Abstract:
The inventive mechanism produces multiple output signals from a two-channel stereo input signal. The mechanism produces a first pair of output signals which retain the monaural information in the stereo input signal. The first pair of signals each comprise a combination of one of the input channels and filtered signal produced from a difference of both input channels. The mechanism produces a second pair of output signals which lacks the monaural information. The second pair of output signals each comprise a combination of one of the input channels and an inverse filtered signal produced from a difference of both input channels. Q-filters are used to provide the various filtered signals.

Description:
RELATED APPLICATIONS 
     The present application is a continuation in part of co-pending and commonly assigned U.S. application Ser. No. 08/858,586, entitled FULL SOUND ENHANCEMENT USING MULTI-INPUT SOUND SIGNALS filed May 19, 1997, which is incorporated herein by reference. The present application is related to co-pending and commonly assigned U.S. application Ser. No. 08/511,788, entitled STEREO ENHANCEMENT SYSTEM INCLUDING SOUND LOCALIZATION FILTERS, filed Aug. 7, 1995, which is incorporated herein by reference, which is a continuation in part of U.S. Pat. No. 5,440,638. 
    
    
     TECHNICAL FIELD OF THE INVENTION 
     This application relates in general to audio signal processing, and in specific to synthesizing multiple output channels from two-channel, stereo input signals. 
     BACKGROUND OF THE INVENTION 
     A recent trend in the audio industry is the purchase and installation of home theater systems. Consumers have been purchasing multiple speaker sound systems which are integrated with a video system which uses VCR tapes and/or DVD disks. A similar trend is occurring in the automobile audio industry, wherein multiple speaker sound systems are being installed in automobiles and trucks. 
     In both of these cases, the input signal typically comprises a stereo or two-channel signal, which is being outputted on five or more speakers, each of which is capable of receiving a separate channel. Since there are more speakers than signals, the same signal is sent to multiple speakers. Thus, these audio systems are under utilized. Although there are a small number of recorded movies and/or sound CDs that are available, which have been recorded with the full five channel system, the vast bulk of audio/visual (A/V) entertainment information (including music CD&#39;s, VHS movies, television broadcasts) is recorded in the stereo or two-channel format. 
     Such systems typically handle stereo signals by sending the same signal to the front and rear speakers. For example, the front left and rear left speakers would receive the same left input channel. The amplitude of the signal can be controlled through a fader button which defines the portion of the signal going to the front speakers and the portion going the rear speakers. A sub-woofer channel can be created by summing the left and right channels and filtering out the high frequency information. Consequently, the multiple speaker systems are being under utilized when using stereo two-channel A/V information. 
     Therefore, there is a need in the art for a mechanism which will synthesize multiple channels of audio signals from a two-channel stereo input signal. This would allow an existing multiple channel audio system to output unique synthesized channels to each speaker. 
     SUMMARY OF THE INVENTION 
     These and other objects, features and technical advantages are achieved by a system and method which synthesizes multiple output channels or signals from a two-channel stereo signal. 
     The inventive mechanism uses several sub-systems to generate output signals from the stereo input signals. A first sub-system synthesizes the front left and front right signals, which include monaural information. A second sub-system synthesizes the surround (or rear) left and surround right signals, which have the monaural information canceled or greatly diminished. A third sub-system synthesizes the center signal and the sub-woofer low frequency signal. Thus, using a stereo input signal, the inventive mechanism can synthesize six different output signals. Each of the output signals can be directed to a different speaker. 
     A technical advantage of the present invention is to allow multiple channel audio systems to utilize their multiple channel capabilities and playback four or more channels synthesized from input materials recorded in two-channel stereo. 
     Another technical advantage of the present invention is that the center or monaural information is delivered by the front speakers or by a center speaker. 
     A further technical advantage of the present invention is that the center or monaural information is removed from the rear speakers. 
