Abstract:
A system and method for optimizing audio transfer resolution including a digitally controlled analog gain control element receiving an incoming analog input, an analog to digital converter receiving an adjusted analog signal from the analog gain control element, a digital level measurement element receiving a digital signal from the analog to digital converter; and a processor receiving a level measurement control signal from the digital level measurement wherein the processor supplies a gain control signal to the analog gain control element.

Description:
TECHNICAL FIELD  
         [0001]    The present invention relates generally to the field of analog to digital conversion, and more particularly, to optimizing audio transfer resolution through the analog to digital conversion process.  
         BACKGROUND OF THE INVENTION  
         [0002]    Digital transmission and storage systems have a limited capacity of accurately transferring and storing the magnitude of digital signals. Digital signals are based upon a finite range of values including a maximum digital value. When an analog signal is converted to a digital signal, any analog signal that corresponds to a digital value higher than the maximum value cannot be accurately represented. In other words, Analog-To-Digital (A-D) converters clip input signals when the magnitude of the input signal exceeds the upper limit of the A-D converter. A level higher than the maximum allowable value is “clipped” to the maximum digital value, which is known as digital clipping or digital overload. These digital errors cause audible distortions. Methods to prevent digital overload include processing the signal before the A-D conversion with automatic gain devices. These devices modify the analog input level within a predetermined range to limit the maximum input level and reduce or eliminate digital clipping. Unfortunately, these devices also alter the overall characteristics of the audio signal relative to time. Another common method for reducing the probability of digital overload in digital recording or transmission is to operate at a reduced level to provide more capacity for signal peaks to pass without encountering the maximum allowable value. Operating at a reduced level means the incoming analog signal is reduced to eliminate or decrease subsequent clipping.  
           [0003]    Digital signals also have a finite resolution of discrete values. As converter resolution increases, the steps used to quantize the analog waveform become “finer.” When incoming analog signals are reduced to eliminate clipping, the granularity or fineness of the steps to represent the analog waveform is also reduced. Reducing the input signal results in a reduction in the difference between the lowest level and highest level signal (the dynamic range). Low level signals that could have been accurately represented using the entire range of values may be lost in the conversion noise from the reduction in granularity.  
           [0004]    One method to prevent overload when converting an audio signal to a digital signal and to prevent distortion is by ensuring the level of the analog signal is kept below the corresponding maximum allowable digital signal. This is typically accomplished by injecting the analog signal into a converter and checking for any overloads. If overloads occur, the level is manually reduced by the operator and a second pass is made. This process may take several iterations before the maximum analog signal level is identified and passed without creating a digital overload or distortion.  
           [0005]    Another method to reduce or eliminate digital overloads is to input analog signals at a reduced level and then add gain in the digital domain after the A-D conversion has been completed with a technique called normalization. Normalization is where a digital signal is sampled to find the highest amplitude digital samples. Once the highest amplitude digital signals are identified, the gain of the digital signals would be increased to achieve the desired maximum level. Typically this level is a fraction of a decibel below the maximum allowable digital level. This process does not consider the incoming signal and will modify all resulting digital samples by the same calculated amount. Any noise will have the same increase in gain (amplitude multiplication), thereby raising the noise floor of the “normalized” signal as well. In addition, raising the digital level in this manner (multiplication) will not improve the ultimate resolution of the signal. This occurs because the entire dynamic range of the digital signal path was not used by the incoming analog signal.  
           [0006]    U.S. Pat. No. 5,821,899, entitled “Automatic Clip Level Adjustment For Digital Processing,” purportedly automatically adjusts the analog input and output signal levels to optimize transfer resolutions through a digital processor. This system alters both the input and output gains of a digital process to theoretically maximize the resolution and reduce the probability of digital overloads. This system is designed to operate dynamically, changing the input gain and output gains together in a predetermined fashion, to maintain unity gain through the system so no perceived volume change is noted by maintaining an overall gain of unity in the two adjustments made.  
         SUMMARY OF THE INVENTION  
         [0007]    The present invention is directed to a system and method for optimizing audio transfer resolution including: a digitally controlled analog gain control element receiving an incoming analog input, an analog to digital converter receiving an adjusted analog signal from the analog gain control element, a digital level management element receiving a digital signal from the analog to digital converter; and a processor receiving a level measurement control signal from the digital level measurement wherein the processor supplies a gain control signal to the analog gain control element.  
           [0008]    The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention are described hereinafter and form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions does not depart from the spirit and scope of the invention as set forth in the claims. The novel features that are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying figures. It is to be expressly understood, however, that each of the figures is provided for the purpose of illustration and description only and is not intended as a definition of the limits of the present invention.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    [0009]FIG. 1 is a waveform illustrating an example of an analog input signal received by the system of FIG. 3;  
         [0010]    [0010]FIG. 2 is a waveform illustrating an example of the digitized audio signal values of the analog signal resulting from the A-D conversion of the waveform of FIG. 1 in the system of FIG. 3;  
         [0011]    [0011]FIG. 3 is a simplified block diagram of an analog to digital processing system;  
         [0012]    [0012]FIG. 4 illustrates the dynamic range and resolution results from a digital normalization process;  
         [0013]    [0013]FIG. 5 illustrates the use of the dynamic range resulting from one embodiment of the present invention; and  
         [0014]    [0014]FIG. 6 is a flow chart of the operation of one embodiment of FIG. 3. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0015]    One primary objective of the present invention is to provide a circuit configuration that employs automatic adjustment of an analog gain stage. This automatic adjustment is based upon an analysis, of a digital representation of the analog gain amplitude. The dependency of the automatic adjustment of the analog stage on the digital representation may be used to optimize the analog to digital transfer through the analog and digital signal paths.  
