Note: Descriptions are shown in the official language in which they were submitted.
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The present invention relates to a headphone system for
reproducing a 2-channel stereo signal recorded in a program
source and listening to the reproduced signal through a head~
phone.
The invention provides a headphone system which enables
an audiophile, when he merely connects a simple electric circuit
between a reproducing system and a usual headphone, to obtain an
auditorial state such as if he were listening directly to loud-
speakers Witll the auditory image located outside of his head in
spite in his actual state clad witll a headphone and continue to
listen with the headphone for a long time without feeling fati-
gue.
According to the present invention there is provided
a headphone system for reproduction of 2-channel stereophonic
signals with a pair of headphones, which system comprises a 2- ~-
channel stereophonic signal source, two low-pass filters each E
positioned at a cross-talk channel cross-coupling between a
respective channel of said 2-channel stereophonic signal source E
and a respective one of said headphones and each provided with t
`O a cut-off frequency not greater than 400 Hz and an attentuation
characteristic of -1.5 to -6 dB/oct and two adders each provided t
for receiving a cross-talk channel signal from the output of one ~;
of said low pass filters and a direct channel signal from said E
stereophonic signal source, the outputs of said adders being
respectively coupled to said headphones.
In one embodiment of the present invention the system
includes two delay circuits each connected between said stereo-
phonic signal source and said low-pass filter and each provided i-
for providing a delay time corresponding to the difference be- t~
tween the arrival times of the cross-talk channel and direct F:
channel signal at said headphone.
In another embodiment of the present invention the
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system includes two notch filters each connected between said
adder and said headphone for correcting sound pressure versus
frequency characteristic occurred due to the use of said head-
phone into the substantially same characteristic as that ob-
tained by actually listening to sound waves radiated from two
loudspeakers of said stereophonic signal source. L
The present invention will be illustrated by way of the
accompanying drawings in which:
Fig. 1 is a view schematically illustrating an auditorial
10state of a person listening directly to loudspeakers;
Fig. 2 is a view schematically illustrating an auditorial
state of a person listening with a headphone;
Fig. 3 is a view schematically illustrating an auditory
path in case of a direct auditorial state;
Figs. 4A to 4F are graphs showing sound pressure versus
frequency characteristics with respect to left and right ears of
the audiophile in case of Fig. 3 when the angle ~ is set to 15,
30, 45, 60, 75 and 90 respectively; [
Figs. 5A to 5F are graphs showing the (SL - SR) charac-
teristics regarding the characteristics in Figs. 4A to 4F; t
Fig. 6 is a view schematically illustrating one
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ofthe stereophonic channels in case of the direct auditorial
state;
Fig. 7 is a block diagram showing an embodiment of -
the headphone system according to the invention; and ~;
Fig. 8 is a graph showing a fre~uency characteristic -
of a notch filter used in the system according to the invention.
Fig. 1 is a view showing an auditorial state of a
person listening directly to loudspeakers and where a 2-
channelstereophonic signal recorded on a program source
such as a record disc or magnetic tape is being reproduced.
In Fig. 1, designated at 1 is a reproducing system re-
producing the program source, at 2 and 3 loudspeakers
converting the respective left and right signals reproduced
from the reproducing system 1 into sound,at4 an audiophile,
and at 5 and 6 the ears of an audiophile 4. In this case,
the sound wave radiated from the loudspeakers 2 and 3 are
directly received by the ears 5 and 6 of the ~udiophile 4, and
he recognizes the auditory image at a point outside his head,
for instance at point A.
Fig. 2 shows an auditorial state in which the reproduc-
tion of the same program source as in the case of Fig. 1
enters the ears of the audiophile via a headphone 7. In this
case of using the headphone 7, the audiophile 4 recognizes the
auditorial image at a point within his head, for instance at
point B. Designated at 8 and 9 in the Figure are right and
left sound converters of the headphone 7.
The invention features a headphone system,
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which permits a person to recog~ize an auditory image at
a point outside his head like the case of the direct
auditorial state as shown in Fig. 1 while he actually uses
the headphone 7 for listening as shown in Fig. 2.
Fig. 3 schematically shows a direct auditorial
state where sound wave issuin~ from one of the loudspeakers,
namely loudspeaker 3, is directly received hy the right
and left ears 5 and 6 of the audiophile 4. In this
situation, the sound wave of sound pressure S emerging
from the loudspeaker 3 and entering the right and left
ears 5 and 6 is guided through the respective auricles
to reach the eardrums and give sound pressures SR and SL
to the eardrums of the respective right and left ears 5
and 6.
