Audio bit rate

Bit rate


Bit rate refers to the amount of data, specifically bits, transmitted or 
received per second.


One of the most common bit rates given is that for compressed audio files.
For example, an MP3 file might be described as having a bit rate of 
160 kbit/s or 160000 bits/second. This indicates the amount of compressed
data needed to store one second of music.


The standard audio CD is said to have a data rate of 44.1 kHz/16, meaning
that the audio data was sampled 44,100 times per second, with a bit depth
of 16. CD tracks are usually stereo, using a left and right track, so the 
amount of audio data per second is double that of mono, where only a 
single track is used. The bit rate is then 44100 samples/second/track 
x 16 bits/sample x 2 tracks = 1,411,200 bit/s or 1.4 Mbit/s.


This explains why, for example, a Minidisc recorder, which uses ATRAC 
compression, can store files lasting twice as long on a disc, if the 
default, recording in 2 channel stereo, is set to single channel 
mono recording.


To fully define a sound file's digital audio bit rates: the format of 
the data, the sampling rate, word size (bit depth), and the number of 
channels (e.g. mono, stereo, quad), must be known.


Calculating values
An audio file's bit rate can be calculated given sufficient information. 
Given any three of the following four values, the fourth can be calculated.


Bit rate = (sampling rate) × (bit depth) × (number of channels)
E.g., for a recording with a 44.1 kHz sampling rate, a 16 bit depth, 
and 2 channels (stereo):


44100 samples per second per channel × 16 bits per sample × 2 channels 
= 1411200 bits per second = 1411.2 kbit/s