Patent Publication Number: US-7907923-B2

Title: Method and apparatus for squelch gating a receiving signal

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to an apparatus and a method for squelch gating a receiving signal. 
     2. Related Technology 
     In digital receivers the possibility to mute an audio output signal in case of disappearance of the carrier of the received audio signal or in case of a drop of the signal-noise-ratio in the received audio signal below a threshold is known from U.S. Pat. No. 6,047,170 A. 
     The technical realization for determining the measure of noise in a received signal is called squelch and is based on the continuous measuring of phase noise in the audio signal and the gating of the demodulated audio signal according the measured level of phase noise in comparison with specified thresholds. 
       FIG. 1  shows an apparatus for gating a received audio signal on the basis of a squelch signal characterizing the measure of phase noise in the audio signal. 
     The received sampled RF audio signal x n  with its sampled cartesian components—sampled real component Re{x n } and sampled imaginary component Im{x n }—is converted in a Cordic-unit  1  to its polar components, sampled amplitude m n  and sampled phase φ n . In a differentiating unit  3  of a squelch detector  3  a sampled phase difference Δφ n  between a sampled phase φ n  and its succeeding sampled phase φ n-1 —determined in a delay unit  4  of the differentiating unit  3  on the basis of the sampled phase φ n-1 —is determined in subtracting unit  5  of the differentiating unit  3 . 
     The sampled phase difference Δφ n  contains signal portions of low frequency, caused by frequency offset or frequency drift of the carrier in the received RF audio signal, and phase noise as signal portions of high frequency. To eliminate the signal portions of low frequency in the sampled phase difference Δφ n  a subsequent highpass filter  6  of the squelch detector  2 —typically a 4th order IIR highpass filter—produces sampled phase differences Δφ n  comprising only phase noise relevant signal portions. 
     The signed sampled phase differences Δφ n  corresponding to phase noise are rectified in a subsequent squaring unit  7 . The squaring unit  7  can alternatively be replaced by a device performing the absolute value of its input signal. For averaging the sampled unsigned RF phase differences p n  over time a digital filter  8  having a proportional-integral characteristic concerning equation (1) represents the last signal processing unit of the squelch detector  2  delivering the sampled squelch signal s n  at its output. In equation (1) f s  represents the sampling rate and τ represents the time constant of the integral characteristic of the digital filter  8 . 
     
       
         
           
             
               
                 
