5
Sound IF Input:
The sound IF amplifier consists of three AC-coupled differential amplifier stages each with approximately 20dB gain.
At the output of each stage is a multiplier for gain controlling. The overall control range is approximately -6 to +60dB and the
frequency response (-3dB) of the IF amplifier is approximately 6 to 70MHz. The steepness of gain control is approximately 10mV/dB.
IF AGC:
The automatic gain control voltage to maintain the AM demodulator output signal at a constant level is generated by a mean
level detector.The AGC-detector charges and discharges the capacitor at pin 3 controlled by the output signal of the
AM-demodulator compared to an internal reference voltage.The maximum charge/discharge current is approximately 5 mA.
AM-demodulator
The IF amplifier output signal is fed to a limiting amplifier (two stages) and to a multiplier circuit.
However the limiter output signal (which is not any more AM modulated) is also fed to the multiplier, which provides AM
demodulation (in phase demodulation). After lowpass filtering (fg @ 400kHz) for carrier rejection and buffering,
the demodulator output signal is present at pin 6.
Audio Switch
This circuit is an operational amplifier with three input stages and internal feedback network determining gain (0dB) and
frequency response (fg @ 700kHz). Two of the input stages are connected to pin 7 and pin 9, the third input stage to an
internal reference voltage. Controlled by the switching pins 10 and 12, one of the three input stages can be activated and a
choice made between two different AF signals or mute state. The selected signal is present at pin 8. The decoupling capacitors
at the input pins are needed, because the internally generated bias voltage for the input stages must not be influenced
by the application in order to avoid DC-plop in case of switching.
Reference Circuit:
This circuit is a band gap stabiliser in combination with a voltage regulation amplifier, which provides an internal reference voltage
of about 3.6V nearly independent from supply voltage and temperature. This reference voltage is filtered by the capacitor at
pin 4 in order to reduce noise. It is used as a reference to generate all important voltages and currents of the circuit.
For application in 12V power supply concepts, there is an internal voltage divider in combination with a Darlington transistor in
order to reduce the supply voltage for all IC function blocks to approximately 6V.
6. DIGITAL TV SOUND PROCESSOR TDA9875
The TDA9875 is a single-chip Digital TV Sound Processor.
Supported standards are M, B/G, D/K, I and L.
Description of the demodulator and decoder section:
SIF Input:
Two input pins are provided, SIF1 e.g. for terrestrial TV and SIF2 e.g. for a satellite tuner. The selected signal is passed through an
AGC circuit and then digitized by an 8-bit ADC operating at 24.576MHz.
AGC:
The gain of the AGC amplifier is controlled from the ADC output by means of a digital control loop employing hysteresis.
The AGC has a fast attack behaviour to prevent ADC overloads and slow decay behaviour AGC oscillations.
For AM demodulation AGC must be switched off.
Mixer
The digitized input signal is fed to the mixers, which mix one or both input sound carriers down to zero IF. A 24-bit control word for
each carrier sets the required frequency.
FM and AM Demodulation
An FM or AM input signal is fed via a band-limiting filter to a demodulator that can be used for either FM or AM demodulation.
Apart from the standard (fixed) de-emphasis characteristic, an adaptive de-emphasis is available for encoded satellite programs.
A stereo decoder recovers the left and right signal channels from the demodulated sound carriers.
FM Identification:
The identification of the FM sound mode is performed by AM synchronous demodulation of the pilot signal and narrow-band detection
of the identification frequencies. The result is available via the I²C-bus interface.
NICAM Demodulation:
The NICAM signal is transmitted in a DQPSK code at a bit rate of 728 kbit/s. The NICAM demodulator performs DQPSK
demodulation and feeds the resulting bitstream and clock signal onto the NICAM decoder and, for evaluation purposes, to PCLK
(pin 1) and NICAM (pin 2).
NICAM Decoder:
The device performs all decoding functions in accordance with the EBU NICAM 728 specification. After locking to the frame
alignment word, the data is descrambled by applying the defined pseudo-random binary sequence; the device will then synchronise
to the periodic frame flag bit C0.
The status of the NICAM decoder can be read out from the NICAM status register by the user. The OSB bit indicates that the
decoder has locked to the NICAM data. The VDSP bit indicates that the decoder has locked to the NICAM data and that the
data is valid sound data.
7. SOUND OUTPUT STAGE TDA2614/TDA2615/TDA2616Q
TDA2614 is used as the AF output amplifier for mono applications. It is supplied by ±12VDC coming from a separate winding in the
SMPS transformer. An output power of 2*6W (THD=0.5%) can be delivered into an 8ohm load.
TDA2615 is used as the AF output amplifier for stereo applications. It is supplied by
±12VDC coming from a separate winding in the SMPS transformer. An output power of 2*6W (THD=0.5%)
can be delivered into an 8ohm load.
TDA2616Q is used as the AF output amplifier for stereo and dolby prologic applications. It is supplied by ±16VDC coming from
a separate winding in the SMPS transformer. An output power of 2*12W (THD=0.5%) can be delivered into an 8ohm load.
