Patent Publication Number: US-6711268-B2

Title: Automatic stereo/monaural headphone

Description:
RELATED APPLICATION 
     This application claims the benefit of Provisional Application No. 60/212,807 filed Jun. 19, 2000 entitled AUTOMATIC STEREO/MONAURAL HEADPHONE. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to headphones designed primarily for classroom use in the education environment. A great variety of audio and audio-visual equipment is now commonly employed for instructional purposes in school classrooms. The use of headphones for students to listen to instructional audio material is often desirable in order to provide a noise-free private environment for individual and small group instruction. These audio sources include study carrels, phonographs, tape players, compact disk players, video displays, film projectors, and computers. The audio sources commonly employed in the education environment may be monaural or stereo and are often accessed by a single jack that accepts only a ⅛-inch or a ¼-inch headphone plug. This has resulted in difficulty for teachers and equipment custodians to stock and maintain control of headphone equipment for use with all of these various audio sources. 
     PRIOR ART 
     Existing headphone designs for accessing the various audio sources described above are shown in FIG.  1  and FIG.  2 . FIG. 1A discloses a headphone  10  that includes left and right earpieces  12  and  14  respectively, a connection cord  18  terminated in a stereo plug  16  and an “in-line” switch  20  to select a stereo or monaural source. FIG. 1B is a circuit diagram showing the switch  20  set for a stereo source. It shows a common connection from the plug sleeve-contact  22  to like identified terminals (+) of the two earpiece drivers  13  and  15 . The plug ring-contact  24  is connected to the other identified terminal (−) of the left earpiece driver  13  and the plug tip-contact  26  is connected to the corresponding identified terminal (−) of the right earpiece driver  15 . This setting provides normal stereo listening from a stereo source. FIG. 1C shows the circuit diagram with the switch  20  set to access a monaural source. There is no connection to the plug  16  ring-contact  24 . The plug  16  tip-contact  26  is connected to the (−) terminals of the earpiece drivers  13  and  15 . This allows a monaural source to be heard in both ears. A plug adapter  28  provides access to audio sources that accept only a ¼-inch plug. 
     Although a headset with a mono/stereo selection switch would require only the addition of a single stereo plug adapter to change the headset plug size, the use of such a switch for selecting a monaural or stereo source has not found favor in the education environment for the following reasons: 
     1. The selection switches are likely to be played with by the students and have not proved to have the ruggedness and reliability required for classroom use, and; 
     2. The source must be positively identified prior to making a switch selection. 
     3. Teachers prefer to configure the headset for the proper source when used by younger children so they cannot easily change it to the wrong configuration. 
     FIG. 2 discloses additional prior art for a headphone system that can access either a monaural or stereo audio source that accepts only a ¼-inch or ⅛-inch plug. FIG. 2A shows a stereo headset  30  consisting of an adjustable headband  32 , left and right earpieces  34  and  36  respectively, connection cord  38  terminated in a molded plug assembly  40  that includes a ⅛-inch stereo plug  42 . The plug  42  includes external screw threads  58  that mate with internal screw threads of three plug adapters  44 ,  46  and  48 . A plastic holder  50  for the plug adapters is secured to the connection cord  38  to store them when not in use. These plug adapters are shown enlarged in outline drawings in FIG.  2 C. FIG. 2B is a schematic diagram of the headset for accessing a stereo source with the ⅛-inch stereo plug  42 . The ring-contact  54  is connected to the (−) terminal of the left earpiece driver  35  and the tip-contact  56  is connected to the (−) terminal of the right earpiece driver  37 . A connection to the (+) terminal of each earpiece driver is made to the sleeve-contact  52  of the plug  42 . 
     Plug adapter  44  shown in FIG. 2C converts the headset  30  for listening to a monaural source equipped with a ¼-inch output jack. It connects the tip-contact  60  to both the tip-contact  56  and to the ring-contact  54  of the stereo plug  42  shown in FIG. 2B thus allowing the audio source to be heard at both earpieces  34  and  36  of the headset. Sleeve-contact  62  connects the source sleeve circuit to sleeve-contact  52  of plug  42 . Similarly plug adapter  46  provides for listening to a monaural source equipped with a ⅛-inch output jack. It connects the tip-contact  64  to both the tip-contact  56  and to the ring-contact  54  of the stereo plug  42  shown in FIG. 2B thus allowing a monaural audio source equipped with a ⅛-inch output jack to be heard at both earpieces  34  and  36  of the headset. 
     Plug adapter  48  shown in FIG. 2C adapts the ⅛-inch stereo plug  42  of the headset to a stereo source equipped with a ¼-inch stereo jack. It connects the tip-contact  68 , the ring-contact  70 , and the sleeve-contact  72  to their respective contacts  56 ,  54  and  52  of plug  42  thus providing for normal stereo listening to the source. 
     SUMMARY OF THE INVENTION 
