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TDA9875AH データシート(PDF) 11 Page - NXP Semiconductors |
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TDA9875AH データシート(HTML) 11 Page - NXP Semiconductors |
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11 / 96 page  1999 Dec 20 11 Philips Semiconductors Product specification Digital TV Sound Processor (DTVSP) TDA9875A 6 FUNCTIONAL DESCRIPTION 6.1 Demodulator and decoder section 6.1.1 SIF INPUT Two input pins are provided: SIF1 e.g. for terrestrial TV and SIF2 e.g. for a satellite tuner. For higher SIF signal levels the SIF input can be attenuated with an internal switchable −10 dB resistor divider. As no specific filters are integrated, both inputs have the same specification giving flexibility in application. The selected signal is passed through an AGC circuit and then digitized by an 8-bit ADC operating at 24.576 MHz. 6.1.2 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 a slow decay behaviour to prevent AGC oscillations. For AM demodulation the AGC must be switched off. When switched off, the control loop is reset and fixed gain settings can be chosen (see Table 15). The AGC can be controlled via the I2C-bus. Details can be found in the I2C-bus register definitions (see Chapter 10). 6.1.3 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. Access to the mixer control word registers is via the I2C-bus. When receiving NICAM programs, a feedback signal is added to the control word of the second carrier mixer to establish a carrier-frequency loop. 6.1.4 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. Both the European and Korean stereo systems are supported. 6.1.5 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. A selection can be made via the I2C-bus for B/G, D/K and M standard and for three different modes that represent different trade-offs between speed and reliability of identification. 6.1.6 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 pins PCLK and NICAM. A timing loop controls the frequency of the crystal oscillator to lock the sampling rate to the symbol timing of the NICAM data. 6.1.7 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 and the device will then synchronize to the periodic frame flag bit C0. Bit VDSP (see Section 10.4.1) indicates that the decoder has locked to the NICAM data and that the data is valid sound data. The status of the NICAM decoder can be read out from the NICAM status register by the user (see Section 10.4.2). Bit OSB indicates that the decoder has locked to the NICAM data. Bit C4 indicates that the sound conveyed by the FM mono channel is identical to the sound signal conveyed by the NICAM channel. The error byte contains the number of sound sample errors, resulting from parity checking, that occurred in the past 128 ms period. The Bit Error Rate (BER) can be calculated using the following equation: BER bit errors total bits ----------------------- error byte 1.74 × 10 5 – × ≈ = |
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