About the benefits of phantoms. The art of audio

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About the benefits of "phantoms"

The art of audio

Well, that's it! Either we sanctimoniously grumble at the Hi-Fi amplifier for its unswitchable timbre, or we seriously think: “Shouldn’t we classify expensive multi-channel-encoded-electronic-processing home theaters as untouchable High End ... Extremes not only in opinions, but and in equipment, it opposes a saving middle with a more relaxed approach to sound and a democratic price.Having thus outlined the boundaries of the influence of this material, we can calmly talk about the device and principles of operation of electronic surround sound processors.Moreover, the topic was prompted by editorial mail with your questions, dear readers.

We are not talking about real "theatre" processors (for example, developments from Dolby Laboratries), but about the numerous Surround'ax that are found in modern micro-mini-midi systems and portable audio under this name or, for example, under the abbreviation DSP ( Digital Signal Processor). Here it is time to mention for the first time "what is not in the world": imaginary sound sources, virtual images, if you like, phantoms or sound ghosts - "cute and cute" (curious play on words: I dreamed of a sound ...). In short, the vast majority of electronic systems that modify the stereo sound of music programs are based on the idea of creating such an acoustic field using a conventional stereo pair (two loudspeakers separated by a certain distance), which can be felt in a completely different environment (instead of a kitchen - the space of a stadium roaring with delight ), or when increasing the number of sound sources (cinema), or when expanding the sound space (desktop radio with the voice of a system with a two-meter stereo base).

A little history on the issue of surround sound. Stereo, based on the binoural (two ears!) principle, is self-sufficient. Two channels have long and firmly established themselves in household equipment, they draw a spatial picture well, allow you to localize musical instruments by ear. The striking difference between stereo and mono will not force you to change your stereo system to a sadly lonely point source of sound. And if you use four information channels and four loudspeakers? Quadrophony (Q-sound) was not an easy nut to crack: the rise in price was clearly inadequate to the effect achieved. Stereo has held its own, but its position is currently under attack from multi-channel Home Cinema enthusiasts.

And next to the problems of surround sound, there was just a place for simple electronic sound processing tools: DSP, 3D-sound, Surround, Sound Retrieval System, Incredible Sound ... As a rule, such systems, using one or another circuit and technical solution, designed, for example, in the form of a microcircuit, one or more algorithms for frequency-phase processing of the original electrical signal are implemented. At the dawn of portable stereo recorders, the simplest schemes for artificial expansion of the stereo base were already used, operating according to the subtraction method. And here are examples of modern "sound phantom generators": Digital controlled audio processor (SGS-Thomson, tdA7344), Matrix surround IC (NEC, PC1853), Spatializer (Desper Products), RSS (Roland), SRS or Vivid 3D (Hughes), Incredible Sound (Philips Research Laboratories).

The existence of these tools is justified because of the relative cheapness and versatility. And most importantly - due to the fact that for the manifestation of additional sound effects, one stereo pair is enough. The element base of various processors is currently actively used by developers of consumer audio equipment. So, for example, many modern models of Sharp and Kenwood mini-systems are equipped with Sound Retrieval System (SRS) processors from the American corporation Hughes; Dutch Philips produces a wide range of equipment (from portable audio to TV) with the Incredible Sound function; the Spatializer is installed in the Panasonic-"cobra" radio tape recorder. I must say that for the most part DSPs are quite close to each other in terms of the final result.

Let's look at a specific example of how DSPs are used. Based on SRS technology, Sharp engineers have developed a virtual surround sound system - Virtual Surround, which works with a stereo signal of any source (cassette, CD, LD, TV, video, radio) and does not require additional speakers. The designers made two gradations of three-dimensional sound with a virtual expansion of the stereo base: music mode (Virtual I - it seems that the speakers are moved apart from their original position), cinema mode (Virtual 2 - there is an illusion of additional side speakers). In addition, it is possible to recreate the acoustic environment of a medium-sized concert hall (Hall, reverberation is noticeable), a cathedral with a dome (Dome, a long reverberation time manifests itself as the echo of multiple reflections) and the "live" space of a concert venue (Live).

For the sake of truth, it should be noted that artificially acquired bulk costs not only money, but is also associated with unwanted artifacts that can manifest themselves, for example, in violation of tonal balance. However, this kind of distortion just forms spatial sensations. And now, for the most inquisitive and patient, I would like to propose to deal with the question: "How exactly do you achieve surround effects on a conventional stereo with the help of electronics?" Let's take as a basis materials about the system that is widely used in Philips equipment - Incredible Surround Sound, which is the subject of an invention by Ronald Aarts and Robert Dekkers (the authors are employees of the Philips research laboratory).

It is worth emphasizing once again that the stereo sound quality of most portable systems, multimedia systems and TV is limited by the small distance between the speakers, so electronic expansion of the stereo base often becomes one of the few, relatively inexpensive ways to make surround sound. This circumstance, apparently, was the reason for the search for a new technical solution. The essence of the invention "Extension of the stereo base when playing sound - Incredible Sound (incredible sound)" will help to understand the scheme. It shows: a listener, a signal processing unit, a stereo pair, imaginary loudspeakers ("phantoms") and a picture of the propagation of direct (not reflected) sound waves. It is shown that the virtual sources actually expand the distance and angle between the left and right speakers.

The authors, summarizing the data of psychoacoustic studies, conducted including on real people, obtained the dependence of the change in sound pressure in the left and right ear when the real loudspeakers are sequentially moved from the initial position to the extended one. It follows from the illustration that the sound waves that come to the listener from the displaced speakers will differ significantly from the initial ones in terms of phase and frequency parameters, depending on the time of arrival of the wave front to the eardrums of the left and right ear, the effects of shading and wave diffraction on the head listener.

The found functional dependence of the sound transformation, which was called the Head Related Transfer Function (HRTF), was in a certain way embedded in the electronic circuit that processes the output stereo signal. The introduction of appropriate changes in the left and right channels leads to the reconstruction by the existing loudspeakers in the listener's area of exactly the structure of the acoustic field, which is characteristic of the modified position of the emitters. Thus, the listener gets a real feeling that the sound space opens up before him. It can be assumed that the information obtained by the authors was exhaustive, and the function had a linear structure, since in practice it is controlled using a simple two-channel potentiometer. Therefore, in audio equipment (for example, a portable radio AZ8070) it became possible to smoothly change the base extension mode from ordinary stereo (0% extension) to the maximum manifestation of the effect - 100% "incredible sound".

By the way, a group of foreign experts, having made a comparison using the method of "double-blind testing" of the artificial stereo expansion systems mentioned in our article, noted the best tonal balance and natural sounding in Incredible Sound, despite the fact that this technical solution is very easy to implement in practice and it's pretty cheap...

And in the research labs, new ghost hunters continue to search for natural sound, and don't be surprised if tomorrow a huge sound "phantom" materializes in a room with the help of a tiny radio tape recorder.

Author: Sergey Klobukov

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