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TEA6880H/V2,518

Up-level Car radio Analog Signal Processor (CASP)

NXP Semiconductors. 

Philips Semiconductors

Up-level Car radio Analog Signal

Processor (CASP)

Product specification

TEA6880H

7 FUNCTIONAL DESCRIPTION

7.1 Stereo decoder

The MPX input is the null-node of an operational amplifier

with internal feedback resistor. Adapting the stereo

decoder input to the level of the MPX signal, coming from

the FM demodulator output, is realized by the value of the

input series resistor RIN. To this input a second source

(AM detector output) can be fed by current addition.

The input amplifier is followed by an integrated 4th order

Bessel low-pass filter with a cut-off frequency of 80 kHz.

It provides necessary signal delay for FM noise blanking

and damping of high frequency interferences coming to

the stereo decoder input.

Output of this filter is fed to the soft mute control circuitry,

the output is voltage to current converted and then fed to

phase detector, pilot detector and pilot canceller circuits,

contained in the stereo decoder PLL block. For

regeneration of the 38 kHz subcarrier, a PLL is used.

The fully integrated oscillator is adjusted by means of a

digital auxiliary PLL into the capture range of the main PLL.

The auxiliary PLL needs an external reference frequency

(75.4 kHz) which is provided by the TEA6840H.

The required 19 and 38 kHz signals are generated by

division of the oscillator output signal in a logical circuitry.

The 19 kHz quadrature phase signal is fed to the 19 kHz

phase detector, where it is compared with the incoming

pilot tone. The DC output signal of the phase detector

controls the oscillator (PLL).

The pilot presence detector is driven by an internally

generated in-phase 19 kHz signal. Its pilot dependent DC

output voltage is fed to a threshold switch, which activates

the pilot indicator bit and turns the stereo decoder to stereo

operation. The same DC 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 pilot cancelled signal is fed to the matrix. There, the

side signal is demodulated and combined with the main

signal to left and right audio channel. Compensation for

roll-off in the incoming MPX signal caused by IF filters and

FM demodulator is typically realized by an external

compensation network at pin MPXIN, individual alignment

is achieved by I2C-bus controlled amplification of the side

signal (DAA). A smooth mono to stereo takeover is

achieved by controlling the efficiency of the matrix with

help of the SNC peak detector.

The matrix is followed by the FM noise suppression gates,

which are combined with FM single poles and High Cut

Control (HCC).

The single pole is defined by internal resistors and external

capacitors. From the gate circuits audio is fed to the

switchable de-emphasis, where the demodulated AM

stereo signal can be fed in. After de-emphasis the signal

passes to the output buffers and is fed to the radio input of

the source selector. For HCC, the time constant of the

single pole contained in the output buffer can be changed

to higher values. This function is controlled by an average

detector contained in the multipath and fading detector.

7.2 FM noise blanker

The input of the ignition noise blanker is coupled to the

MPXRDS input signal and to the LEVEL input. Both

signals are fed via separate 120 kHz filters and rectifiers to

an adder circuit. The output signal of the adder circuit is fed

in parallel to the noise detector and the interference

detector. The noise detector is a negative peak detector.

Its output controls the trigger sensitivity (prevention to

false triggering at noisy input signals) and the gain of the

MPX high-pass filter. The output of the interference

detector, when receiving a steep pulse, fires a monoflop,

contained in the pulse former circuitry. The time constant

of the monoflop is defined by an internal capacitor and its

output activates the blanking gates in the audio.

7.3 AM noise blanker

The AM noise blanking pulse is derived from the AM audio

signal which is fed into pin AMNBIN with the help of a

peak-to-average comparator. The blanking time is set by a

pulse former with external capacitor. The blanking pulse is

fed to the gate in the AM audio path and out to

pin AMHOLD to operate the gate built into the external

AM stereo processor.

7.4 Multipath/fading detection and weak signal

control

For FM signal quality dependent controls there is built-in a

combination of six detectors driven by the level information

direct, by the AC components on the level via a 20 kHz

band-pass filter (AM wideband) or the high notes present

at the FM demodulator output via a 60 kHz high-pass filter

(ultrasonic noise). The relation between DC level and the

AC components is programmable by the I2C-bus (2 bits

each). Output of level buffer, AM wideband detector and

ultrasonic noise detector are analog-to-digital converted

and readable by the I2C-bus.

2003 Feb 04

10

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