| Revision as of 11:49, 19 October 2003 editDysprosia (talk | contribs)28,388 editsm Reverted to last edit by 64.53.168.228← Previous edit | Revision as of 11:57, 19 October 2003 edit undoMuriel Gottrop~enwiki (talk | contribs)8,859 edits multiple copyviosNext edit → | ||
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| : ccc.inaoep.mx/~cferegrino/cursos/ comprcrip/Lossy_Methods.pdf | |||
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| '''Pulse-code ]''' ('''PCM''') is a digital representation of an analog ]. | |||
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| The magnitude (height or strength) of the signal (with respect to a fixed reference) is ] regularly at a frequency ''f<sub>s</sub>''. This means the value of the signal, a sample, is captured at uniform intervals of duration ''T ''(= ''1/f<sub>s</sub>''). Every sample is ] to a series of symbols in a digital code, which is usually a ] code. | |||
| It also should be noted that the posting of copyrighted material that does ''not'' have the express permission from the copyright holder is possibly illegal and is a violation of our ]. Those with a history of violations may be temporarily ] from editing pages. If this is in fact an infringement of copyright, '''you are still welcome to make any original contribution'''. | |||
| PCM is used in digital ] systems and for digital audio recording on ]s. | |||
| Thanks, ] 11:57, 19 Oct 2003 (UTC) | |||
| In conventional PCM, before being digitized, the ] may be processed (e.g. compressed), but once digitized, the PCM signal is not subjected to further processing (e.g. ] compaction). In telephony, several PCM streams may be multiplexed into a larger aggregate ]. | |||
| '''Differential (or Delta) pulse-code modulation''' ('''DPCM''') encodes the PCM values as differences between the current and the previous value. For audio this type of encoding reduces the number of bits required per sample compared to PCM by about 25%. A variant of DPCM, '''Adaptive DPCM''' ('''ADPCM''') varies the size of the quantization step, to allow futher reduction of the required bandwidth for a given signal-to-noise ratio. | |||
| Pulse-code modulation can be either Return to Zero (RZ) or ] (NRZ). For a NRZ system to be synchronized using in-band information, there must not be long sequences of identical symbols, such as ones or zeroes. For ] PCM systems, the density of 1-symbols is called 'ones-density'. | |||
| Ones-density is often controlled using precoding techniques where the PCM code is expanded into a slightly longer code with a guaranteed bound on ones-density before modulation into the channel. In other cases, extra 'framing' bits are added into the stream which guarantee at least occasional symbol transitions. | |||
| Another technique used to control ones-density is the use of a 'scrambler' ] on the raw data which will tend to turn the raw data stream into a stream that looks ], but where the raw stream can be recovered exactly by reversing the effect of the polynomial. In this case, long runs of zeroes or ones are still possible on the output, but are considered unlikely enough to be within normal engineering tolerance. | |||
| In other cases, the long term ] value of the modulated signal is important, as building up a DC offset will tend to bias detector circuits out of their operating range. In this case special measures are taken to keep a count of the cumulative DC offset, and to modify the codes if necessary to make the DC offset always tend back to zero. | |||
| Many of these codes are ]s, where the pulses can be positive, negative or absent. Typically, non-zero pulses alternate between being positive and negative. These rules may be violated to generate special symbols used for ] or other special purposes. | |||
| === History === | |||
| In the 1930s the necessity to convert analogue data to digital format for computer use was unknown - there were no computers capable of using it. However the idea was seen by Alec Reeves as a means of communicating with perfect fidelity - no errors could arise in a system based on ones and noughts. In 1938 he filed a patent in France that introduced the concept of digital communication which he called Pulse Code Modulation. It is now commonplace for computers, radio, television, CDs, recording and so on. More about Alec Reeves can be found on http://www.alecharleyreeves.com, including on line versions of his writings and lectures. There are also some Active-X online versions of the experiments he performed in PCM. | |||
| '''See also''': | |||
| * ] | |||
| * ] | |||
| * ] | |||
| * ] for other examples of modulation techniques. | |||
Revision as of 11:57, 19 October 2003
Removed possible copyright infringement. Text that was previously posted here is the same as text from these webpages:
- www.derivaz.fsnet.co.uk/ahr/pcm.htm
- ccc.inaoep.mx/~cferegrino/cursos/ comprcrip/Lossy_Methods.pdf
This page is now listed on Misplaced Pages:Votes for deletion.
To the poster: If there was permission to use this material under terms of our license or if you are the copyright holder of the externally linked text, then please indicate so on this page's talk page (to access the talk page, click on Discuss this page in the sidebar). If there was no permission to use this text then please either replace this message with at least a good stub and an external link or leave this page to be deleted. (NOTE: unless a stub replaces this text, deletion will occur about one week from the time this page title was placed on the Votes for deletion page).
It also should be noted that the posting of copyrighted material that does not have the express permission from the copyright holder is possibly illegal and is a violation of our policy. Those with a history of violations may be temporarily suspended from editing pages. If this is in fact an infringement of copyright, you are still welcome to make any original contribution.
Thanks, Muriel Gottrop 11:57, 19 Oct 2003 (UTC)