Audio bit depth
Encyclopedia
In digital audio
, bit depth describes the number of bit
s of information recorded for each sample. Bit depth directly corresponds to the resolution of each sample in a set of digital audio data
. Common examples of bit depth include CD
quality audio, which is recorded at 16 bits, and DVD-Audio
, which can support up to 24-bit audio.
, provides the necessary information to reproduce the sound wave. In pulse-code modulation
(PCM) sampling, the bit depth will limit signal-to-noise ratio
(S/N). The bit depth will not limit frequency
range, which is limited by the sample rate.
By increasing the sampling bit depth, quantization noise is reduced so that the S/N is improved. The 'rule-of-thumb' relationship between bit depth and S/N is, for each 1-bit increase in bit depth, the S/N will increase by 6 dB
. 24-bit
digital audio has a theoretical maximum S/N of 144 dB, compared to 96 dB for 16-bit; however, digital audio converter technology is limited to a S/N of about 124 dB (21-bit) because of real world limitations in integrated circuit
design. Still, this approximately matches the performance of the human ear.
Technically speaking, bit depth is only meaningful when applied to pure PCM devices. Non-PCM formats such as lossy compression systems like MP3
, have bit depths that are not defined in the same sense as PCM. In lossy audio compression, where bits are allocated to other types of information, the bits actually allocated to individual samples are allowed to fluctuate within the constraints imposed by the allocation algorithm.
is the difference between the largest and smallest signal a system can record or reproduce. With the proper application of dither
digital systems can reproduce signals with levels lower than their resolution would normally allow. Therefore there is not a direct connection between bit depth and dynamic range.
Audiophile-spec recording resolutions extend this to a theoretical -120dB (20-bit) or -144dB (24-bit), the latter of which exceeds the dynamic range between complete silence (signal energy below that which can be detected by the human ear) and noise of high enough intensity to cause almost immediate ear injuries, with an ideal 24-bit DAC and associated amplified being able to accurately output signal values from 0, 1, 2 through 16777213, 16777214, 16777215.
is the abbreviation for a single binary
digit, represented by a 0 or a 1. A word is a binary number with more than one digit. Binary numerics are base-2; thus, each digit can only be a 0 or a 1. In comparison, traditional decimal numerics are base-10, having digits that can only be 0 through 9. For example, the 16-bit binary number 0110111110111010 is equivalent to the 5-digit decimal number 28602. The number of bits per word is simply how many digits there are in the corresponding number. The words in commonly used PCM digital audio formats are 8, 16 or 24 bits long. Larger words have higher resolution. The resolution of a 16-bit system can be calculated by using 216
which gives a value of 65,536. A 24 bit system (224) has a resolution of 16,777,216.
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 kbps or 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 x 16 bits/sample x 2 = 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, four-track), must be known.
E.g., for a recording with a 44.1 kHz sampling rate, a 16 bit depth, and 2 channels (stereo):
The eventual file size of an audio recording can also be calculated using a similar formula:
E.g., a 70 minutes long CD quality recording will take up 740880000 Bytes, or 740MB:
Digital audio
Digital audio is sound reproduction using pulse-code modulation and digital signals. Digital audio systems include analog-to-digital conversion , digital-to-analog conversion , digital storage, processing and transmission components...
, bit depth describes the number of bit
Bit
A bit is the basic unit of information in computing and telecommunications; it is the amount of information stored by a digital device or other physical system that exists in one of two possible distinct states...
s of information recorded for each sample. Bit depth directly corresponds to the resolution of each sample in a set of digital audio data
Data
The term data refers to qualitative or quantitative attributes of a variable or set of variables. Data are typically the results of measurements and can be the basis of graphs, images, or observations of a set of variables. Data are often viewed as the lowest level of abstraction from which...
. Common examples of bit depth include CD
Compact Disc
The Compact Disc is an optical disc used to store digital data. It was originally developed to store and playback sound recordings exclusively, but later expanded to encompass data storage , write-once audio and data storage , rewritable media , Video Compact Discs , Super Video Compact Discs ,...
quality audio, which is recorded at 16 bits, and DVD-Audio
DVD-Audio
DVD-Audio is a digital format for delivering high-fidelity audio content on a DVD. DVD-Audio is not intended to be a video delivery format and is not the same as video DVDs containing concert films or music videos....
, which can support up to 24-bit audio.
Digital audio
A set of digital audio samples contains data that, when converted into an analog signalAnalog signal
An analog or analogue signal is any continuous signal for which the time varying feature of the signal is a representation of some other time varying quantity, i.e., analogous to another time varying signal. It differs from a digital signal in terms of small fluctuations in the signal which are...
, provides the necessary information to reproduce the sound wave. In pulse-code modulation
Pulse-code modulation
Pulse-code modulation is a method used to digitally represent sampled analog signals. It is the standard form for digital audio in computers and various Blu-ray, Compact Disc and DVD formats, as well as other uses such as digital telephone systems...
(PCM) sampling, the bit depth will limit signal-to-noise ratio
Signal-to-noise ratio
Signal-to-noise ratio is a measure used in science and engineering that compares the level of a desired signal to the level of background noise. It is defined as the ratio of signal power to the noise power. A ratio higher than 1:1 indicates more signal than noise...
(S/N). The bit depth will not limit frequency
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...
range, which is limited by the sample rate.
By increasing the sampling bit depth, quantization noise is reduced so that the S/N is improved. The 'rule-of-thumb' relationship between bit depth and S/N is, for each 1-bit increase in bit depth, the S/N will increase by 6 dB
Decibel
The decibel is a logarithmic unit that indicates the ratio of a physical quantity relative to a specified or implied reference level. A ratio in decibels is ten times the logarithm to base 10 of the ratio of two power quantities...
. 24-bit
24-bit
Notable 24-bit machines include the ICT 1900 series and the Harris H series.The IBM System/360, announced in 1964, was a popular computer system with 24-bit addressing and 32-bit general registers and arithmetic...
digital audio has a theoretical maximum S/N of 144 dB, compared to 96 dB for 16-bit; however, digital audio converter technology is limited to a S/N of about 124 dB (21-bit) because of real world limitations in integrated circuit
Integrated circuit
An integrated circuit or monolithic integrated circuit is an electronic circuit manufactured by the patterned diffusion of trace elements into the surface of a thin substrate of semiconductor material...
design. Still, this approximately matches the performance of the human ear.
Technically speaking, bit depth is only meaningful when applied to pure PCM devices. Non-PCM formats such as lossy compression systems like MP3
MP3
MPEG-1 or MPEG-2 Audio Layer III, more commonly referred to as MP3, is a patented digital audio encoding format using a form of lossy data compression...
, have bit depths that are not defined in the same sense as PCM. In lossy audio compression, where bits are allocated to other types of information, the bits actually allocated to individual samples are allowed to fluctuate within the constraints imposed by the allocation algorithm.
Dynamic range
Dynamic rangeDynamic range
Dynamic range, abbreviated DR or DNR, is the ratio between the largest and smallest possible values of a changeable quantity, such as in sound and light. It is measured as a ratio, or as a base-10 or base-2 logarithmic value.-Dynamic range and human perception:The human senses of sight and...
is the difference between the largest and smallest signal a system can record or reproduce. With the proper application of dither
Dither
Dither is an intentionally applied form of noise used to randomize quantization error, preventing large-scale patterns such as color banding in images...
digital systems can reproduce signals with levels lower than their resolution would normally allow. Therefore there is not a direct connection between bit depth and dynamic range.
Performance
8-bit resolution, as found in older computers and audio samplers offers up to a 48dB dynamic range under perfect recording and reproduction conditions (roughly equivalent to standard-grade audio cassette tape, but with more obvious quantisation errors at low volumes unless a deliberate 1-bit background noise "dither" is introduced, which provides a greater perceived dynamic range despite the noise floor being at approx -45dB), and 16-bit, as used in CD and modern equipment, can provide up to 96dB of dynamics (again, a deliberate noise floor may be introduced to soften perceived quantisation error; however in this case, the floor is still below -90dB, which is quiet enough to become lost in circuit distortion in cheap players, or environmental background noise in all but the quietest rooms with the loudest playback volume). The 12- and 14-bit DV/NICAM standards (-72 and -84dB respectively) were thought to be perfectly adequate for televisual and video camera applications at the time of their inception, particularly compared to VHS and Hi-8.Audiophile-spec recording resolutions extend this to a theoretical -120dB (20-bit) or -144dB (24-bit), the latter of which exceeds the dynamic range between complete silence (signal energy below that which can be detected by the human ear) and noise of high enough intensity to cause almost immediate ear injuries, with an ideal 24-bit DAC and associated amplified being able to accurately output signal values from 0, 1, 2 through 16777213, 16777214, 16777215.
Applications
Standard DV audio is 12-bit (4096 levels), NICAM pseudo-14-bit (10-bit data + 4-bit gain signal, with 14-bit output DAC), CD and DAT audio is 16-bit (65536 levels), and enhanced CDs, SACDs and DVD-Audio can use 20 or even up to 24-bit sampling (>16 million levels). CD Audio has also left a lasting impression on computer and other digital audio applications, where 16-bit is the default "hi-fi" sample resolution (as opposed to earlier 8, 6 or even 4-bit efforts), with higher precision often considered the reserve of audiophiles as the representable range of intensities rapidly exceeds the theoretical limits of human perception, particularly when environmental noise is considered.What is a 'bit' of data?
In computing parlance, bitBit
A bit is the basic unit of information in computing and telecommunications; it is the amount of information stored by a digital device or other physical system that exists in one of two possible distinct states...
is the abbreviation for a single binary
Binary numeral system
The binary numeral system, or base-2 number system, represents numeric values using two symbols, 0 and 1. More specifically, the usual base-2 system is a positional notation with a radix of 2...
digit, represented by a 0 or a 1. A word is a binary number with more than one digit. Binary numerics are base-2; thus, each digit can only be a 0 or a 1. In comparison, traditional decimal numerics are base-10, having digits that can only be 0 through 9. For example, the 16-bit binary number 0110111110111010 is equivalent to the 5-digit decimal number 28602. The number of bits per word is simply how many digits there are in the corresponding number. The words in commonly used PCM digital audio formats are 8, 16 or 24 bits long. Larger words have higher resolution. The resolution of a 16-bit system can be calculated by using 216
Power of two
In mathematics, a power of two means a number of the form 2n where n is an integer, i.e. the result of exponentiation with as base the number two and as exponent the integer n....
which gives a value of 65,536. A 24 bit system (224) has a resolution of 16,777,216.
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
MP3
MPEG-1 or MPEG-2 Audio Layer III, more commonly referred to as MP3, is a patented digital audio encoding format using a form of lossy data compression...
file might be described as having a bit rate of 160 kbps or 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 x 16 bits/sample x 2 = 1,411,200 bit/s or 1.4 Mbit/s.
This explains why, for example, a Minidisc recorder, which uses ATRAC
ATRAC
Adaptive Transform Acoustic Coding is a family of proprietary audio compression algorithms developed by Sony. MiniDisc was the first commercial product to incorporate ATRAC in 1992. ATRAC allowed a relatively small disc like MiniDisc to have the same running time as CD while storing audio...
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
Sampling rate
The sampling rate, sample rate, or sampling frequency defines the number of samples per unit of time taken from a continuous signal to make a discrete signal. For time-domain signals, the unit for sampling rate is hertz , sometimes noted as Sa/s...
, word size (bit depth), and the number of channels (e.g. mono, stereo, four-track), 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) x (bit depth) x (number of channels)
E.g., for a recording with a 44.1 kHz sampling rate, a 16 bit depth, and 2 channels (stereo):
- 44100 x 16 x 2 = 1411200 bits per second, or 1411.2 kbit/s
The eventual file size of an audio recording can also be calculated using a similar formula:
- File Size (Bytes) = (sampling rate) x (bit depth) x (number of channels) x (seconds) / 8
E.g., a 70 minutes long CD quality recording will take up 740880000 Bytes, or 740MB:
- 44100 x 16 x 2 x 4200 / 8 = 740880000 Bytes