TEF6901A_3 © NXP B.V. 2008. All rights reserved.
Product data sheet Rev. 03 — 20 March 2008 10 of 107
NXP Semiconductors
TEF6901A
Integrated car radio
7.21 FM stereo decoder
A low-pass filter provides additional suppression of high frequency interferences at the
stereo decoder input and the necessary signal delay for FM noise blanking.
The MPX signal is decoded in the stereo decoder part. An integrated oscillator and pilot
PLL is used for the regeneration of the 38 kHz subcarrier. The required 19 kHz
and 38 kHz signals are generated by division of the oscillator output signal in logic
circuitry.
By means of a 19 kHz quadrature detector the pilot PLL oscillator frequency is locked to
the incoming 19 kHz stereo pilot. A pilot level voltage derived from a 19 kHz in-phase
detector is used for stereo detection and for generation of an anti-phase 19 kHz signal to
remove the pilot tone from the audio signal.
The signal is then decoded in the decoder part. The L-R side signal is demodulated using
the 38 kHz subcarrier and combined with the main signal to the left and right audio
channel. A fine adjustment is done by adjusting the gain of the L-R signal. A smooth mono
to stereo takeover is achieved by controlling the efficiency of the matrix by the Stereo
Noise Control (SNC) signal from the weak signal processing block.
7.22 FM and AM AF noise blanker
The FM or AM tuner operation selects between two noise blanker operations optimized for
FM or AM ignition noise suppression.
In FM mode the noise blanker operates as a modified sample and hold circuit with
ultrasonic noise detection on MPX and detection of noise spikes on level.
In AM mode the audio signal is muted during the interference pulse triggered by slew-rate
detection of the audio signal.
7.23 Fixed high cut and high cut control
The high cut part is a low-pass filter circuit with seven bandwidth settings. The cut-off
frequencies of the filter curves can be selected to match different application requirements
(fixed high cut).
The high cut circuit also provides a dynamic control of the filter response, the High Cut
Control (HCC). This function is controlled by the HCC signal from the weak signal
processing.
7.24 De-emphasis
The signal passes the low-pass filter de-emphasis block and is then fed to the source
selector. The de-emphasis time constant can be selected between the standards of 50 µs
and 75 µs.
7.25 Weak signal processing
The weak signal processing block detects quality degradations in the incoming signal and
controls the processing of the audio signal accordingly. The weak signal processing block
has three different quality criteria: The average value of the level voltage, AM components
on the level voltage (WAM = wideband AM) and high frequency components in the MPX
signal (USN = ultrasonic noise).
