Abstract:
A method and apparatus for decoding digital quadrature phase shift keying data includes converting and intermediate frequency signal from an analog signal to a digital signal and digitally processing the digital signal to detect and decode the digital quadrature phase shift keying and extract encoded data.

Description:
TECHNICAL FIELD OF THE INVENTION 
   The technical field of this invention is decoding NICAM encoded audio data. 
   BACKGROUND OF THE INVENTION 
   It is desirable to provide a Near Instantaneous Companded Audio Multiplex (NICAM) system that it can be integrated into a large SOC (system on a chip) efficiently. The method of this invention allows the demodulation and decoding of NICAM without requiring any phase locked loops (PLLs) or feedback. This invention can be built using only a standard A/D converter, a digital signal processor (DSP) core and logic gates. Because this method does not include a PLL or additional analog demodulation circuitry, it permits efficient implementation in an advanced digital process. 
   At the receiver, the tuner converts the video carrier and the F.M. sound inter-carrier to respective intermediate frequencies (IF) of 39.5 MHz and 33.5 MHz in the normal way. The NICAM carrier (which is 6.552 MHz away from the video carrier) is converted to an intermediate frequency of NICAM IF of 39.5 MHz-6.552 MHz=32.948 MHz or approximately 32.95 MHz. 
   This IF signal is demodulated by a digital quadrature phase shift keying (DQPSK) detector and applied to the NICAM decoder which reverses the transmitter encoding to recreate the 14-bit sample code words for each channel. A digital-to-analog converter reproduces the original analog two-channel, left and right sound waveforms. 
     FIG. 1  illustrates the basic elements of NICAM sound reception in a TV receiver. Antenna  110  receives radio frequency signals and supplies them to tuner  120 . Tuner  120  selects the desired radio frequency signal and supplies an intermediate frequency (IF) signal to special surface acoustic wave (SAW) filter  130 . SAW filter  130  separates the video and sound IF outputs. A sharp cut-off removes the two sound IFs, 33.5 MHz for mono and 32.95 MHz for NICAM, from the video 39.5 MHz carrier. SAW filter  130  generates separate outputs for the video and NICAM IF carrier. SAW filter  130  also provides for a very narrow peak at 39.5 MHz. Sound. IF demodulator  140  uses the 39.5 MHz pilot frequency to beat with the FM sound IF signal to produce 6 MHz FM mono audio signal and with the NICAM IF to produce 6.552 MHz DQPSK carriers. Sound IF demodulator  140  uses sharply tuned filters to separate the two sound carriers. The FM carrier goes to a conventional FM processing channel  155  for mono sound. The 6.552 MHz NICAM phase modulated carrier goes to a NICAM processing section. This includes three basic parts. DQPSK decoder  160  recovers the 728 kbit per second serial data stream from the 6.552 MHz carrier. NICAM decoder  170  de-scrambles, de-interleaves, corrects and expands the data stream back into 14-bit sample code words. Finally, digital-to-analog converter  180  reproduces the original analog signals for each channel. 
   DQPSK demodulator  160  works on the same principle as a frequency modulation (FM) detector. A variation in phase or frequency produces a variation in the direct current (DC) output. In the case of two-phase modulation, the DC output of the detector has two distinct values representing logic 1 and logic 0. However, in the case of quadrature, i.e. four-phase modulation, the output of the detector is ambiguous. The same output for a 90° phase shift is obtained as that for a phase shift of 270°. This is similar for phase shifts of 0° and 180°. In order to resolve the ambiguities, a second phase detector operating in quadrature (90°) is typically used. 
     FIG. 2  illustrates the main elements of a DQPSK demodulator  160  of the prior art. The input NICAM 23.95 MHz IF signal in input to band pass filter  150 . The output of band pass filter  150  supplies the inputs of both in-phase phase detector (PDI)  210  and quadrature phase detector (PDQ)  220 . The output of in-phase phase detector  210  supplies the input of low pass filter  215 . The output of quadrature phase detector  220  supplies the input of low pass filter  225 . The filtered outputs from the two phase detectors are the data I and data Q. These feed data recovery circuit  170  which reproduces the original serial data stream. In the NICAM standard this is a 728-bit serial bit stream. Carrier recovery block  230  recovers the 6.552 MHz reference carrier frequency from the in-phase filtered signal. Carrier recovery block  230  supplies this recovered carrier to in-phase phase detector  210  to beat with the input. Carrier recovery block  210  supplies 90° phase shifter  235  which supplies this phase shifted signal as the beat carrier to quadrature phase detector  220 . 
     FIG. 3  illustrates an example of the quadrature encoding of the NICAM standard. Two bits of data are encoded in a phase shift of the carrier. A 0° phase shift encodes the binary pair “00.” A 90° phase shift encodes the binary pair “01.” A 180° phase shift encodes the binary pair “10.” Finally, a 270° phase shift encodes the binary pair “11.” The decoding task is to unambiguously determine the transmitted phase shift to recover the encoded bit pair. 
   This prior art technique has disadvantages making construction of low-cost systems difficult. A DQPSK demodulator such as illustrated in  FIG. 2  typically requires a phase locked loop (PLL) in carrier recovery. In current technology it is difficult to construct a single integrated circuit including both the analog components needed for such as PLL and digital data processing circuits. A typical TV requiring NICAM demodulation will include a high-performance digital signal processor (DSP) for many image and audio tasks. Because of the difficulty of constructing a single integrated circuit including both a PLL and a DSP, more circuits are needed for the TV system. This results in increased cost. 
