Electrical audio signal processing systems and devices – Binaural and stereophonic – Stereo speaker arrangement
Reexamination Certificate
1999-09-22
2003-10-28
Isen, Forester W. (Department: 2644)
Electrical audio signal processing systems and devices
Binaural and stereophonic
Stereo speaker arrangement
C381S307000, C381S058000, C381S103000
Reexamination Certificate
active
06639989
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for loudness calibration of a multi-channel sound systems: The present invention also relates to a multichannel sound system.
2. Description of Related Art
The following terminology is used in the document. The reproduction level of a sound system is controlled by volume control, which changes the channel gains equally. The channel gain is a channel specific control with respect to initial level to be used for compensating various differences between loud-speakers e.g. in sensitivity. The level calibration is used to adjust the channel gains to give equal physical measure at the listening position using a test signal. The loudness calibration is used to adjust the channel gains to give equal loudness at the listening position using test signal. The loudness is an auditory sensation and as such it can not be directly measured. It depends on acoustical intensity, frequency, duration and spectral complexity. These are physical attributes that can be measured and the loudness can be estimated from those using existing models [3,4,5].
Domestic multichannel sound systems, with or without pictures, are becoming increasingly popular. A sound system has to be calibrated to ensure the best possible aural environment. A traditional stereo system usually has two identical loudspeakers. When they are set-up symmetrically in a room and the listener stays with equal distance to both of them, the level calibration is quite simple. The system is provided with balance control, which can be set to middle; equal gains to both channels. If the listening position is closer to one of the loudspeakers or the loudspeakers are set-up asymmetrically to the room, the balance must be re-adjusted. This provides the listener with a means of level control.
The current trend in the field of domestic sound system is towards multichannel systems having more that two loudspeakers, like the 5 channel system shown in
FIG. 1
a
. With multichannel system the calibration situation can be far more complex than with traditional stereo system. The loudspeakers often have different characteristics; they differ in bandwidth, sensitivity, directivity etc. Furthermore the positioning of a loudspeaker has a great effect on room coupling. The loudspeaker in a corner of the room or just close to one wall may have very different amplitude response characteristics than one located away from the walls.
In the ideal situation such as specified in e.g. ITU-R BS.775-1, shown in
FIG. 1
a
, the central loudspeaker
102
, the left and right loudspeakers
104
a
and
103
a
as well as left and right surround loudspeakers
105
a
and
106
a
have an equal distance to the listening position
101
. In
FIG. 1
b
a more realistic loudspeaker placement is shown. The loudspeakers
102
,
103
a
,
104
a
,
105
a
,
106
a
are normally placed near the walls. When the shape of the room
110
b
is not ideal from the viewpoint of aural environment, it is typical that the distances from the loudspeakers
102
,
103
a,
104
a
,
105
a
,
106
a
to the listening location
101
are not equal. With these circumstances even matching the reproduction level of centre channel from the loudspeaker
102
to usually identical left and right channels from the loudspeakers
104
a
and
103
a
is difficult. And further the situation with surround channels from loudspeakers
106
a
and
105
a
is even more problematic. The situation becomes even more problematic when the room coupling effects are taken into account. These problems relate to bandwidth, sensitivity, directivity, and distances of the loudspeakers and room interaction.
The object of the sound system calibration is to calibrate the loudspeakers
102
,
103
a
,
104
a
,
105
a
and
106
a
so that in the listening position
101
it seems, or rather sounds, like the sound is coming from the virtual loudspeakers
103
b
,
104
b
,
105
b
and
106
b
, all at equal distances from a listening position
101
. This sensation of virtual loudspeakers is achieved mainly by the two methods. First, by changing delay times of each loudspeaker
102
,
103
a
,
104
a
,
105
a
,
106
a
so that sound meant to be heard simultaneously are transmitted at different times by each loudspeaker so that the sounds arrive to the listening position
101
simultaneously. Secondly, by adjusting the gain of each loudspeaker so that they produce equal loudness at the listening position
101
.
There are basically two methods for calibrating a multichannel sound system. The calibration can either be done automatically without human perception or subjectively when the person calibrating the system calibrates the system according his personal subjective audio perceptions.
An automatic calibration is quite an accurate method for calibrating delay times for each loudspeaker, but not as good for loudness calibration. The loudness is a auditory sensation, and as such it cannot be directly measured in the same manner as acoustic pressure or intensity, which are physical attributes and as such straightforward to measure. Therefore a subjective calibration is mainly used for loudness calibration. So called “pink noise” [1] is most often used as a test signal in subjective calibration, because its spectrum correlates well to statistical properties of natural sound. Bandlimited test sounds are normally used in subjective loudness calibration, to avoid problems with room coupling on lower frequencies and location sensitivity with the higher frequencies.
In
FIG. 2
a flow chart of the prior art method
200
for automatic sound system calibration is shown. In step
201
a test signal is generated. The test signal is preferably some pseudorandom signal allowing the calculation of the periodic impulse response of the aural environment under study. Said aural environmental includes the actual multichannel sound system as well as loudspeakers and the listening space as they give a considerable contribution to the aural environment. One possible test signal type is a maximum-length sequence (MLS) [2].
In the step
202
the test signal is transmitted via a sound source i.e. loudspeaker to the listening space. In the step
203
the test signal is received by a microphone at the preferred listening position.
In step
204
a cross correlation between the original signal generated in step
201
and the signal received in step
203
is carried out. If the test signal is an MLS or similar signal, this gives in step
205
the periodic impulse response of the aural environmental. In step
207
various parameters giving information about aural properties the aural environment in the time domain, like arrival times, early reflection and room reverberation information are calculated from the periodic impulse response.
In step
206
the periodic impulse response of the system is transformed to the frequency domain using a fast fourier transform (FFT) algorithm. In step
208
various frequency domain properties of the aural environment, like phase and amplitude response, are calculated from FFT transform of the periodic impulse response.
In step
209
an automatic calibration is carried out according to the time and frequency domain information calculated in steps
207
and
208
. By applying similar calibration for each sound source, the whole system can be calibrated.
The problem of the above stated prior art is that with automatic calibration the achieved calibration is not sufficiently good due the subjective nature of the loudness. The calibration according only to physical terms does not necessarily provide optimum calibration in perceptual terms. On the other hand, when using subjective loudness calibration the test signals do not excite the room or the listener to the extent the programme material does. In addition some frequency ranges are more dominant at the perceptual level, thus making the calibration based on only to these ranges. Therefore the calibration according to the prior art does not give sufficiently accurate calibra
Suokuisma Pekka
Zacharov Nick
Isen Forester W.
Nokia Display Products Oy
Pendleton Brian
Ware Fressola Van Der Sluys & Adolphson LLP
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