Reductio Ad Ambiosurdum

Since it is very difficult to mount three microphones so that they are really coincident, and although there are electronic position shifters that correct for this, we are still left with six outputs when we want Ambiophonics to work with two. The trick is to see how the Ambisonic microphone arrangement can be simplified without sacrificing anything vital to realistic music reproduction. The first thing we should remember is that we do not need to pickup any sound from the rear half circle since that ambience is computer generated. Thus if our microphone is baffled, we can ignore the rear half lobe of the f microphones and the rear halfcircle of the o and s signals. For angles close to the middle, the f and o signals would be very much the same so let us assume that we delete the two f signals altogether. We also know from our earlier discussions that a coincident microphone requires crosstalk in reproduction in order to provide low frequency phase shift differential localization. But the binaural arrangement of the two coincident microphone assemblies eliminates this concern. If we use two coincident s,o microphones, (of course, if two microphone sets are used they cannot be coincident in Ambisonic lingo, but bear with me.) spaced the average distance between the ears, we restore the natural low frequency phase cues. Remember that in Ambisonics, the problem was that if the wave was correct at the center of the head it might not be exact enough at the sides of the head. Well, in our Ambiophonic version of Ambisonics, the right and left ear signals are isolated by the panel or panels extending from the listeners between the speakers, so now we can independently generate a wave for each ear. This means that for the central 90° or so sound sources, the left half of a right side o microphone circle and the right half of a left o circle are not really required. Thus the decoding equations become even simpler.

If the mics were now mounted in a dummy head with pinnae, then the signal would be modified by pinnae squared. (bad) But if the mics were just mounted on a head wide boom, then on playback, since the speakers are close together, there would be no head related response even for signals from the sides. Therefore the best arrangement is to use a dummy head without pinna and place either an o or an o-s pair at the entrance to the dummy head ear canal. The dummy head will of course not match the listeners head exactly, but the effect is slight, since in this case the pinna are not involved and this discrepancy only effects side sources. Someday, one will be able to get his head response measured and correct for the difference between his head and the standard dummy head. Looked at this way, Ambiophonics is just a subset of Ambisonics or a superset of Binaural.

I haven't forgotten about the s signals. Since the half exposed s microphones point to the extreme front sides, if there is no music there, they are not always needed. Depending on the exact aperture of the o microphones, the shadowing effect of the dummy head, the loudspeaker angle and the lower frequency cues present, stage widths of 120° are obtainable without an s signal. However, if four channels are available and two more front side speakers are used, the stage width can be expanded. In this case interaural crosstalk is not as much of a spoiling factor because, by definition, extreme side sound sources engender neither pinna angle distortion nor loudspeaker crosstalk but there is a likelihood of localization to a single side speaker, and to avoid this a full fledged Ambisonic decoder could be used with a pair of side speakers. Another possibility is to modify the s signal with a pinna equalizer and add it to the o signal so that when it comes from the front speakers it will sound as natural as possible even in the absence of any interaural low frequency cues.. Again, for best results, this would require having your pinna response measured and applying a correction to the hopefully published recording company pinna equalizer. There is at least one company that maintains that 25 pinna response curves describe over 90% of the worlds population and they have put such pinna responses in a box so you can select the one that works best with your own ears. Thus the personal pinna era may be here sooner rather than later.

Nothing New Under the Sun

After completing the above text on recording, I began to see if the recordings that were easy to playback Ambiophonically had anything consistent or unusual about them. Not being a recording engineer or a microphone aficionado, it took me awhile to notice that many of the easy to match CDs were made with something called a Schoeps KFM-6. A picture of this microphone in a PGM Recordings promotional flyer showed a head sized but spherical ball with two omnidirectional microphones one recessed on each side of the ball where ear canals would be if we had an exactly round head. The PGM flyer also included a reference to a paper by Günther Theile describing the microphone, entitled On the Naturalness of Two-Channel Stereo Sound, J. Audio Eng. Soc., Vol 39, No. 10, 1991 OCT.

