Stereo Microphone Techniques
When recording a stereo signal of a sound source there are a number of techniques and setup’s concerning the microphones positioning relative to the source which affect the result in playback. The dynamics here being manipulated in the mix imaging are the stereo width, this is an effect of the angle of the microphones determining the recordings SRA (stereo recording angle), these will place the sound source in the stereo field and this is received by the listener who then determines psychoacoustically where the source is placed from the subtle differences in time and / or level difference depending on the setup.
The two types of stereo techniques involve adjusting the level difference or the slight time difference, setups utilizing only level difference will always be coincident as this will prevent any time difference from occurring, this then leads to the different levels of the signal across the stereo field in front of the setup being the determining factor with the directional imaging of the source in the recording, as with louder signals on either side and less on each corresponding opposite the signal will appear more present on either L / R side of the recording. Stereo mic setups utilizing time difference will contain some distance between the microphones (microphone base) which will then lead to the sound source arriving at each diaphragm at very slightly different times depending on which side of the stereo field the sound source is placed. These times usually consist of only milliseconds (double figure sample count difference in total at most between peaks)
Along with the time and level variables there is also the aspects of coincident placement (microphones which are atop the other placed with diaphragms crossing resulting in ideally 0 time difference) and mono compatibility (were playback of the stereo signal can be in mono without phasing issues) of the setup to consider, the different stereo microphone setups utilizing these variables are :
XY - These being placed coincident to each other, the setup consists of two cardioid microphones with an angle of 90 degrees between them and an SRA of +/-98 degrees. This setup is mono compatible.
Blumlein - This setup similarly to XY is coincident, consisting of two mics set to figure-eight polar patterns, with a fixed angle of 90 degrees and a SRA of +/- 38 Degrees. This setup also is mono compatible, however the proximity effect with this setup must be considered as it is accentuated due to the two figure-eight patterns present.
Both the XY and Blumlein setups utilize the level difference between each stereo channel (L/R) to create the imaging effect, with louder signals on one side leading the listener to perceive the audio source’s positioning to be on that side.
AB - Differing from the previous this setup has the time difference also at play, being a space pair and with distance positioning required between the two microphone bases. The space between the microphone mimics the space between the ears and therefore those slight differences in arrival time from the sound source to the diaphragm also mimics to the listener the positioning of the source directionally in the stereo field, with one side of the signal arriving ahead in playback leading to the listener to perceive the source to be further to that side. The AB setup also do contain a phase difference due to this distance between microphones, however it lends to a good spatial impression in the recording, the setup is less mono-compatible due to this however as when converted to mono this phasing effect becomes much more obvious in a negative way, leading to comb filtering changing the tone and timbre of the source. There are no specifically suitable microphone or polar pattern types for this setup, however whichever is chosen the diaphragms are typically with both diaphragms parallel and in-line with each other.
