typically pointing at an angle between 90° and 135° to each other. The stereo effect is achieved through differences in sound pressure level between two microphones. A difference in levels of 18 dB (16 to 20 dB) is needed for hearing the direction of a loudspeaker. Due to the lack of differences in time-of-arrival/phase ambiguities, the sonic characteristic of X-Y recordings has less sense of space and depth when compared to recordings employing an A-B setup. When two figure-eight microphones are used, facing ±45° with respect to the sound source, the X-Y setup is called a Blumlein Pair. The sonic image produced is realistic. M/S technique: mid/side stereophony This coincident technique employs a bidirectional microphone facing sideways and another microphone at an angle of 90°, facing the sound source. The second microphone is generally a variety of cardioid, although Alan Blumlein described the usage of an omnidirectional transducer in his original patent. The left and right channels are produced through a simple matrix: Left = Mid + Side; Right = Mid − Side (the polarity-reversed side signal). This configuration produces a completely mono-compatible signal and, if the Mid and Side signals are recorded (rather than the matrixed Left and Right), the stereo width can be manipulated after the recording has taken place. This makes it especially useful for film-based projects. Near-coincident technique: mixed stereophony These techniques combine the principles of both A-B and X-Y (coincident pair) techniques. For example, the ORTF stereo technique of the Office de Radiodiffusion Télévision Française (Radio France) calls for a pair of cardioid microphones placed 17 cm apart at a total angle between microphones of 110°, which results in a stereophonic pickup angle of 96° (Stereo Recording Angle, or SRA). In the NOS stereo technique of the Nederlandse Omroep Stichting (Dutch Broadcasting Organisation), the total angle between microphones is 90° and the distance is 30 cm, thus capturing time-of-arrival stereo information as well as level information. It is noteworthy that all spaced microphone arrays and all near-coincident techniques use a spacing of at least 17 cm or more. 17 cm roughly equals the human ear distance and therefore provides the same interaural time difference (ITD) or more, depending on the spacing between microphones. Although the recorded signals are generally intended for playback over stereo loudspeakers, reproduction over headphones can provide remarkably good results, depending on the microphone arrangement. Pseudo-stereo In the course of restoration or