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Bit rate

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In telecommunications and computing, bit rate (sometimes written bitrate or Rbit) is the number of bits that are conveyed or processed per unit of time. In digital multimedia, bit rate is the number of bits used per unit of time to represent a continuous medium such as audio or video. It is quantified using the bit per second (bit/s) unit or some derivative such as Mbit/s.

While often referred to as "speed", bit rate does not measure distance/time but quantity/time, and thus should be distinguished from the "propagation speed" (which depends on the transmission medium and has the usual physical meaning).

Usage notes

The formal abbreviation for "bit per second" is "bit/s" (not "bits/s"). In less formal contexts the abbreviations "b/s" or "bps" are often used, though this risks confusion with "bytes per second" ("B/s", "Bps"). 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 correct only when each modulation transition of a data transmission system carries exactly one bit of data (something not true for modern modem modulation systems, for example). Similarly, hertz, the SI unit of frequency, is not precise without some context, such as the number of bits carried per cycle.

For large bit rates, SI prefixes are sometimes used:

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)

however, most ISP, and IT software companies use base 2, and therefore:

1,024 bit/s = 1 kbit/s (one kilobit or roughly one thousand bits per second)
1,048,576 bit/s = 1 Mbit/s (one megabit or roughly one million bits per second)
1,073,741,824 bit/s = 1 Gbit/s (one gigabit or roughly one billion bits per second)

With the exceptions of bus transfer rates, binary prefixes are almost never used with units of bit/s. Such usage is most often seen if the unit is the byte/s, and is not typical for communication system channels (e.g., for telecommunication). Sometimes it is necessary to seek clarification of the units used in a particular context.

Bit rates in multimedia

In digital multimedia, bit rate represents the amount of information, or detail, that is stored per unit of time of a recording. The bit rate depends on several factors:

  • The original material may be sampled at different frequencies.
  • The samples may use different numbers of bits.
  • The data may be encoded by different schemes.
  • The information may be digitally compressed by different algorithms or to different degrees.

Generally, choices are made about the above factors in order to achieve the desired trade-off between minimizing the bit rate and maximizing the quality of the material when it is played.

If lossy data compression is used on audio or visual data, differences from the original signal will be introduced; if the compression is substantial, or lossy data is decompressed and recompressed, this may become noticeable in the form of compression artifacts. Whether these affect the perceived quality, and if so how much, depends on the compression scheme, encoder power, the characteristics of the input data, the listener’s perceptions, the listener's familiarity with artifacts, and the listening or viewing environment.

Experts and audiophiles may detect artifacts in many cases in which the average listener would not. Some musicians enjoy the distinct artifacts of low bit rate (sub-FM quality) encoding and there is a growing scene of net labels distributing stylized low bit music.

The bit rates 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 (MP3)

  • 4 kbit/s — minimum necessary for recognizable speech (using special-purpose speech codecs)
  • 8 kbit/s — telephone quality
  • 32 kbit/s — MW (AM) quality
  • 96 kbit/s — FM quality
  • 128 kbit/s — Typical "acceptable" music quality
  • 256 - 320 kbit/s — Near audio CD quality

Video

  • 16 kbit/s — videophone quality (minimum necessary for a consumer-acceptable "talking head" picture)
  • 128 – 384 kbit/s — business-oriented videoconferencing system quality
  • 1 Mbit/s — VHS quality
  • 5 Mbit/s — DVD quality
  • 15 Mbit/s — HDTV quality

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 what is listed above. For example:

References

See also

Bandwidth conversion

Allow easy conversion from kbit/s to MB/h to GB/day to TB/month to ...

Bandwidth calculator online

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