     The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and the specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
     FIGS. 1A and 1B depict the inventive system which outputs four-channels from a two-channel stereo input; 
     FIG. 2 depicts the inventive system which outputs six channels from a two-channel stereo input; 
     FIGS. 3A and 3B each depict an alternate sub-system of FIGS. 1 and 2 which create the left front and right front output channels; 
     FIG. 4 depicts a sub-system of FIGS. 1 and 2 which creates the left rear and right rear output channels; 
     FIG. 5 depicts a sub-system of FIG. 2 which creates the center and sub-woofer output channels; and 
     FIGS. 6A and 6B depict the effects a switch in the sub-system of FIG.  4 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1A depicts the inventive arrangement  10  wherein a two-channel stereo input, from an A/V source, is converted into four-channel output signals, each of which can be sent to a different speaker. Left input  11  and right input  12  are sent through summers  35  and  36 , respectively, to front left output  31  and front right output  32 . FIG. 3 shows this more clearly. Note that summer  36  includes an inverter, however, this inverter could reside within Q-filter  34 . Similarly, summers  45  and  46  also are shown to have inverters, however, these inverters could reside with in Q-filters  44  and  43 , respectively. Optionally, Q-filter  34  is switched into the circuit  10  by a user via switches  21 ,  22 . A single switch could be placed between Q-filter  34  and summers  35 , 36  instead of switches  21 ,  22 . The Q-filter  34  could be a QXpander filter, wherein QXpander is a registered trademark of QSound. The QXpander is described in U.S. Pat. No. 5,440,638 to Lowe et al., which is hereby incorporated by reference. The Q-filter  34  could be a Q1 filter, which is described in U.S. Pat. Nos. 5,105,462 and 5,208,860 both to Lowe et al., wherein each of these patents are hereby incorporated by reference. The output of summer  33  is L−R which is inputted into Q-filter  34 , which adjusts the amplitude and phase of the signal on a frequency dependent basis. Note that summer  33  includes an invertor on the R input side, however, the invertor does not have to be a part of summer  33 , but can be separate from the summer and be applied such that only that portion of the R input side going to summer  33  is inverted. This note applies to the other summers as well. The inversion multiplies the signal by −1, and therefore the polarity of the amplitude is changed. Any positive amplitude becomes negative and any negative amplitude becomes positive. This applies to the other summers as well. 
     The signal output from Q-filter  34  is then added to L input  11  by summer  35 , which results in a (Q{L−R}+L) signal as front left output  31 . The signal output of Q-filter  34  is also subtracted from R input  12  by summer  36 , and results in (R−Q{L−R}) signal as front right output  32 . Note that summer  33  could have the invertor on the L input side, which would require an inverter on summer  35 , instead of summer  36 , to invert the output from Q-filter  34 . Switches  21 , 22  allow for a user to switch off the Q-filtering on the front channels and to have the Q-filter enhanced signals only on the rear or surround channels, thereby receiving the standard left and right stereo channels in the front speakers. 
     Each of the output signals  31  and  32  retains monaural or center information. For example, if the same sounds were in both the left and right channels, then an output of L−R would equal zero, because L=R. Therefore no monaural or center information is passed to the Q-filter  34 , and the outputs  31  and  32  would be the inputs  11  and  12 . This means that the monaural or center information is retained. In today&#39;s music, voice and drums tend to be in center information, with guitar and piano to the side information. Thus, the mid-panned or center panned sounds in the recorded mix appear in the front speakers. 
     Instead of the single Q-filter as shown in FIG. 3A, an arrangement having two Q-filters can be used, as shown in FIG.  3 B. As stated earlier, Q-filter could be a QXpander filter, and thus both filters of FIG. 3B could be QXpander filters. A difference of L input signal  11  and R input signal  12  is created by summer  38  and provided to Q-filter  34 ′. The output of Q-filter  34 ′ is then inverted and added to R input signal  12  by summer  36 ′. The output from summer  36 ′, R−Q{L−R}, is the R front output signal  32 . Note that the R front output signal  32  of FIG. 3B is the same as FIG.  3 A. Similarly, a difference of R input signal  12  and L input signal  11  is created by summer  39  and provided to Q-filter  34 ″. The output of Q-filter  34 ″ is then inverted and added to L input signal  11  by summer  35 ′. The output from summer  35 ′, Q{L−R}+L, is the L front output signal  31 . Note that the L front output signal  31  of FIG. 3B is the same as FIG.  3 A. FIG. 3B also retains the monaural information. Note that the input signal to each Q-filter,  34 ′ and  34 ″ is either L−R or R−L, if the scale multipliers  37  are set to 1. If the same sounds were in both the left and right channels, then L-R and R-L would equal zero, and thus, monaural information is not processed by the Q-filter, and the outputs of arrangement  30 ′ is merely equal to the respective inputs. The arrangement of FIG. 3B could include switches  21  and  22  as shown in FIG.  3 A. FIG. 3B includes scale multipliers  37 , each independently operable, for introducing an attenuation in the signal going into the negative input of summers  38  and  39 . The scale multipliers control how much monaural or center information is passed to the Q-filters. In the extreme case where the attenuation is set to infinity, there is no signal sent to the summers  38  and  39  from the opposite input signal. Hence, all of the monaural information is passed to the Q-filters. This results is a severe loss of monaural energy at the outputs  31  and  32 . The arrangement of FIG. 3B can be substituted for the arrangement of FIG. 3A shown in FIGS. 1 and 2, if the scale multipliers  37  are set to 1. This arrangement will duplicate the effects of FIG. 3A, however the scale multipliers  37  can be adjusted to provide control over the balance of the center information. The arrangement of FIG. 3A can also be fitted with scalers. Note that a scale multiplier could be placed before each of the summers in FIGS. 1A,  1 B,  2 ,  3 A,  3 B,  4 , and  5 , and would be used to control the amount of signal energy reaching the summer. 
