Abstract:
A wireless microphone system for the transmission and reception of high fidelity analog or digital audio information uses the 7 GHz frequency spectrum. In particular, the system has ultra-low latency and is capable of being used within existing receiver infrastructure allocated to program making and special events (PMSE). The system incorporates packet diversity in order to provide wider area coverage than conventional wireless microphone systems.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention concerns an ultra-low latency wireless microphone operating within the 7 GHz frequency spectrum, with primary use for program making and special events (PMSE). The wireless microphone integrates within current broadcast infrastructure as used for wireless camera video systems, or it may be used as a standalone wireless microphone system for live events. 
       BRIEF DESCRIPTION OF THE PRIOR ART 
       [0002]    Professional wireless microphones predominately operate within the ultra-high frequency (UHF) spectrum, typically over the frequency range 440-790 MHz. Working within this part of the spectrum poses a number of operational limitations as described below. 
         [0003]    UHF spectrum is shared with many other wireless systems. At medium or large broadcast events, this spectrum becomes extremely congested and requires intense co-ordination to avoid interference issues. 
         [0004]    As the allowable occupied bandwidth for a wireless microphone channel in the UHF spectrum is limited to around 100-200 kHz, this implies that for high quality digital audio transmission, compression techniques must be applied. Most audio compression techniques, such as MPEG, introduce time delays that are not desirable for live events. 
         [0005]    Some wireless microphone systems have used multi-level modulation techniques to permit the transmission of digital audio within the bandwidth of a UHF channel. Such implementations have an unfavorable impact on the wireless link and the efficiency of the microphone transmitter. 
         [0006]    Wireless camera systems, used to relay video images for live broadcast events, have recently transitioned from 2 GHz spectrum to 7 GHz. As a result, infrastructure is now available and already deployed to receive and process information over this frequency range. A 7 GHz wireless microphone would be able to use this common infrastructure which would simplify and subsequently reduce operational costs associated with live broadcast events. 
         [0007]    At large broadcast events multiple receive locations must be deployed to provide reliable wireless coverage. Wireless camera systems employ packet diversity techniques to allow automatic and seamless transfer between different receive locations. Current UHF based wireless microphone systems have to be manually switched. Using the same infrastructure will allow for package diversity techniques to be applied in a wireless microphone system. 
         [0008]    The present invention was developed in order to overcome these and other drawbacks of prior wireless transmission systems by providing a 7 GHz wireless microphone system. 
       SUMMARY OF THE INVENTION 
       [0009]    Accordingly, it is a primary object of the present invention to provide a wireless microphone system which operates over the 7 GHz spectrum and which has ultra-low latency and employs package diversity to permit the use of multiple receive sites for wide area coverage. A particular property of the invention is that by operating at higher frequencies than traditional UHF wireless microphone systems, the usual limitations of spectrum congestion, frequency allocation, bandwidth limitation and latency are overcome. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0010]    Other objects and advantages of the invention will become apparent from a study of the following specification when viewed in the light of the accompanying drawings, in which: 
           [0011]      FIG. 1  is a block diagram of a 7 GHz wireless microphone transmitter according to the invention; 
           [0012]      FIG. 2  is a block diagram of a 7 GHz diversity wireless microphone down-converter according to the invention; 
           [0013]      FIG. 3  is a block diagram of a wireless microphone receiver according to the invention; and 
           [0014]      FIG. 4  is a block diagram of a package diversity receiver according to the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]    In accordance with the present invention, the components of a handheld, battery powered microphone transmitter  2  are shown in  FIG. 1 . Since each country allocates different frequencies for wireless broadcast links, the transmitter is capable of operating over a wide frequency range, 6.0-8.0 GHz, to allow usage worldwide. The transmitter interfaces with a professional microphone capsule  4  which provides power to the capsule and an analog audio input to the transmitter unit. 
         [0016]    There are different opinions regarding the use of digital techniques in relation to audio fidelity. Accordingly, the transmitter includes digital  6  or analog  8  audio processing paths that are selectable by the program maker via a switch  10 . In the analog path  8 , a gain control device  12  and a pre-emphasis device  14  are applied in a conventional manner and the resultant analog signal is used to directly modulate a voltage controlled, temperature stable reference oscillator (VTCXO)  16 . A high frequency voltage controlled oscillator (VCO) and phase locked loop (PLL) combination  18  is phase locked to the modulated VTCXO  16  using a frequency synthesizer. The frequency modulated (FM) VCO output is then amplified by an amplifier  20  and filtered by a filter  22  prior to transmission. A 1/16 th  wavelength microstrip directional coupler  24  and a power level monitor circuit  26  are used to maintain a stable transmission power by dynamically adjusting the gain of the amplifier  20 . 
         [0017]    If the digital audio path  6  is selected, then the audio signal must be digitized by a digital encoding modulation device  28 , at which stage error correction, interleaving, non-linear companding and auxiliary information may be added to enhance the performance of the audio and wireless link Digital modulation is applied to a digital to analog converter (DAC)  30  which generates in-phase (I) and quadrature phase (Q) signals. A reconstruction filter  32  removes DAC alias frequencies, while an IQ modulator  34 , driven by the high frequency VCO  18 , directly up-converts the IQ signals to 7 GHz. The output of the IQ modulator  34  is then amplified by the amplifier  20  and filtered by the filter  22  using common components to the analog path. The directional coupler  24  and power level monitor  26  are used to maintain a stable transmit power by dynamically scaling the modulation level in the digital domain encoding modulation device. Finally, a bi-conical antenna element  36  is used as a wideband omni-directional radiating element. 
         [0018]    Since current professional wireless microphone transmitters operating in the UHF band are channel bandwidth limited to around 200 kHz, and digitized audio suitable for professional use typically requires &gt;1 Mbit/s, some form of audio compression and/or multi-level modulation must be applied. Audio compression techniques such as MPEG introduce time delays which are undesirable for live broadcasting. The use of multi-level modulation (e.g., 16-QAM) will have a detrimental impact on the robustness of the wireless link and furthermore, on the efficiency of the active radio frequency (RF) components in the transmitter chain due to the envelope of the modulation not being constant. By operating at higher frequencies, the channel bandwidth limitations are relaxed and the transmission of uncompressed audio is achievable. One drawback to use of higher frequencies is the reduction in the range of the wireless link and the increased occurrence of fading due to reflections and multi-path effects. These shortcomings are mitigated through the use of receive infrastructure incorporating spatial and packet diversity, which also form part of the present invention. 
         [0019]    A dual diversity down-converter  38  is shown in  FIG. 2 . It is used to receive and transfer the 7 GHz wireless microphone transmissions to UHF spectrum, at which stage the UHF signals may be integrated into already available distribution and receive infrastructure. The down-converter is also suitable for the reception of wireless video transmissions, commonly deployed at broadcast events. The down-converter includes two receive antennas  40  that are physically separated to provide spatial diversity in order to counteract channel fading issues. The down-converter is a wide-IF architecture in which a local oscillator  42  is at a fixed frequency and is supplied to both down-converter chains. The local oscillator is generated by phase locking a high frequency VCO  46  to a temperature stable reference oscillator (TCXO)  44  whose output is delivered to a band-pass filter  48 . The received signals from the antennae  40  are passed through a 7 GHz band-pass filter and limiter  50  which provides protection against damage or overload from high-level transmissions. The receive signal is amplified by an amplifier  52  and further filtered by a filter  54  prior to delivery to a down-conversion mixer  56  which mixes the receive signal with the output from the local oscillator  42  and converts the 7 GHz signals to UHF. A diplexer  58  is used to filter unwanted mixing components and reduce reciprocal mixing. The UHF signal is further amplified by an amplifier  60  and filtered by a filter  62  prior to distribution. 
         [0020]    The dual diversity down-converter  38  is usually located within ˜200 m of the wireless microphone transmitter and effectively forms a receive point for the wireless microphone system. Multiple dual-diversity down-converter units can be deployed to form a network and provide wider area coverage. When used in a UHF distribution system, the diversity down-converters may be powered via the UHF distribution cable to avoid the need for remote power. UHF distribution is provided to allow the use of existing UHF based analog audio infrastructure and to be compatible with wireless video systems. 
         [0021]    Where UHF distribution is not used, a UHF receiver block  64  shown in  FIG. 3  can be docked to the down-converter in order to provide an ethernet based interface. In the UHF receiver block, an IQ demodulator  66  performs a direct down-conversion of the UHF signals to baseband. The IQ demodulator  66  is driven by a VCO/PLL combination  68  that is phase locked to a TCXO  70 . The VCO/PLL operates at twice the incoming frequencies. The IQ baseband signal which includes audio data from multiple wireless microphones is filtered by a filter  72  and delivered to a gain control device  74  which applies gain to optimize the IQ level presented to the analog-to-digital converter (ADC)  76 . The output of the ADC passes through a digital processing block  78  where the data is converted to an internet protocol (IP) compliant format. 
         [0022]    As shown in  FIG. 4 , the resultant IP packets from n receive locations  64  are distributed to a packet diversity switch  80  where packet diversity is applied. The multiple receive locations are used to provide wide area coverage. Each of the receive locations, through the use of standard error checking techniques, identifies the integrity of the received digital audio information and accordingly marks which audio data packets are valid. The packet diversity switch, usually located within a broadcast production area, selects only valid data packets from each of the audio steams arriving from the different receive locations. The resulting signal is delivered to an audio processor  82  which reconstructs the audio information into an appropriate format for integration within standard audio broadcast infrastructure and produces an audio output  84 . 
         [0023]    While the preferred forms and embodiments of the invention have been illustrated and described, it will become apparent to those of ordinary skill in the art that various changes and modifications may be made without deviating from the inventive concepts set forth above.