Audio Bit Depth
From Wikipedia, the free encyclopedia
In digital audio, bit depth describes the amount of data contained in each sample, using the unit bits (not to be confused with bytes). Common examples of bit depth include CD audio, which is recorded at 16 bits, and DVD audio which can support up to 24-bit audio.
Contents |
[edit] Digital Audio
Each sample of audio contains data that, when converted into an analog signal, provides the necessary information to reproduce the sound wave as accurately as possible with details such as dynamic range and different frequencies. As one would expect, the lower the bit depth, the lower the overall quality of the recording. By reducing the bit depth, data is lost in each sample using a method of 'a little of everything'; rather than completely strip away, for example, the dynamic range or the frequency range, a compromise is made and each part of that sample drops an amount of data.
One might ask why an audio file would be recorded at a lower bit depth. There are different reasons, such as lack of storage space (such as on a portable digital audio player) or the necessity to transfer the file over a network with the most efficiency where quality is not a top priority.
[edit] What is a 'bit' of data?
Bits refer to binary; 16-bit means there are sixteen digits, all ones or zeroes. Binary is base-2 (each column can only be one or zero. Decimal is base 10, since each column can be any one of 0-9). Make the bit value, in this scenario we'll use 16, the exponent:
216 (or) 2x2x2x2x2x2x2x2x2x2x2x2x2x2x2x2 = 65,536
This means that each sample can contain any one of 65,536 unique values made up of sixteen ones and zeroes.
[edit] Calculating Values
There is an easy way to determine a file's bit rate when given sufficient information. (In fact, as long as you are given any three of the following four values, you can calculate the missing number.)
Bit rate = (bit depth) x (sampling rate) x (number of channels)
For a recording with a 44.1 kHz sampling rate, 2 channels (stereo) and a 16 bit depth:
16 x 44100 x 2 = 1411200 bits per second, or, 1411.2 kBps
[edit] Bit depth versus bit rate
The generally-accepted definition of bit rate: The amount of data (bits) per second being processed.
In all digital audio, there is a bit rate that can be calculated. There is confusion amongst many who believe bit rate only applies to compressed audio (such as MP3). This is probably because the term bit rate often only appears when compressing audio, and bit depth seems to only appear when digitizing a recording.
The use of the term bit rate is possibly more for the sake of ease; someone who isn't very knowledgeable in audio can easily get to know numbers like 128 kBps, 192 kBps and 256 kBps (common bit rates for compressed audio). Additionally, if one was to encode, say an mp3 (sample rate of 44.1 kHz, 2 channels), with two copies at 192 and 256 kBps, the bit depth would then be 2.1 and 2.9 bits, respectively. The difference between these two values seems very small and to the unknowledgable, might give the impression that this difference is negligible.