                   
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     The sampled amplitudes m n  of the RF receiving audio signal x n  are demodulated in a demodulator  9 . The demodulated audio signal y n  is gated in a subsequent gating unit  10  on the basis of the determined sampled squelch signal s n  and a specified threshold delivering a muted audio signal a n  to the outputting unit  11 . 
     In an off→on→off-cycle of an audio transmission  FIG. 2  displays the DC component of the carrier DC(m n ) (curve  40 ), the rectified RF phase noise p n  (curve  20 ) and the squelch signal s n  (curve  30 ) in the apparatus for gating a received audio signal according to  FIG. 1 , whereby the time constant τ of the integrating digital filter  8  ms is 5 ms. 
     The time constant of the digital filter is typically between 5 ms and 10 ms. Thus random peaks in the RF phase noise p n  being shorter than 5 ms to 10 ms are not averaged by means of the digital filter  8  with proportional-digital characteristic leading to a squelch signal s n  with random peaks. Such peaks in the squelch signal s n  drops below respectively rises above the specified threshold(s) and disadvantageously mutes the received audio signal for a short period. Especially in case of a squelch signal s n  rising above respectively dropping below a specified threshold leading to switch off respectively switch on of the received audio signal random peaks in the squelch signal s n  occurring shortly after the switch reverses the switch of the received audio signal. 
     Prolongation of the time constant τ of the integrating digital filter  8  for avoiding such unwanted reversals of switches in the audio signal in case of peaks in the squelch signal would deteriorate the time behavior of the integrating digital filter and thus the time behavior of the squelch detector. The worse reaction of the integrating digital filter would undesirably delay the squelch signal s n  in comparison with an on→off-step or an off→on-step of the carrier in the audio transmission (see in  FIG. 2  the delay of the squelch signal s n  (curve  20 ) in comparison with an off→on-step of the DC component of the carrier DC(m n ) (curve  40 )). 
     SUMMARY OF THE INVENTION 
     The invention provides an apparatus and a method for squelch gating a received audio signal and thereby avoiding the occurrence of short-time reversal of the audio signal&#39;s switching without deteriorating the time behavior of the squelch detector. 
     The invention provides a method for squelch gating a receiving signal depending on the level of phase noise in the receiving signal comprising following steps: determining sampled amplitudes and sampled phases of the receiving signal, determining sampled phase noise by differentiating subsequent sampled phases, determining a squelch signal by rectifying and averaging the sampled phase noise, determining a start point of gating after a rising of the squelch signal above a higher threshold delayed by a first delay time, determining an end point of gating after a dropping of the squelch signal below a lower threshold delayed by a second delay time, and gating the sampled amplitudes from the start point of gating until the end point of gating. 
     The invention also provides an apparatus for squelch gating a receiving signal depending on the value of phase noise in the receiving signal including a unit for determining sampled amplitudes and sampled phases of the receiving signal, a unit for determining sampled phase noise by differentiating subsequent sampled phases, a unit for determining a squelch signal by rectifying and averaging the sampled phase noise, and a switching unit for gating the sampled amplitudes from a rising of the squelch signal above a higher threshold delayed by a first delay time until a dropping of the squelch signal below a lower threshold delayed by a second delay time. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       An embodiment of the inventive apparatus and four embodiments of the inventive method for gating a receiving signal are described in detail with reference to the drawings. The figures of the drawings show: 
         FIG. 1  is a block diagram of an apparatus for gating a receiving signal. 
         FIG. 2  is a time diagram of RF phase noise, of squelch signal and of DC component of the carrier in an apparatus for gating a receiving signal according to  FIG. 1 . 
         FIG. 3  is a block diagram of an inventive apparatus for gating a receiving signal. 
         FIG. 4  is a flowchart of an inventive method for gating a receiving signal. 
         FIG. 5  is a time diagram of starting a gating of a receiving signal in a first embodiment of the invention. 
         FIG. 6  is a time diagram of starting a gating of a receiving signal in a second embodiment of the invention. 
         FIG. 7  is a time diagram of starting a gating of a receiving signal in a third embodiment of the invention. 
         FIG. 8  is a time diagram of starting a gating of a receiving signal in a fourth embodiment of the invention. 
         FIG. 9  is a time diagram of ending a gating of a receiving signal in a first embodiment of the invention. 
         FIG. 10  is a time diagram of ending a gating of a receiving signal in a second embodiment of the invention. 
         FIG. 11  is a time diagram of ending a gating of a receiving signal in a third embodiment of the invention. 
         FIG. 12  is a time diagram of ending a gating of a receiving signal in a fourth embodiment of the invention. 
         FIG. 13  is a time diagram of RF phase noise, of squelch signal and of DC component of the carrier in an inventive apparatus for gating a receiving signal. 
     
    
    
