Method of and arrangement for adjusting the transfer characteristic to two listening position in a space

In the method of adjusting the transmission of an electric signal, applied to an input terminal (1, 1'), via two transducer devices (8, 9), to an acoustic signal at two positions (P.sub.1, P.sub.2) in a space (10), (a) the levels of the electric signals applied to the two transducer devices are adjusted in such a way that the two transducer devices provide substantially equal contributions to the acoustic signal at the first position (P.sub.1), (b) the second filter unit (4) is adjusted so as to obtain a desired frequency-response characteristic (50, FIG. 5) for the transmission from the input terminal (1, 1') to the second listening position (P.sub.2), (c) the first filter unit (3) is adjusted so as to obtain a desired frequency-response characteristic (50, FIG. 5) for the transmission from the input terminal (1, 1') to the first listening position (P.sub.1). Also described are an arrangement for carrying out the method (FIG. 1) and an arrangement for amplifying an electric signal adjusted on the basis of the measurement results obtained in accordance with the method (FIG. 3).

The invention relates to a method of adjusting the frequency-response 
characteristic of the transmission of an electric signal which is applied 
to an input terminal of an amplifier unit and which is converted to an 
acoustic signal at at least one listening position in a space via two 
electro-acoustic transducer devices, the electric signal being applied, 
via the input terminal, to a first and a second filter unit belonging to 
the amplifier unit and having an output coupled to the first and the 
second transducer device respectively, and also relates to an arrangement 
for carrying out the method and to an arrangement for amplifying an 
electric signal, comprising a first and a second filter unit adjusted in 
response to the measurement results obtained by means of the method. The 
method of the type defined above is known from European Patent Application 
No. 165,733 which has been laid open to public inspection and which 
relates to an equalizing method for use in a car. Although the invention 
is not limited to automotive use, the description is mainly based on the 
use of the invention in a car. Moreover, as will become apparent 
hereinafter, the invention is not limited to the mere equalization of a 
frequency-response characteristic, but is more generally intended for 
adjusting a frequency-response characteristic so as to obtain a desired 
(not necessarily flat) frequency-response characteristic. 
In the known method and the known arrangement an electric signal in the 
form of a stereo signal is applied to two electro-acoustic transducer 
devices. One transducer device (for example the first one) comprises two 
loudspeakers arranged at the front left and front right in the passenger 
compartment of the car to reproduce the left-hand and right-hand signal 
components of the stereo signal. The other transducer device (for example 
the second one) also comprises two loudspeakers arranged at the rear left 
and rear right in the passenger compartment, also for reproducing the 
stereo signal. 
The known method provides equalization for one listening position. At the 
listening positions detection means are placed in the form of two 
microphones arranged in a artificial head, which microphones measure the 
acoustic signal at this listening position, convert this acoustic signal 
into an electric signal which is a measure of the acoustic signal detected 
at this listening position, and feed it to an output coupled to an input 
of a control unit. Under the influence of this signal the control unit 
supplies a control signal to two filter units in the form of equalizers, 
which are each coupled between the input terminal of the arrangement and 
one of the two transducer devices. Under the influence of the control 
signal the two equalizers are adjusted in such a way that a substantially 
flat frequency-response characteristic is obtained for the transmission of 
the electric signal at the input terminal to the acoustic signal at the 
listening position. 
It is the object of the invention to provide a method of and device for 
carrying out the method, enabling a desired frequency-response 
characteristic for the transmission to at least two non-coincident 
listening positions in the space to be obtained. In particular, it may 
provide equalization for these two listening positions. 
