Stereo Recording Techniques

The stereo recording techniques are used when we want to register and / or reproduce the feeling of space that surrounds our auditory perception of an acoustic event. All elements must be considered, since the reproduction of images side and center, passing through the sense of depth or distance and even the acoustic feeling of involvement with the environment, which is usually represented by reverberation. Surround makes this experience even more real and has the help of psychoacoustic effects mainly caused by lags.

There are four main groups of techniques:

• Coincident pair (X, Y, M and S-Blunlein)
• Spaced pairs (A / B, 3x1, Deca Tree)
  
• Coincident pair almost (ORTF)
• Artificial Head (Dummy Head)

X-Y

This technique requires two identical directional microphones, usually cardioid. The capsules should be very close - preferably overlap - that can turn forming angles between 90 and 130 °. When fitted properly, the result of the sum to mono must suffer an increase of up to 6 dB in signal amplitude, without phase cancellation.

M-S

It also consists of two microphones, a directional points at the center toward the sound source and another, Figure 8 with its axis rotated 90° relative the microphone to the center . Figure 8 The microphone should have doubled his signal to the control PAN complete opposites, a 100% to 100% left and one right. The channel has completely overturned the PAN to the right should undergo a polarity reversal for stereo image when there is added to the central microphone. Thus the encoding will MS: M + S = L, while MS = R.

Blumlein

Blumlein was the forerunner of all harvesting techniques we know today. It was the first to perform experiments with quadraphonic uptake which in turn is the predecessor of the surround. The technique uses double August obviously two bi-directional microphones (figure 8) at the same point, only rotated 90 ° apart. It uses the same condition except that the MS signals for two microphones. This technique has good results when using very close distances. It has a channel separation greater than the technical and xy is a cornerstone of the surround.

Spaced Pair

A-B

Two identical microphones are positioned the same distance from the center of the image. The stereo image comes from the gap and differences in amplitude and frequency response of the signals arriving at two microphones. They are not compatible in mono, but can produce great images according to the interaction with the sound source.

3x1

Basically uses the same principle as AB but with one rule for positioning and distance between the capsules. If one of the microphones are one meter from the sound source, the other must also be, but the distance between them should correspond to three times that amount in this case, three meters.

Deca Tree

Created by the studios Deca, uses directional microphones in a combination of distance and positioning while providing the feeling of depth and stereo. It is a technique that takes advantage of more funding for the surround and guard the same distance between microphones in an array "tree"

Coincident pair almost (ORTF - Office de Radiodiffusion - Television Francaise)

Developed by ORTF especially for the transmission of classical music concerts, this technique has both the name of their creators, usually uses two cardioid microphones that can be angled between 70 ° and 140 ° and spaced at 17 cm to 21 cm. The idea is to reproduce the gap between our ears.

Dummy Head

The goal is to simulate the most of the processes involved in our hearing, literally copying a human head and placing microphones in their "ears". The head artificial simulation including the phase cancellations caused by reflections of the ear. In the frequency response of this type of microphone you can notice a strong cancellation in the treble region. When we listen to headphones, this technique shows an impressive reality "tricking" well our brain.

Model of Dummy Head