Abstract:
A pitch adaptive circuit ( 200 ) includes an equalizer control circuit ( 206 ) that evaluates the pitch of the speech signals that are being processed and depending on the pitch information, the equalizer control circuit ( 206 ) selects an equalizer ( 208, 210 ) to shape the decoded speech signals. By selecting the best equalizer ( 208  or  210 ) to use based on the pitch information, improvements in audio quality are provided automatically without user intervention.

Description:
TECHNICAL FIELD 
     This invention relates in general to the field of electronics, more specifically to a method and apparatus for providing pitch adaptive equalization for improved audio. 
     BACKGROUND 
     Communication devices such as digital cellular telephones use low bit rate vocoders to encode and decode the users&#39; speech signals. Modeling and compressing of the speech signals achieves increased capacity in a communication system. The end product of modeling and compressing of the speech signals is sometimes unnatural sounding reproduced speech. Added to this problem is the constant pressure to keep manufacturing costs low in electronic devices, which leads to the use of lower quality audio circuitry, microphones, speakers, etc. 
     Equalization of the audio signal, which can be done either in hardware and/or software, can help increase the intelligibility of the decoded speech and counteract some of the limitations of the audio circuitry. However, the problem with equalization is that it is very difficult to provide equalization for a broad group of users such as female and male voices. 
     In  FIG. 1 , there is shown a prior art audio circuit having an equalizer. Coded speech is presented to a vocoder  102  that provides decoded speech at its output. The decoded speech is sent to an audio equalizer  104  for equalization prior to being converted to analog by digital-to-analog converter  106 . The analog speech is then presented to a speaker (or earpiece)  108 . Although useful, the equalizer  104  can not provided optimum equalization for speech signals having different pitch. An equalization curve that may provide good results for higher pitched voices may leave lower pitched voices sounding muffled. While an equalization setting that may sound good for lower pitched voices may leave higher pitched voices sounding harsh or thin. 
     One solution to the equalization problem above is to provide simple “bass” and “treble” controls that the user can adjust manually or by providing a multi-band equalizer as found in some audio equipment. However, such controls are not typically found in cellular telephones and even if they were, the cellular telephone user may do more harm than good, since proper equalization setting can be tricky. Users may end up blaming poor sound quality on the cellular telephone and associate poor sound quality with the particular cellular telephone manufacturer even if the poor sound quality is caused by improper equalizer settings. A need thus exists in the art for a better method of providing different equalization setting for different voice types in order to improve the overall sound quality in communication devices such as cellular telephones. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The features of the present invention, which are believed to be novel, are set forth with particularity in the appended claims. The invention, may best be understood by reference to the following description, taken in conjunction with the accompanying drawings, in the several figures of which like reference numerals identify like elements, and in which: 
         FIG. 1  shows a prior art equalization technique. 
         FIG. 2  shows a block diagram of a pitch adaptive equalization circuit in accordance with the invention. 
         FIG. 3  shows a block diagram of an alternate embodiment of a pitch adaptive equalization circuit in accordance with the invention. 
         FIG. 4  shows an electronic device such as a cellular telephone in accordance with the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     While the specification concludes with claims defining the features of the invention that are regarded as novel, it is believed that the invention will be better understood from a consideration of the following description in conjunction with the drawing figures. 
     Referring now to  FIG. 2 , there is shown a pitch adaptive equalization circuit  200  in accordance with one embodiment of the invention. Coded speech is provided to an input port of a vocoder  202  for decoding of the speech. In this embodiment it is assumed that vocoder  202  provides pitch information  204  such as an estimate of the pitch of the speech it is decoding. The pitch information  204  is provided to an equalizer control circuit  206 . Equalizer control circuit  206  can be for example a simple threshold circuit where pitch information below a certain frequency result in a first state such as a “low pitch” decision being made, while pitches above the certain frequency result in a second state such as a “high pitch” decision being made. A more sophisticated equalizer control circuit  206  may average the pitch estimates received from vocoder  202  over a predetermined period of time in order to prevent a possible scenario where the equalizer control circuit  206  based on the received pitch information  204  starts toggling between the “low pitch” decision and the “high pitch” decision. In still another design of equalizer control circuit  206 , the equalizer control circuit  206  could support several threshold levels and the equalizer control circuit  206  could select from amongst more than two equalizer circuits. 
     The equalizer control circuit  206  based on the pitch determination the circuit has made provides a control signal  218 . In the embodiment shown, the control signal  218  controls a switch  216  that selects between a first or low-pitch equalization circuit  208  or a second or high-pitch equalization circuit  210 . Although, two equalization circuits are shown, in other designs more than two equalization circuits can be supported with the equalizer control circuit  206  providing the extra control signal information to make the equalization decision. The equalization circuits  208 ,  210  shape the decoded speech signal provided by the vocoder  202  and is each set to equalize for a different pitched signal. The equalization circuits or equalizers  208 ,  210  can be formed from just hardware or just software or a combination of both. In the case the equalizers  208 ,  210  are formed using software, switch  216  represents the selection of the appropriate equalizer software routine or equalizer coefficients from memory. 
     After equalization, the equalized speech is converted into analog by a digital-to-analog (D/A) converter  212 . The analog signal is then presented to a speaker or earpiece  214 . Although not shown, typically an audio amplifier is provided to amplify the analog speech signal prior to being presented to speaker or earpiece  214 . 
     In  FIG. 3  there is shown another embodiment of a pitch adaptive equalization circuit  300 , in circuit  300  the vocoder  302  does not provide a pitch estimate as in circuit  200 , so a pitch detection/estimation and equalization control circuit  304  is provided. Circuit  304  can use one of a number of well-known pitch detection methods known in the art and again based on its pitch detection/estimation an equalization control signal  306  is provided to control switch  308 . Switch  308  selects from amongst a first or low-pitch equalization circuit  310  or a second or high-pitch equalization circuit  312 . Again, although two equalization circuits are shown, any number of equalizers can be used, depending on the design. If more than two equalization circuits are used, pitch detection/estimation circuit  312  can have as many pitch threshold circuits, as there are equalization circuits. After the appropriate equalization circuit  310  or  312  provides the proper shaping, the equalized speech is converted into analog by D/A circuit  314  prior to being presented to speaker  316 . Again, equalizers  310  and  312  can be formed from hardware circuitry, software, or a combination of the two. In  FIG. 4 , there is shown a electronic device such as a cellular telephone  400  that uses a audio circuit having an adaptive equalization circuit such as circuit  200  or  300 . 
     By adapting the audio equalization to the voice characteristics of the received speech signal, improvements in audio quality and intelligibility can be provided by the automatic equalization technique provided by the present invention. Selecting an equalization circuit based on the estimated pitch of the speech signal that is being equalized, helps provide better audio performance in electronic devices such as cellular telephones, etc. 
     While the preferred embodiments of the invention have been illustrated and described, it will be clear that the invention is not so limited. Numerous modifications, changes, variations, substitutions and equivalents will occur to those skilled in the art without departing from the spirit and scope of the present invention as defined by the appended claims.