TDA4481-D

This TDA4481-D configuration permits high-quality processing of audio carriers for all standards, providing separate inputs for the video and audio carriers. The audio carrier signal (FM or AM) is passed to two multiplying mixer arrangements via a 3-stage variable wide-band amplifier with levelled output signals. One mixer demodulates AM  and generates the gain control signal. The second mixer operates as an intercarrier demodulator and supplies the intermediate AF carrier.

The TDA4481-D required to form the intercarrier is decoupled in a prelimited and selected form from the demodulator tank of the intermediate video frequency circuit (TDA 4453 or TDA 4439) and led to the intercarrier mixer via a limiting amplifier. By virtue of the system, the Nyquist range of the IF input filter in the video channel affects the attainable AF signal-to-noise ratio. The audio FM IF carrier reaches a quadrature demodulator via an interconnected IF-filter and subsequent limiting amplifier. The resulting AF signal is led via a low-pass amplifier with increased level and the AF standard switch to the SCART switch. It is available at both AF outputs, regulated (pin 7) and unregulated (pin 9).

Pin 14 is provided to connect a de-emphasis capacitor (10 nF for 50 s). The de-emphasis is effective for both AF outputs during FM operation, but deactivated for AM operation.

A frequency TDA4481-D converter is inserted to mix the different possible sound frequency standards (4.5, 6.0, 6.5 MHz) into a uniform intermediate audio frequency, e.g. 5.5MHz. The VCO frequency (pin 10) can be controlled from the unlevelled AF output at pin 9 or the de-emphasis connection at pin 14.

The intercarrier and converter circuits as well as the FM demodulator are deactivated during AM modulation. FM  muting is also possible if a switching voltage is applied to pin 17.

All switching functions (standard, SCART, mute) can take  place with TTL-equivalent level.