To this end the method in accordance with the invention is characterized in 
that (a) the levels of the electric signals applied to the first and the 
second transducer device are adjusted in such a way that the acoustic 
signals delivered by the first and the second transducer device provide 
substantially equal contributions to the acoustic signal at the location 
of the first listening position; (b) the second filter unit is adjusted so 
as to obtain a desired frequency-response characteristic for the 
transmission of the electric signal, applied to the input terminal, to the 
acoustic signal at the second listening position; (c) the first filter 
unit is adjusted in such a way that in the presence of an electric signal, 
applied to the second transducer device a desired frequency-response 
characteristic is obtained for the transmission of the electric signal, 
applied to the input terminal, to the acoustic signal at the first 
listening position, in that the first and the second listening position 
are situated at least substantially between the first and the second 
transducer device in the space, and in that the distance between the first 
transducer device and the first listening position is smaller that the 
distance between the first transducer device and the second listening 
position, and vice versa. The method is very suitable for equalizing two 
listening positions, for example, in the passenger compartment of a car 
and in that case it is characterized in that in the steps (b) and (c) the 
second filter unit and first filter unit respectively are adjusted so as 
to obtain a substantially flat frequency-response characteristic for the 
transmission of the electric signal, applied to the input terminal, to the 
acoustic signal at the second and the first listening position 
respectively. The first listening position is for example the front 
position and the second listening position is then the rear position in 
the passenger compartment. In that case the first transducer device 
comprises the two front loudspeakers and the second transducer device 
comprises the two rear loudspeakers. 
The invention is based on the recognition of the following facts. After the 
contributions of the first and the second transducer devices to the 
acoustic signal at the first listening position have been equalized in 
step (a), this means in fact that the level of the electric signal applied 
to the first transducer device will be lower than the level of the 
electric signal applied to the second transducer device. This is because 
the first listening position is situated closer to the first than to the 
second transducer device. As a result of this, the acoustic signal at the 
second listening position is mainly determined by the acoustic signal 
delivered by the second transducer device. In step (b) this enables the 
frequency-response characteristic of the transmission of the electric 
signal, at the input terminal to the acoustic signal at the second 
listening position to be adjusted in conformity with a desired curve, for 
example to be equalized, by means of the second filter unit. It is 
irrelevant whether this is effected when the first transducer device is 
operative or inoperative. 
Adjusting the transmission between the electric signal at the input 
terminal and the acoustic signal at the location of the first listening 
position is subsequently effected in step (c) by adjusting the first 
filter unit. Since the acoustic signal at the first listening position is 
determined inter alia by the acoustic contribution provided by the second 
transducer device, this second transducer device must be operative during 
step (c) of the method. 
The method may be characterized further in that in step (a) the level of 
the electric signal which, via the associated transducer device, provides 
the larger contribution to the acoustic signal at the location of the 
first listening position is reduced until the contributions to the 
acoustic signal at the first listening position by the acoustic signals 
delivered by the first and the second transducer device are substantially 
equal to one another. This ensures that a manually preset level (selected 
by the driver) which is acceptable for both listening positions is not 
exceeded after step (a) has been carried out, so that this cannot start 
the persons in the space. 
Changes in conditions affecting the acoustic characteristics of the space 
(for example a passenger gets out of the car) and a change in balance 
between the left-hand and the right-hand signal component of the stereo 
signal influence the transmission from the input terminal to the two 
listening positions. In order to restore the desired frequency-response 
characteristics in such a case the steps (a), (b) and (c) should therefore 
be repeated at least once. This repeating of the method can be effected 
for example automatically after a control unit in the arrangement for 
carrying out the method has detected a (manual) change of the balance 
setting. If, moreover, during repetition of step (b) the first transducer 
device is operative, a kind of iteration can be realized by repeatedly 
carrying out the method, leading to an optimum setting of the filter 
units. 
An arrangement for carrying out the method, comprising: 
an input terminal for receiving an electric signal; 
an amplifier unit having an input coupled to the input terminal, a first 
and a second output, a first and a second filter unit coupled between the 
input and the first and the second output respectively, and a 
control-signal input for receiving a control signal; 
a first and a second transducer device respectively coupled to the first 
and the second output of the amplifier unit; 
detection means for detecting an acoustic signal at the listening position 
in the space, for generating an electric signal which is a measure of the 
acoustic signal detected at said position, and for applying said electric 
signal to an output; 
a control unit comprising a frequency-analyser unit, having an input 
coupled to the output of the detection means, and an output for supplying 
the control signal, which output is coupled to the control-signal input of 
the amplifier unit, 
is characterized in that the detection means are further adapted to detect 
an acoustic signal at the second listening position in the space, to 
generate an electric signal which is a measure of the acoustic signal 
detected at said second position, and to apply said electric signal to an 
output, which output is coupled to an input of the control unit, in that 
the control unit is adapted: 
(a) to adjust the levels of the electric signals applied to the first and 
the second transducer device by the amplifier unit in such a way that the 
acoustic signals supplied by the first and the second transducer device 
provide substantially equal contributions to the acoustic signal at the 
location of the first listening position and detected by the detection 
means; 
(b) to adjust the second filter unit so as to obtain a desired 
frequency-response characteristic for the transmission of the electric 
signal, applied to the input terminal, to the acoustic signal detected by 
the detection means at the second listening position; 
(c) to adjust the first filter unit, while the second transducer device is 
operative, so as to obtain a desired frequency-response characteristic for 
the transmission of the electric signal, applied to the input terminal, to 
the acoustic signal detected by the detection means at the first listening 
position. This arrangement enables the transfer characteristics to two 
listening positions in a space to be adjusted, for example to be 
equalized. 
