Abstract:
A phase reversal detector comprises a circuit for receiving an input signal subject to occasional phase reversals, a circuit for generating signals representing the quadrature components thereof, and a circuit responsive to a migration in the quadrature plane of the position of the quadrature components by an amount greater than a predetermined threshold to generate a signal indicative of a valid phase reversal. The detector is capable of exceeding the requirements of ITU standard G.165, and yet is simple to implement and works over a large dynamic range.

Description:
FIELD OF THE INVENTION 
     This invention relates to the field of telecommunications, and more particularly to a method of detecting a valid phase reversal. 
     BACKGROUND OF THE INVENTION 
     There are certain situations in telecommunications where a phase reversal is used to signal a particular condition. For example, in ITU (International Telecommunications Union) standard G.165 entitled “General Characteristics of International Telephone Connections and International Telephone Circuits”, a 2100 Hz tone signal that has a phase reversal every 450±25 ms is used to disable an echo canceller. A valid phase reversal is defined as a phase variation in the range of 180°±25° of a 2100 Hz (±21 Hz) tone. An invalid phase reversal is defined as a phase variation in the range of 0°±110°. 
     Some means must be provided for detecting valid phase reversals and rejecting invalid phase reversals. The detector employed must operate perfectly on signals having a level of −31 dBmø to 06 dBmø in conditions of white noise less than or equal to 11 dB below the level of the 2100 Hz tone signal. For white noise levels between 11 dB and 5 dB below the level of the tone signal, the percentage of correct operation should fall by no more than 1% for each dB. 
     The tone disabler is required to operate (disable the echo canceller) within one second of the receipt of the disabling signal. 
     While the G.165 standard specifies additional requirements, reliable phase reversal detection is essential for the proper functioning of the echo canceller tone disabling detector. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a reliable phase reversal detector. 
     According to the present invention there is provided a phase reversal detector comprising means for receiving an input signal subject to occasional phase reversals, means for generating signals representing the quadrature components thereof, and means responsive to a migration in the quadrature plane of the position of said quadrature components by an amount greater than a predetermined threshold to generate a proportional signal indicative of a valid phase reversal. 
     The detector preferably includes means for adjusting the frequency of a local signal used in deriving the above-mentioned quadrature components. 
     The detector may also include means for bandpassing the incoming signal, which may be hard limited to eliminate the need for an automatic gain control circuit while ensuring the functionality of the detector over a large dynamic range of the input signal. The hard limiting operation is equivalent to applying the sign function to the band passed signal. 
     The invention also provides a method of detecting valid phase reversals in an input signal subject to occasional phase reversals, comprising the steps of generating signals representing the quadrature components thereof, determining the distance between said signals at different times in the quadrature plane, and generating a signal indicative of a valid phase reversal when said distance, or a value dependent thereon, exceeds a predetermined threshold value. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described in more detail, by way of example only, with reference to the accompanying drawings, in which: 
     FIG. 1 is a general block diagram of an echo canceller tone disabler detector circuit employing a phase reversal detector in accordance with the invention; and 
     FIG. 2 is a detailed block diagram of a phase reversal detector in accordance with the invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring now to FIG. 1, an input signal  1  S(KT), which is an input sample expressed a 16 bit linear value (1 sign bit and 15 magnitude bits) is applied to a bandpass filter  2  having the following characteristics: Type: Chebyshev II; Order: 6 (three 2 nd  order sections); sampling rate: 8000 Hz; Center Frequency: 2100 Hz; Passband bandwidth: 96 Hz; Passband ripple: 0.5 dB (maximum); Stop band ripple: 35 dB (minimum). The bandpass filter  2  extracts the 2100 Hz tone from the incoming signal S(KT). 