         [0016]    An embodiment of the present invention may also automatically optimize analog to digital transfer of an audio signal based upon an analyzed digital representation of the analog signal and a predetermined set of dynamic range parameters applied to the signal in the analog domain.  
         [0017]    Another objective of this invention is to maintain an analog signal that presents a consistent level range to an analog to digital conversion stage based upon a set of parameters.  
         [0018]    [0018]FIG. 1 is a waveform illustrating an example of an analog input signal  100  with maximum positive  101  and negative  102  amplitudes. This waveform may be applied to an analog to digital converter resulting in an output of a string of digits that represents the waveform in the digital domain.  
         [0019]    [0019]FIG. 2 is a waveform illustrating an example of the digitized audio signal values  200  of the analog signal resulting from the A-D conversion of the waveform of FIG. 1. The digital measurement typically consists of signed integers with a finite maximum value. The maximum positive  201  and maximum negative  202  values are shown that correspond to the maximum positive  101  and negative  102  values of the analog waveform of FIG. 1.  
         [0020]    [0020]FIG. 3 is a simplified block diagram of one embodiment of an analog to digital processing system  300  of the present invention. An analog input signal  301  is applied to the input of a gain element such as analog gain control  302 . Analog gain control  302  is variable and computer controlled by Central Processing Unit (CPU)  309  through control line  312 . In its simplest form analog gain control  302  applies a linear multiplier to analog input signal  301  to increase or decrease the amplitude by a fixed amount. Note that many other adjustments are possible and within the present invention. The resulting gain adjusted analog signal at  303  is applied to analog to digital converter  304  where the signal is changed from an analog signal to a digital representation of the analog signal. A digital representation of the signal is available at  305  and applied to a digital level measurement device such as Digital Signal Processor (DSP)  306 . Note that other forms of digital level measurement could be substituted in place of dedicated DSP. Measurement of the amplitude of the digital signal is made by DSP  306  and the digital signal is available at digital output  307 . Digital output  307  may be sent to a digital transmitter or other circuits for further manipulation.  
         [0021]    A level measurement control signal is available at  308  and is sent to CPU  309 . A user using user interface  311  may supply the desired control parameters through bi-directional control interface  310 . Note that CPU  309  may be programmed to automatically apply predefined control parameters. CPU  309  calculates the correct gain control parameters using level measurement control signals applied at  308  and user interface signals at  310 . These signals are available from DSP  306  and user interface  311 . The correct parameters are sent from CPU  309  to analog gain control  302  by gain control signal path  312 .  
         [0022]    [0022]FIG. 4 illustrates the dynamic range and resolution resulting from a digital normalization process. FIG. 4 also shows the effect of the normalization scheme on the dynamic range and noise floor. The digital representation of the signal includes a dynamic range  401 . Dynamic range  401  is the difference between digital minimum level  402  and digital maximum level  403 . An input signal includes an input maximum level  404  indicating that the input signal is not using the total available dynamic range  401 , with unused dynamic range  405  remaining. Adding gain  409  to input signal  406  in the digital domain results in a new noise floor  407  and no change to the input signal&#39;s dynamic range  408 .  
         [0023]    [0023]FIG. 5 illustrates the use of the dynamic range resulting from one embodiment of the present invention. Dynamic range  401 , maximum level  403  and minimum level  402  remain unchanged from FIG. 4. Input signal  406  is applied to the system of FIG. 3 and converted to digital. DSP  306  (FIG. 3) measures the difference between the maximum level  403  and input maximum level of  404  of input signal  406 . This measurement indicates the amount of unused gain  405 . This measurement is used to calculate the desired gain by CPU  309  (FIG. 3) that is sent via gain control path  312  to analog gain control  302 . The desired gain may also be dependent upon the value received from user interface  311 . This can be any level below the maximum input level  403 . The resulting signal  501  uses more of dynamic range  401  of the system. A maximum input level is now  502  leaving much less unused dynamic range  503 .  
         [0024]    [0024]FIG. 6 represents a flow chart  600  of the operation of one embodiment of the present invention. Before an input signal is applied  406  (FIG. 4), maximum digital level (also called maximum level)  403  is stored in step  601 . This maximum digital level  403  allowed by CPU  309  is compared to input level  406  in step  602 . In step  603 , if input level  406  is less than maximum digital level  403  step  610  prompts the user to reset gain lower.  
         [0025]    In step  603 , if input maximum  404  is below maximum level  403  and the system is set to automatically adjust the gain in step  604 , CPU  309  will increase the analog gain using the analog gain control  302  in step  609 . The gain is then changed in step  611 . If the system is not set to automatically adjust the gain, the user will be prompted in step  605 , through user interface  311 , to accept or reject the change in step  605 . Upon receipt of user input in step  606 , the user will either accept the gain change or reject the change in step  607 . If the gain change is rejected by the user in step  607 , the gain will not be changed as shown in step  608 . If the system is set to automatically adjust the gain in step  604 , then the gain will be increased in step  609 . If the input is at the maximum level  403  the invention will prompt the user to set the analog gain lower in step  610 .  
         [0026]    While the invention has been described in connection with a preferred embodiment, it is not intended to limit the scope of the invention to the particular form set forth, but on the contrary, it is intended to cover such alternatives, modifications, and equivalents as may be included within the spirit and scope of the invention defined by the appended claims.  
         [0027]    The drawings constitute a part of this specification and include exemplary embodiments to the invention, that may be embodied in various forms. It is to be understood that in some instances various aspects of the invention may be shown exaggerated or enlarged to facilitate an understanding of the invention.