Patent Publication Number: US-2001000313-A1

Title: Direct digital synthesis of FM signals

Description:
FIELD OF THE INVENTION  
       1. The present invention relates to the generation of composite stereo signals for broadcasting in the FM frequency band. More particularly, the present invention relates to a novel circuit for direct digital synthesis of composite stereo signals for broadcast in the FM frequency band.  
       BACKGROUND OF THE INVENTION  
       2. In FM broadcasting, left and right stereo base band signals are low-pass filtered and combined to produce a composite stereo signal. The circuit that combines the left and right component signals and produces the composite stereo signal is called an exciter.  
       3. Once generated, the composite signal is used to drive an FM modulator which modulates a carrier wave in accordance with the composite signal. The modulated carrier wave is then broadcast using an FM antenna.  
       4. To be broadcast from an antenna, the modulated carrier wave must be an analog signal. For this reason, conventional systems have generated the composite stereo signal using analog equipment. However, there are a number of difficulties that arise in generating the composite stereo signal in the analog format. For example, low-pass filtering and sub-carrier stereo modulation are very complicated for an analog system. Mechanical filters may be used, but are large and bulky. Additionally, analog filters introduce phase distortions and group delay distortions into the resulting signal. These distortions are very difficult to correct.  
       5. The alternative is to generate the stereo composite signal in the digital format and then, eventually, convert the signal to an analog signal for broadcasting. With recent advances in the quality of digital signal processing hardware, including high speed, high precision A/D and D/A converters, an FM exciter using digital signal processing has a far superior performance than the counterpart analog system and costs much less.  
       6.FIG. 1 shows a typical digital signal processing system for a digital FM exciter. In FIG. 1, the left channel  101  provides a left analog audio signal which becomes the left component of the composite stereo signal. Similarly, the right channel  102  provides a right analog audio signal which becomes the right component of the composite stereo signal.  
       7. The left and right analog signals are respectively processed by anti-aliasing filters  104  and  105 . After filtering, the left and right signals are respectively converted from analog into digital signals by A/D converters  107  and  108 . The converted digital signals are provided to a digital signal processor (DSP)  109 .  
       8. Generally speaking, the DSP  109  combines the left and right signals into a composite digital signal. More specifically, the DSP  109  performs band limiting filtering, pre-emphasizing, left and right channel mixing, sub-carrier generation, sub-carrier modulation and Sin(x)/x compensation for the D/A converter. Additionally, the DSP  109  provides soft level limiting (soft clipping), loudness signal monitoring for analog and digital automatic gain control, and spectrum analysis for optimized system control and operation.  
       9. The composite digital signal output by the DSP  109  is then converted to an analog signal by D/A converter  111  and filtered through low pass filter  150 . The result is a composite analog base-band stereo signal  151  which may be used to modulate a carrier wave which is then broadcast by an FM antenna.  
       10. The drawbacks of this system result from the fact that the D/A converter  111  and the external analog FM modulator (not shown) must be of the highest quality, and therefore are very expensive. The high quality processing achieved by the front end A/D converters  107  and  108  and the DSP  109  will be lost if the D/A converter  111  and analog FM modulator (not shown) cannot match the performance of the DSP  109 .  
       11. Accordingly, there is a need in the art for a system that digitally generates a high quality analog stereo signal without making excessive demands on the D/A converter and analog FM modulator which must receive and prepare the stereo signal for broadcasting.  
       SUMMARY OF THE INVENTION  
       12. It is an object of the present invention to meet the above-described needs and others. Specifically, it is an object of the present invention to provide a signal generator which digitally modulates a carrier signal to produce a digital modulated signal which can be converted to an analog signal for broadcasting without the need for an analog modulator.  
       13. Additional objects, advantages and novel features of the invention will be set forth in the description which follows or may be learned by those skilled in the art through reading these materials or practicing the invention. The objects and advantages of the invention may be achieved through the means recited in the attached claims.  
       14. To achieve the stated and other objects of the present invention, the present invention may be embodied as a digital modulated signal generator having a digital signal processor for recieving and processing left and right signals from left and right signal channels to produce a composite base band signal; and a numerically controlled oscillator for recieving the composite base band signal and generating a modulated digitial carrier signal which is modulated in accordance with the composite base band signal. Preferably, the frequency of the numerically controlled oscillator is updated at a fraction of a clock signal of the numerically controlled oscillator.  
       15. The present invention may further include a digital-to-analog converter for converting the modulated digital carrier signal into a modulated analog signal. A band pass filter may be used for filtering the modulated analog signal to remove harmonic distortions created by the numerically controlled oscillator.  
