Patent Document

BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    This invention relates to microphones, particularly for use in a one-bit digital audio system.  
           [0003]    2. Description of the Prior Art  
           [0004]    A microphone generally produces a low-level electrical signal in response to audible sound levels around the microphone. This low level electrical signal is then conducted along an electrical cable to subsequent processing apparatus-for example, a digital signal processing device such as an audio mixing console, where it is converted into a digital signal for further processing.  
           [0005]    The low-level electrical signal is subject to induced noise and interference during transmission along the cable.  
         SUMMARY OF THE INVENTION  
         [0006]    This invention provides a microphone comprising:  
           [0007]    a housing;  
           [0008]    an acoustic-to-electrical signal transducer disposed within the housing, the transducer being operable to generate an analogue audio signal; and  
           [0009]    a one-bit analogue-to-digital converter disposed within the housing for converting the analogue audio signal into a one-bit digital audio signal for transmission to other, external, audio processing apparatus.  
           [0010]    The invention can provide a microphone for use in a one-bit digital audio system having reduced noise and distortion because the low-level microphone output signals are supplied directly to the ADC without having to pass along long lengths of cable.  
           [0011]    If the microphone is used in an audio system employing one-bit signals throughout, it avoids the need to carry out multi-bit PCM encoding at the system&#39;s input, with the attendant loss of bandwidth, added time delay and truncation errors in conversion. In particular, the time delay resulting from a PCM encoding stage is particularly troublesome for singers wearing headphones, where some part of the sound they hear is from the microphone/ADC combination; the time delay of a typical PCM ADC is about 1 millisecond. This delay corresponds to 30 cm propagation in air, which is similar enough to the mouth-ear distance to lead to comb filtering effects.  
           [0012]    A further feature can also alleviate any problems due to feedback, given that the audio band component of a one-bit digital audio signal is highly correlated with the input supplied to the ADC and the output of the line driver could easily be picked up by the sensitive input circuitry of the ADC.  
           [0013]    To alleviate any such feedback problems, the one-bit signal for transmission on the coaxial cable is decorrelated from the audio signal by scrambling the digital data before supplying it to the cable driver. At the other end of the cable, a corresponding descrambler can be used.  
           [0014]    As a further preferred feature, the scrambling process could be arranged so that if the microphone is powered down or unplugged, the descrambler (which now receives data representing a constant stream of zeroes) would generate a substantially equal distribution of ones and zeroes—a one-bit representation of digital silence.  
           [0015]    The above, and other objects, features and advantages of this invention will be apparent from the following detailed description of illustrative embodiments which is to be read in connection with the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    [0016]FIG. 1 is a schematic diagram of a microphone;  
         [0017]    FIGS.  2  to  4  are more detailed schematic diagrams of respective embodiments of a microphone connected to an input stage of a digital signal processing apparatus; and  
         [0018]    [0018]FIGS. 5 a  and  5   b  illustrate a scrambler and complementary descrambler respectively. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0019]    Referring now to the drawings, FIG. 1 is a schematic diagram of a microphone comprising a housing  10  in which an audio transducer (e.g. a microphone insert)  20  and a one-bit digital signal processor  30  are disposed. A transmission line such as a coaxial cable  40  carries signals from (and, in some embodiments, to) the signal processor  30 .  
         [0020]    FIGS.  2  to  4  are more detailed schematic diagrams of respective embodiments of the microphone connected to an input stage  50  of a digital signal processing apparatus  60 .  
         [0021]    In FIGS.  2  to  4 , the following reference numerals are used to denote similar, though not identical, parts:  
         [0022]    [0022] 30 ,  30 ′,  30 ″ digital signal processor within the microphone housing  
         [0023]    [0023] 50 ,  50 ′,  50 ″ input stage  
         [0024]    [0024] 60 ,  60 ′,  60 ″ digital signal processing apparatus  
         [0025]    The digital signal processing apparatus  60  could be, for example, an audio mixing console or effects unit operable to process one-bit digital audio signals.  
         [0026]    Starting therefore with FIG. 2, the signal processing apparatus  60  includes a clock generator  110  which generates a clocking signal to which the one-bit digital audio signal from the microphone is to be synchronised. The clock generator supplies the clock signal to the input stage  50  and also, via the coaxial cable  40  (but in a “reverse” direction), to a clock recovery and power supply unit  120  within the signal processor  30  of the microphone.  
         [0027]    The clock recovery and power supply unit  120  generates two output signals: one is a straightforward clocking signal supplied to a one-bit analogue-to-digital converter (ADC)  130 , and the other is a power output which supplies operating power to the one-bit ADC  130 , a line driver  140  and (if necessary) the audio transducer  20 .  
