Abstract:
An audio transmission system of an FM carrier frequency signal conveys an analog audio signal with minimum distortion, high signal to noise ratio, and minimum phase distortion. The system employs digital data processing blocks to generate a VHF FM carrier signal which is subsequently converted to an UHF FM carrier frequency signal for transmission wirelessly to a receiver for reproducing the audio signal.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the invention 
         [0002]    This invention relates to an audio transmission system for modulating an analog signal to generate an UHF FM radio frequency carrier signal. More particularly, it relates to an audio transmission system for providing an FM carrier frequency signal conveying analog audio signal with minimum distortion, high signal-to-noise ratio, and minimum phase distortion. 
         [0003]    2. Background Art 
         [0004]    Commonly in the transmission of a UHF FM carrier frequency signal, the left and right channel are firstly compressed and then multiplex (MPX) by using either a microprocessor, or a specific solid state integrated circuit to form the stereo composite signal. The multiplex signal contains information of the L+R, L−R and the 19 KHz pilot tone. This composite signal is further modulated into the carrier frequency by an oscillator. 
         [0005]    In order to provide stability to the transmission system, it is a common practice to use a free running dielectric resonator oscillator (DRO), a VCXO, or simply a VCO together with a phase lock loop (PLL) circuit. The merit of a DRO is that it has a high Q-factor, thus it provides a high signal-to-noise ratio (SNR); however, if the Q-factor of the oscillator is too high, it will deteriorate the audio frequency response and it is also most unfavorable for the producing the low frequencies in the signal. A further drawback of a DRO is that it is too susceptible to external interferences, such as temperature change, and/or ambient capacitance or inductance change, which causes the carrier frequency to drift, and thus adding noise to the system. 
         [0006]    More and more systems now employ a PLL system to stabilize the carrier frequency, which contains a VCO (K VCO /s), a loop filter (LF, Z(s)) and a phase detector (PD, Kφ), and a reference crystal clock. However, the stability of a PLL system requires careful tuning of the loop filter&#39;s phase margin and loop bandwidth. Furthermore, matching of the PLL can be time consuming and difficult to obtain. Therefore, designing  30  a PLL system for audio transmission is demanding in that the loop filter would limit the quality of its performance. It is a normal practice to tune the loop filter to a phase margin at 40-55 degrees. For an audio signal of 15 KHz or higher, a narrow loop bandwidth (ωc) is required. However, a narrower loop bandwidth will cause a higher error in the RMS phase, or sometimes the residual FM, in the signal. For the above reason, modulation of the audio signal to the carrier becomes difficult without at the expense of the signal-to-noise ratio (SNR). There is always a tradeoff among the signal-to-noise ratio (SNR), the lock time, the loop stability, and the frequency response. If the loop filter has a high stability to external interference i.e. having a wide loop bandwidth, the low frequency interference will not be modulated because the PLL system will track out the signal. This will result in a dull frequency response when it is received by the receiver. On the other hand, if loop bandwidth is too narrow, the RMS phase error will degrade the SNR. For the application of modulating an audio signal below 5 KHz, such as the traditional FM system, for example, of a telephone system, it does not create a major concern, since it is mainly employed for voice communication. Unfortunately, if a system is required to broadcast an audio signal up to 15 KHz or higher, as usually in the upper end frequency in an FM system, this type of system is inadequate. 
         [0007]    Another drawback of employing a PLL system in this application is that group delay of different frequency components contained in the audio signal would occur due to the loop response characteristic. As it is commonly known, the relative phase difference of an audio signal has an impact on the quality of the sound. Furthermore, the linearity of the VCO is also important in that the design of a very linear VCO is a difficult task and the cost for providing such a system is high. 
         [0008]    U.S. Pat. No. 5,272,525 to Robert L. Borchardt et al describes a wireless transmission system with particular emphasis on the use of an FM radio in the receiver to receive a signal above 900 MHz. U.S. Pat. No. 5,666,422 to Robert N. Harrison et al teaches the use of IR for the wireless transmission of audio signal for surround speakers. U.S. Pat. No. 5,299,264 to Larry Schatz et al describes the usage of a varactor modulator for transmission in the 900 MHz band. U.S. Pat. No. 5,666,658 and No. 6,215,981 both to Robert L. Borchardt et al describes the use of a ceramic resonator stabilized FM transmitter for audio signal transmission. U.S. Pat. No. 6,466,832 to Benjamin Zuqert et al describes the transmission in using digital modulation in which the audio signal is reconstructed in the receiver. U.S. Pat. No. 7,212,787 to Eric Wu et al further describes a wireless audio transmission system employing pulse width modulation. 
         [0009]    There are various methods developed for modulating a mono or stereo audio signal to a carrier frequency signal for transmission via a wireless media. However, due to the reasons described above, the performance of these methods is not satisfactory. Moreover, there are transmission systems using digital methods, but the structure of such digital systems is complex. Some systems employ a higher transmission frequency such as 2.4 GHz which is better in performance, but the cost of the material for constructing such systems is high so that they are not commercially viable in price. 
       SUMMARY OF THE INVENTION 
       [0010]    It is the object of the present invention to provide a transmission system of an FM carrier frequency signal by conveying an analog audio signal with minimum distortion, high signal-to-noise ratio, and minimum phase distortion. The transmitter of this system includes a single solid state device with a built-in digital signal processor (DSP) for direct digital synthesis (DDS) which is a technique for using digital data processing blocks as a means to generate an output signal having a frequency and tunable with reference to a clock having a fixed frequency. A solid state device is utilized as a synthesizer. It facilitates tuning under complete digital control; and it offers a high signal-to-noise ratio in the output signal. The transmitter also includes a local oscillator, a mixer for up-conversion, and at least a radio frequency power amplifier. The solid state device will first convert the analog signal to a digital signal. The DSP within the device will then digitally generate the stereo composite signal which is synthesized to a low frequency FM signal. The low frequency FM signal is then up-converted to a VHF FM carrier signal having a frequency range typically of 76 to 108 MHz. The VHF FM carrier signal is then further up-converted to an UHF FM carrier frequency signal of either 863 to 865 MHz or 902 to 928 MHz by a local oscillator. 
         [0011]    Change of channel in the transmission is achieved by either changing the frequency of the VHF FM carrier signal of the solid state integrated circuit or the local oscillator or both. 
         [0012]    A band pass filter or alternatively a low pass filter is added in the circuit to remove any undesired harmonics or inter-modulation signals during the up-conversion process. At least a radio frequency amplifier is also included for maximum allowable power delivered to the antenna of the transmitter. 
         [0013]    The transmission system of the present invention provides a means of transmitting a mono or stereo audio signal using UHF to offer a better performance than the conventional system, as well as less cost such that it is commercially lower in price. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    Other objects and advantages of the present invention will become apparent from the following detailed description of the preferred embodiments thereof in connection with the accompanying drawings in which 
           [0015]      FIG. 1  is a schematic block diagram showing the overall construction of the audio transmission system of the present invention. 
           [0016]      FIG. 2  is a graphical illustration showing the up-conversion process of the system of the present invention of utilizing the upper side frequency in which the local oscillator is oscillating at a frequency lower than the desired UHF FM carrier signal. 
           [0017]      FIG. 3  is a graphical illustration showing the method of changing channel in the present system by changing the transmitted frequency of the single chip IC in the up-conversion process. 
           [0018]      FIG. 4  is a graphical illustration showing another up-conversion process employing the lower side frequency in which the local oscillator is oscillating at a frequency higher than the desired UHF FM carrier signal. 
           [0019]      FIG. 5  is a graphical illustration showing the method of changing channel in the present system by varying the transmitted frequency of the single chip IC when using the lower side frequency. 
           [0020]      FIG. 6  is a schematic block diagram showing the single chip RF integrated circuit in the present system having a digital signal processor (DSP) operative for generating a VHF radio frequency carrier signal. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0021]    With reference to the drawings, the system of the present invention is best shown in  FIG. 1  illustrating the exemplary embodiment of the transmission of a stereo signal having a left audio channel input signal  10  and a right audio channel input signal  11 . The amplitude of the high frequencies of the left audio channel input signal  10  are increased in an optional pre-emphasis circuit  12  to provide a left channel pre-emphasized signal  13 , similarly the amplitude of the high frequencies of the right audio channel input signal  11  are increased in an optional pre-emphasis circuit  14  to provide a right channel pre-emphasized signal  15 . The left channel pre-emphasized signal  13  and the right pre-emphasized signal  15  are inputted into an RF circuit  16  to form a VHF FM carrier signal  17 . The left channel pre-emphasized signal  13  is refined in a low pass filter  18  which permits low frequency signal to pass but attenuates signals with frequencies higher than a pre-determined cut-off frequency. The filtered left channel pre-emphasized signal  13  is converted to a left channel digital signal  19  by an analog to digital converter (ADC)  20 . Similarly, the right channel pre-emphasized signal  15  is filtered by a low pass filter  21  which passes low frequency signal but attenuates signals with frequencies higher than the same pre-determined cut-off frequency. The filtered right channel pre-emp signal  15  is converted to a right channel digital signal  22  by another ADC  23 . Both the left channel digital signal  19  and the right channel digital signal  22  are inputted into a pre-emphasis multiplexer (MPX)  24  for multiplexing the two digital signals as well as increasing the amplitude of the higher frequency of the mixed signal to provide a composite digital signal with 19 KHz pilot tone. The composite digital signal is further processed by an internal digital signal processor (DSP) in a system-on-chip (SOC) IC  25  using direct digital synthesis (DDS) as a means to generate a frequency tunable output with reference to a fixed frequency clock generated by a crystal  26 . The DSP is utilized as a synthesizer to make the tuning under complete digital control and it offers a high signal to noise ration to the output signal. The signal is then converted to an analog signal by a digital to analog converter  27 . The digital signal processor (DSP) digitally converts the stereo signal to a digital composite signal and also synthesizes it to a low frequency FM carrier. The low frequency FM carrier is then up-converted by mixer  28  for converting it to a VHF FM carrier, typically at 76-108 MHz by a local oscillator  29  and frequency range of the VHF FM carrier is restricted by a band pass filter  30 . The VHF FM carrier is amplified by an internal amplifier  31  to produce the VHF FM carrier signal  17  which is further filtered by a band pass filter or low pass filter  32  before it is passed to an up-converting circuit  33 . The band pass or low pass filter  32  filters out any unwanted harmonics or inter-modulation products due to the non-linear behavior of the up-conversion by the mixer  28  and the amplifier  31 . 
