TEF6894H datasheet(9/55 Pages) PHILIPS

9 / 55 page

2003 Oct 21

Philips Semiconductors

Product speciﬁcation

Car radio integrated signal processor

TEF6894H

The pilot detector is driven by an internally generated

in-phase 19 kHz signal. Its pilot dependent voltage

activates the stereo indicator bit and sets the stereo

decoder to stereo mode. The same voltage is used to

control the amplitude of an anti-phase internally generated

19 kHz signal. In the pilot canceller, the pilot tone is

compensated by this anti-phase 19 kHz signal.

The signal is then decoded in the decoder part. The side

signal is demodulated and combined with the main signal

to the left and right audio channels. A fine adjustment of

the roll-off compensation is done by adjusting the gain of

the L-R signal in 16 steps. A smooth mono to stereo

takeover is achieved by controlling the efficiency of the

matrix by the FMSNC signal from the weak signal

processing block.

7.2

FM and AM noise blanker

The FM/AM switch selects the output signal of the stereo

decoder (FM mode) or the signal from the AM input for the

noise blanker block. In FM mode the noise blanker

operates as a sample and hold circuit, while in AM mode it

mutes the audio signal during the interference pulse. The

blanking pulse which triggers the noise blanker is

generated in the noise detector block.

7.3

High cut control and de-emphasis

The High Cut Control (HCC) part is a low-pass filter circuit

with eight different static roll-off response curves. The

cut-off frequencies of these filter curves can be selected by

I2C-bus to match different application requirements. The

HCC circuit also provides a dynamic control of the filter

response. This function is controlled by the AMFMHCC

signal from the weak signal processing.

The signal passes the de-emphasis block with two

de-emphasis values (50 and 75

µs), which can be selected

via I2C-bus, and is fed to the input selector.

7.4

Noise detector

7.4.1

FM NOISE DETECTOR

The trigger signal for the FM noise detector is derived from

the MPXRDS input signal and the LEVEL signal. In the

MPXRDS path a four pole high-pass filter (100 kHz)

separates the noise spikes from the wanted MPX signal.

Another detector circuit triggers on noise spikes on the

level voltage. The signals of both detectors are combined

to achieve a reliable trigger signal for the noise blanker.

AGC circuits in the detector part control the gain

depending on the average noise in the signals to prevent

false triggering. The sensitivity of the triggering from the

MPXRDS signal can be adjusted in four steps, the

triggering from the LEVEL signal in three steps.

7.4.2

AM NOISE DETECTOR

The trigger pulse for the AM noise blanker is derived from

the AM audio signal. The noise spikes are detected by a

slew rate detector, which detects excessive slew rates

which do not occur in normal audio signals. The sensitivity

of the AM noise blanker can be adjusted in four steps.

7.5

Multipath/weak signal processing

The multipath (MPH)/weak signal processing block

detects quality degradations in the incoming FM signal and

controls the processing of the audio signal accordingly.

There are three different quality criteria:

• The average value of the level voltage

• The AM components on the level voltage

[Wideband AM (WAM)]

• The high frequency components in the MPX signal

[Ultrasonic Noise (USN)].

The level voltage is converted to a digital value by an 8-bit

analog-to-digital converter. A digital filter circuit (WAM

filter) derives the wideband AM components from the level

signal. The high frequency components in the MPX signals

are measured with an analog-to-digital converter (USN

ADC) at the output of the 100 kHz high-pass filter in the

MPXRDS path.

The values of these three signals are externally available

via the I2C-bus.

In the weak signal processing block the three digital

signals are combined in a specific way and used for the

generation of control signals for soft mute, stereo blend

(stereo noise control, FMSNC) and high cut control

(AMFMHCC).

The sensitivities of the detector circuits (WAM and USN)

are adjustable via the I2C-bus.

Also the start values and the slopes of the control functions

soft mute, stereo blend and high cut control can be set via

the I2C-bus.

Soft mute, stereo blend and HCC are set on hold during

the AF updating (quality check of alternative frequency) to

avoid an influence of the tuning procedure on the weak

signal processing conditions.

In AM mode the soft mute and high cut control are

available too, the weak signal block is controlled by the

average value of the level voltage.

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