[78-L] Binaural, duophonic, etc.

[DAVID BURNHAM]

Dan Van Landingham wrote:

"Binaural" refers to a method of recording that mimics the human
ear. - "Two Sounds." The microphones are literally mounted on a Styrofoam
head in some cases, or a single crossed-8 cardoid stereo mic is used.  The
listener wears headphones.  

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As dl has pointed out, Duophonic was a Capitol term for simulated stereo. 
 Binaural recordings have to be made using microphones spaced approximately the 
same distance apart as the distance between a pair of ears, just like stereo 
photography uses two cameras which are eye-spaced apart.  In both cases the 
signal for the left ear/eye cannot reach the right ear/eye or it won't work. 
 That's why headphones or some sort of glasses must be employed respectively

BUT the second half of the above statement is not only wrong, but is confusing. 
 There are several different kinds of mikes, based on polar patterns;  two of 
these are Cardioid and figure of 8.  Crossed figure of 8, (or bidirectional) 
mics will yield superb stereo recordings reproduced on speakers.  This is known 
as an XY system and was regularly used by BBC and EMI in the early days of 
Stereo.  The system was invented by Alan Blumlein and bears his name.  A 
bidirectional mike is also known as a cosine mike because the level of pickup is 
proportional to the cosine of the angle of incidence on the mike.  This may 
sound confusing but what it means is that this particular pickup is ideal 
because when your position to the mike is 90 degrees, which in an XY system is 
the direction from which the right signal will be arriving to the left channel 
mike, the output is theoretically zero, (the cosine of 90 degrees is "0").  At 
45 degrees, which is the angle at which each mike will be picking up a centre 
sound source, the output of each mike is reduced by half.  If a speaker walks 
from the extreme left to the extreme right of an XY pair of figure 8s, his voice 
level will not vary and the direction of the image will be identical to his 
position on the sound stage.  

Crossed cardioids, on the other hand, produce a less than ideal stereo image.  A 
cardioid mike is only down a couple of dBs at 90 degrees so a signal from the 
right side of the sound stage will produce a considerable signal to the left 
channel mike.  If the same speaker walks across a XY pair of cardioid mikes, the 
image will never be at the extreme left or right and the voice will be louder 
when he is at the centre, (called centre channel build up).  

In any case, neither of these situations will produce binaural recordings 
because the inter-channel time difference which is required for binaural 
reproduction is totally absent when using a coincident stereo mike.

db