         [0022]    The VHF FM carrier signal containing the frequency modulated information is further up-converted by a mixer  34  in the up-converting circuit  33  to an UHF FM carrier signal with its frequency controlled by a local oscillator  35 . The local oscillator  35  is preferably designed with a high Q factor so as to minimize excess noise in the up-conversion, although lower Q factor will not seriously affect the performance of the system. 
         [0023]    The up-converting circuit  33  is stabilized by a PLL system consisting a PLL synthesizer IC  36  a loop filter  37 , a local oscillator  35 , and a buffer amplifier  38  to optimize the circuit performance. In case of a PLL system for the local oscillator, wide loop bandwidth is preferred so that a minimum RMS phase error will be added in the up-conversion process. The oscillator can easily be designed to have the optimal performance at only a particular fixed frequency. 
         [0024]    As shown in  FIG. 2 , the up-conversion process utilizes the upper side frequency, and the local oscillator is oscillating at a frequency lower than the desired UHF FM carrier signal. 
         [0025]    The up-converted UHF FM carrier signal passes through a band pass filter  39  which removes unwanted harmonics and inter-modulation products. The desired UHF FM carrier signal is amplified by at least a RF tuned power amplifier  40  to provide the maximum allowable power to be delivered by the system. The amplified UHF FM signal is purified by a band pass filter  41  and is fed through a matching network  42  the antenna  43 . 
         [0026]    The operations of the RF circuit and the up-converting circuit are controlled by a microprocessor  48  having a user interface  49  in the form of LCDs, LEDs, or keys. 
         [0027]    If the intended transmitting frequency or the desired UHF FM carrier signal is f, the VCO oscillator  33  can conveniently be designed to oscillate at a frequency of f LO , therefore the up-converting circuit  31  will be programmed by the microprocessor  48  to transmit at f-f LO . Change of channel is achieved by either changing the frequency of the VHF FM carrier signal of the IC chip or the local oscillator  33  or both. 
         [0028]    As shown in  FIG. 3 , change of channel by changing the transmitted frequency of the IC chip can be achieved by simply programming the IC chip at a different frequency. In this process, the oscillating frequency of the local oscillator can be kept unchanged. 
         [0029]    As shown in  FIG. 4 , another up-conversion process can be carried out by using the lower side frequency. In this case, the local oscillator will be oscillating at a frequency higher than the desired UHF FM carrier signal. 
         [0030]    As shown in  FIG. 5 , change of channel can be achieved by changing the transmitted frequency of the IC chip using the lower side frequency. The process is similar to the up-conversion process of using the upper side frequency as shown in  FIG. 2 . 
         [0031]    An embodiment of the digital signal processor (DSP)  25  is shown in  FIG. 6  for generating the VHF frequency carrier signal in reference to a crystal  50 . The VHF carrier signal is fed to an LC network  51  consisting of a series of capacitors and inductors as shown. A band pass filter is used in this case to filter any unwanted frequency from reaching the mixer  34 . The left and right audio signals  10  and  11  are directly fed to the digital signal processor (DSP)  25 . 
         [0032]    In the case of a mono audio signal, multiplexing is not required at the pre-emphasis multiplex  24  and the mono audio signal may be inputted directly into the DSP  16 . 
         [0033]    The UHF FM signal resulted from the present invention can be acquired by a receiver tuned to the particular frequency and with the necessary bandwidth. 
         [0034]    It can be understood from the above description of the preferred embodiment that the FM signal generated by the present invention is synthesized completely by digital means and it has a much higher signal to noise ratio and it can provide a much better sound quality than the signal generated by the conventional system. 
         [0035]    Various modifications can be made without departing from the spirit of the invention or the scope of the appended claims. The illustrated embodiment set forth in this disclosure is given as an example and is in no way final or binding. In view of the above, it will be seen that several objects of the invention are achieved and other advantages are obtained. As many changes could be made in the exemplary embodiment without departing from the scope of the invention, it is intended that all matter contained in the above description shall be interpreted as illustrative and not in a limiting sense.