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Binaural Vs. Pseudo-binaural

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There are many microphones out there that claim to be binaural. Most of them are not, but can be called pseudo-binaural.

True binaural microphones take into account what are often referred to as HRTFs, or head-related transform functions. They are also placed differently from pseudos, in that both mics face straight out from the sides of the head, the same as your ears do.

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binaural mic

Pseudo-binaural mics basically follow what is known as the A-B stereo pattern:

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pseudo-binaural mic

The biggest difference between these is that the pseudo-binaural mics usually face directly forward, with a separation about the same as the average person's ears [around 17cm]. Binaurals face out, directly away from each other.

The reason this is important is that if you compare the phase in each channel of sounds from different locations, the binaurals represent more or less what your ears are used to, where:

* sounds coming from directly ahead are in-phase [as they hit the mics at the same angle]; the same goes for sounds from behind

* sounds directly to one side of the head are 180 degrees out of phase [exactly the opposite phase] at each mic [as the sound hits each mic from exactly the opposite direction], as well as being slightly delayed

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in-phase signals

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180-degree out-of-phase phase signals

In contrast, Pseudo-binaurals, where the mics usually face forward, should be in-phase more often [keeping in mind that the distance between them causes a phase-delay, though less-often resulting with the outright opposite phase] regardless of where the sound is coming from.

As an example, A-B stereo mic'ing is often used for recording orchestras with the expectation of listening to the recording on a home stereo system, where the speakers stand in front of the listener and face the same direction. The idea is that phase be maintained throughout the recording [since the mics face one way, and the speakers basically face the opposite], with the distance between the mics providing a tiny [phase] delay that gives a sense of space or dimension.

Binaural recordings are usually made to be listened to on headphones, where the speakers themselves are pointed straight at the ears, exactly the opposite from the way the mics faced when recording. This can provide a truly stunning sense of dimension, and if done well, can even give a sense of front/back and up/down placement when listening. This is part of why true binaural recordings can sound odd or spacey when played back through speakers.

The other half of binaural recording is that the microphones are often placed inside a dummy head, or in the case of some of the more recent stealth mics, inside the recordist's outer ears. The dummy head technique also often uses silicon ears, to reproduce the effect of a real person's ears.

Why bother going so far as to make fake ears? This goes back to HRTFs - that the pinnae [outer ears] affect the equalisation and phase of the sound depending on what direction it's coming from. Sound coming from straight ahead of you follows a different response curve from sound coming from behind or directly to the side. Also, the fact that the head sits between the mics changes the intensity of the sound at each mic, since the head blocks sound more from certain directions than others [i.e. sound can't go through your head].

The upper body also plays a part in levels at each ear and equalisation; your lungs, for instance, are resonators, and sound [particularly bass] can come from the inside of your head through your lungs, sinus cavities, and mouth, reaching the inner ear through the eustachian tubes [which connect your throat and sinuses to your inner ear, to let your ears equalise when air pressure changes].

Pseudo-binaurals take part of HRTFs into account, but since the mics are usually facing forward, they do not record things in the same way as true binaurals. There's still a rather stunning sense of dimension, but part of the information is lost to give better playback compatibility with normal stereo systems.

In the end, which one is better depends entirely on what you want to do with your recordings:

* Those intended for playback through speakers are usually best recorded with pseudos, or with other mics following any of the established coincident patterns [Y, X-Y, decca tree, &c.].

* Those intended for playback over headphones are usually best recorded with binaural mics, which, because of their orientation, help to eliminate that "inside your head" effect so common with normal stereo recordings that are mastered using speakers.

I realise that I have grossly simplified things in trying to explain some of the differences, but hopefully this will provoke some useful discussion between us.

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For the true experimenters out there, try doing some reading on Ambisonics - a 4-track recording method that uses a multi-point [often 6-point] microphone and both pre-and post-processing to record fully 3-dimensional sound, including left-right, front-back, and up-down. This with a technique invented in the late 1960s and early 70s when the analogue processing equipment cost thousands of dollars to build.

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Coincidentally, phase is also how matrix surround-sound [like Dolby Stereo] works. Dolby Stereo uses in-phase signals in both channels to represent "mono" or front-centre, and 180 degree out-of-phase signals [like the diagram above, where the level is the same in both channels but one channel is the opposite phase] to represent the rear channel.

Different mic'ing techniques will give different results when played through a matrix surround decoder [whether it be Dolby Surround, Prologic, or Prologic-II; the encoding is basically the same for all of them].

Binaural recordings will stick sounds that were originally directly to the recordist's [or the dummy head's] side in the rear channel, since they're out of phase, with sounds that were originally directly in front or behind showing up in the front centre channel.

Pseudo-binaurals [and A-B stereo recordings] will have a less-pronounced effect when played through surround decoding. The effect can be noticeable but, as with binaural mic'ing, does not correspond to the original locations of the sounds compared to the mics.

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That's a good introduction if I might say so!

From a binaural CD review at http://www.binaural.com/bins.html-

"Mike Skeet is a binaural-positive recording engineer. He has fashioned some of his own mike systems, including a dummy head into the back of which a digital MiniDisc recorder has been installed for a completely portable package that goes anywhere. And Mike went just about everywhere with it to put together this unique collection of music, locations, voices, events, happenings and even history... He designed it for a high quality headphone listening experience. If you think MiniDisc is just a consumer digital format, experiencing this sampler might change your mind. The 60 different tracks include such startling binaural demos as bell-ringing in the belfry, a steam engine passing right over your toes, a thunderstorm that beats any of the others in our catalog, a party with a jazz group that easily outclasses the "you are there" feeling of the famous Jazz in the Pawnshop , some choir excerpts in which you can easily locate each singer in the ensemble, and some of the usual walking-around-the dummy-head-and-laughing demos. "

I introduced Mike to minidisc many years ago, and he's never looked back. He has a pair of Aiwa MD recorders built into a special case with preamps he designed and built himself, together with a custom built level and phase display. Last time I saw him he was recording a classical concert in London's prestigious Wigmore Hall with this rig, fed from a complex surround sound mic array suspended above the stage. He's built many different binaural setups and you'll find further references to these via Google.

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