Patent Publication Number: US-6661861-B1

Title: Voice-channel frequency synchronization

Description:
This application is a continuation of U.S. patent application Ser. No. 09/083,865 filed May 21, 1998, now U.S. Pat. No. 6,118,770 issued Sep. 12, 2000, which is incorporated herein by reference, and which claims priority from Israeli Patent No. 120996, filed Jun. 4, 1997. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a method and a system for synchronizing related functions in a communication system apparatus and to a method and a system for synchronizing between a transceiver and a voice encoder-decoder unit in frame based communication in particular. 
     BACKGROUND OF THE INVENTION 
     In conventional communication methods, such as TDMA and CDMA, the link between a mobile unit and a base unit, is frame based. The base conventionally includes a very precise and relatively expensive crystal clock which is used to time its operations and synchronize every unit therein. A mobile unit which communicates with this base often includes a low quality crystal clock which is of lower precision than the crystal clock of the base unit. 
     Therefore, the mobile unit includes correction mechanisms which are used to synchronize the frequency produced by the mobile unit crystal to the frequency produced by the base unit crystal. 
     Reference is made now to FIG. 1 which is a schematic illustration of a mobile unit and a base unit known in the art. Base unit  2  includes a data interface  14 , an FEC unit  16  connected to the data interface  14 , a transmitter  18  connected to the FEC unit  16  and an antenna  20  connected to the transmitter  18 . 
     The base unit  2  also includes clock mechanism which includes a crystal  13  for generating a basic high frequency and a PLL unit  15  connected thereto. PLL unit  15  is further connected to the transmitter  18  and provides it with a frequency which is        m   n                   
     of the basic frequency. It will be appreciated by those skilled in the art, that the values set for m and n can be any natural number (including the number 1). 
     Mobile unit  4  includes an antenna  30 , a channel sampler  26  connected to antenna  30 , a demodulator  27  connected to the channel sampler  26 , a time tracking unit  28  connected to demodulator  27 , a voice decoder  24  connected to the demodulator  27 , a digital to analog converter (DAC)  22  connected to voice decoder  24  and a speaker  32  connected to DAC  22 . 
     Mobile unit  4  also includes a timing mechanism which includes a crystal oscillator  23  and  2  PLL units  21  and  25 . PLL unit  25  is further connected to channel sampler  26  and provides it with a frequency which is        m   n                   
     of the basic frequency provided by crystal oscillator  23 . PLL unit  21  is further connected to the DAC  22  and provides it with a frequency which is        l   k                   
     of the basic frequency provided by crystal oscillator  23 . 
     The mobile unit  4  also includes a sampling and transmitting section, including a microphone  42 , a voice sampler  34  connected to the microphone  42 , a voice encoder  36  connected to the voice sampler  34 , a modulator  38  connected to the voice encoder  36  and transmitter DAC  40  connected to the modulator  38 . 
     The timing mechanism of the mobile unit is also connected to the transmitting section so that PLL  21  also times and controls voice sampler  34  and PLL  25  also times and controls the transmitter DAC  40 . 
     Accordingly, transmitter DAC  40  is affected by PLL unit  25  and sampler  34  has to adjust accordingly. The sampler  34  has to provide a block of a predetermined number of samples N BLOCK , for each frame produced by modulator  38 . Since the timing of transmitter DAC  40  is dynamic, wherein its frequency may increase or decrease, the voice sampler  34  may provide more or less than N BLOCK  samples in a block, for each frame. 
     According to frame based communication standards such as TDMA and CDMA, each frame includes a predetermined number of information bits which may be the information data, voice data and the like. According to TDMA standard, each frame lasts 20 ms. A voice information frame includes 160 voice samples. 
     Accordingly, the data interface  14  provides blocks to the FEC unit, which include N BLOCK  samples, in each block, every 20 ms. The FEC unit  16  produces a frame which is then provided to transmitter  18 . The transmitter  18  transmits this frame via antenna  20 . 
