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MC145532

ADPCM Transcoder

Motorola => Freescale 

DEVICE DESCRIPTION

An Adaptive Differential PCM (ADPCM) transcoder is used

to reduce the data rate required to transmit a PCM encoded

voice signal while maintaining the voice fidelity and intelli-

gibility of the PCM signal.

The transcoder is used on 64 kbps data streams which

represent either voice or voice band data signals that have

been digitized by a codec (e.g., MC145557). The transcoder

uses a filter to attempt to predict the next PCM input value

based on previous PCM input values. The error between the

predicted and the true PCM input value is the information

that is sent to the other end of the line. Hence the word differ-

ential, since the ADPCM data stream is the difference be-

tween the true PCM input value and the predicted value. The

term “adaptive” applies to the filter that is performing the pre-

diction. It is adaptive in that its transfer function changes

based on the PCM input data. That is, it adapts to the statis-

tics of the signals presented to it.

PIN DESCRIPTIONS

ENCODER INPUT

EDI

Encoder Data Input (Pin 12)

PCM data to be encoded are applied to this input pin which

operates synchronously with EDC and EIE to enter the data

in a serial format.

EDC

Encoder Data Clock (Pin 13)

Data applied to EDI are latched into the transcoder on a

falling edge of EDC and data are output from EDO on a rising

edge of this input pin. The frequency of EDC may be as low

as 64 kHz or as high as 5.12 MHz.

EIE

Encoder Input Enable (Pin 11)

The beginning of a new PCM word is indicated to the

transcoder by a rising edge applied to this input. The fre-

quency of EIE may not exceed 8 kHz.

ENCODER OUTPUT

EDO

Encoder Data Output (Pin 15)

ADPCM data are available in a serial format from this out-

put, which operates synchronously with EDC and EOE. EDO

is a three–state output which remains in a high–impedance

state, except when presenting data.

EOE

Encoder Output Enable (Pin 14)

Each ADPCM word is requested by a rising edge on this

input, which causes the EDO pin to provide the data when

clocked by EDC. One EOE must occur for each EIE.

DECODER INPUT

DDI

Decoder Data Input (Pin 5)

ADPCM data to be decoded are applied to this input pin,

which operates in conjunction with DDC and DIE to enter the

data in a serial format.

DDC

Decoder Data Clock (Pin 4)

Data applied to DDI are latched into the transcoder on the

falling edge of DDC and data are output from DDO on the ris-

ing edge of DDC. The frequency of DDC may be as low as

64 kHz or as high as 5.12 MHz.

DIE

Decoder Input Enable (Pin 6)

The beginning of a new ADPCM word is indicated by a ris-

ing edge applied to this input. Data are serially clocked into

DDI on the subsequent falling edges of DDC following the

DIE rising edge. The frequency of DIE may not exceed

8 kHz.

DECODER OUTPUT

DDO

Decoder Data Output (Pin 2)

PCM data are available in a serial format from this output,

which operates in conjunction with DDC and DOE. DDO is a

three–state output that remains at a high–impedance state

except when presenting data.

DOE

Decoder Output Enable (Pin 3)

Each ADPCM word is requested by a rising edge on this

input which causes the DDO pin to provide the data when

clocked by DDC. One DOE must occur for each DIE.

CONTEXT

MODE

Mode Select (Pin 1)

A logic 0 applied to this input makes the transcoder com-

patible with Mu–255 companding and D3 data format. A

logic 1 applied to this pin makes the transcoder compatible

with A–Law companding with even bit inversion data format.

SPC

Signal Processor Clock (Pin 10)

This input is typically clocked with a 20.48 MHz clock sig-

nal which is used as the digital signal processor master

clock. This pin has a CMOS compatible input.

RESET

Reset (Pin 7)

A logic 0 applied to this input forces the transcoder into a

low power dissipation mode. A rising edge on this pin causes

power to be restored and the optional transcoder RESET

state (specified in the standards) to be forced. Valid data is

available at the output pins four input enables after a rising

edge on this pin. This pin has a CMOS compatible input.

MC145532

2

MOTOROLA

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