     The headphone design of this invention accesses a monaural or stereo source with a connection cord preferably terminated in a ⅛-inch stereo plug. In a first embodiment of the invention the right and left signals from a stereo source are connected in a normal manner directly to each earpiece driver; that is, the right channel is connected to an identified terminal of one (usually the right) earpiece driver, and the left channel is connected to a similarly identified terminal of the other earpiece driver. A common connection from the stereo source is connected to the other terminal of each earpiece driver. In this embodiment of the invention a connection is also made between the right and left channel identified terminals of the earpiece drivers through an impedance element. The magnitude of the impedance element is chosen with respect to the impedance of the earpiece drivers such that an audio signal appearing at only one earpiece driver terminal, as would be the case with a monaural source, is coupled to the other earpiece driver with a reduction in signal strength at the other earpiece driver terminal of the order of two decibels. This slight reduction in loudness in one ear is virtually imperceptible to the listener, probably aided by a psycho-acoustical effect that masks the slight reduction in loudness in one ear if the other ear suffers no reduction in loudness. 
     The impedance element results in a slight mixing of right and left channel audio signals from a stereo source. The amount of channel mixing described above is so small that stereo separation is virtually unaffected and no reduction of stereo imaging is perceptible to the listener. The reason for this small amount of channel mixing is the typical low impedance of the audio sources compared to the value of the impedance element; a ratio of about one hundred. This results in the crosstalk between channels caused by the impedance to be about 40 decibels below the level of either channel, a level well below the channel separation requirement for good stereo performance. 
     In a second embodiment of the invention the stereo channel that is connected to the ring contact of the stereo plug is connected directly to one earpiece driver and the other channel that is connected to the tip of the stereo plug is connected to each earpiece driver through a an impedance element. A monaural source is always accessed by the plug tip, and thus each earpiece driver is connected to the source through one of the impedance elements. If each impedance element is one-half the value of the impedance element used in the first embodiment described above the monaural signal will be heard at each earpiece with equal loudness at a level of the order of one decibel less than if a direct connection to each earpiece driver had been made as in a conventional stereo headset. With a stereo source one channel will be heard with no reduction in loudness compared to a conventional stereo headset and the other channel will be heard with reduced loudness of the order of one decibel. This slight difference in the level of the stereo channels is not perceptible and can be easily balanced, if desired, by a balance control available with many stereo sources. Again, because of the typical low impedance of the stereo source, the stereo crosstalk due to the coupling impedance elements is about −34 decibels, providing excellent stereo performance. 
     Thus the headset can be used with either a monaural or stereo source without requiring any action to identify the nature of the source or configure the headset for the type of audio source, while providing virtually the same efficacy of use as a separate headphones dedicated to either a monaural or stereo source, or a headphone provided with a mono/stereo switch selector, or a headphone employing stereo-to-monaural plug adapters. 
     Accordingly, it is an object of the invention to provide a headphone that can be used for automatic listening to a monaural or stereo audio source, that does not have to be configured for the type of audio source, and that provides for hearing with substantially equal loudness at both earpieces either a monaural signal or equal magnitude right and left channel stereo signals and with which the stereo source is heard with right and left channel separation and stereo imaging substantially as afforded by the stereo source. 
     It is a further object of this invention to provide at a lower cost than heretofore possible a headphone that requires no switching means and only one plug adapter to access the four configurations of sources represented by a monaural or stereo source with either a single ⅛-inch or single ¼-inch output jack. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a drawing of prior art showing a headphone that employs a Monaural-Auto selection switch to select a monaural or stereo audio source. 
     FIG. 2 is a drawing of prior art showing a headphone that employs plug adapters to select a monaural or stereo audio source. 
     FIG. 3 is a drawing of the present invention. 
     FIG. 4 is a schematic drawing of a first embodiment the present invention. 
     FIG. 5 is a schematic drawing of a first embodiment the present invention showing its connection to a monaural audio source. 
     FIG. 6 is a schematic drawing of a first embodiment the present invention showing its connection to a stereo audio source. 
     FIG. 7 is a schematic drawing of a second embodiment the present invention. 
     FIG. 8 is a schematic drawing of a second embodiment the present invention showing its connection to a monaural audio source. 
     FIG. 9 is a schematic drawing of a second embodiment the present invention showing its connection to a stereo audio source. 
    