     At the mobile unit  4 , the channel sampler  26  detects the transmitted frame via antenna  30  and provides it to demodulator  27 . The demodulator  27  analyzes the frame, extracts voice coded data and provides it to voice decoder  24 . The voice decoder  24  decodes this data and provides the decoded signal to DAC unit  22 . The DAC unit  22  converts the decoded signal into analog signal and provides it to speaker  32  which in turn produces sounds. 
     In the mobile unit  4 , the channel sampler  26  and the DAC  22  have to be synchronized too. Thus, for every frame detected by channel sampler  26 , the DAC  22  has to convert N BLOCK  samples. It will be appreciated that the synchronization mechanisms, of both base unit  2  and mobile unit  4 , have to be completely synchronized. Accordingly, the channel sampler  26  has to be synchronized with FEC  16  so that each frame produced by FEC  16  will be received as such in channel sampler  26 . 
     Conventional communication systems include a calibration mechanism which constantly calibrates PLL  25  so as to synchronize it according to the transmitting rate of the base unit  2 . It will be appreciated that since DAC  22  is dependent on channel sampler  26 , changing the frequency on which channel sampler  26  operates will affect DAC  22 . For example, if the frequency of channel sampler  26  increases, then the DAC  22  may be provided with more than N BLOCK  samples in 20 ms. 
     On the other hand, if the frequency of channel sampler  26  is decreased then the DAC  22  might be provided with less than N BLOCK  samples per frame. Both of these situations are not allowed in TDMA and for that matter in most frame base communication standards. Another method known in the art deletes or estimates the last samples. 
     If the current number of samples which are to be provided for a block exceeds N BLOCK  samples than N BLOCK  samples are selected and provided as a block and the rest of the samples are deleted. If, on the other hand, there are less than N BLOCK  samples and time comes to provide them to the voice encoder then samples are estimated according to the existing samples so as to provide complementary samples which will complete the block to N BLOCK  samples. It will be appreciated that according to this method, the quality of speech is reduced since an additional element of distortion or noise is introduced, which did not exist in the original sampled or the decoded signal. 
     SUMMARY OF THE PRESENT INVENTION 
     It is an object of the present invention to provide a novel method and a novel system for providing synchronized sampling frequencies, which overcomes the disadvantages of the prior art. 
     In accordance with the present invention there is thus provided a synchronized frequency generating system which includes a main crystal clock, for producing a basic frequency F B , a channel sampling phase locked loop (PLL) unit, connected to the main crystal clock, for converting the basic frequency F B  into a channel sampling frequency F CS , a voice sampling PLL unit, connected to the main crystal clock, for converting the basic frequency F B  into a voice sampling frequency F VS , a time tracking unit, connected to the channel sampling PLL unit, for detecting signal characteristics so as to determine a channel sampling frequency phase change value Δφ CS  and a frame timing phase change value Δφ FRAME  and a frequency controller, connected to the voice sampling PLL. 
     The frequency controller receives channel sampling frequency phase adjustment data and determines a voice sampling frequency phase change value Δφ VS . The frequency controller provides the voice sampling frequency phase change value Δφ VS  to the voice sampling PLL. 
     Then, the frequency controller receives channel sampling frequency phase adjustment data from the time tracking unit and the frequency controller receives channel sampling frequency phase adjustment data from the channel sampling PLL. 
     The system according to the invention may further include a primary voice sampling PLL connected between the voice sampling PLL and the main crystal clock. The primary voice sampling PLL converts the basic frequency F B  into primary channel sampling frequency F CS1  and the voice sampling PLL converts the primary channel sampling frequency F CS1  into a channel sampling frequency F CS . 
     According to one aspect of the invention, the primary voice sampling PLL converts the basic frequency F B  into primary channel sampling frequency F CS1 , by changing the voice sampling phase. 
