Stereo Recording Techniques

MS (or Mid-Side) recording

Mid-side recording utilises either an omni or cardioid microphone facing the middle of the stereo sound stage (this is fed to one channel of the mixer) and at right angles to this is a figure of eight mic (being fed to the other channel), its front faces the left and the rear faces the right of the stereo sound stage, a decoder converts the two signals into conventional left and right channels within the mixer for recording or at a later stage if the two microphone signals are recorded separately as mid and side; M+S = Left channel, M-S = Right channel.

The two capsules should be as close to each other as possible, usually one mic is directly above the other.

The advantage of M-S recording is that there is perfect mono compatibility since the mid microphone is looking at the middle of the stereo soundstage, there are no slight time differences between the left and the right sound sources (at the same distance away) arriving at the microphone, also there is no need to have closely matched frequency responses in both microphones.

Coincident (or XY) stereo recording

This technique (also called AB stereo recording) usually relies on two cardioid microphones ideally with identical frequency responses, if figure of eight mic's are used then there will be more of the room's acoustics picked up or other sound sources behind the microphones.

They are placed very close to each other, one above the other and at a 90 degree angle. You can use a wider angle but the wider you go the more of a ‘hole in the middle’ effect you will get and mono compatibility gets worse.

The XY system doesn’t have perfect mono compatibility because the sound coming from the right hand side of the stereo sound stage arrives at the left hand microphone a fraction of a second later than the right hand microphone.

Purpose built XY microphones such as the Schoeps CMXY 4V consist of two microphones  mounted very close to one another and the capsules are rotatable from 0 - 180 degrees.

Spaced Omni's

This is where two cardioid or omni microphones are placed about 1 metre apart facing forwards, sometimes a third microphone is used in between them and the two original microphones can be placed further apart upto 3 metres, the third centre microphone allows for greater mono compatibility and the orchestra and / or choir sounds more spacious than M-S or XY recordings would but the detail and mono compatibility is reduced. This is often called the Decca Tree system after the company that pioneered this method of classical music recording.

Stereo recording techniques vary between sound recordists, Coincident and Mid-Side recordings tend to give the most technically accurate stereo recordings but some recordists prefer to use spaced omni’s and figure of eight mic’s for aesthetic reasons.

Mono compatibility is probably less of an issue nowadays so you can use the techniques that will cause problems in mono if you are certain that the end user wont be listening in mono, however it would be wise to keep checking in mono to make sure you are not making it un-usable should it be broadcast at a later date in mono.

If you have enough microphones and tracks to record them with you will probably want to eaperiment with several techniques at the same time in addition to using ‘spot’ mics on individual instruments or sections of the orchestra and some ambient microphones further back in the hall which you can then mix to taste at a later date.

Although you can use dynamic microphones for stereo recordings they will never give a full frequency response. You should use condenser, ribbon or electret microphones.