FI102715B - Enhanced stereo effect expansion - Google Patents

Description

102715

IMPROVED STEREO IMPACT EXTENSION - FÖRBÄTTRAD STEREOEFFEKTENS UTVIDGNING

The present invention relates to audio equipment, more particularly to signal processing, to expand the stereo effect produced by the equipment.

Stereo audio is designed to provide the listener with a true-to-life audio image through two audio channels. In traditional stereo sound, the direction of the audio input is determined by the volume and phase ratios of the audio signal across different channels. In this case, the direction from which the listener detects the sound is always in the area between the pair of speakers or in the direction of either speaker.

The stereo effect is particularly limited when the left and right channel speakers are close to each other, such as a television or a ste-15 portable radio. When both loudspeakers are in almost the same direction to the listener, very obvious differences in the direction of the perceived sounds will not occur.

One known way of expanding the directional performance of an audio reverberation system is to use multiple audio channels and speakers, as is known, e.g. movie announcement-20 thighs.

The human perceives the direction of the incoming sound mainly through the difference in the travel time and amplitude of the sound sensed by the different ears. In principle, a dual-channel audio system can simulate all directions of audio input by changing the factors mentioned above. In this way, the sound can even be heard from outside the speaker pair.

In this method, the so-called. HRTF (Head Related Transfer Function) filters. HRT-30 F-Filters are defined as measuring or computationally determined transfer functions that describe the directional filtering of incoming sound due to head and outer ear shapes.

HRTF filters can be used to create an artificial sound-35 image for stereo speaker reproduction if cross-ears from the speakers are taken into account in the calculation. The filters Haja Hv shown in Fig. 1 consist of two parts: a stereo expansion filter and a crossover filter. Figure 2 shows a similar system in a more efficient embodiment. In this embodiment, 102715 2 filters Haja Hv are replaced by Ha and Hv filters, so that the extension can be implemented with only two filters.

The problem with such a method is the complex delay and frequency response characteristics that distort the signal of the HRTF filters. In stereo audio, this does not pose a problem as these distortions produce the desired stereo widening. In the monophonic portion of the signal from the center of the loudspeakers, the filters cause harmful distortions - the monophonic portion of the signal does not need to change direction of reception. In such systems, the monophonic portion 10 sounds colored. In principle, distortion of a portion of the monophonic signal could be corrected by adding an additional filter degree to the system output, but this in turn would distort the desired amplification of the stereo effect.

The difference between a mono signal and a stereo signal can be made by examining the inter-channel correlation. If the signals are fully correlated, the signal is monophonic. The separation of the stereophonic portion from the two-channel sound is in principle successful by the generation of a separation signal, since the correlating Monophonic portions cancel each other out. The sum signal generated from the two-channel audio can be considered as some approximation to the monophonic sound, because the proportion of the correlating sig-20 signals is significantly higher than that of the non-correlating ones.

EP-A-0 354 519 B1 discloses a coupling to eliminate a distortion of a monophonic signal. This connection has a mono signal detector which controls the signal going to the stereo effect amplifier circuit. If the input signal is ste-25 rheophonic, the switching input signal is normally applied to the stereo effect amplifier circuit. If the input signal is monophonic, the detector circuit guides the signal past the stereo amplifier expansion circuit, whereby the monophonic signal is applied to the speakers as such, without distortion. The problem with this method, however, is that the principle disclosed in the publication only detects a monophonic signal. However, the same audio signal may have both mono and stereo components: the method does not correct distortions due to the mono portion of such a signal.

It is an object of the invention to provide a method and a device which produces less distortion in the produced stereo sound than is possible with prior art solutions.

It is also an object of the invention to correct distortions caused by filters produced by an amplified stereo effect on a monophonic signal portion.

102715 3 This improvement is achieved by generating a sum signal from the original stereo material, which is passed past the expansion filters. Specifically, the system of the invention is characterized in that the system comprises means for generating sum and difference signals, and means for generating a delay corresponding to the delays of the filters of the difference signals, the output of which is coupled to the summing means for combining the edited sum signals.

The invention also relates to a method of generating a sum signal from a 10 stereo signal, passing a sum signal past the filters forming the stereo effect expansion, adding a substantially similar delay to the difference signals due to the filtering forms forming the stereo effect expansion, and adding the modified sum signals.

BRIEF DESCRIPTION OF THE DRAWINGS In the following, the invention will be described in more detail with reference to the accompanying drawings, in which: Figure 1 illustrates a known stereo widening principle (referred to above), Figure 2 illustrates a simpler known stereo widening principle (referenced above) Figure 4 illustrates a system according to the invention as applied to the basic structure shown in Figure 2, and Figure 5 illustrates a preferred embodiment of the invention.