     According to another aspect of the invention, the voice sampling PLL converts the primary channel sampling frequency F CS1  into a channel sampling frequency F CS , by changing the voice sampling phase. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be understood and appreciated more fully from the following detailed description taken in conjunction with the drawings in which: 
     FIG. 1 is a schematic illustration of a prior art system, for controlling the internal frequency of a transceiver; 
     FIG. 2 is a schematic illustration of a transceiver which includes a synchronized frequency generating system, constructed and operative in accordance with a preferred embodiment of the present invention; 
     FIG. 3 is a schematic illustration of a synchronized frequency generating system, constructed and operative in accordance with another preferred embodiment of the present invention; 
     FIG. 4 is a schematic illustration of a synchronized frequency generating system, constructed and operative in accordance with a further embodiment of the present invention; 
     FIG. 5 is a flow chart illustration of a method for synchronizing channel sampling frequency and voice sampling frequency, operative in accordance with yet another embodiment of the present invention; and 
     FIG. 6 which is a schematic illustration of the transmitting and voice sampling timing diagram. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The present invention overcomes the disadvantages of the prior art by controlling the frequency of the voice sampling mechanism. 
     Reference is now made to FIG. 2 which is a schematic illustration of a transceiver, generally referenced  80 , which includes a synchronized frequency generating system, generally referenced  100 , constructed and operative in accordance with a preferred embodiment of the present invention. 
     Transceiver  80  includes an antenna  81 , a channel sampler  82  connected to the antenna  81 , a demodulator  112  connected to the channel sampler  82 , a voice decoder  83  connected to the demodulator  112 , a DAC unit  84  connected to the voice decoder  83  and a speaker  85  connected to the DAC unit  84 . The transceiver  80  also includes a transmitter DAC  90  connected to the antenna  81 , a modulator  91  connected to the transmitter DAC  90 , a voice encoder  92  connected to the modulator  91 , a voice sampler  93  connected to the voice encoder  92  and a microphone  94  connected to the voice sampler  93 . The transceiver unit  80  further includes synchronized frequency generating system  100 . 
     System  100  includes a main crystal clock  102 , a channel sampling PLL  104  connected to the main crystal clock  102 , a time tracking unit  106  connected to the channel sampling PLL  104 , a voice sampling PLL  108  connected to the main crystal clock  102  and a frequency controller  110  connected to voice sampling PLL  108  and time tracking unit  106 . 
     The time tracking unit  106  is also connected to the demodulator  112 . The channel sampling PLL  104  is also connected to the channel sampler  82  and the transmitter DAC  90 . The voice sampling PLL  108  is also connected to a voice sampling unit  114  and to the DAC  84 . 
     The main crystal clock  102  produces a basic frequency F B  and provides it to PLL&#39;s  104  and  108 . PLL  104  reduces this frequency by a ratio of          m   n     ,                   
     thereby producing a channel sampling frequency F CS . PLL  108  reduces this frequency by a ratio of          l   k     ,                   
     thereby producing a voice sampling frequency F V . 
     Basically, the demodulator  112  acquires an initial symbol phase value as well as frame timing. Then, the demodulator  112  initializes the PLL  104  so that the channel sampling phase is aligned with the channel symbols at the antenna  81  and the framing phase is aligned with the boundary between information frames at the antenna  81 . 
     Reference is also made to FIG. 6 which is a schematic illustration of the transmitting and voice sampling timing diagram. Frame  400  defines a time period which commences at t 1  and ends at t 4 . N BLOCK  voice samples, generally designated  402 , have to be sampled during time from t 1  to t 4 , starting at voice samples  402   1 , which is sampled at t 2  and ending at voice sample  402   N , which is sampled at t 3 . 
     It will be appreciated that the first voice sample  402   1  has to be sampled after the frame has commenced, wherein T START =t 2 −t 1 &gt;0. Similarly, the last voice sample  402   N  has to be sampled before the frame has ended, wherein T END =t 4 −t 3 &gt;0. 
