TDA7405_04 Datasheet, PDF (31/57 Pages) STMicroelectronics – EQUALIZER CARRADIO SIGNAL PROCESSOR

TDA7405
The stereodecoder-part of the A631 (see Figure 28) contains all functions necessary to demodulate the MPX-
signal 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.
10.1 Stereodecoder-Mute
The A631 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. Figure 30 shows the output-signal VO as well as the in-
ternal 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.
Figure 30. Signals during stereodecoder's SoftMute
Figure 31. Signal-Control via SoftMute-Pin
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