Sound processing circuit

Details Sound processing circuit Sound processing circuit

1998-01-16

2002-04-30

Isen, Forester W. (Department: 2644)

Electrical audio signal processing systems and devices

Binaural and stereophonic

Pseudo stereophonic

C381S080000, C381S098000

Reexamination Certificate

active

06381333

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to the field of sound processing circuits which handle low frequency signal components in multichannel audio signals.
BACKGROUND OF THE INVENTION
With recent progress in audio signal compression technology and faster signal processing, recording and reproduction of multichannel audio signals, which have more channels than the conventional two channel stereo signals, are now being adopted in commercial equipment. Typical multichannel systems include the AC-3 system developed by Dolby Laboratories (hereafter referred to as the discrete digital multichannel system) and MPEG2. Optical disks on which audio signals are recorded employing the discrete digital multichannel system are already on the market. Decoders for converting signals recorded in the discrete digital multichannel format into ordinary signals are also available. Furthermore, at the end of 1996, software and hardware for digital video disks adopting the discrete digital multichannel system as one audio recording format were released.
The characteristics of these multichannel audio signal recording systems are (1) Audio signals for each channel can be recorded as completely independent audio signals without any correlation between channels and (2) Audio signals of a broad frequency band ranging from low frequency to high frequency, limited only by sampling frequency, can be recorded in each channel. For example, in the discrete digital multichannel system, there are five independent channels with frequency bands from 20 Hz to 20 kHz, and one channel exclusive to low frequencies up to 120 Hz.
The conventional processing method used in commercial equipment is to first encode the above multichannel audio signals and record them as 2-channel stereo signals. These stereo signals can be decoded during reproduction to reconstitute multichannel audio signals. The Dolby surround system adopts this method. This system is most frequently used for recording multichannel audio signals in movies.
The chief characteristic of this method is its feasibility to record and reproduce multichannel audio signals in a format completely compatible with two-channel stereo signals. Using this method, however, the signals in each discrete channel lose their independence since signals are produced for each channel by signal processing such as the addition and subtraction of the stereo signals recorded on the recording medium. This converts previously independent multichannel audio signals, before encoding, into completely different signals.
To reduce the above disadvantage, an active matrix circuit called the Dolby ProLogic circuit has been developed. This circuit secures the independence of each channel by reducing the sound level of the other channels when signal components of a certain channel are dominant in multichannel audio signals processed by the addition and subtraction of stereo signals, and reproducing the signals only in the dominant channel. This circuit is effective when only one channel is dominant, but much less efficient when all channels have about the same signal level.
New multichannel systems including the discrete digital multichannel system completely assure the independence of each channel during recording in the conventional two-channel stereo signal format. These new multichannel systems are used mainly for recording and reproducing sound in movies. Assurance of independence of each channel improves the clarity of spoken word, movement and direction of sound and spatial impression, allowing viewers an enhanced impression of live sound performance.
For reproducing these multichannel audio signals, speakers which can cover a broad range of frequency bands from low to high bands are preferably used. In the above active matrix system, for example, audio signals of four channels at the left, center, right and rear are decoded from input stereo signals. Audio signals for the rear channel have a frequency range from about 100 Hz to 7 kHz, and audio signals of other three channels at the left, center, and right have a broad frequency range from 20 Hz to 20 kHz.
Accordingly, it is preferable to employ the same type of speaker for at least three channels, i.e. at the left, center, and right, for covering the frequency range from 20 Hz to 20 kHz. In the above discrete digital multichannel system, it is preferable to employ speakers to cover the frequency range of 20 Hz to 20 kHz for all five channels, i.e., at the left, center, right, left back, and right back, because the signals for all five channels range from 20 Hz to 20 kHz.
However, if this type of reproduction system is introduced for home use, a large speaker for broad reproduction bands can be employed for the left and right speakers but it is generally difficult to use this type of speaker in the center because there is a display monitor for displaying video images. Also for back speakers, smaller speakers are often used due to limitations in installation space. These smaller speakers generally have less reproduction capability for low frequencies compared to large speakers.
When multichannel audio signals are reproduced in unmodified form in a system employing speakers with both good and poor low frequency reproducibility, the relative volumes of low and high frequencies may be unbalanced. The volume of low frequency sound may be insufficient if audio signals are concentrated in channels with poor low-band reproducibility. In particular, listeners may have a sense of incongruity when the sound moves from one side to the other.
To reduce these disadvantages, equipment exists which features an active matrix circuit which further employs a sound processing circuit for distributing low frequency signal components of the center channel to the left and right channels.
FIG. 7
shows an example of a sound processing circuit of the active matrix system. Audio signals input from two channels to an active matrix circuit
51
are decoded into signals for four channels: left (Lch), center (Cch), right (Rch), and back (Sch). A high-pass filter (HPF)
52
receives decoded signals for the center channel, allows through only high-band signals, and outputs them as signals for the center channel.
At the same time, signals for the center channel are input to a low-pass filter (LPF)
53
. The cut-off frequency of the LPF
53
is set at almost equivalent to the cut-off frequency of the HPF
52
, and it allows through only low frequency signals for the center channel. The output here is attenuated by about 3 dB by a coefficient multiplier 54, and then supplied to adders
55
L and
55
R for the left and right channels. The adder
55
L adds the low frequency signal components of the center channel to the audio signals for the left channel, and the adder
55
R adds the low frequency signal components of the center channel to the audio signals for the right channel. Consequently, these low frequency signal components are distributed to the left and right channels by the two adders
55
L and
55
R. The cut-off frequencies for the HPF
52
and LPF
53
are both set to about 100 Hz.
The above sound processing circuit enables the diversion of low frequency signals, originally destined for the center channel, to the left and right speakers and avoids insufficient low frequency signal components even when the center channel speaker has poor low frequency reproducibility. It is difficult to specify the position of sound source of frequency signal components lower than 100 Hz which are distributed to the left and right channels. This avoids a sense of incongruousness as to sound source direction even though the sound source is split between the left and right channels.
The active matrix circuit
51
suppresses the supply of audio signals to the center and right channels when the left channel receives large audio signals. On the other hand, when the center channel receives a large portion of audio signals, the active matrix circuit
51
suppresses the supply of audio signals to the right and left channels. This makes it unnecessary to set a surplus am

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