ST75C540 datasheet(53/84 Pages) STMICROELECTRONICS

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IX - TONE DETECTORS

Z -1

OUT

Figure 17 : Biquadratic IIR Filter

IX.1 - Overview

The general purpose ST75C530/540 tone detec-

tors block is a powerful module that covers a lot of

applications :

- call progress tone detection, fully programmable

for all countries,

- FAX, voice, data automatic detection,

- call waiting detection, while in vocoder or data

mode.

IX.2 - Description

The primary tone detector block is a set of 16

identical Cells. Each cell is composed of a Double

Biquadratic Filter, a Power estimator section, a

Static level and a Level comparator.

Each Biquadratic Filter, Power Estimator and Static

Level can be programmed using a complete set of

commands (TDRC, TDRW, TDWC, TDWW, TDZ).

The wiring between the different Cells can be de-

fined by the user, using the associated command

allowing a wide range of applications.

The sampling frequency is 7200Hz, allowing detec-

tion of signals less than 3300Hz.The level of detec-

tion is programmable from -6dBm down to -51dBm.

The 16 Comparator Outputs give, on a baud basis,

the information into two 8 bits words TONEDET0

(for cells number 0 to 7) and TONEDET1 (for cells

number 8 to F). These TONEDET variables can be

accessed using a MRI command or, more easily,

monitored on a baud basis using the DOSR com-

mand.

The 16 primary tone detectorsare initializedeach time

entering the tone mode. However the previous coeffi-

cient values could be kept using a MW command.

The secondary tone detector have been added to

the ST75C530/540. The filter structure is the same

as the primary tone detector.

The sampling rate is 4800Hz allowing detection of

signal less than 1800Hz by default programming or

with a MODC command, the sampling rate is

9600Hz allowing detection of signal less than

3300Hz. The level of detection is programmable

from -6dBm down to -51dBm. In order to increase

the reliability of the detection, using a TDWW com-

mand, 2 comparisonsare provided,one with a fixed

level (absolute) or with the receive signal (relative).

The 4 secondary tone detectors are initialiazed

each time entering the tone mode. However the

previous coefficient values could be kept using a

CONF command.

The commandTDRC, TDWC, TDWW, TDRW, TDZ

with the TD_CELL parameter of 0x10, 0x11, 0x12

or 0x13 can be used to program these filters.

IX.2.1 - Biquadratic Filters

Each Biquadratic Filter is a double regular section

that can perform any Transfer function with 4 Poles

and 4 Zeros.

This routine is run on a sample basis.

The corresponding transfer function is :

Out

Input

= C0 ⋅

+ 2 ⋅ C3 ⋅ z−1 + 2 ⋅ C4 ⋅ z−2

− 2 ⋅ C1 ⋅ z−1 − 2 ⋅ C2 ⋅ z−2

⋅ C6 ⋅

+ 2 ⋅ C9 ⋅ z−1 + 2 ⋅ CA ⋅ z−2

− 2 ⋅ C7 ⋅ z−1 − 2 ⋅ C8 ⋅ z−2

⋅ z−1

Note :

All coefficients are coded on 16 bits 2’s complement in the range +1, -1 (Q15). To avoid the possibility of overflow the user must check

that the internal node must not be higher that 0.5 (in Q15 representation).

ST75C530 - ST75C540

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