TDA7402 Datasheet, PDF (32/59 Pages) STMicroelectronics – CARRADIO SIGNAL PROCESSOR

TDA7402
3.0 FUNCTIONAL DESCRIPTION OF STEREODECODER
Figure 25. Block diagram of Stereodecoder
The stereodecoder-part of the A619 (see Fig. 25) contains all functions necessary to demodulate the MPX-sig-
nal like pilottone-dependent MONO/STEREO-switching as well as ”stereoblend” and ”highcut”. Adaptations like
programmable input gain, roll-off compensation, selectable deemphasis time constant and a programmable
fieldstrength input allow to use different IF-devices.
3.1 Stereodecoder-Mute
The A619 has a fast and easy to control RDS-Mute function which is a combination of the audioprocessor’s Soft-
Mute 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 con-
dition after the audio-signal has been softmuted. Hence a checking of alternate frequencies could be performed.
Additionally the PLL can be set to ”Hold”-mode, which disables the PLL input during the mute time. To release
the system from the mute condition simply the unmute-command must be sent: the stereodecoder is unmuted
immediately and the audioprocessor is softly unmuted. Fig. 26 shows the output-signal V_O as well as the internal
stereodecoder mute signal. This influence 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 selected the decoder remains muted and the MPX-pin is connect-
ed to V_ref to avoid any discharge of the coupling capacitor through leakage currents. No further mute command
should be applied.
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