Daewoo DTR-14D3VG, Service Manual

Page: 35 / 73

33
Sound
The main functions are :
- Limiter
- PLL-Demodulator
- Pre-amplifier and mute
- Audio switch
- Volume controlled amplifier
• 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 amplifi-
er stages. The minimum input amplitude for limiting is typical 1mVrms. The input impedance is 8K
Ω
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 optimised 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 500m Vrms for a AM 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
Anaudio switch has been provided for full SCART function. The audio switch is controlled by the I
2
C 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.
In the Fixed Audio Volume mode, FAV, the deemphasis signal is fed directly to the sound output buffer. The gain then
is 1(0dB). This feature is mainly for further stereo processing where a fixed audio level is required. This saves an
external buffer at the deemphasis pin.
• Automatic Volume Levelling (AVL)
The Automatic Volume Levelling(AVL) circuit is an automatic gain control on the volume which regulates the audio
level to a constant sound level with non-standard TV signals (i.e. advertisement). The regulated output level can be
adjusted by means of the volume control. The AVL function can be activated via I2C-bus and is available for the
TDA8840/1/2/6/6A. By means of the external AVL capacitor the time constant can be chosen.
The AVL circuit works properly for an AVL input signal range of approx. 75-750m Vrms. (In external mode thus 150-
1500mV). This is the so called “boost range” and is 20dB. Within the “boost range” any desired stabilised output level
can be adjusted by means of the volume control. The characteristic is automatically adapted for selected crystals as
4.43MHz (PAL) and 3.57MHz(NTSC) standards by means of the 0/6dB attenuator controlled by the XA bit.
An external AVL capacitor, connected to pin 45, acts as a “memory” capacitor and integrates the audio peak signal.
Circuit Description