Abstract:
Interference in a receiver caused by a beat frequency arising from mutual interference of local oscillation signals from other, proximately located receivers is prevented by changing over between an externally supplied clock signal and internal clock signal. The selected clock signal is used as the local oscillation signal of the receiver, thereby removing a difference between the mutually interfering local oscillation signals and preventing the beat interference from arising.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to preventing beat interference between receiving devices and, more particularly, to a receiving system which prevents beat interference between proximately situated receivers receiving a common radio signal. 
     An example of a receiver which receives radio frequency signals from a transmitter and converts them into an audio signal suitable for audio reproduction by, for example, a speaker, is shown in FIG. 3, wherein the radio signal Si is received via an antenna  2  of receiver  1 . The radio signal Si is filtered by band-pass filter  3 , amplified by high frequency amplifier  4  and filtered again by band-pass filter  5 . A mixer  6  generates a fixed band signal by mixing the filtered signal with a lower frequency local oscillation signal S 2  provided by a local oscillator  7 . The fixed band signal S 3  is further filtered by band-pass filter  8  and amplified by high-frequency amplifier  9 . This signal is, then, mixed in mixer  10  with another local oscillation signal S 4  generated by local oscillator  7  to produce an intermediate frequency signal of an intermediate frequency. An IF amplifier  11  amplifies this intermediate frequency signal and a detector  12  detects the amplified IF signal to generate the audio signal S 6  suitable for audio reproduction. 
     An example of local oscillator  7  is shown in FIG. 4, wherein an internal oscillator  13  generates an internal clock signal S 7  which is frequency divided by frequency dividers  14 ,  15  and coupled to respective phase locked loop (PLL) circuits  16 ,  17 . It will be appreciated that the internal clock signal S 7  may be supplied additionally to a control circuit (not shown) of a CPU or other processor as a reference clock. Each of the two PLL circuits in FIG. 4 have the same construction in this example and each includes a phase comparator  18  ( 21 ) which generates a phase difference signal indicating a difference in phase between the internal clock signal and a local oscillation signal generated by a voltage controlled oscillator (VCO)  20  ( 23 ) and fed back through a frequency divider  21  ( 24 ). A low pass filter  19  ( 22 ) band-passes this phase difference signal to VCO  20  ( 23 ) which generates the local oscillation signal S 2  (S 4 ). 
     Problematically, if two supposedly identical receivers include local oscillator  7 , the local oscillation signal generated by the local oscillator in, for example, receiver  1 A of FIG.  5  and interferes with the local oscillation signal generated by the local oscillator of proximately situated receiver  1 B, such as when receivers are used to provide useful information during a “walking tour”. This situation arises, for example, when the conductive housing of the receiver acts as an antenna and transmits the local oscillation signal. Attempts to shield the housing, or to insert an isolator between the receivers, or to split the antenna signal between the receivers  1 A,  1 B (FIG. 6) has been found inadequate to prevent the interference because some leakage of the local oscillation signal always occurs. 
     The problem described above is particularly noticeable when two local oscillation signals from different receivers have slightly different frequencies, which may be due to the physical characteristics of each receiver or the difficulty arising out of adhering to very close manufacturing tolerances. As shown by the frequency-domain graph of FIG. 7, for example, two local oscillation signals associated with receivers  1 A and  1 B may be set at the slightly different frequencies of 850 and 851 MHz. These local oscillation signals ( 1 A,  1 B) combine to create high frequency interference signals (a, b) near the frequencies of 852 and 849 MHz, respectively. When a third receiver ( 1 C) has a local oscillation signal at the frequency of 852 MHz, a difference (d) between this frequency and the frequency of the nearby interference signal (a) creates interference which manifests in the reproduced audio sound as a beat. 
     This “beat” interference arises conspicuously when receivers are situated proximate enough to receive the interfering signals. An example of this proximate situation is a tour group of people shown in FIG. 8A touring through a museum and carrying wireless receiver head-sets which receive a common radio signal of audio information about a viewed museum piece. Since these local receivers are tuned to receive the common radio signal, the generated local oscillation signals of slightly different frequencies are combined to generate mutual interference which is picked up by the other receivers, thus, causing beat interference. It will be appreciated that shielding the receivers by inserting an isolator or by splitting the antenna between a plurality of receivers in such a tour group is impractical because such measures would restrict the user&#39;s movement. 
