Control circuit for a spatial stereo expander

Abstract: A stereophonic audio processing system (20,26,24,28) having left and right stereophonic sound channels with respective loudspeakers therefor is presented. The system is provided with spatial expansion of the stereophonic sound so that a first pair of spaced-apart loudspeakers (30) will acoustically appear to be spaced further apart then they actually are. The audio system is provided with provisions (40) for connecting a second pair of loudspeakers which can be spatially located at a distance between the loudspeakers which is larger than the first pair of loudspeakers. The audio system is provided with apparatus for providing stereo sound without spatial expansion when such secondary loudspeakers are used.

Description: Background The present invention relates to spatial expansion of the apparent spacing between two stereophonic loudspeakers, and more particularly, to providing stereo sound without spatial expansion when the spaced-apart distance between a pair of loudspeakers is sufficiently large so that expansion is not desired. Spatial stereo expansion in audio systems is well known in the art and has been available for many years. In such systems, the left and right channel signals are processed in a manner so that it appears to the listener that the distance of separation of the loudspeakers is much greater than the actual physical separation of the loudspeakers. This is called psycho-acoustic expansion. Examples of spatial stereo expansion are shown in U.S. Patent Nos. 4,495,637 of Bruney, and 4,831,652 of Anderson. In such spatial expansion systems, a portion of the signal from one channel is added in reverse phase to the signal of the other channel such that an ambience of spaciousness is introduced between the left and right channels. This ambience of spaciousness is independent of any separation in the original signal of the two channels. This feature has the desirable characteristic of making the acoustic perceived stereo image seem wider than the actual locations of a pair of stereophonic loudspeakers. This is particularly desirable for the normal spacing of loudspeakers built into a cabinet of a television receiver or small radio, where the stereo speaker spacing between loudspeakers is typically only about 26-80 cm. apart. In such an arrangement, the perceived stereo effect is greatly improved with the perceived spatial expansion of the loudspeakers. One well known method of increasing the perceived spatial expansion between stereophonic loudspeakers is by increasing the (L-R) signal level with respect to the (L+R) signal level when the signals are applied to the decoder matrix to form the L and R signals. An undesirable effect of the spatial "expanded stereo" is that the (L-R) ambiance sound can be too loud compared to the (L+R) sound, often resulting in a center-stage vocalist between "drowned out" by the ambiance sound, or by the tonal variations between stereo and monophonic modes being large in extent. These objections, particularly the latter, can be more noticeable when using the auxiliary hi-fi output jacks sometimes provided in a TV sound system where the L and R signals can be applied to a high quality stereophonic reproduction system For the above reasons, it is desirable to provide a stereo signal without spatial expansion in a system which provides for the spatial stereo expansion when auxiliary loudspeakers are used. When auxiliary loudspeakers are used, i.e., loudspeakers disposed external of the television cabinet, these auxiliary loudspeakers can be placed at spaced-apart distances much greater in extent than the loudspeakers of the television receiver cabinet. For such a case, the rationale for using spatial expanded stereo is no longer applicable. An example of a switchable stereo expander is described by Anderson et al. in US patent 4,831,652 entitled STEREO EXPANSION CIRCUIT SELECTION SWITCH which issued 16 May 1989. In this system left L and right R stereo signals are coupled to respective output terminals via respective operational amplifiers. The system includes cross coupling between the amplifiers which, when activated by a user operable key, adds stereo expansion components to the amplifier output signals. This system operates on the baseband stereo signals L and R after matrixing and requires two operational amplifiers and a switched cross coupling network to control the stereo expansion of the baseband left and right channel signals and is thus relatively complex. Another example of a switchable stereo expander is described by Hahner in EP-A-0 412 260 entitled AUDIO PLAYBACK ARRANGEMENT WITH FREQUENCY-SELECTIVE AMPLIFIERS FOR DRIVING LOUDSPEAKERS INTERNAL AND EXTERNAL TO THE DEVICE which was published 13 February 1991. This system includes low-pass and high-pass filters for driving woofers and tweeters in a receiver. When external speakers are plugged in, the system disables the filters to provide wideband signals to the external speakers. Audio signal processing apparatus embodying the present invention comprises a source (20) of stereo difference (L-R) and sum (L+R) signals; the magnitude of said stereo difference signal (L-R) being greater than that of said sum signal (L+R);matrixing means (24) for generating left and right channel