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3DQDVRQLF
1.1.3 Sound
See also the related block diagram as well as the diagrams at the end of the report.
The main functions are:
* Limiter
* PLL-Demodulator
* Pre-amplifier and mute
* Audio switch
* Volume controlled amplifier
These functions will be described next.
* Limiter
The sound carrier signal is supplied to this limiter input via an external bandpass filter. This external
bandpass filter is used for selectivity, the internal filter at the pin is used for noise reduction. The limiter
consists of AC coupled amplifier stages. The minimum input amplitude for limiting is typical 1mVrms.
The input impedance is 8K5 in parallel with 5pF.
* PLL-Demodulator
Sound demodulation is achieved by a PLL FM-demodulator and does not need any external
alignment.
The PLL has been optimized for a low S/N ratio with still an acceptable power consumption. The PLL
catching range is 4.2 - 6.8MHz, which is suitable for all multistandard applications.
* Pre-amplifier and mute
The pre-amplifier output signal available at the deemphasis pin can be used for SCART application. At
this pin the deemphasis capacitor has to be connected. The output level is 500mVrms for a FM swing
of 50kHz.
A pre-amplifier with DC feedback has been provided. The DC component of the deemphasis signal is
always 3V, also during sound mute. Sound mute plop is therefore minimized.
When no video signal is identified the deemphasis output is automatically muted.
* Audio switch
An audio switch has been provided for full SCART function. The audio switch is controlled by the I2C
bus Source Select INA/INC. For positive modulation, MOD, the external sound input is automatically
selected for an external AM sound demodulator.
* Volume controlled amplifier
The volume control is active for both internal and external audio signals. The nominal gain is +9dB and
minimal -71dB, which gives a total control range of 80dB minimal.
8
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