TDA7338_05 Datasheet, PDF (6/12 Pages) STMicroelectronics – STEREO DECODER

TDA7338
Figure 5. High Cut Control
f_c
(KHz)
3.18
(=50µs)
2
VR=3.6V
D95AU366
Figure 6. Stereo Blend
SEP
(dB)
50
40
V_R=3.6V
30
D95AU367
1
0
-0.5 -0.4 -0.3 -0.2 -0.1 0.0 V_HCC-V_R(V)
20
10
0
-0.40 -0.32 -0.24 -0.16 -0.08 V_SB-V_R(V)
Furthermore the conditions of the stereo separation (see fig.6) can be controlled through the signal applied
to Pin 16 (VSB). Both signal levels (VSB and VHCC) are referred to Pin 17 (VR), with the characteristic
that the control range is 11% of VR. By modifying the feedback resistor value of the output stages (Pin 9
- 10, Pin 11 - 12) the total gain of the stereodecoder can be modified.
Pin 7 and Pin 8 have an additional function. By pulling them to ground the VCO-OFF (Pin 7) and the MUTE
(Pin 8) function are activated. The MUTE signal disconnects the MPX-signal from the circuit, while in com-
bination with VCO-OFF also the output buffers are disconnected from the circuit. In this mode the output
buffers can be used for AM-stereo, cassette play back and other purposes.
4.2 AM Mono Mode
By selecting VCO-OFF (Pin 7 to GND) the VCO is switched off and the SB and HCC are disabled. The
deemphasis time constant is changed to 40µs (f_c = 4KHz).
4.3 DESCRIPTION OF THE NOISE BLANKER
In the normal automotive environment the MPX signal is disturbed by ignition spikes, motors and high fre-
quency switches etc.
The aim of the noise blanker part is to cancel the influence of the spikes produced by these components.
Therefore the output of the stereodecoder is switched off for a time of 40µs (average spike duration).
In a first stage the spikes must be detected but to avoid a wrong triggering on high frequency noise a com-
plex trigger control is implemented. Behind the trigger stage a pulse former generates the 40ms "blanking"
pulse. This duration of 40µs can be varied by changing the capacitor at pin 7.
4.3.1 Trigger Path
The incoming MPX signal is highpass-filtered, amplified and rectified (block RECT-PEAK). The second
order highpass-filter has a corner-frequency of 140KHz.
The rectifier signal, RECT, is used to generate by peak-rectification a signal called PEAK, which is avail-
able at the PEAK pin 8.
Also noise with a frequency >100KHz increases the PEAK voltage. The value of the PEAK voltage influ-
ences the trigger threshold voltage Vth (block ATC). The higher the noise level the higher the threshold.
Both signals, RECT and PEAK+Vth are fed to a comparator (block PEAK-COMP) which outputs a saw-
tooth-shaped waveform at the TBLANK pin 7. A second comparator (block BLANK-COMP) forms the in-
ternal blanking duration of 40µs. The noise blanker is supplied by his own biasing circuit (block BIAS-
MONO) to avoid any cross talk to the signal path (block BIAS-MONO).
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