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ZyXEL: Voice Data Compression
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<B><FONT SIZE=+3>V</FONT>OICE <FONT SIZE=+2>D</FONT>ATA <FONT SIZE=+2>C</FONT>OMPRESSION</B>
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The main issue in digitized voice is the amount of storage required. A good
phone-quality voice digitization will produce about 64 Kbits of data for
each second of voice. The hard disk will quickly be filled up by digitized
voice at this digitization rate. Speech compression is needed to reduce
the digitization data rate. A relatively simple ADPCM (Adaptive Delta Pulse
Code Modulation) algorithm can reduce the speech data rate to half and
maintain about the same voice quality. This algorithm can also be used to
reduce the speech data rate to 1/3 or 1/4 of the original rate but with voice
quality degradation. Reducing the speech data rate further and maintaining
good voice quality requires a sophisticated and complicated signal processing
algorithm. It also requires a lot of digital signal processing computation
power. We call this sophisticated speech data compression capability advanced
voice capability.
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The U-1496 series modems support five voice digitization schemes. Four
schemes use the ADPCM algorithm and one uses the advanced CELP (Code Excited
Linear Prediction) algorithm to achieve near-phone-quality voice at 9.6 kbps
speech data rate. A summary of these four schemes is listed below:
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Digitization Speech Compression Data Rate
Scheme Algorithm
CELP Code Excited Linear Prediction 9600 bps
2-ADPCM ADPCM, 2 bits/sample 19200 bps
3-ADPCM ADPCM, 3 bits/sample 28800 bps
3-ADPCM(NEW) ADPCM. 3 bits/sample 30720 bps
4-ADPCM ADPCM, 4 bits/sample 38400 bps
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where the sampling rate used is 9600 samples per second.
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The ADPCM voice modes are supported on all U-1496 series modems. However, the
advanced CELP scheme is supported only on the plus version and the U-1496 LCD
model.
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Silence detection is implemented to eliminate voice data coding in silence
period.
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The modem also supports simultaneous DTMF (Dual Tone Multiple Frequency) tone
detection that allows a user to use the keypad of a tone-dialing, push-button
telephone set to control, instruct, or answer the modem.
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<A HREF="@HTMLPATH@/Modems/ZyXEL/autodetect.html"><IMG SRC="@HTMLPATH@/icons/next.gif"> Automatic detection of voice, fax, and data</A>.<BR>
<A HREF="@HTMLPATH@/Modems/ZyXEL/voice.html"><IMG SRC="@HTMLPATH@/icons/back.gif"> ZyXEL Advanced Voice Capability</A>.
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<A HREF="@HTMLPATH@/sam.html">Sam Leffler</A> / <A HREF="mailto:sam@engr.sgi.com">sam@engr.sgi.com</A>.
Last updated $Date: 1998/10/12 20:47:51 $.
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