ugly cousins were part of an internet driven reconstruction of the industry. Music became unfathomably accessible. It seemed you could find almost any album on the internet. New music you’d never even heard of and it was even free if you felt like stealing from musicians and the labels that stole from them. You didn’t even have to leave the house.
Music seemed to become infinitely portable; scratched CDs scattered about a car’s floorboard became a small thing in your pocket or they didn’t even seem to physically exist at all. Instead existing in some mysterious wonderland they call the cloud. People traded entire music collections and found new music. But most importantly, the business was thrown into seeming chaos. A computer company became the most powerful music distributor in the world and any musician could release their own album on the web and possibly hit it big (though probably not). My point is that the MP3 was part a revolution in the industry and that revolution brought some bad and some good changes. So get used to the term bit rate. It’s here to stay…at least for now.