Figs 4A to 4F show sound pressure versus fre~uency
characteristics with respect to the left and right ears
when the angle ~ of the loudspeaker 3 in Fig. 3 with
respect to the line directed right forward from the front
of the audiophile 4 is set to 15, 30, 45, 60, 75 ;
and 90, respectively.
Figs. 5A to 5F show the difference between the
sound pressure SL of the left ear 6 and the sound pressure
SR of the right ear 5 in the respective cases of Figs. 4A
to 4F. It will be seen that for ~ = 15 , 30, 45, 60,
75 or 90 , the value of (SR - SL) begins to decrease with
frequency from the neighborhood of 400 Hz. In Figs. 5A
to 5F, the plots A15 to Ago represent the approximation of
the value (SL - SR). The cut-off frequencies and
attenuation characteristics of Al5 to Ago are as follows.
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Cut-off frequency Attenuation characteristic
A15 250 (Hz) -l.S (dB/oct)
A30 220 ~3
A45 200 -3.5
A60 200 -4
A75 350 -6
Ago 400 ~4-5
The time lag T between the instants of arrival of
the sound wave issued from the loudspeaker 3 at the ears
5 and 6 is determined by the angle 0, the distance d
between the ears of the audiophile 4 and the speed C of
sound and is given as -
T = d sin ~
If ~ = 30, d = 17 cm and C = 340 m/s, the time lag
T 8 250 ~ sec.
In the case of the auditorial state shown in Fig. 6,
where the audiophile 4 uses the headphone 7, one of the 2-
channel signals, for instance, the left channel signal, is
converted by the left-hand sound converter 9 of the headphone
7 and is transmitted to the left ear only. Also, since
the sound converter 9 directly bears on the ear 6, a peculiar
sound pressure versus frequency characteristic is provided
due to the presence of the auricule between the sound converter
9 and the eardrum of the left ear. More particularly, when
the headphone bears on the ear a peculiar sound pressure
versus frequency characteristic (SH) shown by the dashed curve
in Fig. 4B, with a resonant frequency fl, is provided due to
the presence of the auricule.
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It will be seen from a comparison of Figs. 3 and
6 that in the auditorial state with a headphone a peculiar
sound pressure versus frequency characteristic which is
absent in the case of the direct auditorial state prevails
due to the presence of the headphone and also that in the
state with a headphone the right channel signal is transmitted
only to the left ear; that is, in the auditorial state with
a headphone the intelligence that would reach from the right-
hand speaker 3, for instance, to the right ear 5 in the case
of the direct auditorial state is lacking. Due to this
difference the auditory image is located within the audiophile's
head when he listens with the headphone. The invention is
predicated on the above considerations and permits realization
of practically the same auditorial state as the direct one
while wearing the headphone by making the peculiar charac-
teristic to the auditorial state with the headphone coincide
with that in the direct auditorial state and mixing each ~
channel signal after passing it through a delay circuit and ~-
a filter with the other channel signal. ~
An embodiment of the invention will now be described -
with reference to Fig. 7. In the Figure, designated at 10
and 11 are delay circuits each providing the afore-mentioned
delay time T, at 12 and 13 low pass filters with a cut-off
frequency not greater than 400 Hz and an attenuating charac-
teristic of -1.5 to -6 dB/oct as shown in Figs. 5A to 5F,
at 14 and 15 adders, and at 16 and 17 notch filters attenua-
ting only a particular frequency fl as shown in Fig. 8. The
notch filters 16 and 17 are provided to correct the character-
istic due to the use of
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1 the headphone as shown by the dashed curve in Fig. 4~
for obtaining the same sound pressure versus frequency
characteristic (S~) as in the case of the direct auditorial
state.
As has been shown in the foregoing, in the head-
phone system according to the invention notch filters
are used to couple the signals to the headphone so that
the characteristic peculiar to the auditorial state using
the headphone is removed. Also, since each of the 2-
10 channel signals is mixed with the other channel signal :-
after passing through the delay circuit and filter, it
is possible to obtain the same auditorial state as the
direct one while actually using the headphone for listen-
;~ -.
A ing~ ~ is, in spite of the auditorial state of a person
wearing the headphone, he can recognize the auditory
image located outside his head without feeling any oppres-
sive sensation peculiar to the auditorial state using
the headphone and continue this state for a long time
without feeling fatigue.
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