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Q89879: Differences Between PCM/ADPCM Wave Files Explained

Article: Q89879
Product(s): Miscellaneous Windows Products
Version(s): 
Operating System(s): 
Keyword(s): 
Last Modified: 11-SEP-1999

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The information in this article applies to:

- Microsoft Windows Sound System, versions 1.0, 1.0a 
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SUMMARY
=======

Pulse Code Modulation (PCM) and Adaptive Delta Pulse Code Modulation (ADPCM) are
subclasses of the Microsoft waveform (.WAV) file format. In PCM, data for .WAV
files is stored using linear samples, while ADPCM uses deltas between samples.

MORE INFORMATION
================

PCM and ADPCM are techniques for storing analog audio data in a digital format,
the Microsoft .WAV file. Other storage methods (for example, Mu- Law, A-Law,
Transform coding, CELP, and so on), are not supported by Windows Sound System
1.0.

How PCM Works
-------------

PCM works by taking discrete samples at even intervals (called the sampling
rate). Common intervals are 11 kHz, 22 kHz, and 44 kHz. The higher the sampling
rate, the better the representation of the original analog wave and the better
the sound quality. Each sample is a real number with infinite resolution from
+1.0 of full-scale value to -1.0 of full scale value. Because these must be
stored as finite-precision digital numbers, the data is truncated to either
16-bit PCM or 8-bit PCM, commonly called 8- and 16-bit samples. 16-bit data has
more resolution, so the digital waveform sounds better. 8-bit PCM has less
resolution, causing audible hiss in the waveform. It also requires less disk
space.

How ADPCM Works
---------------

ADPCM, commonly termed as a form of compression, is a more efficient way of
storing waveforms than 16-bit or 8-bit PCM. It only uses 4 bits per sample,
taking up a quarter of the disk space of 16-bit PCM. However, the sound quality
is inferior. Because the Windows Sound System hardware only understands 8/16-bit
PCM, the computer must compress and decompress the ADPCM into/from PCM, which
requires CPU time. 22 kHz mono ADPCM can be decompressed real-time (that is,
while playing) on a 386SX/16 megahertz CPU. Higher sampling rates (44 kHz) or
stereo files will take too long for a 386SX/16 to decompress, which causes
skipping in the audio. 11 kHz mono ADPCM can be compressed real-time on a
386SX/16 computer. To do ADPCM, the computer must have the Audio Compression
Manager (ACM) installed.

ADPCM stores the value differences between two adjacent PCM samples and makes
some assumptions that allow data reduction. Because of these assumptions, low
frequencies are properly reproduced, but any high frequencies tend to get
distorted. The distortion is easily audible in 11 kHz ADPCM files, but becomes
more difficult to discern with higher sampling rates, and is virtually
impossible to recognize with 44 kHz ADPCM files.

Additional query words: 1.00

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Keywords          :  
Technology        : kbWinSoundSysSearch kbWinSoundSys100 kbWinSoundSys100a

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