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IDT821034

QUAD PCM CODEC WITH PROGRAMMABLE GAIN

Integrated Device Technology 

IDT821034 QUAD PCM CODEC WITH PROGRAMMABLE GAIN

INDUSTRIAL TEMPERATURE RANGE

Bit

7 Register Indicator

6 --

5 --

4 O_4 Data

3 O_3 Data

2 O_2 Data

1 I/O_1 Data

0 I/O _0 Data

Name

Description

Always ‘0’

Reserved, always ‘0’

Reserved, always ‘0’

Output data on O_4 pin of the selected channel

Output data on O_3 pin of the selected channel

Output data on O_2 pin of the selected channel

Output data on I/O_1 pin (if defined as an output) of the selected channel

Output data on I/O_0 pin (if defined as an output) of the selected channel

Table 4. Definition of SLIC Control Register

Bit

7 I/On_0 Image

6 I/On_1 Image

5 On_2 Image

4 On_3 Image

3 On_4 Image

2 I/O1_0 Image

1 I/O2_0 Image

0 I/O3_0 Image

Name

Description

Mapped to I/On_0 pin of the selected channel n

Mapped to I/On_1 pin of the selected channel n

Mapped to On_2 pin of the selected channel n

Mapped to On_3 pin of the selected channel n

Mapped to On_4 pin of the selected channel n

Always mapped to the I/O1_0 pin

Always mapped to the I/O2_0 pin

Always mapped to the I/O3_0 pin

Table 5. Definition of SLIC Status Register

APPLICATION NOTE

The IDT821034 is mainly used in line card application. Figure 5 shows

a typical system with telephony line interface.

The IDT821034 offers not only encoding/decoding function, but also a

signaling channel, which can simplify the circuit design of the control

interface. In addition, the dynamic time slot assignment of IDT821034

reduces the hardware requirement for PCM interface. The device also

supports 8.192 Mbps PCM data rate, which can increase the time slot

density up to 128.

Signal to total

over -55 dBm0 to

distortion

+3 dBm0

rraantigoe(wboitthhaSsTpDeXciaficndgaSinTsDeRt)tinagre(0gduBarfaonr tbeoethd

transmit path and receive path). Since there is a finite noise floor associated

with the quantization effect of both data converters and digital filter

coefficients, the overall signal to total distortion ratio of each path is a function

of the gain setting. In system design, attention should be paid to the gain

setting for the best signal to total distortion performance.

Generally, a channel gain of a line-card system is contributed

by both SLIC and CODEC. In a system design using IDT821034, the

SLIC gain should be taken into account to optimize the SNR. In the transmit

path of IDT821034, there are two resistors (R1 and R3 in Figure 5)

which enable the analog gain to be adjusted around 0 dB. Further gain

adjustment can be obtained by programming the DSP filters. Since this

adjustment is close to 0 dB, the SNR remains at the optimum value. In

the receive path of IDT821034, analog gain adjustment is not available.

Thus, the adjustment of CODEC gain will be performed only by

programming the DSP filters. In this way, the SLIC gain should be such

that the DSP gain is closest to 0 dB. This will maximize the achievable

SNR in the overall system. For example, if the design target for receive

path gain is -3.5 dB and -7 dB for local and long distance calls

respectively, the recommended solution is to set SLIC gain at -3.5 dB.

As a result, the gain of CODEC, which is adjusted by programming DSP

coefficients, will be 0 dB and -3.5 dB.

8

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