A codecfilter is used for digitizing and reconstructing the human voice. These devices were developed primarily forthe telephone network to facilitate voice switching and transmission. Once the voice is digitized, it may be switched by digital switching methods or transmitted long distance (T1,microwave, satellites, etc.) without degradation. The namecodec is an acronym from "COder" (for the A/D used to digitize voice) and "DECoder" (for the D/A used for reconstructing voice). A codec is a single device that does both the A/D and D/A conversions.
To digitize intelligible voice requires a signaltodistortion ratio of about 30 dB over a dynamic range of about 40 dB.This can be accomplished with a linear 13bit A/D and D/A, but will far exceed the required signaltodistortion ratio at amplitudes greater than 40 dB below the peak amplitude. This excess performance is at the expense of data per sample. Methods of data reduction are implemented by com-pressing te 13bit lnear scheme to companded 8bit schemes. There are two companding schemes used:Mu255 Law specifically in North America, and ALawspecifically in Europe. These companding chemes are accepted world wide. These companding schemes follow asegmented or "piecewiselinear" curve formatted as sign bit, three chord bits, and four step bits. For a given chord, all sixteen of the steps have the same voltage weighting. As the voltage of the analog input increases, the four step bits incre-ment and carry o the three hord bits which increment.
The D/A process reconstructs a staircase version of thedesired inband signal, which has spectral images of the inband signal modulated about the sample frequency and its harmonics. These spectral images, called al
|
|
Fully Differential Analog Circuit Design for Lowest Noise Performance Specified for Extended Temperature Range of 40 to + 85°C Transmit BandPass and Receive LowPass Filters OnChip Active RC PreFiltering and PostFiltering MuLaw Companding MC145554 ALaw Companding MC145557 OnChip Precision Voltage Reference (2.5 V) Typical Power Dissipation of 40 mW, Power Down of 1.0 mW at ± 5V
|
| Rating |
Symbol |
Value |
Unit |
DC Supply Voltage VCC toVBB
VCC to GNDA
VBB to GNDA |
|
0.5 to + 13
0.3 to + 7.0
7.0 to + 0.3 |
V |
| Voltage on Any Analog Input or Output Pin |
|
VBB 0.3 to
VCC+ 0.3 |
V |
| Voltage on Any Digital Input or Output Pin |
|
GNDA 0.3
toV+ 0.3 |
V |
| Operating Temperature Range |
TA |
40 to + 85 |
°C |
| Storage Temperature Range |
Tstg |
85 to + 150 |
°C |
Post a Buying Lead
Required Field 