     The bandpass filter  2  outputs a signal bpf_S(KT), which is sampled at time t=KT, where T is the sampling period (125 μs for the 8000 Hz telephony rate). Signal bpf_S(KT) is applied to the input of phase reversal detector  4 , which will be described in more detail below, and also to circuit  5  for protection against false operation due to speech. Circuit  5  also receives the input signal  1  S(KT). 
     The input signal  1  S(KT) is also applied to the input of a circuit  6  for protection against false operation due to a data signal, and the outputs of circuits  4 ,  5  and  6  are connected to a control logic unit  7 , which produces an echo canceller disabling signal when a valid phase reversal is detected. 
     The operation per se of circuits  5 ,  6  and  7  is conventional and is well understood by a person skilled in the art. 
     The phase reversal detector circuit is shown in more detail in FIG.  2 . This comprises a quadrature component calculating circuit  10  and a local frequency control circuit  11 . 
     The band limited input signal bpf_S(KT) is applied to the (sign function) hard limiter  12  which derives signal sign (bpf_S(KT)). The band limiting function is equivalent to applying the sign function to the bandpass signal.                  sign                     (   x   )       =     {               -   1                     when                   x                   is       &lt;   0                 0                   when                   x                   is       =   0                 1                   when                   x                   is       &gt;   0                     (   1   )                                
     The quadrature components are calculated by multiplying the input signal with a locally calculated sine/cosine signal of 2100 Hz (initially). The components of the terms are A sin(ω L KT) and A cos(ω L KT), where A is a scaling constant, and ω L =2πf L  where f L  is the local sine wave frequency initialized to 2100 Hz. 
     The signal A sin ω L (KT) is generated in sine wave generator  13  and passed through a 90° phase shifter  14  to multiplier  15 , and directly to multiplier  16 . 
     The outputs of multipliers  15  and  16  are respectively passed through low pass filters  17  and  18  to derive the quadrature components Q b (KT), and I b (KT). The inputs to the filters  17  and  18  are thus respectively sign (bpf_S(KT)×Asin (w L KT) and sign (bpf_S(KT)×A cos (w L KT) respectively. 
     The low pass filters  17  and  18  have the following characteristics in one embodiment: Type: elliptic, low pass; Order: 2; Sampling frequency: 8000 Hz; Passband bandwidth: 100 Hz; Passband ripple: 0.1 dB; and Stopband attenuation :40 dB. 
     The output of low pass filters  17 ,  18  is then presented to averaging circuits  19  and  20 , which derive the average quadrature components over 4 ms (i.e. 32 samples at 8000 Hz). 
     The invention is based on the idea that a migration of the coordinates in the quadrature plane occurs at each phase reversal. While it would be possible to determine the Euclidean distance between the position of the quadrature point (I b (KT), Q b (KT)) before and after phase reversal in the quadrature plane, due to filtering effects in the telephone network, the analog-to-digital converter, etc., the migration of the quadrature point when a phase jump occurs does not generally happen instantaneously. It can take at least 5 ms before the position of the quadrature point stabilizes in a new region after a phase jump (assuming that f I -f L  is very small or 0, otherwise the new position starts to shift around a circle in the quadrature plane). 
     Although this problem could be overcome by calculating the Euclidean distance between the current quadrature point and a previous one with a given delay, for example, a delay of 6 ms (48 samples at 8000 Hz), a difficulty arises from the fact that the incoming signal is not always clean. It is generally affected by severe noise, quantization effects, etc. 
     In the preferred embodiment, these effects are coped with by the averaging circuits  19  and  20  since the detection of the phase reversal is based on the average position in the quadrature plane over a predetermined period, in this case 4 ms. These average values are presented to the inputs of delay circuits  21 ,  22  in the local frequency control circuit  11 . 
     The local sine and cosine signals are generated in the sine wave generator  13 . This is controlled by the local frequency control circuit  11 . These signals are calculated using the following recursive equations: 
     
       
         sin( nw   L   T )=2×sin(( n− 1) w   L   T )×cos( w   L   T )−sin(( n− 2) w   L   T )  (2) 
       
