DESCRIPTION OF STEREODECODER
The stereodecoder part of the TDA7403 (see Fig.
16) contains all functions necessary to demodu-
late the MPX signal like pilot tone dependent
MONO/STEREO switching as well as
"stereoblend" and "highcut" functions.
Adaptations like programmable input gain, roll-off
compensation, selectable deemphasis time con-
stant and a programmable fieldstrength input al-
low to use different IF devices.
Stereodecoder Mute
The TDA7403 has a fast and easy to control RDS
mute function which is a combination of the
audioprocessor softmute and the high-ohmic
mute of the stereodecoder. If the stereodecoder
is selected and a softmute command is sent (or
activated through the SM pin) the stereodecoder
will be set automatically to the high-ohmic mute
condition after the audio signal has been soft-
muted.
Hence a checking of alternate frequencies could
be performed. To release the system from the
mute condition simply the unmute command must
be sent: the stereodecoder is unmuted immedi-
ately and the audioprocessor is softly unmuted.
Fig. 14 shows the output signal V
O
as well as the
internal stereodecoder mute signal. This influ-
ence of Softmute on the stereodecoder mute can
be switched off by setting bit 3 of the Softmute
byte to "0". A stereodecoder mute command (bit
0, stereodecoder byte set to "1") will set the
stereodecoder in any case independently to the
high-ohmic mute state.
If any other source than the stereodecoder is se-
lected the decoder remains muted and the MPX
pin is connected to Vref to avoid any discharge of
the coupling capacitor through leakage currents.
Input Stages
The Ingain stage allows to adjust the MPX signal
to a magnitude of about 1Vrms internally which is
the recommended value. The 4.th order input fil-
ter has a corner frequency of 80kHz and is used
to attenuate spikes and noise and acts as an anti-
aliasing filter for the following switch capacitor fil-
ters.
Demodulator
In the demodulator block the left and the right
channel are separated from the MPX signal. In
this stage also the 19 kHz pilot tone is cancelled.
For reaching a high channel separation the
TDA7403 offers an I2C bus programmable roll-off
adjustment which is able to compensate the low-
pass behaviour of the tuner section. If the tuner
attenuation at 38kHz is in a range from 20.2% to
31% the TDA7403 needs no external network be-
fore the MPX pin. Within this range an adjustment
to obtain at least 40dB channel separation is pos-
sible.
The bits for this adjustment are located together
with the fieldstrength adjustment in one byte. This
gives the possibility to perform an optimization
step during the production of the carradio where
the channel separation and the fieldstrength con-
trol are trimmed.
Deemphasis and Highcut
The lowpass filter for the deemphasis allows to
choose between a time constant of 50
µ
s and
75
µ
s (bit D7, Stereodecoder byte).
The highcut control range will be in both cases
t
HC
= 2
⋅
t
Deemp
. Inside the highcut control range
(between VHCH and VHCL) the LEVEL signal
is converted into a 5 bit word which controls the
lowpass time constant between t
Deemp
...3
⋅
t
Deemp
.
There by the resolution will remain always 5 bits
independently of the absolute voltage range be-
tween the VHCH and VHCL values.
The highcut function can be switched off by I2C
bus (bit D7, Fieldstrength byte set to "0").
PLL and Pilot Tone Detector
The PLL has the task to lock on the 19kHz pilo-
tone during a stereo transmission to allow a cor-
rect demodulation. The included detector enables
the demodulation if the pilot tone reaches the se-
lected pilottone threshold VPTHST. Two different
thresholds are available. The detector output (sig-
nal STEREO, see block diagram) can be checked
SOFTMUTE
COMMAND
STD MUTE
V
O
t
D97AU638
t
t
Figure 14. Signals during stereodecoder’s
softmute
TDA7403
13/25
Obsolete Product(s) - Obsolete Product(s)