     DETAILED DESCRIPTION 
     According to the invention, the start point of gating the audio signal is the time point when the squelch signal s n  rises above the higher threshold delayed by a first delay time. The end point of gating the audio signal is the time point, when the squelch signal s n  drops below the lower threshold delayed by a second delay time. Therefore short-time reversal of switching the audio signal in case of occurrence of peaks in the squelch signal shortly after switching the audio signal can be advantageously prohibited. 
     In a first embodiment of the inventive method a third delay time for delaying the sampled amplitudes m n  of the audio signal at the start point of gating and a fourth delay time for delaying the sampled amplitudes m n  of the audio signal at the end point of gating is calculated to compensate the unwanted delay in the muting of the demodulated audio signal y n  caused by the first delay time at the start point of gating or by the second delay time at the end point of gating, respectively. 
     A delaying of the sampled amplitudes m n  of the audio signal by different delay times—third delay time at the beginning of gating, fourth delay time at the end of gating—can not be realized. Thus the maximum delay time of the third and fourth delay time is chosen for delaying the sampled amplitudes m n  of the audio signal both at the start and at the end of gating. The selection of one of the third or fourth delay time for delaying the sampled amplitudes m n  of the audio signal leads to a failure in the correct time point either for starting or for ending the gating. To eliminate this failure at one of the two time points both the first and the second delay time are recalculated by the chosen maximum delay time for delaying the sampled amplitudes m n  of the audio signal. 
     For avoiding the delay between the squelch signal s n  and the demodulated audio signal y n  caused by the time constant τ of the integrating digital filter  8  the user of the receiver or an automatic selection unit can select a first time interval Δt sp  relative to the original start point of gating—the time point, when the squelch signal s n  rises above the higher threshold delayed by the first delay time—for determining a new start point of gating and a second time interval Δt ep  relative to the original end point of gating—the time point, when the squelch signal s n  drops below the lower threshold delayed by the second delay time—for determining a new end point of gating. The new start point of gating and the new end point of gating is determined empirically by the user or the automatic selection unit changing the first time interval Δt sp  or the second time interval Δt ep , until phase noise disappears at the beginning or at the end, respectively, of the audio receiving. 
     In a second embodiment of the inventive method a third delay time for delaying the sampled amplitudes m n  of the audio signal at the start point of gating is calculated as the difference of the original start point of gating and the new start point of gating being the summation of the first delay time and the first time interval Δt sp  relative to the original start point of gating. Consequently a fourth delay time for delaying the sampled amplitudes m n  of the audio signal at the end point of gating is calculated as the difference of the original end point of gating and the new end point of gating being the summation of the second delay time and the second time interval Δt ep  relative to the original end point of gating. 
     To eliminate the failure caused by delaying the sampled amplitudes m n  of the audio signal both at the beginning and also at the end of gating both the first and the second delay time are recalculated each by subtracting the first time interval Δt sp  or the second time interval Δt ep , respectively, from the chosen maximum delay time for delaying the sampled amplitudes m n  of the audio signal. 
     For avoiding unwanted clicks at the beginning and/or at the end of audio transmission the gating is continuously increased from an inactive gating level up to an active gating level during a first fading time T fade1  at the beginning of gating and is continuously decreased from an active gating level down to an inactive gating level during a second fading time T fade2  at the end of gating. In a third embodiment of the inventive method the start point of gating is the start point of fading from the inactive level and the end point of gating is the start point of fading from the active gating level. In a fourth embodiment of the inventive method the start point of gating is the end point of fading at the active gating level and the end point of gating is the end point of fading at the inactive gating level. 
     Optionally for reducing the delay between the squelch signal s n  and an on→off-step or an off→on-step of the carrier signal in the audio transmission the value of said squelch signal s n  is changed to the value of the higher threshold in case of a rising squelch signal s n  being below the higher threshold and the value of said squelch signal s n  is changed to the value of the lower threshold in case of a dropping squelch signal s n  being above the lower threshold. 
     The inventive apparatus and the inventive method for gating a receiving signal are described below with reference to  FIGS. 3 and 4  of the drawings: 
     In the first step S 10  of the inventive method for gating a receiving signal the sampled amplitudes m n  and sampled phase φ n  of the sampled receiving audio signal x n  are determined in an converter  1 —typically a Cordic-unit—on the basis of the sampled real component Re{x n } and the sampled imaginary component Im{x n } of the complex receiving audio signal x n . 
     In the following step S 20  of the inventive method sampled rectified phase noise p n  is determined on the basis of the sample phases φ n . Firstly the phase difference Δφ n  between a sampled phase φ n  and its succeeding sampled phase φ n-1  is determined for each sampling point in a differentiating unit  3 . Secondly the low frequency signal portions of the phase difference Δφ n —caused by frequency offset and frequency drift of the carrier signal—are eliminated in a 4th order highpass filter  6  to obtain a phase difference Δφ n  comprising only high frequency signal portions being phase noise p n . In a squaring unit  7  the phase noise p n  is rectified. Alternatively the rectifying of the phase noise p n  can be performed by means of a absolute value performing unit. 
     In the next step S 30  of the inventive method the squelch signal s n  is determined by averaging the rectified phase noise p n  in a digital filter  9  with proportional-integral characteristic. 
     Optionally in case of a rising squelch signal s n  being below a selected higher threshold S high  the value of the squelch signal s n  is changed to the higher threshold S high  according to  FIG. 13  in a gating unit  10 . Consequently in case of a dropping squelch signal s n  being higher a selected lower threshold S low  the value of the squelch signal s n  is changed to the lower threshold S low  according to  FIG. 13  in the gate unit  10 . By these means the time for starting or for ending the gating, respectively is thus reduced. 
     The following step S 40  of the inventive method comprises the determination of the start point and of the end point of gating. 
     In a gating unit  11  the squelch signal s n  is compared with a selected higher threshold S high  in case of a rising of the squelch signal s n . The start point SP G1  of gating in a first embodiment of the invention occurs according to  FIG. 5 , when the squelch signal s n  rises above the higher threshold S high  and a subsequent first delay time T D1  expires. The starting of gating is characterized by the rising audio gate signal AG—the control signal for gating the amplitudes m n  of the received audio signal in the gating unit  10 —in  FIG. 5 . 
     In a second embodiment of the invention the user of the receiver determines the start point SP G2  of gating by adjusting the start point SP G1  of gating of the first embodiment of invention, until phase noise disappears at the beginning of gating. The start point SP G2  of the second embodiment of the invention corresponds to an adjustment of the start point SP G1  of gating of the first embodiment of invention in the size of a first time interval Δt sp  according to  FIG. 6 . The adjustment in the size of a first time interval Δt sp  can be performed in both directions relative to the start point SP G1  of gating of the first embodiment of invention. 
     In a third embodiment of the invention the gating is started from zero up to an active gating level inside a first fading time T fade1 . Defining the start of fading as the start point of gating the start point SP G3  of the third embodiment of the invention corresponds to the start point SP G2  of gating of the second embodiment of invention according to  FIG. 7 . 
     In a fourth embodiment of the invention the end of fading defines the start point of gating leading to a start point SP G4  of the fourth embodiment of the invention according to  FIG. 8 . 
     For determining the end point of gating the dropping squelch signal s n  is compared with a selected lower threshold S low  in the gating unit  10 . The end point EP G1  of gating of the first embodiment of the invention occurs according to  FIG. 9 , when the squelch signal s n  drops below the lower threshold S low  and a subsequent second delay time T D2  expires. The ending of gating is characterized by the dropping audio gate signal AG in  FIG. 9 . 
     In the second embodiment of the invention the end point EP G2  of gating is determined by adjusting the end point EP G1  of gating of the first embodiment of the invention, until the phase noise disappears at the end of gating. The end point EP G2  of the second embodiment of the invention corresponds to an adjustment of the end point EP G1  of gating of the first embodiment of invention in the size of a second time interval Δt ep  according to  FIG. 10 . 
     In the third embodiment of the invention with fading from an active gating level down to zero inside a second fading time T fade2  at the end of gating according to  FIG. 11  the end point EP G3  of gating corresponds to the end point EP G2  of the second embodiment of the invention, if the start of fading defines the end point of gating. 
     According to  FIG. 12  the end point EP G4  of gating in the fourth embodiment of the invention corresponds to the end point EP G2 , of the second embodiment of the invention in addition to the second fading time T fade2 , if the end of fading defines the end point of gating. 
     In the following step S 50  of the inventive method the third delay time T D3  for delaying the sampled amplitudes m n  of the received audio signal at the beginning of the gating and the fourth delay time T D4  for delaying the sampled amplitudes m n  of the received audio signal at the end of the gating are calculated. 
     The third delay time T D3  at the beginning of the gating in the first embodiment of the invention calculates according to equation (2a):
 