     Since beat interference arises when local oscillation signals exhibit different frequencies, it is theoretically possible to prevent beat interference by using receiver components of precisely the same electronic characteristics for all receivers because this would cause the local oscillation signals in each receiver to have the same frequency. However, it is quite difficult and expensive to manufacture a plurality of receivers with the same electronic characteristics. Typically, an internal oscillator has a precision of ±5 [ppm], meaning that is such an oscillator tuned to generate 850 MHz will, in fact, generate 850 MHz±4.25 KHz. This problem is compounded when there are more than two receivers which need to be precisely manufactured, such as when a large group of users tour a museum with many receiver head-sets. Problematically, beat interference is audibly disturbing to such individuals, seriously diminishing the acceptance of such audio signal reproduction. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     It is an object, therefore, to provide an apparatus and method for preventing beat interference in the aforementioned environment. 
     It is a further object to provide an apparatus and method for preventing beat interference arising from mutual interference of a plurality of receivers. 
     It is an even further object to provide an apparatus and method for preventing beat interference caused by a difference in local oscillation signal frequencies among a plurality of proximate receivers. 
     In accordance with the above objectives, the present invention provides an apparatus and method for preventing beat interference among receivers by selecting either an external clock signal that is transmitted to all receivers or, if no external clock signal exists, selecting an internal clock signal generated internally by each receiver. With this invention, each receiver can receive the same external clock signal as other receivers and the local oscillation signal for that receiver may be set precisely to the local signal oscillation frequency thereby preventing beat interference from arising. 
     In addition, the present invention also provides a receiver system formed of a plurality of receivers coupled in parallel to form a relay. Each of the receivers selects the external clock signal transmitted thereto, and in the absence of the external clock signal selecting the internal clock signal, thereby setting each of the respective local oscillation signals to the same frequency to prevent beat interference arising from a difference of local oscillation frequencies. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above objects, features and advantages of the present invention will become apparent from the following detailed description of illustrative embodiments which is to be considered in connection with the accompanying drawings, in which: 
     FIG. 1 is a schematic diagram of a specialized internal oscillator of a receiver according to the present invention; 
     FIGS. 2A-E are timing diagrams for the signals generated by the circuit shown in FIG. 1; 
     FIG. 3 is a block diagram of a receiver of the prior art; 
     FIG. 4 is a block diagram of a local oscillator; 
     FIG. 5 is an illustration of proximate receivers that result in mutual interference of the local oscillation signals therefrom; 
     FIG. 6 is an illustration of using a common antenna in an attempt to prevent the mutual interference of the two receivers in FIG. 5; 
     FIG. 7 is a frequency-domain graph for the explanation of beat frequency arising from the mutual interference; 
     FIG. 8A is an illustration of a tour group with receiver/head-sets which incorporate the present invention; and 
     FIG. 8B is a schematic representation of a receiver system which incorporates the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, a receiver in accordance with the present invention for preventing beat interference will now be described. In the present invention, the receiver  1  of FIG. 3 generates local oscillator signals either from an internal crystal oscillator which is used to generate an internal clock signal or from an external clock signal that is transmitted to the receiver as part of the radio signal S 1 . The external clock signal S 10  is received with the radio signal at an input terminal  31  of a specialized internal oscillator  30  shown in FIG. 1, which acts as the aforementioned local oscillator. An external clock signal detector  40  detects whether the radio signal supplied to terminal  31  and buffered by buffers  32  and  33  includes the external clock signal S 10 . A voltage comparator  41  compares the output of detector  40  to a reference level and generates a switching signal S 11  as a result of such comparison. The switching signal S 11  triggers a reset signal generator  80  to reset a CPU  90 ; and also controls a selector  50  to switch between the external clock signal received at terminal  31  and an internal clock signal generated by an internal clock generator  60 . The voltage comparator also triggers a stop controller  70  to stop the generation of the internal clock signal by clock generator  60  when the external clock signal is detected. Depending upon the switching, either the external clock signal or the internal clock signal is buffered by buffer  36  to the CPU where the selected clock signal (CK 1 ) is used in timing the signal processing operations; and the selected clock signal is buffered by buffer  37  to the local oscillator generator  7  (FIG. 1) where the selected clock signal is used to generate the local oscillation signal. 