The detection means may comprise two microphone devices, one for each of 
the two position, or one microphone device which means that this device 
has to be transferred continually between the first and the second 
listening position. If the transfer characteristics to two listening 
positions in a car are to be adjusted (equalized), the method may be 
carried out once (for example at the car factory), after which the setting 
of the two filter units may be adapted as standard in a car radio. 
Such a car radio then includes an arrangement which comprises: 
an input terminal for receiving the electric signal, 
an amplifier unit having an input coupled to the input terminal, a first 
and a second output, and a first and a second filter unit coupled between 
the input and the first and the second output respectively, to which first 
and second output a first and a second electro-acoustic transducer device 
can be connected, and which is characterized in that the two filter units 
are adjusted in such a way that, if the two transducer devices are 
connected to the two outputs and the arrangement with the transducer 
devices is arranged in a space, a desired frequency response 
characteristic is obtained for the transmission between the electric 
signal at the input terminal and the acoustic signal at a first listening 
position in the space and the transmission between the electric signal at 
the input terminal and the acoustic signal at the second listening 
position in the space, which second position does not coincide with the 
first position, in that the first and the second listening position in the 
space are situated at least substantially between the first and the second 
transducer device, and in that the distance between the first transducer 
device and the first listening position is smaller than the distance 
between the first transducer device and the second listening position, and 
vice versa.

FIG. 1 shows an arrangement for carrying out the method in accordance with 
the invention. The present arrangement serves for equalizing the 
transmissions to two listening positions P1 and P2 in a space 10. The 
arrangement comprises an input terminal 1-1' for receiving an electric 
signal. This signal is an electric stereo signal whose left-hand signal 
component is applied to the terminal 1 and whose right-hand signal 
component is applied to the terminal 1' of the input terminals 1-1'. The 
arrangement further comprises an amplifier unit in the form of an 
equalizing unit 2 comprising a first and a second filter unit (equalizer) 
3 and 4 respectively. The equalizers 3 and 4 are coupled between an input 
5-5' and a first output 6-6' and a second output 7-7' respectively of the 
equalizing unit 2. The first output 6-6' is coupled to a first transducer 
device 8. The second output 7-7' is coupled to a second transducer device 
9. The two transducer devices 8 and 9 are arranged in the space 10. The 
space 10 is, for example, the passenger compartment of a car, the 
transducer device 8 being arranged at the front of the passenger 
compartment and the transducer device 9 at the rear of the passenger 
compartment, for example on the rear-deck panel. The transducer devices 8 
and 9 each comprise two loudspeakers 8L, 8R and 9L, 9R respectively, for 
converting the electric stereo signal applied to the transducer devices 
into an acoustic stereo signal. 
For equalizing the two listening positions P.sub.1 and P.sub.2 in the space 
the arrangement further comprises detection means 11, 12 and a control 
unit 13. The detection means are adapted to detect the acoustic signal at 
the listening positions 11 and 12, to generate an electric signal which is 
a measure of the acoustic signal at these positions, and to apply said 
electric signal to first and second inputs 14 and 15 of the control unit 
13. 
