TDA7405 datasheet(31/57 Pages) STMICROELECTRONICS

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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 Vref 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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