    
     DESCRIPTION OF THE INVENTION 
     The invention is described with reference to FIGS. 3-9. 
     FIG. 3 shows what appears to be a conventional stereo headset  80  that includes an adjustable headband  82 , left earpiece  84 , right earpiece  86 , a connection cord  88  that is terminated in a ⅛-inch stereo plug  90 . A screw-on ⅛-inch to ¼-inch stereo plug adapter  92  is provided with the headset. The adapter  92  is attached to the headphone cord  88  by bead-chain  91  to prevent it from becoming misplaced and lost. FIG. 4 illustrates the internal construction of a first embodiment of the invention. The tip  98  of the stereo plug  90  is connected to the + terminal of the right earpiece driver  87  and the ring contact  96  of the stereo plug  90  is connected to the + terminal of the left earpiece driver  85 . The sleeve  94  of the stereo plug  90  is connected to the negative terminal of each earpiece driver. These are the normal connections of a conventional stereo headset. In this embodiment of the invention an impedance element consisting of a single resistor  100  is connected between the + terminals of the earpiece drivers, that is, from the plug tip  98  to the plug ring contact  96  of the stereo plug  20 . Therefore, the resistor can be located in the plug assembly  90 . In the construction shown in FIG. 3 the connection cord enters the left earpiece  84  and connection to the right earpiece  86  is made by conductors passing through the headband  82 , thus allowing the resistor to be alternatively located in the left earpiece instead of in the plug assembly. 
     The value of the resistor  100  is selected with respect to the impedance of the earpiece driver  87  shown in FIG. 4 such that the reduction in loudness at the left earpiece is acceptable to the listener. FIG. 5 is a schematic diagram of a typical headphone employed in this invention when connected to a monaural audio source. For the earpiece driver R D  impedance of 300 ohms, the resistor R 1  is given a value of 100 ohms. This reduces the MONAURAL SOURCE signal level at the left earpiece to 75%, or about 2.5 decibels. On the basis of A/B tests with the resistor R 1  switched in or out, there was no perceptible difference in loudness among the listeners tested, and all listeners felt that they were hearing the monaural signal satisfactorily in both ears. Many listeners could detect a loudness reduction of one decibel when loudness is reduced in both ears or when listening with only one ear. Since the loudness ratio of one decibel was originally established as that representing the threshold of loudness difference perceptible to the human ear this is not a surprising result. It is believed that the imperceptibility of loudness reduction of 2 to 3 decibels at just one ear in the tests mentioned above results from a psycho-acoustic effect in which the brain masks this degree of loudness reduction if occurring at only one ear. 
     FIG. 6 is a schematic diagram of the embodiment of the invention shown in FIG. 3 when plugged into a stereo source. It can be seen that the right and left channel stereo sources are connected directly to their respective earpiece drivers. The typical output impedance of the various audio sources cited above is of the order of one ohm, and that value is used in these illustrations. It can be shown that the stereo signals will be delivered to the earpiece drivers with negligible reduction (about 0.1 decibels) in loudness due to the 100-ohm resistor R 1 , compared to dedicated headphones. 
     The 100-ohm resistor R 1  will also couple some signal from each stereo channel into the other channel resulting in a small amount of crosstalk between channels. Such crosstalk tends to reduce the channel separation upon which stereo imaging is dependent. Stereo imaging depends upon both phase and delay differences as well as magnitude differences between channels and satisfactory imaging can be achieved with channel amplitude separation as little as 10 decibels. Because of the large ratio (100:1) of R 1  to the source resistance R S , crosstalk contributed by the resistor R 1  is −40 decibels, and is virtually imperceptible. 
     FIG. 7 shows a second embodiment of the invention. In order to equalize the loudness at both ears when accessing a monaural source, the plug tip  98  of the stereo plug  90  is connected to each of the earpiece drivers  85  and  87  through resistors  102  and  104  that are of equal value. The left earpiece driver  85  is directly connected to the ring contact  96  of the stereo plug  90 . 
     FIG. 8 is a schematic diagram of the second embodiment when accessing a monaural source. If the resistors R 3  and R 4  are each given ½ the value (50 ohms) of the resistor R 1  (100 ohms) used in the first embodiment of the invention, the reduction in loudness at each earpiece is only 1.3 decibels, and the monaural signal is received with equal loudness at both earpieces. 
     FIG. 9 is a schematic diagram of the second embodiment of the invention when accessing a stereo audio source. Since the left channel receives the signal directly from the plug ring and the right channel receives the signal from the plug tip through the 50 ohm resistor R 2  the right channel loudness is 1.3 decibels lower than the left channel. Again, this slight difference in loudness is not perceptible. Many stereo sources have a balance control that can adjust this small difference if it further contributes to a perceptible difference in channel balance. Crosstalk between channels is increased from −40 decibels of the first embodiment to −34 decibels, which still results in an imperceptible difference in stereo imaging. 
     Both embodiments of this invention described above exhibit excellent efficacy in providing a headset with automatic monaural/stereo listening that virtually equals the performance of an individual monaural or stereo headset or a single headset that can be configured by a switch or plug adapters to duplicate the performance of an individual monaural or stereo headset. The table below shows the differences in the performance between the two embodiments and individual headsets. 
     