The detection means 11, 12 may be constructed in such a way that at each 
listening position one microphone is arranged as is shown in FIG. 1 of the 
afore-mentioned European Patent Application No. 165,733. Another 
possibility is that the detection means 11, 12 comprise two microphones 
for each listening position, which microphones are for example arranged in 
an artificial head, as is shown in FIG. 9 of European Patent Application 
No. 165,733. Yet another possibility is that the detection means 11, 12 
comprise only one microphone--or only one set of two microphones (for 
example, again arranged in an artificial head). During the steps (a) and 
(c) of the method the detection means are then situated at the listening 
position P1 and during step (b) of the method the detection means are then 
situated at the listening position P2. 
The two listening positions P.sub.1 and P.sub.2 are situated roughly 
between the two transducer devices 8 and 9, the distance between the first 
listening position P.sub.1 &gt;l and the first transducer device 8 being 
smaller than the distance between the second listening position P.sub.2 
and the first transducer device 8. Conversely, the distance between the 
second listening position P.sub.2 and the second transducer device 9 is 
smaller than the distance between the first listening position P.sub.1 and 
the second transducer device 9. 
The equalizer 3 comprises two filters 16 and 17 whose frequency-response 
characteristics can be adjusted by means of two control signals S.sub.1 
and S.sub.2 respectively, which are generated by the control unit 13 and 
applied to the filters 16 and 17 respectively. The filters 16 and 17 are 
respectively arranged between the terminals 5, 5' and 6,6' of the 
equalizing unit 2. The equalizer 4 also comprises two filters 18 and 19, 
whose frequency-response characteristic can be adjusted by means of two 
control signals S.sub.3 and S.sub.4, generated by the control unit 13 and 
applied to the filters 18 and 19 respectively. The filters 18 and 19 are 
respectively arranged between the terminals 5, 5' and 7, 7' of the 
equalizing unit 2. 
It is assumed that not only the frequency-response characteristics of the 
filters 8 and 16 to 18 can be varied by means of the control signals 
S.sub.1 to S.sub.4 but that it is also possible to adjust a 
frequency-independent gain factor in each of the filters by means of these 
control signals. The control unit 13 further comprises a volume control 20 
and a balance control 21 by means of which the balance between the 
left-hand and the right-hand signal components of the stereo signal can be 
set. The volume control is intended for adjusting a specific 
frequency-independent gain factor for all the four filters 16 to 19. This 
means that by means of the volume control 20 an overall increase or, 
conversely, reduction in the level of the signals applied to the two 
transducer devices 8 and 9 is obtained. By means of the balance control 21 
the frequency-independent gain factors in the filters 16 and 18 can be 
varied relative to those in the filters 17 and 19. This means that by 
actuation of the balance control 21 the level of the signals applied to 
the transducers 8L and 9L can be varied relative to the level of the 
signals applied to the transducers 8R and 9R. 
The operation of the arrangement shown in Figure 1 will now be described 
with reference to the flow chart of FIG. 2. When the arrangement is 
switched on the program is started in block 30. 
Subsequently, the user of the arrangement sets the volume control 20 in 
such a way that the acoustic signals at the location of the two listening 
positions P.sub.1 and P.sub.2 roughly have a desired level: block 32. 
Subsequently, step (a) of the method is carried out in block 34. This 
means that the levels of the electric signals applied to the first and the 
second transducer device 8 and 9 are adjusted in such a way that the 
acoustic signals delivered by the first and the second transducer device 
provide substantially equal contributions to the acoustic signal at the 
location of the first listening position P.sub.1. For the invention it is 
irrelevant how the levels are detected. It is possible, for example, to 
take the RMS value of the output signal of the detection means 10 for this 
level. The transducer device 9 is first switched off, so that the control 
unit 13 can determine the level N.sub.1 of the contribution of the 
transducer device 8 to the acoustic signal at the listening position 
P.sub.1 `Subsequently, the transducer device 8 is switched off and the 
transducer device 9 is switched on. The control unit 13 now determines the 
level N.sub.2 of the contribution of the transducer device 9 to the 
acoustic signal at the listening position P.sub.1. 
Subsequently, the (frequency-independent) gain factors in at least one of 
the two equalizers are varied in such a way that the contributions of the 
two transducer devices to the acoustic signal at the listening position 
P.sub.1 are equal. Correction can be effected in various ways. Two of 
these methods will be discussed. 
It is assumed that N.sub.1 is greater than N.sub.2. 