     According to the present invention, T START  is greater than a predetermined value T ERROR  which is higher than a characteristic frequency error, so that even if there occurred a frequency shift, then the first voice sample will be sampled after t 1 . In a similar manner, T END  is also greater than the predetermined value T ERROR . 
     According to the present invention, the frequency controller  110  calculates T START  and initializes the PLL  108  to control the voice sampler  93  and the DAC, so that the first voice sample  402 , is sampled after T START  has passed since the frame was first detected. The initialization also determines sample  402   N  will be sampled T END  before the sampling of frame  400  is expected to end. 
     The time tracking unit  106  detects the channel at the demodulator  112  and determines if the channel sampling frequency F CS  has to be adjusted so as to synchronize to the received RF signal. 
     When the time tracking unit  106  detects that the channel sampling frequency F CS  has to be changed, it provides a channel sampling frequency phase change command to channel sampling PLL  104  and at the same time it informs frequency controller  110  about the change in phase which is about to occur. Then, the frequency controller  110  defines a voice sampling frequency change, according to the channel sampling frequency phase change and provides a voice sampling frequency phase change command to the voice sampling PLL  108 . 
     As a result, both PLL&#39;s  104  and  108  re-adjust their respective frequency ratios. This way, the frequency provided to the voice sampler  93  is well synchronized to the frequency provided to the channel sampler  82 . 
     This process of detecting and adjusting is continuous, so that over a considerable period of time, a plurality of frequency phase changes results in an average frequency change. 
     Reference is made now to FIG. 3 which is a schematic illustration of a synchronized frequency generating system, generally referenced  150 , constructed and operative in accordance with another preferred embodiment of the present invention. 
     Synchronized frequency generating system  150  includes a time tracking unit  158 , a channel sampling PLL  156  connected to the time tracking unit  158 , a crystal clock  154  connected to the channel sampling PLL  156 , a voice sampling PLL  152  connected to the crystal clock  154  and a frequency controller  160  connected to the channel sampling PLL  156  and the voice sampling PLL  152 . 
     The crystal clock  154  produces a base frequency F B  which is generally a very high frequency form and provides it to channel sampling PLL  156  and voice sampling PLL  152 . Channel sampling PLL  156  reduces the basic frequency F B  according to a ratio        m   n                   
     so as to produce a channel sampling frequency F CS . Voice sampling PLL  152  reduces the basic frequency F B  according to a ratio        l   k                   
     so as to produce a voice sampling frequency F VS . Time tracking unit  158  detects the channel signal and determines from the duration of a received frame and its respective symbols, if the channel sampling frequency F CS  has to be adjusted. If so, then the time tracking unit  158  provides a ratio change command to PLL  156  to change the ratio        m   n                   
     so as to produce a new channel sampling frequency F CS  which will be identical to the frequency of the channel signal. 
     The frequency controller  160  detects when the ratio          m   n     ,                   
     of the channel sampling PLL  156  has changed and provides a ratio change command to the voice sampling PLL, so as to adjust ratio          l   k     ,                   
     accordingly. As a result, the voice sampling PLL  152  reduces the basic frequency F B  to a voice sampling frequency F VS  maintaining the correct reduction ratio          l   k     ,                   
     which is fully synchronized with the reduction ratio          m   n     ,                   
     of the channel sampling PLL  156 , so that the channel sampling frequency F CS  and the voice sampling frequency F VS  are fully synchronized at all times. 
     It will be noted that each ratio change command may be comprised of a plurality of phase changes. 
     Reference is made now to FIG. 4 which is a schematic illustration of a synchronized frequency generating system, generally referenced  200 , constructed and operative in accordance with a further embodiment of the present invention. 
