TDA7405 datasheet(31/56 Pages) STMICROELECTRONICS

31 / 56 page

31/56

TDA7405

The stereodecoder-part of the A631 (see Figure 25) 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.

4.1 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 26 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 26. Signals during stereodecoder's SoftMute

Figure 27. Signal-Control via SoftMute-Pin

TDA7405 датащи(PDF) 31 Page - STMicroelectronics