T D3 ≧T D1   (2a)
 
     The third delay time T D3  at the beginning of the gating in the second embodiment of the invention calculates according to equation (2b), whereby a positive value of the first time interval Δt sp  corresponds to a start point SP G2  of gating of the second embodiment of invention being earlier than the rising of the squelch signal s n  above the higher threshold S high :
 
 T   D3   ≧T   D1   +Δt   sp   (2b)
 
     The third delay time T D3  at the beginning of the gating in the third embodiment of the invention calculates according to equation (2c), whereby a positive value of the first time interval Δt sp  corresponds to a start point SP G3  of gating of the third embodiment of invention being earlier than the rising of the squelch signal above the higher threshold S high :
 
 T   D3   ≧T   D1   +Δt   sp   (2c)
 
     The third delay time T D3  at the beginning of the gating in the fourth embodiment of the invention calculates according to equation (2d), whereby a positive value of the first time interval Δt sp  corresponds to a start point SP G4  of gating of the fourth embodiment of invention being earlier than the rising of the squelch signal s n  above the higher threshold S high :
 
 T   D3   ≧T   D1   +Δt   sp   −T   fade1   (2d)
 
     The fourth delay time T D4  at the end of the gating in the first embodiment of the invention calculates according to equation (3a):
 
T D4 ≧T D2   (3a)
 
     The fourth delay time T D4  at the end of the gating in the second embodiment of the invention calculates according to equation (3b), whereby a positive value of the second time interval Δt ep  corresponds to an end point EP G2  of gating of the second embodiment of invention being earlier than the dropping of the squelch signal s n  below the lower threshold S low :
 