     
     
       
         cos( nw   L   T )=2×cos(( n− 1) w   L   T )×cos( w   L   T )−cos(( n− 2) w   L   T )  (3) 
       
     
     As an initial condition, the sine and cosine values at n=1 and n=2 can be used, and the sine/cosine at n=3 (and so on) calculated using equations (2) and (3) above. 
     If at start-up the frequency of the generated sine wave is exactly 2100 Hz, later on during the process of detecting a phase reversal, the frequency of the generated signal is varied to best match that of the incoming tone, which can vary in the range of 2079 Hz to 2121 Hz, that is 2100±21 Hz. 
     If the frequency of the locally generated sine/cosine wave signal, which is used for calculating the quadrature components, is different from the frequency of the incoming signal, the point I b (KT), Q b (KT) rotates on a circle in the quadrature plane I, Q with a frequency equal to the difference between the two frequencies as discussed above. 
     To avoid this, because of its effect on the probability of detecting a valid phase reversal or rejecting an invalid phase jump, the frequency of the locally generated sine wave must be controlled. 
     Equations (2) and (3) above can be rewritten in a different form as follows: 
     
       
           A× sin( nw   L   T )= A× sin(( n− 1) w   L   T )× FAF−A× sin(( n− 2) w   L   T )  (4) 
       
     
     
       
           A× cos( nw   L   T )= A× cos(( n− 1) w   L   T )× FAF−A× cos(( n− 2) w   L   T )  (5) 
       
     
     The FAF—(Frequency Adjusting Factor) is initialized to: 2×cos(w L T) and A is the scaling factor. 
     A frequency control signal Δf L  is used to modify the FAF (Frequency Adjusting Factor) and through it the frequency of the generated sine/cosine waves. Δf L  is proportional to the signal difference f I −f L , where f I  is the frequency of the incoming tone signal. An increase in the value of FAF results in a decrease in the frequency of the generated signal, while a decrease in the value of FAF has the opposite effect. Experiments have shown that in one embodiment subtracting Δf L /32 from FAF once very 4 ms brings the f L  close enough to make the phase reversal detection meet the requirement of the G.165 standard without a significant increase in complexity. 
     In the local frequency control circuit, the signals I av (nT) and Q av (nT) are queued over 4 samples to ensure reliable functioning of the circuit when a phase reversal results in a transition period of up to 12 ms. In this embodiment the queue is shifted every 4 ms. 
     To determine when phase reversal occurs, the Euclidean distance between the newest point I av (nT), Q av (nT) and the oldest in the queue is determined, and when the distance exceeds a predetermined threshold a valid phase reversal is assumed to have occurred. In order to reduce the computational power required, as shown in FIG. 2, in practice the square of the Euclidean distance is compared with the predetermined threshold in threshold detector  22 . 
     As will be apparent from FIG. 2, the local frequency control circuit  11  calculates the signal Δf L , which is input to the sine wave generator  13  in accordance with the following equations:                    I   m1          (   t   )       =           I   av          (   t   )       +       I   av          (   t   )       -     Z     -   1         2       ,         I   m2          (   t   )       =           I   av          (   t   )       -     Z     -   2       +         I   av          (   t   )            Z     -   3           2               (   6   )                     Q   m1          (   t   )       =           Q   av          (   t   )       +       Q   av          (   t   )       -     Z     -   1         2       ,         I   m2          (   t   )       =           Q   av          (   t   )       -     Z     -   2       +         Q   av          (   t   )            Z     -   3           2               (   7   )                   AE        (   t   )       =         I   m1     ×     Q   m2       -       I   m2     ×     Q   m1           ,     AE        - angle error               (   8   )                 Δ                     f   L          (   t   )         =         AE        (   t   )       +       AE        (   t   )            Z     -   1           2             (   9   )                                
     The described phase reversal detector is simple to implement, works over a much larger dynamic range than required by the G.165 standard, does not require an AGC circuit and withstands high level of noise extremely well.