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SAA7284ZP Datasheet(PDF) 9 Page - NXP Semiconductors |
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SAA7284ZP Datasheet(HTML) 9 Page - NXP Semiconductors |
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9 / 40 page  1996 Oct 24 9 Philips Semiconductors Preliminary specification Terrestrial digital sound decoder for conventional intercarrier PLL-IF systems SAA7284 FUNCTIONAL DESCRIPTION DQPSK demodulation QUADRATURE MIXERS, BASEBAND FILTERS AND AUTOMATIC GAIN CONTROL (AGC) The DQPSK signal is fed into two differential input mixers, where it is mixed with quadrature phases generated by the carrier-loop quadrature VCO. The quadrature signals modulated onto the NICAM carrier are thus recovered. The mixers can be driven by either a single-ended or differential source. In single-ended mode, the device is driven directly from the sound IF down-converter into the DQPSK input pin, with the MIXREF pin decoupled. In differential mode, the signal is applied between the DQPSK and MIXREF pins. The outputs from the mixers are then fed into a pulse-shaping filter, and FM/vision filter stage with improved colour rejection to allow suitable performance with SAW filters. The signal from the filtering stages is then fed into the AGC, which ensures that the phase comparator gain remains constant, irrespective of the input signal level. This is important to maintain the stability of Costas loop PLL. AGC CONTROLLER The AGC controller monitors the I and Q channel signals at the input to the carrier loop-phase comparator and generates a reference voltage to set the AGC output level. EYE BUFFER A differential to the single-ended converter provides the baseband signal as an output at the pins CEYE and SEYE for the I and Q channels respectively for eye-height monitoring. BIT RATE CLOCK RECOVERY The I and Q channels are processed using edge detectors and monostables, which generate a signal with a coherent component at the data symbol rate. The outputs from the I and Q channel monostables are each compared with the clock derived from PCLK (364 kHz nominal), the resultant output is used to derive a 3-state control signal used to control two current sources at the CLKLPF output. This error signal is loop filtered and used to control the master clock oscillator. The bit rate clock, PCLK, and symbol clock are derived from the master clock. NICAM 728 decoding DECODING FUNCTIONS The device performs all decoding functions in accordance with the EBU NICAM 728 specification. After locking to the frame alignment word, the data is de-scrambled by application of the defined pseudo random binary sequence, and the device synchronizes to the periodic frame flag bit C0. The relevant control information and scale factor word is extracted, and with the integrated RAM the data is de-interleaved and the scale factor word is extracted, and expanded to 14 bits. Parity checking on the eleventh bit of each sample word is carried out to reveal any sound sample errors, which if detected are flagged, with the last good sample being held. Automatic muting Enable when AMDIS = LOW. The I2C-bus section has two registers which define an upper and lower limit for the automatic muting function. When the number of errors within a 128 ms period exceeds the number stored in the upper error limit register, then the automatic muting will switch the device output to the FM input, (dependent on the relevant control bits in the I2C-bus) and mute (set to zero) the data input to the filter (in that order). When the error count in a 128 ms period is less than the value stored in the lower error limit register then the data into the filter is uninterrupted, and the device output is switched back to the DAC (dependent on the value of the relevant control bits in the I2C-bus). During the muting operation the open-drain pin MUTE is pulled LOW and the AM bit in the status-byte is set HIGH. Figure 4 shows the dependency of the automatic muting function on error_count, RSSF, C4OV, output state and application mode. The automatic muting function, if enabled, will override user mute via the MUTE pin/bit. When the transmission is DATA format or currently undefined format (C3 = logic 1) the device will automatically switch to the FM inputs regardless of RSSF/C4OV states, and whether the automatic muting function AMDIS is enabled or disabled. |
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