 T   D4   ≧T   D2   +Δt   ep   (3b)
 
     The fourth delay time T D4  at the end of the gating in the third embodiment of the invention calculates according to equation (3c), whereby a positive value of the second time interval Δt ep  corresponds to an end point EP G3  of gating of the third embodiment of invention being earlier than the dropping of the squelch signal s n  below the lower threshold S low :
 
 T   D4   ≧T   D2   +Δt   ep   (3c)
 
     The fourth delay time T D4  at the end of the gating in the fourth embodiment of the invention calculates according to equation (3d), whereby a positive value of the second time interval Δt ep  corresponds to an end point EP G4  of gating of the fourth embodiment of invention being earlier than the dropping of the squelch signal s n  below the lower threshold S low :
 
 T   D4   ≧T   D2   +Δt   ep   +T   fade2   (3d)
 
     For delaying the sampled amplitudes m n  of the received audio signal both at the beginning and also at the end of the gating the maximum delay time T DMax  is chosen of the third delay time T D3  at the beginning of the gating and of the fourth delay time T D4  at the end of the gating in the next step S 60  according to equation (4):
 
 T   DMax =max{0; T   D3   ;T   D4 }  (4)
 
     In step S 60  the sampled amplitudes m n  of the received audio signal are delayed in a delay unit  12  located between the demodulator  9  and the gating unit  10  in the inventive apparatus for gating a received signal according to  FIG. 3  with the maximum delay time T DMax . 
     The failure caused by choosing one delay time of the third delay time T D3  or the fourth delay time T D3  for the delay time T DMax  applied for delaying the sampled amplitudes m n  of the received audio signal both at the beginning and at the end of the gating is compensated in the next step S 70  by recalculating the first delay time T D1  and the second delay time T D2 : 
     For the first embodiment of the invention the first delay T D2  is recalculated according to equation (5a) and the second delay time T D2  is recalculated according to equation (6a):
 
T D1 =T DMax   (5a)
 
T D2 =T DMax   (6a)
 
     For the second embodiment of the invention the first delay T D1  is recalculated according to equation (5b) and the second delay time T D2  is recalculated according to equation (6b):
 
 T   D1   =T   DMax   −Δt   sp   (5b)
 
 T   D2   =T   DMax   −Δt   ep   (6b)
 
     For the third embodiment of the invention the first delay T D1  is recalculated according to equation (5c) and the second delay time T D2  is recalculated according to equation (6c):
 
 T   D1   =T   DMax   −Δt   sp   (5c)
 
 T   D2   =T   DMax   −Δt   ep   (6c)
 
     For the fourth embodiment of the invention the first delay T D1  is recalculated according to equation (5d) and the second delay time T D2  is recalculated according to equation (6d):
 
 T   D1   =T   DMax   −Δt   sp   +T   fade1   (5D)
 
 T   D2   =T   DMax   −Δt   ep   +T   fade2   (6D)
 
     By using the recalculated first delay time T D1  according to one of the equations (5a) to ( 5   d ) and the second delay time T D2  according to one of the equations (6a) to (6d) the new start point SP G1 , SP G2 , SP G3 , or SP G4  of the first, second, third or fourth embodiment of the invention according to  FIG. 5 ,  6 ,  7 , or  8  and the end point EPG G1 , EPG G2 , EPG G3 , or EPG G4  of the first, second, third or fourth embodiment of the invention according to  FIG. 9 ,  10 ,  11 , or  12  is determined in the next step S 80 . 
     In the last step S 90  of the inventive method the delayed sampled amplitudes m n  of the received audio signal are gated in the gating unit  10  between the start point SP G1 , SP G2 , SP G3 , SP G4  in the first, second, third, or fourth embodiment of the invention and the end point EP G1 , EP G2 , EP G3 , or EP G4  of the first, second, third, or fourth embodiment of the invention. 
     The invention is not limited to the embodiment of the inventive apparatus and to the four embodiments of the inventive method for gating a receiving signal specifically disclosed herein. The invention also covers other combinations of the embodiments of the inventive method, for example a level and time trigger of the squelch signal s n  without using any user specific adjustment of the start point and/or end point of gating in combination with fading of the gating. Furthermore, nonlinear characteristic of fading—for example a squared fading characteristic—is also within the scope of the invention.