tda6812-5 ETC-unknow, tda6812-5 Datasheet - Page 5

• Manufacturer Part Number: tda6812-5
• Description: Tv-stereo Processor
• Manufacturer: ETC-unknow

TDA 6812-5
Circuit Description
Signal Section
The dematrixing and switching of multichannel TV-sound signals are performed in the matrix and
switch section by the dual-carrier method. Crosstalk compensation is on the sound 1 input. The
compensation stage has a range of
3 dB with a smallest increment of 0.2 dB, and gain can also
be switched between 0 and 6 dB. In addition to the two inputs for the demodulated sound carriers,
there are three dual-channel SCART-inputs. The two matrix AF-inputs can be bypassed internally
so that decoded stereo signals of other systems (NICAM) can also be processed. The switch
section terminates in the SCART-output and signal paths for the loudspeaker and headphones
outputs. AF-inputs can be randomly switched to AF-outputs (8-6 matrix).
In the loudpeaker signal path there is an inital volume control with a range of 0/– 15 dB and an
increment of 1.25 dB. In conjunction with the main volume control that follows the tone control, very
high overdriving immunity is ensured. The switchable quasi-stereo section that follows produces a
o
stereo listening effect for mono signals through a 180
C phase shift at mid-range frequencies
(approx. 1 kHz) in one channel. The following bass control has an increment of 3 dB in its setting
range of + 15/– 12 dB. The cut-off frequency for each channel is set by an external capacitor. The
circuit for enlarging the stereo sound base can be cut in for stereo signals to make the aural
impression even more stereo-like by frequency-dependent antiphase crosstalk of 55 %. This works
with the same cut-off frequency as the bass control, but the function is largely independent. The
treble control, whose cut-off frequency is also set by an external capacitor, likewise has an
increment of 3 dB in a setting range of
12 dB. The main value control with maximum gain of 10
dB, which can be adjusted separately for L and R, terminates the loudspeaker signal path. 57 steps
with an increment of 1.25 dB mean a setting range of 71.25 dB. Functions like balance or loudness
are implemented by software setting of the appropriate tone and volume controls. In the tone-
2
control section there is a clipping detector that can be read on the I
C Bus and enables automatic
volume correction by the controller. After each reading the clipping bit is reset, which enables a
2
renewed check for clipping with each I
C Bus read operation.
The headphones signal path includes a volume control with joint L/R-setting. 32 increments of 2 dB
produce a range of 62 dB (31 x 2 dB = 62 dB).
Identification-Signal Decoder
The input of the identification-signal decoder consists of an operational amplifier for selectivity of
the pilot tone and its sidebands with an external LC-circuit. The signal is fed to a phase-independent
active bandpass filter of very narrow bandwidth (externally adjustable) that detects the presence of
the lower sideband of the pilot carrier modulated with the identification signal. The center frequency
of the filter is switched back and forth between dual and stereo by a multiplexer (software-controlled
timing). The multiplexer halts when a sideband is detected. This first "detected" criterion is freed
from noise by a digital integrator followed by a comparator and can then be read on the I
2
C Bus
(talker) as stereo or dual mode. The C controls the signal paths. All necesssary timing signals are
derived from a fast settling PLL synchronized by a reference frequency. This reference must be
sufficiently identical to the horizontal frequency, but no phase locking is necessary. This means
that it is possible to use the crystal-controlled frequency of 62.5 kHz that is often found in PLL-tuning
systems. As further alternatives there is an integrated crystal oscillator that requires a 4-MHz
crystal, or it is possible to use a clock frequency of 1 or 4 MHz.
Semiconductor Group
117