     L input  11  is also connected to Q-filter  43  as shown in FIG.  1 . This is more clearly shown in FIG.  4 . Also, R input  12  is connected to Q-filter  44 . Both of these filters may be Q1 filters. The output each Q-filter is subtracted from the opposite input via summers  45  and  46 . For example, the output of Q1 filter  44  is subtracted from L input  11  and used as the left rear or surround output  41 . Right rear or surround output  42  is similarly formed from the output of Q-filter  43  subtracted from the R input. In this instance the outputs are L-Q{R} for L rear output  41  and R−Q{L} for R rear output  42 , and thus the center information is canceled out. If the same sounds were in both the left and right channels, then an output  41  would be nearly zero. This is similar for the right rear output  42 . In today&#39;s music, voice and drums tend to be in center information, with guitar and piano to the side information. With the center information canceled out, the side-panned sounds in the recorded mix appear dominant in the rear speakers. Therefore, the arrangement of FIG. 1A receives a stereo input signal,  11  and  12 , and synthesizes four different output signals,  31 ,  32 ,  41 , and  42 . 
     Switch  47  is a user selectable phase inverter following the output of summer  46 , which allows the user to turn off the expansion effect of the circuit of FIG.  4 . In FIG. 6A the switch is turned on (+1), enabling the expander effect. Note that the portions  63 ,  64  of sound energy of the signal is spread beyond the locations of the speakers  61  and  62 . In FIG. 6B, the switch is turned off (−1), and the energy  65  does not spread beyond the locations of the speakers  61 , 62 . Note that the monaural information is still suppressed, even though the switch is off. Switch  47  could alternatively be placed on the output of summer  45 . Note that the Q-filter processed signals are normally inverted between the two output channels. When the R output signal is inverted, the necessary inversion between the two output channels is lost, and hence the virtual image effects are turned off. Switch  46  inverts or reverses the sign of the amplitude of the signal. 
     The filters of FIGS. 3A,  3 B and  4 , are all IIR or Infinite Impulse Response type. This type of filter has a feedback loop, which cause the output signal to last longer. The filter could alternatively be of the FIR type or Finite Impulse Response. The Q-filters can be implemented as IIR or FIR filters in digital domain. The Q-filters can also be implemented in the analog domain. The Q-filter in FIG. 3A is preferably a two-stage filter. The Q-filters in FIGS. 3B and 4 are preferably a one-stage filter. However all of the filters could comprise one or more stages. 
     The arrangement  10 ′ of FIG. 1B depicts an alternative to the arrangement of FIG.  1 A. However, only two Q-filters are used,  43 ′ and  44 ′. The outputs of these filters are combined with the input signals by summers  33 ′,  35 ′,  36 ′,  45 ′, and  46 ′ to produce output signals  31 ′,  32 ′  41 , and  42 . Note that the output signals  41  and  42  are identical to the output signals  41  and  42  of FIG. 1A, namely L−Q{R} and R−Q{L}, respectively. However, the outputs  31 ′ and  32 ′ appear different than  31  and  32  of FIG.  1 A. The output  31 ′ is Q{L}−Q{R}+L, which is different from Q{L−R}+L output  31 . However, since the Q-filters are linear, then the Q-function is distributive, and thus Q{L−R} equals Q{L}−Q{R}. Therefore, output  31 ′ is the same as output  31 , so long as the Q-filter is operating in a linear fashion. This is also true for output  32 ′ and output  32 . The switch  47  appearing in FIG. 1A could also be used in FIG. 1B. A single switch placed between summer  33 ′ and summers  35 ′, 36 ′ could be used instead of switches  21  and  22 . The arrangement of FIG. 1B could also replace that of FIG. 1A in FIG.  2 . 
     The arrangements of FIGS. 1A and 1B are better suited to four-speaker sound systems. FIG. 2 depicts the arrangement that is preferable for systems having a center speaker and a sub-woofer. Note that the system of FIG. 2 could be modified for a five speaker system, i.e. having either just a center or a sub-woofer. The sub-systems of 
     FIGS. 3A or  3 B, and  4  are present in FIG.  2 . Moreover, FIG. 2 includes the sub-system of FIG.  5 . In FIG. 5, L input  11  and R input  12  are added together by summer  53 , essentially creating a monaural output. The output of summer  53  is filtered by high-pass filter  54  with a cutoff frequency of about 100 Hz, and used as center output  51 . The output is also filtered by low-pass filter  55  with a cutoff frequency of about 100 Hz, and used as sub-woofer output  52 . Note that the recited cutoff frequencies are by way of example only. Therefore, the arrangement of FIG. 2 receives a stereo input signal,  11  and  12 , and synthesizes six different output signals,  31 ,  32 ,  41 ,  42 ,  51 , and  52 . 
     Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims.