AN2113 Freescale Semiconductor / Motorola, AN2113 Datasheet - Page 4

• Manufacturer Part Number: AN2113
• Description: AN2113 Multichannel Voice Coding System on the RTXC Operating System
• Manufacturer: Freescale Semiconductor / Motorola

Voice Coding
2.1 Encoding/Decoding
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Each of the many different voice coding algorithms combines knowledge of the human vocal system with
an understanding of the quality of speech required by the application and the processing power available to
process the given voice coder in a system. The list of algorithm types includes the following:
As Figure 3 shows, the voice coders in the “vocoder” grouping achieve the lowest bit rate but also tend to
have lower subjective quality than the other coders. Perceptual and Waveform coders have high subjective
quality, but the application pays a price in bandwidth since the bit rate is not as low. Hybrid coders may
achieve lower bit rate and higher subjective quality, making them desirable coders for wireless systems.
The voice coder used in the project discussed here is IS-96-A, a type of hybrid coder using the CELP
algorithm. IS-96-A is used in the Code Division Multiple Access (CDMA) wireless standard and was
standardized in 1992 and uses a codebook to assist in the voice compression.
types of voice coders is a table of excitation parameters that define how the vocal tract is stimulated. The
decoder determines the appropriate excitation parameter for a given speech sample and usually transmits
2. Motorola provides a detailed discussion of CDMA on the following website: http://www.motor-
ola.com/NSS/Technology/cdma.html.
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Pulse Coded Modulation (PCM)
Linear Predictive Coding (LPC)
Code Excited Linear Prediction (CELP)
Regular Pulse Excitation (RPE)
Vector Sum Excited Linear Prediction (VSELP)
Hybrid Coders
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Vocoders
Freescale Semiconductor, Inc.
For More Information On This Product,
Figure 3. Comparison of Voice Coders
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Multichannel Voice Coding System
Go to: www.freescale.com
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Bit Rate (kb/s)
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Perceptual Coders
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Waveform Coders
The codebook used in these
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