       16. Preferrably, the digital signal processor includes six digital signal processing units, each of which has a different sampling rate. The first of these digital signal processing units receives and samples the left and right signals. The first digital processing unit then interpolates the signals with a base band filter to eliminate cross talk between the left and right signals; a pre-emphasis filter; a sampling speed-up converter; and an anti-aliasing filter.  
       17. The third of the digital signal processing units computes addition (L+R) and difference (L−R) signals from the left and right signals. The fourth of the digital signal processing units which may receive SCA data and modulate a sub-carrier with the SCA data.  
       18. If SCA data is used, the present invention may include an SCA error control circuit which governs the modulation of the sub-carrier, the SCA error control circuit including: a Reed-Solomon encoder; an inter-leaver connected to the Reed-Solomon encoder; a convolution and differential encoder connected to the inter-leaver; a base band shaping unit connected to the convolution and differential encoder; an RF unit connected to the base band shaping unit; a convolution and differential decoder connected to the RF unit; a de-inter-leaver connected to the convolution and differential decoder; and a Reed-Solomon decoder connected to the de-inter-leaver.  
       19. The present invention may also include a gain control unit and an analog-to-digital converter in each of the left and right signal channels. The gain control units provide a gain control signal to the respective analog-to-digital converters and to the digital signal processor.  
       20. The present invention also encompasses a method of generating a digital modulated signal by digitally modulating a digital carrier signal with a numerically controlled oscillator in accordance with a composite base band signal produced by a digital signal processor from left and right signals received from left and right signal channels. Preferrably, the method includes updating a frequency of the numerically controlled oscillator at a frequency lower than a frequency of a clock signal of the numerically controlled oscillator.  
       21. The method of the present invention may also include converting the modulated digital carrier signal into a modulated analog signal for broadcasting. A further step of filtering the modulated analog signal to remove harmonic distortions created by the numerically controlled oscillator with a band pass filter may also be included.  
       22. If the digital signal processor comprises six digital signal processing units, the method includes sampling with each of the digital signal processing units at a different sampling rate.  
       23. The present method may also include interpolating the left and right signals a plurality of times with the digital signal processor; and modulating a sub-carrier with SCA data with the digital signal processor which receives an input signal containing the SCA data. Where a sub-carrier is modulated with SCA data, the method may include controlling an SCA error with an SCA error control circuit.  
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     24. The accompanying drawings illustrate the present invention and are a part of the specification. Together with the following description, the drawings demonstrate and explain the principles of the present invention.  
     25.FIG. 1 illustrates a conventional system for digitally processing a composite stereo signal prior to modulation.  
     26.FIG. 2 illustrates a system for producing a digital modulated signal according to the present invention.  
     27.FIG. 3 illustrates the DSP of FIG. 2.  
     28.FIG. 4 illustrates the DSP  301  of FIG. 3.  
     29.FIG. 5 illustrates an SCA error control circuit.  
     30.FIG. 6 illustrates a second system for producing a digital modulated signal according to the present invention.  
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     31. Using the drawings, the preferred embodiments of the present invention will now be explained. The present invention provides an all digital FM radio frequency signal synthesizer which produces a composite stereo signal without requiring an analog carrier wave modulator.  
     32.FIG. 2 shows a block diagram of an embodiment of the present invention. As before, left and right stereo signals  101  and  102  are provided through anti-aliasing filters  104  and  105  and A/D converters  107  and  108  to a DSP  209 . The DSP  209  will be described in greater detail below with regard to FIG. 3. As shown in FIG. 3, DSP  209  includes six DSP units  301  to  306  each of which has a different sampling rate.  
     33. DSP  301 , which is shown in greater detail in FIG. 4, receives the right and left signals  300  from A/D converters  107  and  108 . A sampling unit  401  samples the input signals. DSP  301  preferably samples the input signals at a frequency of 47.5 KHz×2.  
     34. DSP  301  then processes the sampled signals  300  by first performing base band filtering with filter  402  to eliminate cross talks between the modulated signals. The base band filter  402  is preferably a 100-tap FIR filter.  
     35. The output of filter  402  is then input to pre-emphasis filter  403  for pre-emphasis filtering. The filter shape is defined by the time constant, either 75 microseconds or 50 microseconds are preferably used.  
     36. Finally, DSP  301  performs two-time sampling speedup conversion with converter  404 . This conversion adds one zero between the existing samples and doubles the sampling frequency to 95 KHz. However, the conversion also creates aliasing components in the image frequency. Accordingly, an anti-aliasing filter, preferably a 40-tap FIR low pass filter,  405  is used to remove the aliasing components. This low pass filter is constructed with a two-phase 20-tap filter to reduce the actual amount of computation. The result is an interpolated signal  406 .  
