Sonic holography

Sonic Holography is a proprietary audio filter design developed by Bob Carver and was used extensively in several preamplifier and receiver units built by Carver Corporation in the 1980s and 1990s.

The goal of sonic holography is to remove distortions in pitch caused by the doppler effect while recording and playing back audio on a sound system. While an audio source is recorded by a microphone, the membrane that picks up sound vibrations moves towards and away from the source. This results in a doppler shift that is preserved in the recording. When that recording is played through a speaker, the cone vibrates back and forth in order to move air particles and create sound. This introduces a second doppler shift, especially when the same cone is producing multiple frequencies at the same time. Both instances of doppler distortion degrade the overall quality of the output. Sonic holography attempts to remove these distortions using a proprietary filter designed by Bob Carver.

Dissenting Opinion: Sonic Holography is Bob Carver's handle for interaural crosstalk cancellation, NOT distortions caused by the doppler effect.

Controversy

Audio purists argue that in order to preserve fidelity, the audio signal should be captured at the highest quality possible and then should not be modified. Some listeners feel that Sonic Holography introduces artifacts into the sound recording while attempting to remove others.

The Original Description

The above article misrepresents what Bob Carver's invention of Sonic Holography is and does. I have taken the following information about Sonic Holography from the user manual of the device where Sonic Holography was first introduced: Carver Model C4000 High Fidelity Control Console ©1981 by Carver Corporation. In that this preamplifier was the first to incorporate Sonic Holography, it contains an extensive explanation of the underlying theory of its operation.

ONIC HOLOGRAM Stereo Image Processor

:"Sonic Holography" is a method of processing stereo signals so as to correct a basic imaging flaw which is inherent in two-channel stereophonic recording and reproduction via loudspeakers. What that flaw is and why it can't be corrected by any conventional recording technique are discussed later in "Stereo Recording and Playback".

:Briefly, the problem is that in stereo listening, both ears hear the outputs of both loudspeakers. When a sonic event such as a musical transient is reproduced by the Left-channel loudspeaker, the sound travels in a straight line from the speaker to your left ear. A tiny fraction of a second later the same Left-loudspeaker sound arrives at your right ear, somewhat filtered by the obstruction of your head. If the same sonic event was recorded in both stereo channels, as normally is the case, then some version of it will be reproduced in the Right-channel speaker, whose sound will arrive at your right ear and then, a tiny fraction of a second later, at your left ear. Thus the single original sonic event is represented by a total of four sound arrivals at your two ears.

:In real life a single sonic event can never cause more than two sonic arrivals: one at your left ear and one at your right ear. (Which ear gets the sound first depends on which direction you are facing, relative to where the sound is coming from. If you are facing the sound, it will arrive at both ears simultaneously.)

:The goal of the Carver Sonic Hologram Generator is to eliminate the "extra" two sonic arrivals that occur in stereophonic playback but do not occur in real life. With these eliminated, the ear/brain system of the listener will receive unambiguous timing and phase information about the original sounds as they struck the recording microphones. Without extra sonic arrivals to confuse it, the ear/brain system will be able to perceive the true location of each sound source in the stereo recording — not only from left to right but also from near to far.

:This is accomplished by canceling the unwanted second arrival of the sound from each speaker to the opposite side ear, so that each ear is free to concentrate its attention on the signal from the speaker on the same side; i.e., the left ear will hear mainly the Left speaker, and the right ear will hear mainly the Right speaker, without the confusing acoustic crosstalk which normally occurs in stereo playback.

:How it works. The circuit produces electronic crosstalk signals from each stereo channel into the opposite-side speaker. These are virtually identical to the unwanted acoustic crosstalk second-arrival signals which flatten the image in stereo. They have essentially the same delays as the second-arrival signals, and about the same filtering as that caused by the blockage of your head, but they are inverted in phase. Consequently, when these mirror-image crosstalk signals are reproduced by your speakers they cancel the acoustic crosstalk signals arriving at each ear from the opposite speaker.

tereo Recording and Playback

:If the sound is recorded and later played back in stereo via loudspeakers, the result will depend on the microphone technique employed. Consider the simplest and most common: the sound is recorded via a single close-up microphone whose signal is "panpotted," i.e., split and recorded in both stereo channels but slightly stronger in the left channel in order to place its image slightly to the left of center. In playback the sound emerges simultaneously from both speakers (a little louder in the left).

:Assume that you are sitting equally distant from the two speakers, facing the midpoint between them. The sound from the left speaker arrives at your left ear, and at the same time the sound from the right speaker arrives at your right ear. A fraction of a millisecond later the sound from the left speaker, after filtering by the acoustic shadow of your head, arrives at your right ear; and similarly the sound from the right speaker arrives at your left ear.

:In the "live" listening experience the single sonic event produces two arrivals at the ear. The delay and frequency spectrum differences between the two ears are the primary cues which the brain uses to determine the direction of the sound source. In the "panpotted" stereo recording and playback, the sonic event has produced a total of four arrivals at the ears, the first two being simultaneous and identical in frequency spectrum — a very different set of cues.

:In an effort at greater realism, some recording engineers attempt to record musical performances with a "coincident pair" of crossed cardioid or figure-8 microphones. The sound from the instrument, regardless of where it is located on the stage, arrives simultaneously at the two mikes and is recorded in both channels, with a difference in intensity which is proportional to the source's angular displacement away from stage center. Thus in playback the sound emerges simultaneously from both loudspeakers, with some difference in strength; but just as with panpotting, the original sonic event generates a total of four sound arrivals at the ears.

:The other common technique for recording large ensembles such as symphony orchestras and choruses is to hang two microphones in front of the stage, separated by about eight feet. Now, if the instrument is located several feet left of stage center, its sound will reach the left microphone first and will get to the right microphone after an extra air-path delay of, say, three milliseconds. As with the previous examples the sound of each instrument is present in both channels of the recording, but in theis case witha time-delay as well as an intensity difference between the two channels.

:In playback the sound emerges for the left speaker, is heard by your left ear, and arrives at your right ear with some head-shadow loss after a small fraction of a millisecond. Meanwhile, three milliseconds after its appearance in the left speaker, the sound emerges from the right speaker and arrives successively at the right and left ears in turn. Not only do four sonic arrivals at the ears arise from the single sonic event — this time they are spread out be several milliseconds in time because of the spacing of the recording microphones. (In life a single event cannot generatearrivals spaced more than one millisecond apart, since no one's ears are spaced more than a foot apart.)

:There are additional stereo miking techniques in common use , but all share the characteristic that every sound is present to some degree in both channels. Therefore every sonic event always produces four sonic arrivals at the ears in stereo playback — instead of two which in life provide the brain's primary cue to localizing the direction of sound. Of course this problem is avoided in a "ping-pong" recording, in which sound emerges only from the left or only from the right; but that's not stereo and cannot present a panoramic image spanning the space between the speakers.

:One successful approach to lifelike sound reproduction is binaural recording, using microphones buried in a dummy head so that the recorded signals already contain the inter-aural delays and head-shadow losses which the listener would experience. The recording is played through headphones, so that each ear hears only what the same-side microphone in the dummy head picked up. This method is not without technical flaws, but its most important limitation is economic; most listeners don't like to be confined to headphone listening, so binaural recordings have very limited sales potential. As a practical matter most recordings must be engineered for loudspeaker playback.

:Therefore, we need a direct solution to the problem of acoustic crosstalk in the listening environment. This is the goal of the Carver Sonic Hologram Generator.

ONIC HOLOGRAPHY: Canceling Acoustic Crosstalk

:As noted earlier, the essence of the problem is that both ears hear the sounds from both loudspeakers. The sound from each speaker reaches the same-side ear directly and then, after a brief delay and loss due to acoustic blockage of the listener's head, reaches the opposite-side ear. Conceptually, the object of sonic holography is simply to cancel out that delayed, attenuated signal from reaching the opposite-side ear, so that each ear will be exposed mainly to just the speaker on the same side.

:In principle it is quite straightforward. We know that the signal form the left-channel speaker arrives first at the left ear, then arrives in a slightly weaker form at the right ear after an added delay of about 0.2 milliseconds with its highs rolled off. All we have to do is to feed the right speaker a sample of the left-channel sound that is delayed 0.2 ms and rolled off in highs. This signal from the right speaker will get to the right ear simultaneously with the unwanted acoustic crosstalk signal from the left speaker. So if we phase-invert our specially-delayed right-speaker sample of the left channel signal, this electrical crosstalk will cancel the acoustic-crosstalk as the two signals arrive at the right ear. A complementary process is used to cancel the acoustic crosstalk from the right speaker into the left ear.

:The actual operation of the Carver Sonic Holography Generator circuit is rather more complex than this, but that is the basic operating principle.

:Why is this process called sonic holography? An optical hologram is a photograph made with a laser whose coherent beam of light is split into two beams and used to illuminate an object; the two beams are recombined, forming alternating rings of constructive and destructive interference. When the photograph is developed and another laser is used to project it, a three-dimensional image of the object is projected into space.

:By analogy, a sonic hologram generator takes the beam of sound produced by each loudspeaker and splits it so that a related beam of sound is produced by the opposite speaker (after delay and filtering) in such a way that the acoustic interference of the sounds occurs in the air near each ear, revealing the true three-dimensional sound image that was hidden in the stereo recording. Recall that "stereo," in the original Greek, means "solid" or three-dimensional, not just wide. Ideally stereo is intended not only to paint a sonic image onto the wall between the loudspeakers but to yield realistic perception of depth as well.

External links

* [http://www.carveraudio.com/ Carver Audio website]
* [http://www.sunfire.com/ Sunfire website]
* [http://www.audioasylum.com/audio/general/messages/14476.html/ Sonic Holography]
* [http://www.soundclick.com/aceholiday Ace Holiday] Artist that re-creates 1960's easy listening genre with newly composed songs. Selected tracks produced with Sonic Holography. * [http://www.soundclick.com/util/streamm3u.m3u?id=5029668&q=hi Listen Here MP3] (free to public).