     System  200  includes a time tracking unit  212 , a channel sampling PLL  208  connected to the time tracking unit  212 , a main crystal clock  206  connected to the channel sampling PLL  208 , a primary voice sampling PLL  202  connected to the main crystal clock  206 , a secondary voice sampling PLL  204  connected to the primary voice sampling PLL  202  and a frequency controller  210  connected to the channel sampling PLL  208  and to the secondary voice sampling PLL  204 . 
     The main crystal clock  206  produces a high frequency and provides it to the channel sampling PLL  208  and to the primary voice sampling PLL  202 . The channel sampling PLL  208  reduces this frequency according to a ratio        m   n                   
     so as to produce a channel frequency F CS . The primary voice sampling PLL  202  reduces the main frequency according to a ratio        l   k                   
     so as to produce a primary voice sampling frequency F V1  and provides it to the secondary PLL  204 . Secondary voice sampling PLL  204  further reduces the frequency F VS1  according to a ratio        o   p                   
     so as to produce a secondary voice sampling frequency F VS2  which then may be provided to a voice sampling unit. The secondary voice sampling PLL  204  is controlled by the frequency controller  210  which may provide it with ratio change commands so as to change the ratio          o   p     .                   
     Such ratio changes enable the system to adjust the voice sampling frequency F VS2 . 
     Time tracking unit  212  detects a channel signal and determines whether the channel sampling frequency F CS  is to be adjusted according to the received channel signal. When the channel detector determines that F CS  should be adjusted it produces a frequency change command and provides it to channel sampling PLL  208 . 
     It will be noted that the channel frequency adjustments and the voice frequency adjustments may be provided through a plurality of phase changes. 
     Reference is now made to FIG. 5 which is a flow chart illustration of a method for operating system  200 , operative in accordance with yet a further embodiment of the present invention. 
     In step  300 , the system  200  receives the incoming signal, thereby detecting the phase and the framing information of the signal. Then, the system  200  analyzes the incoming signal so as to determine a received channel sampling phase φ CSR  (step  302 ), according to various characteristics of the received signal such as frame phase φ FRAME , symbol phase φ SYMBOL  and the like. 
     In step  304 , the system checks if the received channel sampling frequency phase φ CSR  is equal to the current setting of the channel sampling frequency phase φ CS , in the system. If φ CSR  is equal to f CS  then the system proceeds back to step  300 , since the current setting is correct. Otherwise, φ CS  has to be adjusted according to φ CSR  and so the system proceeds to step  306 . 
     In step  306 , the system  200  determines a channel sampling frequency phase correction value Δφ CS , wherein Δφ CS =φ CSR −φ CS . 
     In step  308 , the system  200  determines a received voice sampling frequency phase φ VSR , according to the received channel sampling frequency φ CSR . 
     In step  310 , the system  200  utilizes the received voice sampling frequency phase φ VSR  and the current setting of the voice sampling frequency phase of the system φ VS , so as to determine a voice sampling frequency phase correction value Δφ VS , wherein Δφ VS =φ VSR −φ VS . Then, the system proceeds simultaneously, to steps  312  and  314 . 
     In step  312 , the system  200  adjusts the channel sampling frequency phase φ CS  according to the channel sampling frequency phase correction value Δφ CS  wherein φ CS =φ CS +φ CS . 
     In step  314 , the system  200  adjusts the voice sampling frequency phase φ VS , according to the voice sampling frequency phase correction value Δφ VS  wherein φ VS =φ VS +Δφ VS . 
     Finally, the system repeats the whole process from step  300 . 
     According to the present invention, no voice sample estimating mechanisms are required, since the channel sampling frequency and the voice frequency are well synchronized at all times. It will be noted however that the frequency controller can be connected at any point at the receiving section of the transceiver unit wherein the channel frequency can be detected. 
     It will be appreciated by persons skilled in the art that the present invention is not limited to what has been particularly shown and described hereinabove. Rather the scope of the present invention is defined only by the claims which follow.