     37. DSP modules  302  to  306  continue to interpolate the signal. The sampling rate of DSP  302  is preferably 95 KHz×2. The sampling rate of DSP  303  is preferably 190 KHz×2. The sampling rate of DSP  304  is preferably 380 KHz×3. The sampling rate of DSP  305  is preferably 1.14 MHz×4. The sampling rate of DSP  306  is preferably 4.56 MHz.  
     38. DSP  303  also computes the L+R and L−R signal and adds a 19 KHz pilot sub-carrier and the modulated L−R channel to form the composite stereo signal. The attenuation of the base band filter at 19 KHz is 120 dB. The sub-carrier frequency of the double side band suppressed carrier modulation is 38 KHz. Any base band frequency content above 19 KHz creates cross talk between the sum and difference channels.  
     39. If data is also to be broadcast on the Subsidiary Communication Authorization band (SCA), the SCA data  114  is input to DSP  304 . DSP  304  can process and modulate the SCA data to a sub-carrier up to 99 KHz.  
     40. For high quality data broadcasting, an SCA error control circuit shown in FIG. 5 can be used. The error control circuit includes a Reed-Solomon encoder  501 . The output of the encoder  501  is input to an inter-leaver  502 . The signal from the inter-leaver  502  is passed through a convolution and differential encoder  503 . The encoded signal is input to a base-band shaping unit  504  and then an RF unit  505 . The signal is then decoded by a convolution and differential decoder  506 , passed through a de-inter-leaver  507  and decoded by a Reed-Solomon decoder  508 .  
     41. Returning to FIG. 2, the DSP  209  outputs a base band composite stereo signal  307 . This signal in input to a Numerical Controlled Oscillator (NCO)  110  for direct digital FM modulation. The instantaneous frequency of the NCO  110  is modulated by the composite stereo base band signal  307 . The frequency of the NCO  110  is instantaneously updated at a fraction of the NCO  110  clock speed. This method eliminates the need for expensive, high speed DSP processors and makes the direct digital stereo FM synthesizer practical.  
     42. It should be noted that limiting the frequency update rate to a fraction of the NCO  110  clock rate creates harmonic distortions. However, the harmonic content in the FM signal can be kept well below the main signal level if the sampling rate of the composite stereo signal is in the 1 MHz to 4 MHz range. Such low-level harmonic distortions can be removed by the an analog band pass filter  113 .  
     43. In order to produce a 88 to 108 MHz RF signal, for example, for CATV broadcasting, the clock of the NCO  110  should have a frequency greater than 216 MHz. If the base band signal is updated at the same frequency, the additional up conversion would require extremely fast DSP chips which are very expensive and not practical. Such a high speed frequency update rate can be avoided by using different sampling rates for the NCO  110  and the composite stereo signal  307 .  
     44. In the present invention, the sampling speed of the A/D converters  107  and  108  may be 47.5 KHz. After four times sampling speed up conversion, the clock rate is 10 times the sub-carrier 19 KHz. The generation of the pilot carrier is very convenient with this sampling speed. For high quality A/D conversion, a broadcast quality 64-time over-sampling 20-bit or 18-bit A/D converter can be used to achieve high dynamic range and high signal to noise ratio. One or two bits can also be allocated as head room for digital AGC control and soft clipping.  
     45. The modulated signal output by the NCO  110  is converted to an analog signal by D/A converter  111 . The quantization noise from the D/A converter  111  will be limited by the band pass filter  113  and further reduced when the FM signal is eventually demodulated.  
     46. The signal to noise and distortion performance of the present invention is greatly enhanced by moving the D/A converter  111  from base band processing to the FM RF stage. For a typical FM system, a 38.8 dB signal to noise ratio improvement can be achieved. Thus for a 70 dB output signal to noise ratio, the required D/A output signal to noise ratio is 31.2 dB. In practice, RF D/A converters can achieve 60 dB signal to noise and distortion ratio at very reasonable cost. With an additional analog or digital tunable band pass filter (not shown) following the low pass filter  113  in FIG. 2, a very high signal to noise ratio can be achieved.  
     47.FIG. 6 shows a second embodiment of the present invention. In FIG. 6, a CPU  112  is used to control the functioning of the NCO  110  and the D/A converter  111 . The CPU  112  also receives the SCA data  114  and provides it to the DPS  209 .  
     48. The embodiment of FIG. 6 also includes gain control units  103  and  106  respectively for the left and right signal channels  101  and  102 . These gain control units control the A/D converters  107  and  108 , and provide data to the DSP  209 .  
     49. The preceding description has been presented only to illustrate and describe the invention. It is not intended to be exhaustive or to limit the invention to any precise form disclosed. Many modifications and variations are possible in light of the above teaching.  
     50. The preferred embodiment was chosen and described in order to best explain the principles of the invention and its practical application. The preceding description is intended to enable others skilled in the art to best utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims.