Bit rate

In telecommunications and computing, bit rate (bitrate or as a variable R) is the number of bits that are conveyed or processed per unit of time.

The bit rate is expressed as bits per second (symbol: bit/s), often with an SI prefix such as kilo (1 kbit/s = 1,000 bit/s), mega (1 Mbit/s = 1,000 kbit/s), giga (1 Gbit/s = 1,000 Mbit/s) or tera (1 Tbit/s = 1,000 Gbit/s). The non-standard abbreviation bps is often used: 1 Mbps is 1 Mbit/s, that is, one million bits per second.

The bit rate is different from the transfer rate, measured in transfers per second, when the channel is parallel and thus transfers multiple bits per transfer.

In most computing and digital communication environments, one byte per second (symbol: B/s) corresponds to 8 bit/s (). However if stop bits, start bits, and parity bits need to be factored in, a higher number of bits per second will be required to achieve a throughput of the same number of bytes.

Prefixes

For large or small bit rates, SI prefixes (also known as metric prefixes or decimal prefixes) are used:

|align="right"| 0.001 bit/s ||= 1 mbit/s (one millibit per second, i.e., one bit per thousand seconds) = 1 bit/ks

|align="right"| 1 bit/s ||= 1 bit/s (one bit per second)

|align="right"| 1,000 bit/s ||= 1 kbit/s (one kilobit per second, i.e., one thousand bits per second)

|align="right"| 1,000,000 bit/s ||= 1 Mbit/s (one megabit per second, i.e., one million bits per second)

|align="right"| 1,000,000,000 bit/s ||= 1 Gbit/s (one gigabit per second, i.e., one billion bits per second)

|align="right"| 1,000,000,000,000 bit/s ||= 1 Tbit/s (one terabit per second, i.e., one trillion bits per second)

The binary prefixes defined by International Standard IEC 80000-13 are sometimes used: e.g., 1 KiB/s = 1024 B/s = 8192 bit/s, and 1 MiB/s = 1024 KiB/s.

In data communications

Gross bit rate

In digital communication systems, the physical layer gross bitrate, raw bitrate, data signaling rate, gross data transfer rate or uncoded transmission rate) is the total number of physically transferred bits per second over a communication link, including useful data as well as protocol overhead.

In case of serial communications, the gross bit rate is related to the bit transmission time <math>T_\text{b}</math>

as:

: <math>R_\text{b} = {1 \over T_\text{b,</math>

The gross bit rate is related to the symbol rate or modulation rate, which is expressed in baud or symbols per second. However, the gross bit rate and the baud value are equal only when there are only two levels per symbol, representing 0 and 1, meaning that each symbol of a data transmission system carries exactly one bit of data; this is not the case for modern modulation systems used in modems and LAN equipment.

For most line codes and modulation methods:

: <math>\text{symbol rate} \leq \text{gross bit rate}</math>

More specifically, a line code (or baseband transmission scheme) representing the data using pulse-amplitude modulation with <math>2^N</math> different voltage levels, can transfer <math>N</math> bits per pulse. A digital modulation method (or passband transmission scheme) using <math>2^N</math> different symbols, for example <math>2^N</math> amplitudes, phases or frequencies, can transfer <math>N</math> bits per symbol. This results in:

: <math>\text{gross bit rate} = \text{symbol rate} \times N</math>

An exception from the above is some self-synchronizing line codes, for example Manchester coding and return-to-zero (RTZ) coding, where each bit is represented by two pulses (signal states), resulting in:

: <math>\text{gross bit rate = symbol rate/2}</math>

A theoretical upper bound for the symbol rate in baud, symbols/s or pulses/s for a certain spectral bandwidth in hertz is given by the Nyquist law:

: <math>\text{symbol rate} \leq \text{Nyquist rate} = 2 \times \text{bandwidth}</math>

In practice this upper bound can only be approached for line coding schemes and for so-called vestigial sideband digital modulation. Most other digital carrier-modulated schemes, for example ASK, PSK, QAM and OFDM, can be characterized as double sideband modulation, resulting in the following relation:

: <math>\text{symbol rate} \leq \text{bandwidth}</math>

In case of parallel communication, the gross bit rate is given by

: <math>\sum_{i = 1}^{n} \frac{\log_2 {M_i} }{T_i}</math>

where n is the number of parallel channels, Mi is the number of symbols or levels of the modulation in the ith channel, and Ti is the symbol duration time, expressed in seconds, for the ith channel.

Information rate

The physical layer net bitrate, information rate, payload rate, net data transfer rate, Some operating systems and network equipment may detect the "connection speed" (informal language) of a network access technology or communication device, implying the current net bit rate. The term line rate in some textbooks is defined as gross bit rate,

A theoretical lower bound for the encoding bit rate for lossless data compression is the source information rate, also known as the entropy rate.

The bitrates in this section are approximately the minimum that the average listener in a typical listening or viewing environment, when using the best available compression, would perceive as not significantly worse than the reference standard.

Audio

CD-DA

Compact Disc Digital Audio (CD-DA) uses 44,100 samples per second, each with a bit depth of 16, a format sometimes abbreviated like "16bit / 44.1kHz". CD-DA is also stereo, using a left and right channel, so the amount of audio data per second is double that of mono, where only a single channel is used.

The bit rate of PCM audio data can be calculated with the following formula:

: <math>\text{bit rate} = \text{sample rate} \times \text{bit depth} \times \text{channels}</math>

For example, the bit rate of a CD-DA recording (44.1 kHz sampling rate, 16 bits per sample and two channels) can be calculated as follows:

: <math>44,100 \times 16 \times 2 = 1,411,200\ \text{bit/s} = 1,411.2\ \text{kbit/s}</math>

The cumulative size of a length of PCM audio data (excluding a file header or other metadata) can be calculated using the following formula:

: <math>\text{size in bits} = \text{sample rate} \times \text{bit depth} \times \text{channels} \times \text{time}.</math>

The cumulative size in bytes can be found by dividing the file size in bits by the number of bits in a byte, which is eight:

: <math>\text{size in bytes} = \frac{\text{size in bits{8}</math>

Therefore, 80 minutes (4,800 seconds) of CD-DA data requires 846,720,000 bytes of storage:

: <math>\frac{44,100 \times 16 \times 2 \times 4,800}{8} = 846,720,000\ \text{bytes} \approx 847\ \text{MB} \approx 807.5\ \text{MiB}</math>

where MiB is mebibytes with binary prefix Mi, meaning 220 = 1,048,576.

MP3

The MP3 audio format provides lossy data compression. Audio quality improves with increasing bitrate:

32 kbit/s generally acceptable only for speech
96 kbit/s generally used for speech or low-quality streaming
128 or 160 kbit/s mid-range bitrate quality
192 kbit/s medium quality bitrate
256 kbit/s a commonly used high-quality bitrate
320 kbit/s highest level supported by the MP3 standard

Other audio

700 bit/s lowest bitrate open-source speech codec Codec2, but Codec2 sounds much better at 1.2 kbit/s
800 bit/s minimum necessary for recognizable speech, using the special-purpose FS-1015 speech codecs
2.15 kbit/s minimum bitrate available through the open-source Speex codec
6 kbit/s minimum bitrate available through the open-source Opus codec
8 kbit/s telephone quality using speech codecs
32–500 kbit/s lossy audio as used in Ogg Vorbis
256 kbit/s Digital Audio Broadcasting (DAB) MP2 bit rate required to achieve a high quality signal
292 kbit/s Sony Adaptive Transform Acoustic Coding (ATRAC) for use on the MiniDisc Format
400 kbit/s–1,411 kbit/s lossless audio as used in formats such as Free Lossless Audio Codec, WavPack, or Monkey's Audio to compress CD audio
1,411.2 kbit/s Linear PCM sound format of CD-DA
5,644.8 kbit/s DSD, which is a trademarked implementation of PDM sound format used on Super Audio CD.
6.144 Mbit/s E-AC-3 (Dolby Digital Plus), an enhanced coding system based on the AC-3 codec
9.6 Mbit/s DVD-Audio, 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. These discs cannot be played on a standard DVD-player without DVD-Audio logo.
18 Mbit/s advanced lossless audio codec based on Meridian Lossless Packing (MLP)

Video

16 kbit/s videophone quality (minimum necessary for a consumer-acceptable "talking head" picture using various video compression schemes)
128–384 kbit/s business-oriented videoconferencing quality using video compression
400 kbit/s YouTube 240p videos (using H.264)
750 kbit/s YouTube 360p videos (using H.264)
2.5 Mbit/s YouTube 720p videos (using H.264)
10-40 Mbit/s YouTube 2160p60 (60 FPS) videos (using H.264)
24 Mbit/s max AVCHD (using MPEG4 AVC compression)
25 Mbit/s approximate HDV 1080i (using MPEG2 compression)
250 Mbit/s max DCP (using JPEG 2000 compression)
1.5 Gbit/s 10-bit 4:4:4 uncompressed 1080p at 24 FPS

Notes

For technical reasons (hardware/software protocols, overheads, encoding schemes, etc.) the actual bit rates used by some of the compared-to devices may be significantly higher than listed above. For example, telephone circuits using μ-law or A-law companding (pulse code modulation) yield 64 kbit/s.

Prefixes

In data communications <span class="anchor" id="Bit rates at various protocol layers"></span>

Gross bit rate <span class="anchor" id="UNCODED"></span>