Stereo Miking Techniques (Part.2)

This article is the second part of my serie about stereo recording techniques. Today, we’ll discuss ORFT and AB recording techniques.

THE ORTF RECORDING TECHNIQUE

This ORTF Technique uses also 2 cardioid microphones. However, in this one, the capsules are not stacked but the diaphragms are spaced of 17 cm (6.7 inches) with a 110° angle between the capsules.
Such spacing corresponds to the mean distance between human adult ears and emulates the shadow effect of the human head.

The main advantage of the ORTF technique is that because it respects the phase of the signal it therefore allows a more realistic perception of the stereo image.
The “time delay principle” is respected due to the fact that –because of the capsule are not stacked but positioned at a distance interval of 17 cm- the sound signal can’t reach the 2 capsules without a delay.
While the time delay is minimal, it contributes to a time difference in phase that induces the stereo image.

To perform a quality ORTF technique, care must be taken on the microphones used. They should have a frequency response off-axis as close as possible to the frequency response on-axis to avoid the commonly called “Hole in the middle”.
Indeed, the center of the stereo image results from the sum of the off axis sound signals captured by the 2 microphones (as showed below in green, the right side of the lefty positioned microphone and conversely, the left side of the righty positioned one.

Cheap microphones are often characterized by a difference between the frequency responses between off-axis and within axis.
The off-axis signal having less frequencies than the on-axis one, it will introduce tonal interferences with regards to the signal captured on axis, inducing an imbalance between border and center signals.

THE A-B RECORDING TECHNIQUE

The A-B technique is very easy to set up, it uses 2 parallel microphones positioned at a distance of about 30 to 40 cm apart ; however in some instances the distance can also be of 3 to 4 meters and even more…
This A-B technique is therefore very contributive and efficient to record orchestral ensemble, big bands ...

Microphones can be either cardioid or omnidirectional types ; the latters (as ones can imagine) being able to capture all surrounding sounds.

Due to their sound characteristics, they have a propensity to recreate or being extremely close of the room’s acoustic. Therefore, this technique offers an added tonal value if the room has a very natural pleasant “acoustic” !

Would it not be the case or if the room has too much natural reverb, cardioid microphones would be the best choice due to their ability of capturing less unpleasant frequencies generated by the room where the recording is performed.

The A-B technique (as well as the ORTF one) offers a more refined stereo image and similar “phase properties” as mentioned in the ORTF section above.

That’s all for the second part of the serie about stereo recording techniques. In the third and last part, we’ll talk about the M/S recoding technique which is a little bit different from the other ones in the way it works.