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IDT821024

QUAD NON-PROGRAMMABLE PCM CODEC

Unspecified 

821024 DATA SHEET

FUNCTIONAL DESCRIPTION

The IDT821024 contains four channel PCM CODEC with on chip digital

filters. It provides the four-wire solution for the subscriber line circuitry in

digital switches. The device converts analog voice signal to digital PCM

data, and converts digital PCM data back to analog signal. Digital filters

are used to bandlimit the voice signals during the conversion. Either A-law

or μ-law is supported by the IDT821024. The law selection is performed

by A/μ pin.

The frequency of the master clock (MCLK) can be 2.048 MHz, 4.096 MHz,

or 8.192 MHz. Internal circuitry determines the master clock frequency

automatically.

The serial PCM data for four channels are time multiplexed via two pins,

DX and DR. The time slots of the four channels are determined by the

individual Frame Sync signals at rates from 256 kHz to 8.192 MHz. For

each channel, the IDT821024 provides a transmit Frame Sync signal and

a receive Frame Sync signal.

Each channel of the IDT821024 can be powered down independently to

save power consumption. The Channel Power Down Pins PDN1-4 configure

channels to be active (power-on) or standby (power-down) separately.

Signal Processing

High performance oversampling Analog-to-Digital Converters (ADC) and

Digital-to-Analog Converters (DAC) are used in the IDT821024 to provide

the required conversion accuracy. The associated decimation and interpo-

lation filtering are realized with both dedicated hardware and Digital Signal

Processor (DSP). The DSP also handles all other necessary functions such

as PCM bandpass filtering and sample rate conversion.

Transmit Signal Processing

In the transmit path, the analog input signal is received by the ADC and

converted into digital data. The digital output of the oversampling ADC is

decimated and sent to the DSP. The transmit filter is implemented in the

DSP as a digital bandpass filter. The filtered signal is further decimated

and compressed to PCM format.

Transmit PCM Interface

The transmit PCM interface clocks out 1 byte (8 bits) PCM data out of

DX pin every 125 μs. The transmit logic, synchronized by the Transmit

Frame Sync signal (FSXn), controls the data transmission. The FSXn

pulse identifies the transmit time slot of the PCM frame for Channel N.

The PCM Data is transmitted serially on DX pin with the Most Significant

Bit (MSB) first. When the PCM data is being output on DX pin, the TSC

signal will be pulled low.

Receive Signal Processing

In the receive path, the PCM code is received at the rate of 8,000

samples per second. The PCM code is expanded and sent to the DSP

for interpolation. A receive filter is implemented in the DSP as a digital

lowpass filter. The filtered signal is then sent to an oversampling DAC. The

DAC output is post-filtered and delivered at VOUT pin by an amplifier. The

amplifier can drive resistive load higher than 2 KΩ.

Receive PCM Interface

The receive PCM interface clocks 1 byte (8 bits) PCM data into DR

pin every 125 μs. The receive logic, synchronized by the Receive Frame

Sync signal (FSRn), controls the data receiving process. The FSRn pulse

identifies the receive time slot of the PCM frame for Channel N. The PCM

Data is received serially on DR pin with the Most Significant Bit (MSB) first.

Hardware Gain Setting In Transmit Path

The transmit gain of the IDT821024 for each channel can be set by 2

resistors, RREF and RTXn (as shown in Figure 1), according to the following

equation:

The receive gain of IDT821024 is fixed and equal to 1.

REVISION A 06/25/14

Figure 1. IDT821024 Transmit Gain Setting for Channel 1

5

QUAD NON-PROGRAMMABLE

PCM CODEC

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