       
         
           
               
               
               
               
            
               
                   
               
               
                   
                 RIGHT 
                 LEFT 
                   
               
               
                   
                 EARPIECE 
                 EARPIECE 
                 STEREO 
               
               
                   
                 LOUDNESS 
                 LOUDNESS 
                 CROSSTALK 
               
               
                 AUDIO 
                 EMBODIMENT 
                 EMBODIMENT 
                 EMBODIMENT 
               
            
           
           
               
               
               
               
               
               
               
            
               
                 SOURCE 
                 1 
                 2 
                 1 
                 2 
                 1 
                 2 
               
               
                   
               
               
                 Monaural 
                 0 dB 
                 −1.3 dB 
                 −2.5 dB 
                 −1.3 dB 
                 NA 
                 NA 
               
               
                 Stereo 
                 0 dB 
                 −1.3 dB 
                   0 dB 
                   0 dB 
                 −40 dB 
                 −34 dB 
               
               
                   
               
            
           
         
       
     
     From a theoretical viewpoint embodiment 1 may be preferred by those whose main interest is to maximize stereo performance, whereas others whose primary use involves monaural sources might prefer the slightly better loudness balance of embodiment 2. In either case, the choice would be very difficult to make based upon comparative listening tests. 
     While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that other embodiments are possible. It should be recognized that some variation could be made in the resistor values shown in the above illustrations without materially reducing the efficacy of this invention. Earpiece drivers that vary significantly from the examples shown above will require different impedance element values to preserve substantially the same ratios to the driver impedance as is shown in the illustrations of this disclosure. Depending upon the characteristics of the earpiece drivers it might be desirable to design the impedance element(s)  100 ,  102 , and  103  shown in FIGS. 4 and 7 as complex impedances rather than pure resistors by including inductive or capacitive components in them. Furthermore, while generally specific claimed details of the invention constitute important specific aspects of the automatic monaural/stereo headphone, in appropriate instances even the specific claims should be considered in light of the doctrine of equivalents.