2003 Oct 21 9
Philips Semiconductors Product specification
Car radio integrated signal processor TEF6890H
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
I
2
C-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 I
2
C-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 I
2
C-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 I
2
C-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 I
2
C-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.
