SAA7708H Просмотр технического описания (PDF) - Philips Electronics

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SAA7708H

Car Radio Digital Signal Processor

Philips Electronics 

Philips Semiconductors

Car Radio Digital Signal Processor

Preliminary specification

SAA7708H

10.7.3 AGC SET POINT (1 BIT)

In case the sensitivity and feed forward factor are out of range in a certain application, the set point of the AGC can be

shifted. The set point controls the sensitivity of the other IAC control parameters. See bit 11 of $0FFB (Table 25).

10.7.4 THRESHOLD SENSITIVITY OFFSET (3 BITS)

With this parameter the threshold sensitivity of the comparator in the interfering pulse detectors can be set. It also

influences the amount of unwanted triggering. Settings are according Table 31.

10.7.5 DEVIATION FEED FORWARD FACTOR (3 BITS)

This parameter determines the reduction of the sensitivity of the detector by the absolute value of the MPX signal. This

mechanism prevents the detector from unwanted triggering at noise with modulation peaks. In Table 32 the possible

values are given.

10.7.6 SUPPRESION STRETCH TIME (3 BITS)

This parameter sets the duration of the pulse suppression after the detector has stopped sending a trigger pulse. It can

be switched off by applying the value 0. The duration can be selected in steps of one period of the 304 kHz (3.3 µs)

sample frequency. In Table 33 the possible values are given.

10.7.7 MPX DELAY

With this parameter the delay time between 2 and 5 samples of the 304 kHz sample frequency can be selected. The

needed value depends on the used front end of the car radio. Settings are according Table 34.

10.7.8 LEVEL IAC THRESHOLD (4 BITS)

With this parameter the sensitivity of the comparator in the ignition interference pulse detector can be set. It also

influences the amount of unwanted triggering. The possible values are given in Table 26. The prefix value ‘0000’ switches

the Level IAC function off.

10.7.9 LEVEL IAC FEED FORWARD SETTING (2 BITS)

This parameter allows to adjust for delay differences in the signal paths from the FM antenna to the MPX mute, namely,

via the FM level ADC and level IAC detection and via the FM demodulator and MPX conversion and filtering. These

differences depend on the front end used in the car radio. With a simultaneous appearance of a peak disturbance at the

LEVEL input and the MPX ADC input of the chip, a zero delay setting will make for the level IAC mute pulse to coincide

with the passage of the disturbance in the MPX mute circuit. The setting for the Level IAC Feed Forward allows to

advance the mute pulse by 1 sample period or to delay it by 1 or 2 sample periods of the 304 kHz clock, with respect to

the default. The appropriate I2C bits for each setting are given in Table 27.

10.7.10 LEVEL IAC SUPPRESSION STRETCH TIME (2 BITS)

This parameter sets the time the mute pulse is stretched when the LEVEL input has stopped exceeding the threshold.

The duration can be selected in steps of one period of the 304 kHz (3.3 µs) sample frequency. In Table 28 the possible

values are given

10.7.11 DYNAMIC IAC THRESHOLD LEVELS

If enabled by the lev_en_dyn_iac I2C bit (bit 15, register $0FFC) this block will disable temporarily all IAC action if the

MPX mono signal exceeds a threshold deviation (threshold 1) for a given time with a given excess amount (threshold 2).

This MPX mono signal is separated from the MPX signal with a low-pass filter with the - 3 dB corner point at 15 kHz. The

possible values of the this threshold can be found in Table 29.

1998 May 19

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