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TDA9874AH Arkusz danych(PDF) 10 Page - NXP Semiconductors |
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TDA9874AH Arkusz danych(HTML) 10 Page - NXP Semiconductors |
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10 / 68 page  1999 Dec 03 10 Philips Semiconductors Preliminary specification Digital TV sound demodulator/decoder TDA9874A 6.1.5 FM DECODING A 2-carrier stereo decoder recovers the left and right signal channels from the demodulated sound carriers. Both the European and Korean stereo systems are supported. Automatic FM dematrixing is also supported, which means that the FM sound mode identification (mono, stereo or dual) switches the FM dematrix directly. No loop via the microcontroller is needed. For highly overmodulated signals, a high deviation mode for monaural audio sound single carrier demodulation can be selected. NICAM decoding is still possible in high deviation mode. 6.1.6 FM IDENTIFICATION The identification of the FM sound mode is performed by AM synchronous demodulation of the pilot 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 standards, and for three different time constants that represent different trade-offs between speed and reliability of identification. A pilot detector allows the control software to identify an analog 2-carrier (A2) transmission within approximately 0.1 s. Automatic FM dematrixing, depending on the identification, is possible. 6.1.7 NICAM DEMODULATION The NICAM signal is transmitted in a DQPSK code at a bit rate of 728 kbits/s. The NICAM demodulator performs DQPSK demodulation and passes the resulting bitstream and clock signal to the NICAM decoder and, for evaluation purposes, to various pins. A timing loop controls the frequency of the crystal oscillator to lock the sampling instants to the symbol timing of the NICAM data. 6.1.8 NICAM DECODING 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 then synchronizes 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 (see Section 10.4.2). 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. The C4 bit indicates that the sound conveyed by the FM mono channel is identical to the sound 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) is approximately 0.0000174 times the contents of the error byte: 6.1.9 NICAM AUTO-MUTE This function is enabled by setting bit AMUTE LOW (see Section 10.3.12). Upper and lower error limits may be defined by writing appropriate values to two registers in the I2C-bus section (see Sections 10.3.14 and 10.3.15). When the number of errors in a 128 ms period exceeds the upper error limit, the auto-mute function will switch the output sound from NICAM to whatever sound is on the first sound carrier (FM or AM) or to the analog mono input. When the error count is smaller than the lower error limit, the NICAM sound is restored. The auto-mute function can be disabled by setting bit AMUTE HIGH. In this case clicks become audible when the error count increases. The user will hear a signal of degrading quality. If no NICAM sound is received, the outputs are switched from the NICAM channel to the 1st sound carrier. A decision to enable or disable the auto-mute is taken by the microprocessor based on an interpretation of the application control bits C1, C2, C3 and C4, and possibly any additional strategy implemented by the user in the microcontroller software. When the AM sound in NICAM L systems is demodulated in the 1st sound IF and the audio signal connected to the mono input of the TDA9874A, the controlling microprocessor has to ensure switching from NICAM reception to mono input, if auto-muting is desired. This can be achieved by setting the AMSEL bit HIGH and the AMUTE bit LOW (see Section 10.3.12). BER bit errors total bits ----------------------- error byte 1.74 × 10 5 – × ≈ = |
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