AMBE-2000 Digital Voice Systems, Inc., AMBE-2000 Datasheet - Page 9

• Manufacturer Part Number: AMBE-2000
• Description: 2.0 - 9.6 KBPS Single ic For Toll-quality, Low Data Rate, Real Time Speech Compression
• Manufacturer: Digital Voice Systems, Inc.

2.2.2
In order to ensure proper performance from the voice coder, it is necessary for the vocoder front end to meet a set of minimum
performance requirements. For the purposes of this section the vocoder front end is considered to be the total combined
response between microphone/speaker and the digital PCM interface to the vocoder, as shown in Figure 2-B. This includes any
analog electronics plus the A-to-D and D-to-A converters as well as any digital filtering performed prior to the voice encoder or
after the voice decoder.
The AMBE+™ voice encoder and decoder operate with unity (i.e. 0 dB) gain. Consequently the analog input and output gain
elements shown in Figure 2 are only used to match the sensitivity of the microphone and speaker with the A-to-D converters
and D-to-A converters, respectively. It is recommended that the analog input gain be set such that the RMS speech level under
nominal input conditions is 25 dB below the saturation point of the A-to-D converter (+3 dBm0). This level, which equates to
-22 dBm0, is designed to provide sufficient margin to prevent the peaks of the speech waveform from being clipped by the A-
to-D converter.
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Microphone
Speaker
DVSI Confidential Proprietary, Subject to Change
Analog Speech
Analog Input
Reference Pt.
Analog Output
Reference Pt.
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Vocoder Front End Requirements
-18 dB
-35 dB
-40 dB
+2 dB
-1 dB
-2 dB
Analog
Analog
Output
Input
Gain
Gain
0 200
400
Vocoder Front End
Analog
Analog
Output
Input
Filter
Filter
Figure 2 - C. Front End Input Filter Mask
Figure 2 - B. Vocoder Front End
AMBE-2000™ Vocoder Chip
User’s Manual Version 4.0
Convertor
Convertor
A-to-D
D-to-A
3000
3400 4000
Output
Digital
Digital
Input
Gain
Gain
4600
Digital Speech
(8 kHz sampling)
Digital Output
Reference Pt.
Digital Input
Reference Pt.
8000
AMBE+™
Voice
Encoder
AMBE+™
Voice
Decoder
freq (Hz)
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