SAA7708H Просмотр технического описания (PDF) - Philips Electronics

Номер в каталоге

Компоненты Описание

производитель

SAA7708H

Car Radio Digital Signal Processor

Philips Electronics 

Philips Semiconductors

Car Radio Digital Signal Processor

Preliminary specification

SAA7708H

10.4 Supply of the analog inputs

The analog input circuit has separate power supply connections to allow maximum filtering. These pins are the VSSA1

for the analog ground and the VDDA1 for the analog power supply.

10.5 The DCS clock block

For the digital stereo decoder a clock signal is needed which is the 512 multiple of the pilot tone frequency of the FM

MPX signal. This is done by the DCS clock block, which generates this 512 * 19 kHz = 9.728 MHz clock, the DCS clock,

by locking to the pilot frequency. This block is also able to generate other frequencies and controlling is done via a

number of I2C bits of the registers in Table 21 and Table 22. Default I2C settings of the DCS and the PLL guarantee

correct functioning of the DCS block.

10.6 Synchronization with the DSP core

The system can run in case of I2S input on different audio sample frequencies of Fs=32kHz, 38 kHz, 44.1 kHz or 48 kHz.

After each processing period of one input sample with this signal, the Input flag (I-flag) of the status register of the DSP

core is set on the falling edge of the I2S WS to I=1 during 4 clock cycles. This flag can be tested with a conditional branch

instruction in the DSP. This synchronisation starts really in parallel with the input signal due to the short period that the I

flag is set. It is obvious that the higher the Fs the lower the number of cycles available in the DSP program..

10.7 IAC

10.7.1 GENERAL DESCRIPTION

The Interference Absorption Circuit (IAC) detects and suppresses ignition interference. This hardware IAC is a modified,

digitized and extended version of the analog circuit which is in use for many years already.

The IAC consists of an MPX mute function switched by mute pulses from ignition interference pulse detectors. All IAC

functions must be switched off if there is no FM MPX signal processing.

The input signal of a first IAC detection circuit is the output signal of ADF1. This interference detector analyses the high

frequency contents of the MPX signal. The discrimination between interference pulses and other signals is performed by

a special Philips patented fuzzy logic like algorithm and is based on probability calculations. This detector performs

optimally in higher antenna voltage circumstances. On detection of ignition interference, this logic will send appropriate

pulses to the MPX mute switch.

The input signal of a second IAC detection circuit is the LEVEL signal (the output of the Level AD). This detector performs

optimally in lower antenna voltage circumstances. It is therefore complementary to the first detector.

The characteristics of both IAC detectors can be adapted to the properties of different FM front ends by means of the

predefined coefficients in the IAC control registers. The values can be changed via the I2C bus. Both IAC detectors can

be switched on or off independently of each other. Both IAC detectors can mute the MPX signal independently of each

other.

A third IAC function is the Dynamic IAC circuit. This block is intended to switch off the IAC completely the moment the

MPX signal has a too high frequency deviation which in case of narrow IF filters can result in AM modulation. This AM

modulation could be interpreted by the IAC circuitry as interference caused by the car’s engine.

AM IAC is also implemented. In this case only the AM mono signal is monitored by the on the DSP processor running

program. Input from the LEVEL pin is not used.

10.7.2 PARAMETER SETTING FOR THE MPX INPUT IGNITION DETECTOR

There are in total 5 different coefficients. The settings of these coefficients are described below. On RESET, the nominal

setting for a good performing MPX IAC detector is selected.

1998 May 19

20

Share Link: