Posts Tagged ‘mid-side’

Mid-Side recording and Microphone Sensitivity

November 1, 2009

Small technical backgrounder this time.

When recording stereo it is essential to have equal gain settings for the microphones at the risk of the stereo image shifting to either left or right. More gain on the right mic channel will shift the image to the right and the other way around. Equal gain settings are especially important in XY, ORTF or any other spaced or coincident microphone positioning.

With mid-side recording the gain setting for the mid and side channels allow you control the stereo spread. More gain on the side channel compared to mid will widen the stereo image. More gain on the mid channel will narrow the image. Ultimately, no gain on the side channel will leave you with a centered mono signal from the mid mic.

Reading gain or trim pot settings is easy enough but you usually end up with differences in signal strength if you don’t take into account microphone sensitivity even with an equal gain setting on your recorder or mixer. Unless you have a matched pair, microphones have different sensitivity characteristics.

A typical MS setup is the Sennheiser MKH60/ MKH30 microphone pair.

The MKH60 has a sensitivity of 40mV/Pa, the MKH30 has 25mV/Pa. With equal gain you will end up with a much stronger mid signal and consequently a narrow stereo image.

The calculation to compensate for sensitivity is as follows:

±dB = 20 x log(MmV/SmV)

where

MmV = sensitivity of the mid mic in mV/Pa
SmV = sensitivity of the side mic in mV/Pa

In our example we need 4.1dB more gain on the side channel compared to the mid channel to get equal signal strength on both of the recorded channels.

This is easy to realize on mixers such as the Sound Devices 302. With MS stereo linking set trim pots allow you to set gain for the mid and side channels separately. The input 2 fader controls the overall gain for the pair.

Advertisements

North India Field Recording on SoundCloud

September 26, 2009

Just posted an excerpt of the field recordings I did in North India earlier this year. Using SoundCloud for the first time for this.

Recorded with a Sennheiser MKH8020/ MKH30 MS stereo microphone configuration to a Sound Devices 702 recorder. MS converted to AB stereo at the recorder stage.

SoundCloud uploads are transcoded to 128 kbps mp3 format for streaming. That’s still twice as good as MySpace.

Mid-side and the MKH 8020

November 1, 2008

This is slowly becoming a series.  I posted a recording to Freesound where I am testing the new Sennheiser MKH 8020 omnidirectional microphone as the mid microphone with my MKH 30 as the side mic.

The low end response of the MKH 8020 is very very impressive.  It is more sensitive than the MKH 20 omni (31mV/Pa compared to 25mV/Pa) and adds a few dB to the high end response. Beyond 20 kHz the frequency response goes all the way up to 60 kHz. It’s also very small: diameter 1.9 cm, length 7.4 cm. The pair fitted in my size 3 Rycote windjammer.

Better low end response is what you expect from an omnidirectional microphone as I explained in earlier posts but the MKH 8020 is in a league of it’s own. So, curiosity put to the test I set out in Brussels city center. The “carbon chorus” of the city never fails you if you are looking for those low frequencies. With so much low end energy setting levels proved a bit of a challenge. Dynamic bandwith gets eaten fast and limiters kicked in often.

The sample on Freesound is a recording of the air vent of an airco system. A giant outlet of 2 by 5 meter or so. That night it all came together. Traffic, airplanes, the airco system everything in perfect harmony.

Hear the recording here: Air,

or jump to the Drones Sample pack.

Mid-side recording setup tests

June 11, 2008

Following my previous post on the mid-side recording microphone setup I have posted 6 recordings testing different microphones and configurations to Freesound.

All recordings in this series were done with a Sound Devices 302 field mixer to a Sound Devices 702 recorder. Mixer gain set at +60dB and fader level at +7.5dB across all mic configurations. For the MS+M setup one recording had the fader level set at 0dB. Mixer and recorder line up was done at full scale. No low cut filtering was set on either device. Recordings were matrixed to L-R stereo at the mixer stage.

The differences between recordings are subtle and become more obvious through analyzers. Interesting things to look at are frequency spectrum, stereo correlation and peak and RMS levels. Recordings were done sequentially meaning one setup after the other. Samples presented were cut from the original recordings to get more or less equal soundscapes to make comparing easier. Recordings are presented unmodified. All recordings are approximately one minute in length.

Microphones used:
Pearl MSH-10
DPA 4060
Sennheiser MKH 40
Sennheiser MKH 30

Complete sample pack: “Mid Side test setup

the Freesound Project

Mid/Side + Mid recording

April 29, 2008

No need to introduce the Mid/Side technique developed by Alan Blumlein as a means of obtaining a stereo recording.

While the the technique allows for an omnidirectional microphone to be used for the mid channel one most often sees a microphone with cardioid pick-up pattern being used. This is also the case in my own recordings. For field recordings this usually is a shotgun mic like the Sennheiser MKH60.

Not all cardioids are created equal however. Their frequency response differs greatly and by design cardioids do not pick up low frequencies very well. The same holds true for the figure eight mic used for the side channel. The excellent MKH60 and MKH30 have a frequency response of 50Hz to 20kHz and 40Hz to 20kHz respectively. Some microphones show a more or less important roll-off on the low end.

The relative lack of low frequency information which is the result of this frequency response can seriously influence the recording quality. Recordings might appear particularly thin. In some situations, e.g recording of bird calls, this might not be a problem. In other situations the recording is just not going to be realistic. A typical example is the recording of high waves rolling onto the beach. There is lot of low frequency information in the sound of waves hitting the beach.

To solve this I thought of extending the classic MS technique by adding a low pass filtered third mic. This third mic preferably has an omnidirectional pick-up pattern as an omni captures the low end much better then do cardioids.M/S+M recording

As I said, the trick is to low pass filter the signal. This can be achieved in post production or in the field by inserting a low pass filter such as the Schoeps LP 40 U in the signal chain before recording.  There is of course a requirement to have three input channels available. One for the mid mic, one for the side mic and one for the low pass filtered omni mic. I call this third channel the +M channel because it needs to be panned center in the mix.

In the field I mount a DPA 4060 just below the mid-side pair but since only low frequency information is retained I would think that it can be postitioned anywhere close to the main coincident mid-side mount. At low frequencies the time delay resulting from the distance between the mics becomes irrelevant.

One needs to take care not to turn up de volume trim on the +M channel too high. Low frequencies eat up the available dynamic range very quickly. The whole idea is to get a natural sound character.