Although Theile would probably object to my characterization of his microphone, his design is essentially a simplified dummy head without external ears. He states, It is found that simulation of depth and space are lacking when coincident microphone and panpot techniques are applied. To obtain optimum simulation of spatial perspective it is important for two loudspeaker signals to have interaural correlation that is as natural as possible........Music recordings confirm that the sphere microphone combines favorable imaging characteristics with regard to spatial perspective accuracy of localization and sound color..... Later he states The coincident microphone signal, which does not provide any head-specific interaural signal differences, fails not only in generating a head-referred presentation of the authentic spatial impression and depth, but also in generating a loudspeaker-referred simulation of the spatial impression and depth......it is important that, as far as possible, the two loudspeaker signals contain natural interaural attributes rather than the resultant listener's ear signals in the playback room.

One minor problem with the Theile approach remains. For signals coming from the side, the sphere acts as sort of filter for the shorter wavelengths just as the head does. When this side sound comes from side stereo speakers the listeners head acts as a filter again resulting in HRTF squared. The solution is to move the speakers to fifteen degrees in front of the listener use the barrier and listen to the Theile sphere without the second head response function. I have done this and it works. Eventually, a listener would substitute his own pinnaless HRTF for that of the sphere and the accuracy would be further enhanced.

Theile also "generates artificial reflections and reverberation from spot-microphone signals." He uses the word artificial in the sense that the spot microphone signals will be coming from the front stereo loudspeakers instead of from the rear, the sides, or overhead. While Theile's results rest as much on empirical subjective opinion as they do on psychoacoustic precepts, they certainly are consistent with the premises of Ambiophonics both in recording and reproduction.

Realistic Reproduction of Depth

It is axiomatic that a realistic music reproduction system should render depth as accurately as possible. Fortunately, front stage distance cues are easier to record and/or simulate realistically than most other parameters of the concert-hall sound field. Assuming that the recording microphones are placed at a reasonable distance from the front of the stage, then the high frequency roll-off due to distance and the general attenuation of sound with distance remain viable distance cues in the recording. Depth of discrete stage sound sources is, however, more strongly evidenced in concert-halls by the amplitude and delay of the early reflections and the ear finds it easier to sense this depth if there is a diversity of such reflections. In Ambiophonics, simulated early reflections from 55° make the stage as a whole seem more interesting, but it is only the recorded early reflections coming from the front speakers that provide the reflections that allow depth differentiation between individual instruments. This is why anechoic recordings sound so flat when played back stereophonically or even Ambiophonically, despite the presence of a simulated ambient field. In ordinary stereo, depth perception will suffer if early side and rear hall reflections wrap around to the front speakers or in the anechoic case, are completely missing. Since it is easy to make Ambiophonic recordings that include just proscenium ambience, why not do so and save on synthesis processing power and preserve, undistorted, the depth perception cues?

There remains the issue of perspective, however. When making a live performance recording of an opera or a symphony orchestra the recording microphones are likely to be far enough away from the sound sources to produce an image at home that is not so close as to be claustrophobic. There are many recordings, however, that produce a sense of being at or just behind the conductors podium. This effect does not necessarily impact realism but you must like to sit in the front row to be comfortable with this perspective. Turning down the volume and adding ambience can compensate for this, but at a loss in realism. This problem becomes more serious in the case of solo piano recordings or small Jazz combos. For example, if a microphone is placed three feet from an eight foot piano, then that piano is going to be an overwhelming close-up presence in the listening room and a "They-Are-Here" instead of a "You Are There" effect is unavoidable. This can be very realistic especially with close together speakers and the Ambiophonic barrier, but adding synthesized hall ambience doesn't help much since the direct sound is so overwhelming. The major problem with this type of recording is that you have to like having these people so close in a small home listening room. You may notice that demonstrators of high resolution playback systems in show rooms or at shows, overwhelmingly, use small ensemble, solo guitar, single vocalist etc., close mic'ed, recordings to demonstrate the lifelike qualities of their products and that these demonstrations are mostly of the "They Are Here" variety.

To Probe Further or Try for Yourself