audio signals from said stereo difference (L-R) and sum (L+R) signals; amplitude equalizing means (32) coupled between said source (20) of aid stereo difference (L-R) and sum (L+R) signals and said matrixing means (24) for substantially equalizing the magnitude of said stereo difference (L-R) and sum (L+R) signals; and activation means (40) coupled to said equalization means for selectively activating said equalizing means. In accordance with a feature of a preferred embodiment of the invention said amplitude equalizing means (32) comprises: variable attenuator means (34,36,38) operative when the equalization means is not activated for attenuating the amplitude of the stereo difference signal (L-R) to a first value; the variable attenuator means (34,36) being operative when the equalization means is activated for reducing the amplitude of the stereo difference signal (L-R) to a second value less than the first value. In accordance with a further feature of a preferred embodiment of the invention the variable attenuator means comprises: a first resistor (34) coupled between an output of the source providing the stereo difference signal (L-R) and an input of the matrixing means (24); a second resistor (36) coupled between the input of the matrixing means (24) and a circuit node; a third resistor (38) coupled between the circuit node and a point of reference potential (ground); and wherein the activation means comprises: switch means (46,48) connected in parallel with the third resistor (3 8). In accordance with yet another feature of a preferred embodiment of the invention the switch means (46,48) is in a selected one of two speaker jacks (40) for automatically changing the attenuation of the variable attenuator from the first value to the second value upon insertion of an external speaker plug into the jack. Brief Description of the Drawings Reference can be had to the drawings wherein like members are given like numerals and wherein: FIGURE 1 shows a block diagram of a television receiver having a stereophonic audio system. FIGURE 2 shows a partial block diagram of the television receiver of FIGURE 1 according to aspects of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT FIGURE 1 shows a commonly available television receiver, generally designated 10. A broadcast television signal received by an antenna 12 (or coupled to the receiver via a cable television system) is applied to a tuner 14 for channel selection. Tuner 14 converts a selected one of a plurality of received television signals from its respective broadcast frequency to a common intermediate frequency (IF). The IF frequency television signal is applied to a video and sound IF unit 16 which produces a baseband video signal and a frequency modulated (FM) sound carrier. The video and audio signals are separated, and the video signals are applied to video signal processing circuitry including a video display device (not shown). A stereophonic sound audio signal is detected from the FM sound carrier by an FM detector 18 and applied to a stereo decoder 20. Stereo decoder 20 provides an L+R stereo sum signal and an L-R stereo difference signal. The (L+R) stereo sum signal is de-emphasized in a deemphasis unit 22 (i.e., a low pass filter having an appropriate time constant) and applied to one input of a stereo matrixing unit 24 in which the original left (L) and right (R) stereo signals are reconstructed. The (L-R) stereo difference signal from stereo decoder unit 20 is applied, in the exemplary embodiment, to an expander unit 26. In the exemplary embodiment, expander unit 26 corresponds to a full DBX™ expander. However, such an "expander" is incidental to the present invention, and any expander or no expander can be used. Such a type of "expander" provides a complementary signal processing of a signal compander applied to the audio signal at the signal source in order to provide noise reduction in the (L-R) signal. In the present embodiment, expander 26 is an amplitude and spectral expander which also provides deemphasis for the (L-R) signal. This amplitude and/or spectral expansion is independent of the spatial expansion discussed below which is the subject of the present invention. The amplitude and/or spectral expanded (L-R) stereo difference signal is applied to the other input of stereo matrix unit 24. The reconstructed left (L) and right (R) stereo signals are applied to stereo audio amplifier unit 28 for amplification and ultimate reproduction in a dual speaker unit 30. It is well known that a spatial stereo expanded effect can be achieved by providing a level of the (L-R) signal to matrix 24 which is greater than the (L+R) signal also provided to matrix 24. For the reasons discussed above, it is desirable to defeat this spatial expansion effect when auxiliary loudspeakers are connected to the apparatus since these loudspeakers can be physically spaced at distances greater than the original loudspeakers, which typically are provided within the confines of the cabinet. Also as discussed above, when external loudspeakers are used which can be spaced apart further than the loudspeakers internal to the cabinet, it is desirable to provide these external loudspeaker with a stereo signal having no stereo spatial expansion. Referring now to FIGURE 2, there is shown the