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D6571E

EASYTAD™ Chip for an All- Digital Telephone Answering Machine with Flash Memory Interface

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D6571E Data Sheet

between microphone and speaker, and an adaptive

electrical echo canceller (EEC) which cancels the echo

caused by the 4-to-2 wire telephone line connection.

Automatic Loop Adjustment Mode

When starting the SpeakerPhone mode, both of the

echo cancellers are at their initial point. They are not

trained and provide no cancellation. By default the

D6571E starts the SpeakerPhone with increased level in

order to guarantee that the system will be stable. When

the echo cancellers have converged, the additional loop

attenuation is removed.

The D6571E starts operation with the “target” Loop

Attenuation + 12 dB. This attenuation will be reduced

to “target” value once echo cancellers get trained. The

default setting of the D6571E is: target Loop

Attenuation 8 dB, starting point is 20 dB, which will be

reduced to 8 dB. The Host can set any desired “target”

value of the Loop Attenuation.

If the D6571E is unable to provide efficient echo

cancellation it will (temporarily) introduce an additional

attenuation in order to prevent howling or feedback.

This happens if the D6571E determines that it is unable

to cancel either acoustical echo or electrical echo by

adjusting the echo canceller coefficients. Additional

Loop attenuation will be introduced if the Speaker

Volume or Line Volume is increased or if the target Loop

Attenuation is decreased by Host to ensure the smooth

transaction. The D6571E will restore the target loop

attenuation selected by Host by reducing the additional

attenuation in steps of 4 dB once efficient cancellation

is achieved.

The Automatic Loop Adjustment mode is the default

mode. It can be also selected by the SpeakerPhone

configuration command (Mode = 10, Loop Adjustment =

10).

If the actual Loop Attenuation is more than 0 dB the

SpeakerPhone operates by dynamically distributing the

loop attenuation selected between the receive and

transmit attenuaters providing natural non-interrupted

conversation even with the loop attenuation higher than

0 dB. The D6571E makes a decision as to which

direction is talking and then rapidly shifts the

attenuation to the opposite direction.

Voice Adaptation of the Echo Cancellers

The D6571E has two echo adaptive cancellers. Both of

them are adapting to speech. When microphone side is

speaking the EEC updates its coefficients. The AEC

updates its coefficients when telephone line side is

speaking,

The Voice Adaptation mode is default mode of the

D6571E. Alternatively the D6571E can send the noise

pulse to train echo cancellers. After sending training

pulse the D6571E would return to the Voice Adaptation

mode. The Voice Adaptation mode without any training

pulse is the recommended one.

Also if the user desires to change the volume of the

speaker, this should be done via the D6571E and not by

changing the external speaker amp lification.

Re-Compression Mode

This mode allows to change compression rate of the

recorded message. First command C1 specifies message

to be re-compressed. Second command C2 specifies

new compression rate. Re-compression starts after C2

command. Host can send C3 command continuously to

monitor progress of the re-compression operation.

READY bit will be set when re-compression is complete.

ERROR bit will be set if message to be recompressed

does not exist, or if this a PCM or binary message.

If memo ry full is reached the D6571E will set bit MEM

FULL and terminate re-compression. Old message will

be saved (before recompression), re-compressed

message will be deleted.

Note: Garbage Collection must be executed before re-

compression and after each re-compression.

DS6571E.2

DSP GROUP, INC., 3120 SCOTT BOULEVARD

27

SANTA CLARA, CA 95054 PH: 408 986-4300 FAX: 408 986-4490

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