A first possibility is that the gain factors in the filters 16 and 17 are 
reduced, whilst the gain factors in the filters 18 and 19 are increased 
until the contributions of the two transducer devices are equal to each 
other and have a value between N.sub.1 and N.sub.2, for example (N.sub.1 
+N.sub.2)/2. The disadvantage of this is that the level at the second 
listening position P.sub.2 is raised and may become inconveniently high 
for a listener at said listening position P.sub.2. 
A second possibility, which does not have this drawback, is that only the 
gain factors in the filters 16 and 17 are reduced until the contribution 
of the transducer device 8 to the acoustic signal at the listening 
position P.sub.1 is equal to N.sub.2. 
As a result of the level control in step (a) the acoustic signal at the 
listening position P.sub.2 is mainly dictated by the acoustic signal 
delivered by the transducer device 9. Therefore, in step (b), block 36 in 
FIG. 2, the transmission from the input terminal 1-1' to the listening 
position P.sub.2 is equalized. For this purpose the control unit 13 
generates control signals S.sub.3 and S.sub.4, starting from the signals 
obtained from the detection means 12, so that the filter characteristic of 
the filters 18 and 19 is adjusted to obtain a substantially flat 
frequency-response characteristic for the transmission between the 
electric signal at the input terminal 1-1' and the acoustic signal at the 
second listening position P.sub.2. 
The equalizing method is also irrelevant to the invention. For example, 
European Patent Application No. 165,733 described a few ways of realizing 
equalization for one listening position. The detection means 12 may 
comprise one or two microphones. Further, equalization can be effected 
using an auxiliary signal, for example white or pink noise. The control 
unit 13 then also comprises an auxiliary-signal source for supplying white 
or pink noise to the inputs of the filters 18 and 19. Another possibility 
is that equalization is effected using the desired signals reproduced by 
the arrangement. Such an equalizing method is described, for example, in 
the Applicant's published European Patent Application No. 119,645 (PHN 
10.592). In that case a signal supply line from the input terminal 1-1' to 
the control unit 13 is needed. 
Control may be simplified by adjusting the two filters 16 and 17 to the 
same characteristic. This means that for adjusting these filters only one 
control signal is derived by the control unit 13 and is applied to both 
filters 16 and 17. 
Subsequently, in step (c), block 38 in FIG. 2, the transmission from the 
input terminal 1-1' to the listening position P.sub.1 is equalized. For 
this purpose the control unit 13 derives control signals S.sub.1 and 
S.sub.2 from the signals received from the detection means 11, by means of 
which control signals the filter characteristics of the filters 16 and 17 
is adjusted so as to obtain a substantially flat frequency response 
characteristic for the transmission of the electric signal at the input 
terminal 1-1' to the acoustic signal at the first listening position 
P.sub.1. During this equalization the second transducer device 9 must 
remain operative. 
The method of equalizing in step (c) is the same as that applied in step 
(b). 
Steps (a), (b) and (c) have now been carried out and the transmission to 
two positions P.sub.1 and P.sub.2 in the space 10 is now equalized. If the 
method is applied only once, the program proceeds directly from block 38 
to block 44 and the program has ended. 
It is also possible to repeat the program, for example if the user adjusts 
the balance control 21. This may mean that the transmission 
characteristics between the input terminal and the two listening positions 
are no longer flat. The control unit 13 detects the balance control 21 has 
been adjusted. The program now jumps to block 40, after which the steps 
(a), (b) and (c) are repeated for another balance setting. 
Preferably, step (b) is repeated at least once while the first transducer 
device is operative. By thus repeating the method one or several times an 
iterative process is obtained, which results in an optimum setting 
(equalization) of the transmission characteristics to the two listening 
positions. 
The program is terminated via block 42, for example in that the control 
system is switch off by actuation of a button. 
Alternatively, equalization may be repeated automatically after a certain 
time interval. 
Equalization of a frequency-response characteristic may mean that in fact 
only the amplitude characteristic (i.e. the amplitude of the transmission 
as a function of the frequency) is equalized. This means that the 
amplitude of the total transmission, including the equalizing filter, is 
substantially constant, i.e. frequency-independent. 
However, it is also possible to equalize both the 
amplitude-versus-frequency and the phase-versus-frequency response 
characteristics. 
