MT9196AS datasheet(7/38 Pages) MITEL

7 / 38 page

Preliminary Information

MT9196

7-133

Low and high tones are enabled individually via the

LoEn and HiEN control bits (DTMF/Ringer Control

control over dual tone generation but also allows

single tone generation using either of the enable bits

and its associated coefficient register.

After programming and enabling the tone generators

as described, selection of transmit and/or receive

path destinations are carried out via the Path Control

Registers (see Path Control section). In addition

receive sidetone DTMF must be selected via the

DTMF StEN bit (DTMF/Tone ringer Register, address

18h) so that it replaces the received PCM in the Rx

Filter path.

Table 2 - DTMF Frequencies

DTMF Signal to distortion:

The sum of harmonic and noise power in the frequency band

from 50 Hz to 3500 Hz is typically more than 30 dB below the

power in the tone pair. All individual harmonics are typically

more than 40 dB below the level of the low group tone.

Table 2 gives the standard DTMF frequencies, the

coefficient

required

generate

the

closest

frequency, the actual frequency generated and the

percent deviation of the generated tone from the

nominal.

Tone Ringer

A dual frequency squarewave ringing signal may be

applied to the handsfree speaker driver to generate a

call alerting signal. To enable this mode the Ring En

bit (address 18h) must be set as well as the ringer

function to the loudspeaker via the Receive Path

Control

(address

13h).

Ring

independent of the DTMF enable control bits (see Lo

EN and Hi EN). Since both functions use the same

coefficient registers they are not usually enabled

simultaneously.

The

digital

tone

generator

uses

the

values

programmed into the low and high Tone Coefficient

Registers (addresses 1Ah and 1Bh) to generate two

different squarewave frequencies.

Frequency

(Hz)

COEFF

Actual

Frequency

Deviation

697

59h

695.3

-.20%

770

63h

773.4

+.40%

852

6Dh

851.6

-.05%

941

79h

945.3

+.46%

1209

9Bh

1210.9

+.20%

1336

ABh

1335.9

.00%

1477

BDh

1476.6

-.03%

1633

D1h

1632.8

-.01%

Both coefficients are determined by the following

equation:

COEFF = [32000/Frequency (Hz)] - 1

where COEFF is an integer between 1 and 255. This

produces frequencies between 125 - 16000 Hz with

a non-linear resolution.

The ringer program switches between these two

frequencies at a 5 Hz or 10 Hz rate as selected by

the WR bit in the DTMF/Tone ringer register (address

18h).

Anti-Howl

IDPC includes an Anti-Howling circuit plus speaker

gain control circuit to allow for group listening

operation. Although this is the main function of the

circuit there are additional modes in which it may be

used as defined by the MS1 and MS0 control bits

(address 1Ch).

MS1

MS0

Operational Mode

Tx noise reduction (squelch)

Rx noise reduction (squelch)

switched loss group listening

(anti-howling)

Tx/Rx switched loss

The circuit is enabled by setting the Anti-howl Enable

bit

(address

1Ch)

and

selecting

the

required

operational mode (MS0 & MS1) as described.

For all modes of operation the switching levels and

inserted loss are programmed as follows.

Switching decisions are made by comparing either

the transmit or the receive signal level to threshold

levels

stored

the

High

Threshold

(address 1Dh) and the Low Threshold Register

(address 1Eh). Threshold data is encoded in PCM

sign-magnitude format excluding the sign bit. For

example; THh0 - THh3 encode the PCM step

number while THh4 - THh6 encode the PCM chord

number for the high threshold. Similarily for the THl0

- THl6 bits of the low threshold. The difference

between the high and low threshold levels provides

the circuit with hysteresis to prevent uncontrolled

operation. The low level threshold must never be

programmed to a value higher than the one stored in

the high level threshold. If this occurs the circuit will

become unstable.

Loss is implemented, in the chosen path, by

subtracting the value set by the Pad0 - Pad3 control

bits from the appropriate gain value set by the RxG0

- RxG3 or TxG0 - TxG3 control bits (see Digital Gain

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