exemplary embodiment for selectively providing a stereo signal without spatial stereo expansion when external loudspeakers are provided. Stereo decoder 20 provides an output (L-R) signal at an amplitude greater than the output (L+R) signal out with both of the signals being provided to the stereo matrix 24. The increased amplitude of the (L-R) signal can be provided in many ways, e.g., with additional amplifier gain or decreased feedback (not shown). The emphasis network 22 and expander network 26 are not shown in this drawing, and if used, can be appropriately placed in accordance with FIGURE 1. The relatively increased amplitude (L-R) signal is provided to stereo matrix 24 through an attenuator 32 made up of resistors 34, 36, and 38, with resistor 38 coupled to ground. In the spatial expansion mode, the (L-R) signal provided to stereo matrix 24 is attenuated by the sum of resistors 36 and 38 divided by the sum of resistors 34, 36 and 38. In this manner the increased amplitude of the L-R signal can be precisely determined. In the exemplary embodiment, a second pair of loudspeakers can be used by providing the stereophonic signals to external loudspeakers and/or amplifiers (not shown). A switching jack 40 is provided with a contact 42 for coupling the R signal from the decoder matrix 24 to the external system. Switching jack 40 is arranged with a insulating contactor 44 which mechanically short-circuits contact 46 with contact 48 when a plug (not shown) is physically inserted into jack 40. In such an event, contacts 46 and 48 short circuit resistor 38 changing the attenuation ratio of attenuator 32. In the plug inserted mode, the amplitude of the (L-R) signal provided to stereo matrix 24 is the ratio of resistor 36 divided by the sum of resistors 34 and 36. In this mode, the (L-R) signal provided to stereo matrix 24 is reduced to the extent that the amplitude is equal to the amplitude of the signal (L+R) provided to the stereo matrix. With equal amplitudes of signal (L+R) and (L-R), the spatial expansion effect for the stereophonic sound is not produced. Thus, when the stereophonic signals are provided to external loudspeakers and/or amplifiers with the external loudspeakers capable of being spaced apart at distances sufficient to give a better stereophonic effect, the spatial expansion which is no longer necessary, is not provided. It should be noted that the exemplary attenuator and the switching of the attenuator can just as well be provided in the left channel as well as the right channel. It should also be noted that the external loudspeakers and/or amplifiers can be connected through non-switching jacks or terminals with a panel selector switch being provided for selecting such second pair of loudspeakers and/or amplifiers. In such a case provision can also be made for the providing of stereophonic signals to the second pair of loudspeakers and/or amplifiers without spatial expansion. It should also be noted that the present invention can be used with FM stereo radios or the like, wherein the stereo loudspeakers are closely spaced apart.

Claims: An audio signal processing arrangement comprising: a source (20) of stereo difference (L-R) and sum (L+R) signals, the magnitude of said stereo difference signal (L-R) being greater than that of said sum signal (L+R); matrixing means (24) for generating left and right channel audio signals from said stereo difference (L-R) and sum (L+R) signals; amplitude equalizing means (32) coupled between said source (20) of said stereo difference (L-R) and sum (L+R) signals and said matrixing means (24) for equalizing said magnitude of said stereo difference (L-R) and sum (L+R) signals; and activation means (40) coupled to said equalizing means for selectively activating said equalizing means. Apparatus as recited in Claim 1 wherein said amplitude equalizing means (32) comprises: variable attenuator means (34, 36, 38) operative when said equalization means is not activated for attenuating the amplitude of said stereo difference signal (L-R) to a first value; said variable attenuator means (34, 36) being operative when said equalization means is activated for reducing said amplitude of said stereo difference signal (L-R) to a second value less than said first value. Apparatus as recited in Claim 2 wherein said variable attenuator means comprises: a first resistor (34) coupled between an output of said source providing said stereo difference signal (L-R) and an input of said matrixing means (24); a second resistor (36) coupled between said input of said matrixing means (24) and a circuit node; a third resistor (38) coupled between said circuit node and a point of reference potential; and wherein said activation means comprises: switch means (46, 48) connected in parallel with said third resistor (38). Apparatus as recited in Claim 3 wherein said switch means (46,48) is in a selected one of two speaker jacks (40) for automatically changing the attenuation of said variable attenuator from said first value to said second value upon insertion of an external speaker plug into said jack.

Inventors: HOOVER ALAN ANDERSON, HOOVER, ALAN ANDERSON
Applicants: THOMSON CONSUMER ELECTRONICS, THOMSON CONSUMER ELECTRONICS, INC.
Application year: 1993
Main classification: H04R5
IPC codes: H04S1, H04H5, H04B1, H04N5