In telecommunications and computing, bit rate (sometimes written bitrate) is the frequency at which bits are passing a given (physical or metaphorical) "point". It is quantified using the bit per second (bit/s) unit.
While often referred to as "speed", bit rate does not measure distance/time but number of bits/time, and thus should be distinguished from the "propagation speed" (which depends on the transmission medium and has the usual physical meaning).
The formal abbreviation for "bit per second" is "bit/s" (not bits/s). In less formal contexts the abbreviatons b/s or bps are often used, though this risks confusion with "bytes per second" (B/s). Even less formally, it is common to drop the "per second", and simply refer to "a 128 kilobit audio stream" or "a 100 megabit network".
"Bit rate" is sometimes used interchangeably with "baud rate", which is only the same if each bit occurs in a unit interval.
For large bitrates, SI prefixes are used, not binary prefixes:
- 1,000 bit/s = 1 kbit/s (one kilobit or one thousand bits per second)
- 1,000,000 bit/s = 1 Mbit/s (one megabit or one million bits per second)
- 1,000,000,000 bit/s = 1 Gbit/s (one gigabit or one billion bits per second)
There are typically eight bits in a byte (octet), but communications data rates are almost never expressed in bytes per second, with the notable exceptions of disk and memory I/O transfer rates. To convert from byte/s to bit/s, simply multiply by 8.
If lossy compression is used on audio/visual data, differences from the original signal will be introduced in the form of compression artifacts. Whether these affect the subjective quality, and if so how much, depends on the compression scheme, encoder power, the characteristics of the input data, and the listener's familiarity with artifacts; experts may detect artifacts in a lot of cases where the average listener would not.
The bitrates in this section are approximately the minimum that the average listener (when using state-of-the-art compression) would consider not significantly worse than the reference standard:
- 4 kbit/s – minimum necessary for recognizable speech (using special-purpose speech codecs)
- 8 kbit/s – telephone quality
- 32 kbit/s – MW quality
- 96 kbit/s – FM quality
- 128 kbit/s – CD quality
- 32 kbit/s – videophone quality (minimum necessary for a recognizable talking head)
- 2 Mbit/s – VHS quality
- 8 Mbit/s – DVD quality
- 15 Mbit/s – HDTV quality
Note that because of advancing technology, the actual bitrates used by some of the compared-to devices is significantly higher. For example:
- Telephone circuits using μlaw – 64 kbit/s
- CDs using CDDA – 1.4 Mbit/s
Allow easy conversion from kbit/s to MB/h to GB/day to TB/month to ...
- webair.com (http://www.webair.com/hosting/bw.html)
- forret.com (http://www.forret.com/projects/hizmo/bandwidth.asp)