Audio Bit Depth
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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.
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[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.
24-bit digital recording is believed to improve sound quality substantially, as well as increasing the sampling rate. Currently, only certain music DVDs with a 24-bit/96 kHz DTS track use 24-bit audio. However real world systems can not achieve the theoretical noise floor limits of 24-bit (-144.50dB). Practical dynamic ranges of ~110dB are more realistic after accounting for other noise sources other than 24-bit Quantization noise.
[edit] What is a 'bit' of data?
In computing parlance, bit is the abbreviation used to mean a single 'binary digit', represented by an 0 or a 1. Within the computer, this represents an electronic switch in an 'on' or 'off' state. 16-bit means there are sixteen digits, all ones or zeroes, e.g. 1001011011001010. Binary is base-2; thus, each column can only be one or zero. 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] 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 160kb/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 44.1kHz/16, implying 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 * 16 bits/sample * 2 = 1411200b/s or 1.4Mb/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 sample rate, word size, number of channels, (e.g. mono, stereo, four-track), and format of the data also need to be known.
[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 know any three of the following four values, you can calculate the missing value.
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
