Bitrate

In telecommunications and computing, bitrate (sometimes written bit rate, or as a variable R_bit) is the number of bits that are conveyed or processed per unit of time. In digital multimedia, bitrate 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", bitrate 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". Noted that binary prefixes are used in most operating system and software, and SI prefixes are used in most hardware to that adventage of a bigger number in display to confuse the actual speed that your software tells you. e.g. : Cable high speed internet advertised as 3 Mbit/s and your computer most likely uses Mbytes/s (binary M) and displayed a download rate that never tops 340 Kbytes/s.

"Bitrate" 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 bitrates, SI prefixes are 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)

When describing bitrates, binary prefixes are almost never used and SI prefixes are almost always used with the standard, decimal meanings, not the computer-oriented binary meanings. There are exceptions in some specialty areas such as bus transfer rates. Binary usage is more often seen when the unit is the byte/s, and is not typical for telecommunication links. Sometimes it is necessary to seek clarification of the units used in a particular context.

Bitrates in multimedia

In digital multimedia, bitrate represents the amount of information, or detail, that is stored per unit of time of a recording. The bitrate 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 bitrate 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 bitrate (sub-FM quality) encoding and there is a growing scene of net labels distributing stylized low bitrate music.

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 (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" quality, may be considered low-end by those with discerning ears
256 - 320 kbit/s — Medium-high quality to near audio CD quality
500 kbit/s - 1 mbit/s - Lossless quality, typically in the form of FLAC or .ape format

Audio (wav)

8 bit - FM broadcasting quality
16 bit - CD quality
24 bit - Original Master recording quality

Video (MPEG2)

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 bitrates used by some of the compared-to devices may be significantly higher than what is listed above. For example:

Telephone circuits using µlaw or A-law companding (pulse code modulation) — 64 kbit/s
CDs using CDDA — 1.4 Mbit/s

References

External links

Bandwidth conversion

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

Bandwidth calculator online

VoIP Bandwidth Calculator - Given a codec type and sample period calculate the actual IP and Ethernet bandwidth.
VoIP Bandwidth Calculation White Paper - Companion paper to the above calculator explaining how Voice becomes Voice over IP.
StreamingMarketplace.com( Calculate streaming bandwidth and storage)

Bitrates of DVB-S TV and radio channels

Linowsat - daily updated audio and video bitrates of European satellites.

Data transmission | Physical quantity | Units of measure

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