Electrical audio signal processing systems and devices – Headphone circuits
Reexamination Certificate
2001-05-04
2004-03-23
Harvey, Minsun Oh (Department: 2644)
Electrical audio signal processing systems and devices
Headphone circuits
C381S011000, C381S001000, C381S026000
Reexamination Certificate
active
06711268
ABSTRACT:
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
Colegrave Arthur Barry
Goldstein Richard
Enztec Audio Ltd.
Grier Laura A.
Harvey Minsun Oh
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