ORTF Stereo Microphone Technique
By Phil O'Keefe |
This near-coincident stereo technique offers natural sound and wider stereo imaging than XY
By Phil O'Keefe
ORTF, which stands for "Office de Radiodiffusion-Television Francaise", is a stereo microphone technique that was developed by Radio France in 1960. Unlike other popular stereo techniques such as XY stereo and Blumlein stereo, ORTF isn't a coincident stereo setup where the capsules of the two microphones are positioned as closely together as possible. Instead they are spaced apart from each other in what is called a "near-coincident" arrangement. Near-coincident techniques also usually space the microphones at a distance which is less than true "spaced pair" arrangements such as AB stereo pairs.
Figure 1: An ORTF stereo microphone setup, as viewed from the sound source
THE GEAR YOU'LL NEED, AND HOW TO SET IT UP
You'll need two identical cardioid microphones. A stereo bar and a protractor to measure the angles will greatly help you with setting things up. While it is better if the microphones are a matched pair, you can still experiment with the technique as long as the two microphones are of the same make and model. Just be aware of any level differences between the two mikes and try to compensate with small gain adjustments at the mic preamps. It also helps if the microphones have uncolored off-axis response. For these reasons, small diaphragm condenser models such as the two DPA 2011C microphones shown in Figure 1 are often the preferred choice for use in ORTF stereo configurations.
Mount the two microphones on to a stereo bar, and angle them outwards and away from each other as shown in Figure 1. The centers of the two capsules should be 17 cm apart from each other, and the bodies should be set for a 110 degree angle, as shown in Figure 2. This 17cm spacing of the capsules approximates the distance between a person's ears, and helps to contribute to the increased realism of the ORTF arrangement when compared to XY stereo pairs. Experiment with the distance from the sound source. ORTF tends to pick up less ambience than some of the other stereo microphone techniques, so you can often place the microphones further back from the source without having the ratio of reverberant to direct sound getting out of control.
Figure 2: The ORTF stereo pair, as viewed from above. Note the 110 degree angle of the mics and 17cm spacing between capsules
ADVANTAGES OF ORTF
Unlike ORTF, XY places the microphone capsules as close together as possible, which means that regardless of which direction it is coming from, sound arrives at both capsules virtually at the same instant. However, with an ORTF pair, the 17cm spacing between the microphone capsules means that sounds that are coming from further towards the sides will arrive at one microphone or the other a a fraction of a second before it reaches the other mic. These "time of arrival" differences provide clues that our ears utilize to determine the direction the sound is coming from.
While both techniques typically utilize a pair of cardioid condenser microphones, ORTF stereo gives a wider stereo image than XY. This is due to the fact that ORTF stereo technique is a "mixed stereophony" technique that relies on both time of arrival and sound pressure level differences from the two microphones. The pressure differences are a result of the angle of the microphones and their cardioid directional patterns. The most sensitive part of one microphone's polar pattern is aimed to the left, and the second microphone is angled to more effectively pick up sounds coming from the right. When placed in an ORTF or XY configuration, the "null points" (least sensitive areas of the microphone pickup patterns) are positioned contrary to the other microphone. In other words, the most sensitive pickup of one mic covers one "side" of the stereo image, while the reduced output from the other mike's null point means it will pick up less signal for sounds coming from that same direction. Of course, for the opposite side of the stereo image, the roles of the two microphones are reversed.
Because of the spacing between the microphones, it's often possible to insert a baffle (such as a Jecklin or Schneider Disk) between them to further isolate them and increase stereo separation by mimicking the acoustical shadowing effect of the human head. This certainly isn't required or even typical for ORTF stereo setups, but the possibility is there and some people like to use it occasionally. ORTF has good mono compatibility, although not as good as true coincident stereo microphone techniques such as XY and Blumlein stereo.
I recommend trying ORTF setups for small and large ensembles as well as on individual instruments. Use ORTF instead of XY stereo when you want a wider, more dramatic stereo image than XY provides, or when mono compatibility isn't as crucial. It's also a good alternative to use as a replacement for a Blumlein stereo pair when that configuration results in too much ambient pickup. If mono compatibility is crucial, you may want to do a test recording first to be sure the ORTF recording will sum to mono without suffering from phase cancellation issues. If it does suffer from phase cancellation, either reposition the microphones or consider switching to a coincident stereo pair such as Mid-Side or XY instead.
* A quick footnote about the microphones used in the illustrations for this article: The DPA 2011C microphones pictured in this article are small diaphragm condenser mikes, but their use of "interference tubes" means that the capsules are not within a few millimeters of the front end of the body as with most other small diaphragm condensers; the diaphragms are actually located further back down the length of the tube, under the rearmost side grilles. When using these exact mikes, the ideal configuration would require even greater separation in order to give us the required 17 cm spacing between the capsules. However, this is contrary to the arrangement that is typical for the vast majority of small diaphragm condenser microphones (Neumann KM184, Rode NT5, MXL 604, Oktava MC012, Audio Technica AT4051, AKG C-451B, etc.), so for the sake of clarity, I have used the more typical configuration when setting up the microphones for the illustrations.
Phil O'Keefe is a multi-instrumentalist, recording engineer / producer and the Senior Editor of Harmony Central. He has engineered, produced and performed on countless recording sessions in a diverse range of styles, with artists such as Alien Ant Farm, Jules Day, Voodoo Glow Skulls, John McGill, Michael Knott and Alexa's Wish. He is a former featured monthly columnist for EQ magazine, and his articles and product reviews have also appeared in Keyboard, Electronic Musician and Guitar Player magazines.