   SUMMARY OF THE INVENTION 
   This invention uses a feed-forward technique. This allows for variable and efficient partitioning between gates in the analog front end (AFE) subsection and DSP code. This permits optimizing between gate count of circuits and DSP MIPS. The optimization of this invention is easier than if the decoder used a feedback design. 
   Algorithmically, we will perform the NICAM carrier removal/demodulation and decoding using digital signal processing techniques. There are a couple of ways to implement this, but fundamentally, it will require immediate A/D conversion, logic gates on the front end for any of the high bit rate calculations (such as decimatation filtering) and then DSP software for the low bit rate and decision making calculations. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other aspects of this invention are illustrated in the drawings, in which: 
       FIG. 1  illustrates the basic elements of NICAM sound reception in a TV receiver (prior art); 
       FIG. 2  illustrates the main elements of a DQPSK demodulator of the prior art; 
       FIG. 3  illustrates an example of the quadrature encoding of the NICAM standard; 
       FIG. 4  illustrates in part the construction of a system performing NICAM decoding in accordance with this invention; 
       FIG. 5  illustrates a first example DQPSK demodulation that can be performed via a DSP; and 
       FIG. 6  illustrates a second example DQPSK demodulation that can be performed via a DSP. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   NICAM is a TV audio standard used in Europe and China. It involves digital modulation using DQPSK, very similar to current wireless networking or digital radio modulation. Current methods to demodulate NICAM operate in the analog domain, using a tight PLL feedback loop to recover the carrier. This technique is used because typical NICAM devices use an analog semiconductor manufacturing process. 
   At first glance, it would appear that doing the very high speed carrier removal in the analog domain and outputting the carrier-removed data to an A/D converter to be digitally decoded is a cost efficient DQPSK demodulation implementation. Using a signal processing algorithm to perform the carrier removal and demodulation at radio-frequencies (in the hundreds of MHz or GHz) requires significant DSP processing or very high speed gates. However, the NICAM carrier frequency is only 6.552 MHz. Current circuit densities in digital circuits, such as 130 nm, are sufficient to permit integration of a suitable A/D converter, specialized logic gates (ASIC logic) and a DSP core in the same process. Thus it is more cost effective in silicon to perform this algorithm completely in the digital domain. At the 130 nm process node, it is difficult to perform complex analog integration such as required for a carrier recovery PLL well. The amount of audio post processing required in modern TVs already require a high performance DSP core with large on-chip RAM. A digital signal processing based demodulation technique could perform both the traditional analog functions of carrier removal and audio post processing functions of a higher end TV audio system in a single integrated circuit more cost effectively. Without moving the NICAM demodulation to this single integrated circuit, either multiple devices (one analog, one digital) would be required or the amount of digital audio post processing that could be performed would be severely. 
     FIG. 4  illustrates in part the construction of a system performing NICAM decoding in accordance with this invention. Integrated circuit  300  performs the DQPSK demodulation and the NICAM decoding as well as other post-processing functions. Integrated circuit  300  includes analog to digital converter  310  which receives the 6.522 MHz signal from band pass filter  150 . The digitized sample output from analog to digital converter  310  passes to digital ASIC (Application Specific Integrated Circuit)  320 . Digital ASIC  320  is constructed to handle pre-processing and buffering tasks that occur too fast to be handled by DSP core  330  or are best performed in special purpose hardware. It is contemplated that digital ASIC  320  will have some controllable functions that are set by DSP core  330 . 
   DSP core  330  performs the major signal processing functions including DQPSK demodulation and NICAM decoding. In a typical embodiment of this invention DSP core  330  would handle other signal processing functions. DSP core  330  receives the preprocessed data from digital ASIC  320  in real time. DSP core  330  also sends signals to digital ASIC  320  to set modes, change parameters and the like. DSP core  300  is bidirectionally coupled to memory  340 . Memory  340  includes both read only memory (ROM) storing the program controlling DSP core  330  and random access read/write memory (RAM) which temporarily stores intermediate results during signal processing. DSP core  330  supplies converted digital data to digital to analog converter  180  which produces the two audio channel signals (Ch A and Ch B). Note that digital to analog converter  180  is embodied in integrated circuit  300 . DSP core  330  may be bidirectionally coupled to external memory  340  which is not a part of integrated circuit  300 . 
   There are various methods to do DQPSK demodulation using digital signal processing.  FIGS. 5 and 6  illustrate examples. 
     FIG. 5  illustrates an example DQPSK demodulation that can be performed via a DSP. The digitized NICAM intermediate frequency signal is subjected to a Fast Fourier Transform (FFT) in processing block  410 . Phase compare imaginary component processing block  420  drops the real component of the FFT output and uses the imaginary component to do a phase compare. Processing block  430  receives the detected phase comparison and ties it to the appropriate 2-bit bit pattern. 
     FIG. 6  illustrates a second example DQPSK demodulation that can be performed via a DSP. The digitized NICAM IF signal is subjected to a 16-point FFT in processing block  510 . The resulting complex frequency domain data is transformed from rectangular coordinates to polar coordinates in processing block  520 . Processing block  530  detects phase discontinuities that indicate one of the encoded 2-bit data pairs. Processing block  540  receives the detected phase discontinuity data and ties it to the appropriate 2-bit bit pattern. 
   Other and perhaps much better signal processing algorithms may also exist.