FIG. 3 shows an embodiment of an arrangement 2' for amplifying an electric 
signal, for example a stereo signal in a space, for example the passenger 
compartment of a car. The results of the equalization methods as described 
with reference to FIGS. 1 and 2 may be used for adjusting four filters 16' 
to 19' in the two filter units 3 and 4 of the device 2' so as to obtain 
substantially flat frequency-response characteristics for the 
transmissions between the electric signal at the input terminal 1-1' and 
the acoustic signal at the first listening position P and between said 
electric signal and the acoustic signal at the second listening position 
P.sub.2. The filters 16' to 19' are then fixed filters. However, it 
remains possible of course to adjust the overall level (independently of 
the frequency) by means of a volume control 20. This is achieved in that 
the gain factors of amplifiers 25 and 26 preceding the filters 16' to 19' 
can be varied by means of the volume control 20. 
If desired, it is obviously also possible to adjust the front-rear balance 
as well as the left-right balance by means of associated controls (not 
shown). The left-right balance can be adjusted for example by driving the 
amplifiers 25 and 26 in FIG. 3 with different control signals. 
In general, an arrangement for amplifying an electric signal may be of a 
type as shown in FIG. 4. 
The fixed filters 16' to 19' are preceded by variable filters 30 to 33. 
Further, variable amplifiers 34 to 37 are arranged before the filters 16' 
to 19'. The amplifiers and the variable filters can be adjusted by means 
of control signals generated by means of a control unit 38. The control 
unit 38 comprises controls (not shown) which can be actuated by a user for 
manually adjusting the front-rear balance, the left-right balance and, if 
desired, the frequency-response characteristic. 
A change in frequency-response characteristic means that the filters 30 to 
33 are adjusted to provide another filter characteristic. 
However, the arrangement shown in FIG. 4 comprises fixed filters 16' to 19' 
such that if the setting, of the front-rear balance, the left-right 
balance and the frequency-response characteristics of the filters 30 to 33 
is neutral, the frequency response characteristics of the transmission 
from the input terminal 1-1' to the first and the second listening 
positions P.sub.1 and P.sub.2 respectively, are substantially flat. It is 
obvious that the filters 18' and 32 may be combined to form a single 
(variable) filter. Of course, this also applies to the filters 19' and 33, 
17' and 31, and 16' and 30. 
So far the description only relates to the equalization of two 
frequency-response characteristics for the transmission to the two 
listening positions. However, as already stated, the scope of the 
invention is not limited thereto. In general, the invention is applicable 
to a method and arrangement for adjusting the transmission to the two 
listening positions so as to obtain a desired characteristic. 
In FIG. 5 such a desired characteristic as a function of the frequency is 
illustrated by means of the curve 50. Above a specific frequency 70 this 
curve is flat, i.e. frequency-independent. For frequencies below f.sub.0 
the curve rises towards lower frequencies. The reason why such a curve is 
desired is as follows. The curve 51 in FIG. 5 represents the main noise 
component of the background noise in a car compartment. If for frequencies 
below f.sub.0 the curve 50 would have been flat, the desired sound 
radiated by the transducer devices for frequencies lower than f.sub.1 
would have become (substantially) inaudible on account of the background 
noise. Therefore, an additional increase in gain is needed for frequencies 
below f.sub.0, so that the desired sound level is situated sufficiently 
far above the background noise level, and the desired sound is still 
audible over the background noise. 
The transmission is now adjusted as follows. By means of the detection 
device the control unit determines the actual transmission from the input 
terminal to a listening position. This transmission is represented, for 
example, by the curve 52. 
In order to obtain a transmission in conformity with the curve 50 the 
transmission in the frequency range below f.sub.1 must be raised, that in 
the frequency range between f.sub.1 and f.sub.2 must be reduced, and that 
in the frequency range f.sub.2 must be raised again. This is achieved by 
adjusting the transmission characteristic of the filter unit in conformity 
with the curve 53 in FIG. 5. 
The two desired transmission characteristics may both be in conformity with 
the curve 50 in FIG. 5. 
It is also possible that the transmission characteristics have the same 
shape but differ in amplitude. For example, the other transmission 
characteristic may be a few dB higher than the curve 50 in FIG. 5.