MT9196AS datasheet(6/38 Pages) MITEL

6 / 38 page

MT9196

Preliminary Information

7-132

The receive filter is designed to meet CCITT G.714

specifications. The nominal gain for this filter path is

0 dB (gain control = 0dB). Gain control allows the

output signal to be attenuated up to 7 dB. Filter

response is peaked to compensate for the sinx/x

attenuation caused by the 8 kHz sampling rate.

The Rx filter function can be altered by enabling the

Dial EN control bit in Control Register 1 (address

0Eh). This causes another low-pass function to be

added with a 3 dB point at 1000 Hz. This function is

intended to improve the sound quality of digitally

generated dial tone received as PCM.

Side-tone is derived from the Tx filter before the LP/

HP filter section and is not subject to the gain control

of the Tx filter section. Side-tone is summed into the

receive handset transducer driver path after the Rx

filter gain control section so that Rx gain adjustment

will not affect side-tone levels. The side-tone path

may be enabled/disabled with the Voice sidetone bit

located

the

Receive

Path

Control

(address 13h).

Transmit and receive filter gains are controlled by the

TxFG0-TxFG2

and

RxFG0-RxFG2 control bits,

respectively. These are located in the FCODEC

Control Register 1 (address 0Ah). Transmit filter gain

is adjustable from 0 dB to +7 dB and receive filter

gain from 0 dB to -7 dB, both in 1 dB increments.

Side-tone filter gain is controlled by the STG0-STG2

control bits located in the FCODEC Control Register

2 (address 0Bh). Side-tone gain is adjustable from

-9.96 dB to +9.96 dB in 3.32 dB increments.

Companding law selection for the Filter/CODEC is

provided by the A/µ companding control bit while

the coding scheme is controlled by the sign-mag/

CCITT control bit. Both of these reside in Control

choices.

Table 1

The Filter/CODEC autonull circuit ensures that

transmit PCM will contain no more than

±1 bit of

offset due to internal circuitry.

Code

Sign/

Magnitude

CCITT (G.711)

µ-Law

A-Law

+ Full Scale

1111 1111

1000 0000

1010 1010

+ Zero

1000 0000

1111 1111

1101 0101

-Zero

(quiet code)

0000 0000

0111 1111

0101 0101

- Full Scale

0111 1111

0000 0000

0010 1010

Digital Gain and Tone Generation

The Digital gain and Tone generator block is located,

functionally, between the serial FDI port and the

Filter/CODEC block. Its main function is to provide

digital gain control of the transmit and receive audio

signals and to generate digital patterns for DTMF

and tone ringer signals.

Gain Control

Gain control is performed on linear code for both the

receive and the transmit PCM. Gain control is set via

the Digital Gain Control Register at address 19h.

Gain, in 3.0 dB increments, is available within a

range of +21.0 dB to -24 dB.

DTMF Generator

The digital DTMF circuit generates a dual sine-wave

pattern which may be routed into the receive path as

comfort tones or into the transmit path as network

signalling. In both cases the digitally generated

signal will undergo gain adjustment as programmed

into the transmit and receive gain control registers.

Gain control is assigned automatically as functions

are selected via the transmit and receive path control

registers.

The composite signal

output level in the transmit

direction is -4 dBm0 (

µ-Law) and -10 dBm0 (A-law)

with programmable gains at zero dB. Pre-twist of 2.0

dB is incorporated into the composite signal resulting

in a low tone output level of -8.12 dBm0 and a high

group level of -6.12dBm0 (for

µ-Law, 6 dB lower for

A-Law). Note that these levels will be influenced by

the Anti-Howling circuit when it is enabled (see Anti-

Howling section for more details). DTMF side-tone

levels are set to -28 dBm0 from the generator circuit.

Other receive path gains must be included when

calculating

the

analog

output

signal

levels.

Adjustments to these levels may be made by altering

the settings of the Gain Control register (address

19h).

The frequency of the low group tone is programmed

by writing an 8-bit coefficient into the Low Tone

Coefficient Register (address 1Ah) while the high

group tone frequency uses the 8-bit coefficient

programmed into the High Tone Coefficient Register

(address 1Bh). Both coefficients are determined by

the following equation:

Frequency (in Hz) = 7.8125 x COEFF

Where COEFF is an integer between 0 and 255.

Frequency resolution is 7.8125 Hz in the range 0 to

1992 Hz.

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