Today we are talking about Mid-Side Processing and Recording and lets start with the basics,
What is Mid-Side?
Mid-Side is a method of summing the left and right channels of a stereo signal to produce separate MONO and STEREO channels that can be edited independently. While LEFT and RIGHT can be thought of as two independent channels that can be edited independently, the exact same can be said for MID and SIDE.
This gives the mastering engineer the ability to alter the overall stereo sound without affecting the mono-compatible mix. It’s akin to altering the EQ in the LEFT channel while leaving the RIGHT channel untouched, or vice versa.
In order to convert a Stereo signal into Mid-Side, summing of the audio is necessary. If the RIGHT channel is added to the LEFT channel, the MID is produced.
This is because audio information that is “in-phase” in both channels is added together and audio information that is “out-of-phase” in both channels is subtracted and cancelled out. If the RIGHT channel is subtracted from the LEFT channel, the SIDE channel is produced, this is due to out-of-phase information now ADDING together, and in-phase information cancelling out.
If we ADD the LEFT to the RIGHT, we can see the RIGHT is silent, which won’t add anything, so we end up with exactly the same signal in the MID channel. On subtracting the RIGHT from the LEFT, you can see that we’re subtracting silence and so we end up with exactly the same signal in the SIDE channel. If the sound is panned to the RIGHT, then the LEFT will be silent and RIGHT will contain the sound and when we add those together, the MID is simply the same again.
However, if the LEFT is silent but we SUBTRACT the RIGHT channel, what we’re doing is ADDING an INVERTED copy of the RIGHT, because subtraction is the same as adding an inverted version and so the SIDE channel becomes an INVERTED version. On panning the sound to the CENTRE, then it’ll be equal in both the LEFT and RIGHT channels. When they add together, the MID becomes TWICE the amplitude. This makes sense, because something panned into the CENTRE is going to be EQUAL and IN-PHASE in both channels therefore it will ADD to become twice the amplitude.
How Mid-Side recording is done?
Mid-Side recording is a technique where two separate microphones are used, one to record the MID and one to record the SIDE. The MID microphone usually has a “Cardioid” pickup pattern; the SIDE microphone, which records off-axis sounds, has a “figure-of-eight” pickup pattern. The SIDE microphone is split into SIDE+ [positive] and SIDE- [negative]. This relates to what I talked about before, where when something is panned RIGHT, the SIDE channel is actually INVERTED compared to the LEFT channel being IN-PHASE.
To start recording in Mid-Side, you will need 3 channels on your mixer, Channel 1 for the MID microphone, Channels 2 and 3 for the SIDE microphone. Channel 3 is INVERTED and panned to the RIGHT whereas Channel 2 is IN-PHASE and panned to the LEFT, and the Channel 1 (MID channel) is panned CENTRE. If we reverse the formula, we find that: LEFT is made from MID added to SIDE, and RIGHT is made from MID minus SIDE and this is why Channel 3 is INVERTED.
Tips on Mid/Side Recording, Mixing and Mastering
— The basic setup for recording in Mid/Side makes use of one cardioid microphone (mid) and one bidirectional (figure-eight) microphone (sides).
— The Mid channel is the center of a stereo image. When the Mid channel is boosted, the listener perceives a more centered (mono) sound to the audio.
— The Side channel is the edges of a stereo image. When the Side channel is boosted, the listener perceives a more spacious (wider) sound to the audio.
— The louder the side channel is, the wider the audio is perceived.
— Very gentle Mid-Side processing is generally favorable
— If a track has multiple guitar parts, route them through a bus, using Mid/Side processing on the guitar bus.
— A slight volume boost to the side channel on drum overheads can enhance the room sound.
— A high frequency EQ boost on just the side channel makes the ‘wider’ elements sound brighter.
— If a mix sounds muddy, try reducing low frequencies in the side channel.
— A dry acoustic mix can be warmed up with Mid/Side reverb.
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