30

In the pictures, the same reference numerals and notations are used for like parts.

Figure 3 illustrates the general principle of the invention. In the system of the invention, the mono portion of the signal is not filtered in the same filters as the stereo portion 35 of the signal. The monophonic signal portion is extracted and subtracted from the stereophonic signals in the monophonic signal separator E. Thereafter, the remaining portion of the stereophonic signals is filtered according to prior art, as described above in connection with the description of Figure 1. In turn, the monophonic signal 102715 4 is delayed in the organ Hs so that its delay corresponds to the delays caused by the filters Hv and Ha for the stereophonic signal, whereupon the modified monophonic signal is combined with the filtered signals of the left and right channels in the summing members 1 and 2.

5

Figure 4 illustrates the invention as applied to the basic structure shown in Figure 2. The operation of the invention is similar to that described in the preceding paragraph.

In a preferred embodiment of the invention, the mono signal separation element E 10, as shown in Figure 5, consists of summing elements 3, 5 and 6 and amplifier stages 4 and 7.

The monophonic signal portion is formed by summing the left and right channel signals in the summing member 3, after which the resulting monophonic signal portion is subtracted from the signal of each channel. The monophonic channel weighting can at this stage be adjusted by amplifier stages 4 and 7, wherein the amplification factor k of the amplifier stages 4 is preferably between 0 and 1 and the amplification stage 7 preferably has a gain factor of 1.

According to a preferred embodiment of the invention, the delay element Hs consists of a delay stage 8 and an amplifier stage 9. The monophonic signal is delayed by a delay stage 20 8 so that its delay corresponds to the delay caused by the filters Hv and Ha. After the delay stage 8, the mono signal may be amplified or attenuated by the amplifier stage 9 to compensate for differences in signal levels before combining the mono signal with the filtered signals of the left and right channels in the summing members 1 and 2.

25

However, since the delays caused by the filters are not the same at all frequencies, in another preferred embodiment of the invention, the delay stage 8 is more preferably formed by a filter whose frequency response and delay characteristics substantially correspond to those of the filters Hv and Ha.

30

Filters Hv and Ha, all of said summing members, and all of the amplifier and delay stages mentioned are per se circuit devices known to those skilled in the art.

Claims (8)

A stereo system for expanding a stereo impression, comprising 20. a channel on the right side and a channel on the left side and in each at least one filter (Ha, Hv, Ha ', Hv'), to generate signals that extend the stereo impression and compensate overhang, - a summing means (1) for connecting the output of at least one filter in the left channel to the output of at least one filter in the right channel to generate a first processed stereophonic signal and a summing means (2) for connecting the output of at least one filter in the right channel to the output of at least one filter in the left channel to generate a second processed stereophonic signal, the system being characterized by comprising a means (E) for generating a sum signal and stereophonic difference signals, and means (Hs) for generating a delay corresponding to the delays of the stereophonic delay signals filter (Hv and Ha) in the sum of the sum signal, the output of the means (Hs) being coupled to the summing means (1 and 2) to combine the processed sum signal with the processed stereophonic difference signals. 35 System according to claim 1, characterized in that said means (E) for generating sum and difference signals comprises a summing means (3) for generating a sum signal, gain step (4 and 7) for changing the weighting of the received sum signal, and means (5 and 6) for subtracting the sum signal from the original signals. System according to claim 1 or 2, characterized in that the means (Hs) for processing the sum signal comprises a delay means (8) for generating a delay in the sum signal corresponding to the delays of the filters forming the extension of said stereo sound. . System according to claim 1 or 2, characterized in that the means (Hs) for processing the sum signal comprise a filter (8) whose frequency function and delay characteristics are substantially similar to those of the filters forming the extension of the stereo sound. System according to claim 3 or 4, characterized in that the signal level of the processed sum signal is arranged to be controlled by a gain step (9) before combining the signal with the filtered difference signals in the left and right channels. 6. A method for reducing the errors in the proportion of a monophonic signal generated by the expansion of the stereo pressure, characterized in that the stereophonic signal is formed into a sum signal, the sum signal is passed past the filters forming the extension of the stereo pressure, to the sum signal a substantially similar delay is added. is caused in the difference signals by the filters forming the stereo impression extension, and the processed sum signal is added to the filtered difference signals. 7. A method according to claim 6, characterized in that, at the stage when the sum signal is separated, the weighing of the sum and difference signals is controlled by gain: steps. Method according to Claim 6 or 7, characterized in that the signal level of the processed sum signal is controlled by amplification step before said sum signal 35 is added to the filtered difference signals.