         [0028]    The power supply is derived from the clocking signal carried by the coaxial cable  40  by rectifying and smoothing the clocking signal. This avoids the need for a conventional “phantom power” arrangement, although conventional phantom power could be used instead if desired.  
         [0029]    In operation, therefore, the audio transducer  20  generates an analogue-audio output signal dependant on sound levels in the vicinity of the audio transducer  20 . The one-bit ADC  130  converts the analogue signal into a one-bit digital signal in accordance with the clock supplied from the clock recovery and power supply unit  120 . The line driver  140  then amplifies the output of the one-bit ADC  130  to a suitable level for transmission via the coaxial cable  40 .  
         [0030]    At the digital signal processing apparatus  60 , the input stage (synchronised by the clock generator  110 ) terminates the coaxial cable  40  and “cleans up” the waveform of the digital signal transmitted via the coaxial cable  140  by using a thresholder (e.g. a Schmidt trigger) to detect whether the signal on the coaxial cable  40  is above or below a threshold signal level, thereby generating a “clean” digital output for subsequent processing.  
         [0031]    A second embodiment is illustrated in FIG. 3, where the digital signal processing  30 ′ includes a clock generator  210  which supplies a clocking signal to the one-bit ADC  130  as before. Also, as in FIG. 2, the line driver  140  amplifies the output of the one-bit ADC  130  to a suitable level for transmission along the coaxial cable  40 .  
         [0032]    In FIG. 3, the clock generator  210 , the one-bit ADC  130 , the line driver  140  and (if necessary) the audio transducer  20  are powered either by batteries or by conventional phantom powering.  
         [0033]    At the recipient digital signal processing apparatus  60 , the signal on the coaxial cable  40  is passed to a clock recovery unit  220  which recovers the clocking rate of the one-bit digital signal by synchronising a phase-locked-loop to the bit rate of the one-bit signal. The input stage  50 ′ is synchronised by the output of the clock recovery unit  220 .  
         [0034]    A further synchronising stage may be required if the one-bit signal from the microphone is to be processed along with one-bit signals synchronised to other clocking sources (e.g. from other microphones).  
         [0035]    [0035]FIG. 4 illustrates a third embodiment which addresses three potential problems with the embodiment of FIG. 3.  
         [0036]    These problems are (i) it not always easy to recover a clocking signal from a one-bit digital audio signal; (ii) since a low-pass filtered version of a one-bit digital audio signal can be considered as a representation of the analogue audio signal, there is the danger that the relatively high signal levels output from the line driver  140  will be fed back (e.g. by induction) to the relatively low signal level input of the one-bit ADC  130 , leading to possible feedback problems potentially causing non-linear distortion; and (iii) if the microphone is unplugged or powered down, the thresholder in the input stage  50  could output a continuous sequence of the same bit value (e.g. zero)—which represents a very large signal level indeed in the one-bit digital domain.  
         [0037]    These potential problems are addressed in the embodiment of FIG. 4 by incorporating a status scrambler  310  in the microphone and a corresponding de-scrambler  320  at the recipient digital signal processing apparatus.  
         [0038]    The scrambler  310  and de-scrambler  320  will be described in detail below with reference to FIG. 5, but, briefly, their purpose is to ensure that the data transmitted along the coaxial cable  40  is relatively de-correlated from the audio signal supplied to the one-bit ADC  130 . This can reduce the problems of feedback between the output of the line driver  130  and the input to the one-bit ADC  130 . Also, the digital content of the data signal can be changed so that it is easier for the clock recovery circuit  220  to recover a clocking signal from the scrambled signal.  
         [0039]    [0039]FIG. 5 a  schematically illustrates one embodiment of the scrambler  310 , and FIG. 5 b  schematically illustrates one embodiment of the complementary descrambler  320 .  
         [0040]    In FIG. 5 a,  the signal to be scrambled is supplied as one input to a two-input exclusive-OR gate  500 . The output of the exclusive-OR gate  500  is fed through a series of n one-bit delays  510 —where n could be, for example, between 8 and 16. The output of the final delay of the chain forms the scrambled data output and is also fed back to provide the second input to the exclusive-OR gate  500 .  
         [0041]    Similarly, in FIG. 5 b,  the input data to be descrambled is supplied in parallel to the first of a chain of m one-bit delays (where m is the same as the value of n in FIG. 5 a ) and to one input of a two-input exclusive-OR gate  530 . The other input of the exclusive-OR gate  530  receives the output of the chain of delays  520 . The output of the exclusive-OR gate  530  forms the descrambled data.  
         [0042]    Although illustrative embodiments of the invention have been described in detail herein with reference to the accompanying drawings, it is to be understood that the invention is not limited to those precise embodiments, and that various changes and modifications can be effected therein by one skilled in the art without departing from the scope and spirit of the invention as defined by the appended claims.

Technology Category: h