     The operation of the specialized internal oscillator of the present invention will be described with reference to the timing diagram of FIGS. 2A-2E. At the beginning of a cycle (time T 0 ), it is assumed the external clock signal S 10  is not transmitted to terminal  31 , as indicated by the low level state “L” of FIG.  2 A. Accordingly, the signal buffered to the external clock signal detector by buffers  32 ,  33  is also at the low level state “L”. At time T 1  it is assumed the external clock signal is present as a train of clock pulses and will be detected as will be explained. 
     The external clock signal detector  40  detects whether the external clock signal is present in the buffered signal supplied by buffer  33 . In the example of FIG. 1, the external clock signal detector includes a rectifying section including a capacitor C 1  which acts as a filter and rectifying diodes D 1  and D 2  which rectify the filtered signal to produce a DC signal therefrom. The DC signal is, then, smoothed by smoothing capacitor. C 2 , voltage divided by resistors R 1  and R 2  and smoothed by smoothing capacitor C 3  to be output as voltage V 1 . At time TO, no external clock signal is received and the external clock signal detector transfer a low level state “L” to its output as voltage V 1 . At time T 1 , on the other hand, the external clock signal formed of a train of clock pulses is detected and smoothed into a steady-state DC signal which is applied to the voltage comparator as voltage V 1 . 
     The voltage comparator compares the voltage V 1  to a reference voltage derived from voltage source VCC to produce the switching signal S 11 . In this example, the voltage comparator may be comprised of an operational amplifier with its noninverting input connected to a voltage divider formed of resistors R 3  and R 4 . The inverting input of the operational amplifier is coupled to receive the voltage V 1 ; and as is shown, feedback is provided via resistor R 5 . A load resistor R 6  is provided because the operational amplifier in this example is of the open-collector type and performance is improved by a load resistance pulled up by the reference voltage VCC. With such a voltage comparator, the switching signal S 11  is normalized to the reference voltage VCC when an external clock signal is present and to ground when there is no external clock signal. 
     At time T 0 , no external clock signal is present and the switching signal S 11  is set to ground (i.e., a low level state “ L”) as shown in FIG.  2 B. On the other hand, the switching signal S 11  is pulled up to the normalized value of VCC as a high level state “H” when the external clock signal is present at time T 1 . 
     The switching signal S 11  is, then, coupled to reset signal generator  80  that generates the reset pulse signal S 13  shown in FIG. 2C which resets the CPU at a time when the clock signal is switched between the external and internal clock signals. It is considered that switching between different clock signals disrupts the signal processing operations of the CPU. By providing the reset signal generator to reset the CPU at the precise time of switching, the present invention prevents this disruption from occurring. 
     The reset signal generator is, in effect, a pulse trigger formed of an exclusive NOR circuit which emits a pulse whenever its input logic signals change. As shown in FIG. 1, for example, whenever the switching signal S 11  switches from a low level state “L” to a high level state “H” (time T 1 ) the input of the exclusive NOR (XNOR) circuit connected to a capacitor C 6  remains low for a charging period while the input connected to a resistor R 11  (which receives the switching signal S 11 ) is set high almost immediately. During this time, the XNOR logic gate senses that neither of the inputs are the same and outputs a high level “H” as the reset pulse signal S 13  (FIG.  2 C). When the capacitor C 6  charges up to the high level, the XNOR inputs then are at the same state again and the XNOR logic gate outputs a low level state. The same is true for the opposite case where the switching signal switches back to a low level state “L”; i.e., the input connected to the resistor R 11  drops almost immediately to the low level state while the other input is still charged high by the capacitor C 6  and the XNOR logic gate outputs the reset pulse signal S 13 . After the capacitor C 6  discharges, both XNOR inputs are at the same level again and the reset pulse is set low. The resulting XNOR output signal is the reset pulse signal shown in FIG. 2C which resembles a trigger pulse. Capacitor  6  and resistor R 11  determine the width of reset pulse S 13 . 
     The switching signal S 11  is further coupled as a control signal to a switch  50  that couples the external clock signal S 10  buffered from buffer  34  and couples the internal clock signal generated by internal clock generator  60  (explained later) to output terminal  53 . The switch is controlled by the switching signal S 11  to switch to the external clock signal when such signal is detected by the external clock signal detector and to switch to the internal clock signal when the external clock signal is not present. The switch outputs the selected clock signal from output terminal  53  to both the CPU  90  via buffer  36  and to the local oscillator  7  (FIG. 3) via buffer  37  as the clock signal (CK 1 ). It will be appreciated that the clock signal output from buffer  37  is supplied to frequency dividers  14  and  15  shown in FIG.  4 . 
     When the external clock signal is not present, the internal clock signal is generated by the internal clock generator  60 . The internal clock generator includes a crystal oscillating element X 1  which is connected across grounded capacitors C 4  and C 5  to form a crystal oscillator. An inverting amplifier  61  provides amplification of the crystal oscillator output and a resistance R 7  is connected as a feedback resistor. A resistor R 8  limits the excitation level of the crystal oscillator element X 1 . The internal clock signal S 12  generated by this generator is supplied to switch  50 , via buffer  35 , for selection by the switch depending upon the level of the switching signal S 11 . 
     It will be appreciated that the internal clock generator would otherwise oscillate indefinitely whether or not the external clock signal is present. This could be problematic because the internal oscillation signal can be transmitted erroneously if it is not sufficiently shielded, thereby causing beat interference in other receivers. To prevent this from occurring, the present invention further provides a stop controller  70  that stops the operation of the internal clock generator when the switching signal S 11  indicates that the external clock signal is present. 
     According to the example of FIG. 1, the stop controller generates a stop signal when the switching signal S 11  is present and is supplied to resistors R 9  and R 10 . The switching signal is applied to the base of a transistor Q 1  which turns “on” when the switching signal is high “H” indicating that the external clock signal is present (time T 1 ). At this time, the transistor Q 1  is “on” to supply ground potential to the input of the inverter  61 , thereby turning “off” the inverter  61  and, consequently, the internal clock, shown at time T 1  in FIG.  2 D. 
     With the present invention, the external clock signal is selected over the internal clock signal to generate the local oscillation signal and to time the signal processing operations of the CPU when the external clock signal is present. In this manner, the local oscillation signal frequency for all receivers is set precisely to the external clock signal such that the receivers are not subject to beat interference. In addition, the CPU is reset each time the selected clock is changed between external and internal clocks in order to prevent a disruption in the signal processing operations. Furthermore, when the external clock signal is detected, the internal clock generator is suspended from generating the internal clock signal to ensure that this signal is not transmitted erroneously which would cause the beat interference. 
     The present invention is particularly suited for audio tour arrangements which employ a plurality of local receivers in the form of receiver/head-sets (FIG.  8 A). Each receiver/head-set may be coupled to a transmitter forming a transmitter-receiver pair. The transmitters are, in turn, coupled in common to the external clock signal to form a relay system. With this arrangement, the present invention provides a receiver system for tour systems with proximately situated head-sets that receive a common radio signal without interference of a beat frequency. It will be appreciated that the present invention is applicable not only to tour systems, but to other arrangements where a plurality of receivers in close proximity are tuned to receive a common radio signal, such as airplane transmission of entertainment programs, seminars, and cellular phone reception. 
     Although the present invention has been described in particular detail to the preferred embodiments, it is to be noted that the invention is not limited thereby, and that various changes and modifications may be affected therein by one skilled in the art without departing from the scope of the invention which is intended to be defined by the appended claims.