Surround recording techniques: Double M/S and IRT Cross

17 Jan

As part of the tasks to complete for our Digital Studio Production module at the Master in Audio Production in Salford University, we were asked to do two surround location recordings with portable equipment if possible. These recordings would be later mixed into 5.1 Surround format in the studios as part of our assessment… and it’s also good fun too!

The first recording I did in this fashion was not properly “on location” but in our University’s recording studios instead, using their equipment in full. It all happened before the Christmas break, when some of us Master fellows joined there to make a “jam session” (some noise with instruments  such as guitars, bongos, a battered drumset) and record the resulting soundscape. The goal was to set ourselves as far in the corners of the live room as possible so as to create a wide soundscape to recreate via surround recording techniques.

My setup of choice for this surround recording was the Double M/S technique. But not the usual one with two Mid microphones and one Side microphone for both of them, all sharing the same space in place. Instead of that, I decided to use Curt Wittig’s techniquue, shown in ‘Spatial Audio’ book by Francis Rumsey. In this one, two separated MS pairs of mics are used: one of them will give the front signal and the other one will record the rear signal.  According to Mr. Wittig, the rear MS pair should be placed back in the room at a distance from the front pair further than the room’s Critical Distance (DC).

Double MS Diagram

Double MS setup diagram (taken from “Spatial Audio” by Francis Rumsey)

As usual, we can use either a cardioid or omnidirectional microphone for the Mid signal. Although the omni achieves better low frequency response and more accurate off-axis mic response, it may be too wide for our taste. I personall prefer using a cardiod mic as the mid one. The levels we set on M and S channels of both front and rear will also give a wider or tigther sonic image. The procedure for this setting is as simple as usual: once we have recorded the signal from both M and S pairs we have to convert them into Left and Right signals by setting the adequate levels on both M and S signals and doing some matrixing in our DAW of choice. Given that

Mid = Left + Right
Side = Left – Right

We can solve the arithmethics, which yield that:

L= (M+S) / 2
R= (M-S) / 2

This means that to get the Left Channel we have to create a bus in which we sum the M and S signal, attenuating each of them by 6 dB. To get the Right channel, we will sum M signal plus a phase inverted version of the S signal, both of them attenuated by 6 dB too. Then we will route each front and back Left and Right signals to the square 5.1 panner in our DAW to produce that surround mix.

Double M/S microphone setup

Double M/S setting in the studios. You can see both M/S pairs in the room: one in front of the drums and the other one behind it. Two Rode NT-2A and two Rode NT-2 microphones were used. For the M signal, the cardioid mode was used.

The other surround technique that I used was, as I told in my previous post, after the new year back in Spain recording Sweet Q‘s new song Lovesong #13 in their rehearsal room. This was a true location recording with my own equipment: 2 Rode NT-2As and 2 Beyerdynamic MCE530 microphones were used. The technique I used is the IRT Cross (also known as ‘atmo-cross’), which has been widely used by researchers Theile and Hamasaki as part of a complete Surround array as the OCT system developed by Klepko and Theile, but only for the ambient mics. Instead of that, I used this technique as the main and only surround sound setup for the room.

I used this technique because of the convenience of having to carry only 4 mics with their assorted hardware (cables, mic stands, mic clips, etc). It is based in 4 cardioid microphones, arranged in the edges of a virtual square of 25 to 40 centimeters in length. Each of the microphones points away 90º from each other and each of them should be aimed at the edges of the room if possible. In an ideal configuration, each of them should have the same sensitivity and frequency response, which of course couldn’t be achieved on my case because of the microphone availability.

IRT Cross configuration diagram

IRT Cross configuration diagram,as shown in “Spatial Audio” book by Francis Rumsey

I recorded myself on the rehearsal room explaining this technique right on the spot, if you fancy a watch at our cramped rehearsal room!

Unfortunately, I can’t upload here the results in full surround format unless I give you a link to a hosting space with the 5 WAV files for you to decode on your home, or I send you a DVD-Video with it encoded. But what I did was to create a fairly accurate stereo mix from the 4 microphones and use it as the background track for a video shot while they were recording. This way, they also can use it as a promotional video as part of their videodiary. Here is the result of the recording, and here you are Sweet Q performing “Lovesong #13” live in their rehearsal room.

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