Creating High Impact Video With Stereo Sound

There is a simple way — without spending a lot of money — for small scale video producers to gain a significant advantage in production value over many broadcast competitors. It involves audio — the part of every video production that offers the highest impact with viewers.

Stereo audio was invented long before the video recording device or even the portable television camera, but its use today can add significant impact and realism to any video production. Since few small video crews record in true stereo, I can only suspect they don't understand how it works.

There are many varieties of stereo and immersive audio now available. Each has its own strengths and weaknesses. But one method — mid-side stereo — is simple enough for anyone to use in news gathering, documentary, sports, broadcast remotes and other live events.

Alan Blumlein.

Alan Blumlein.

M/S stereo audio was created by Alan Blumlein, an EMI engineer and pioneer in stereophonic and surround sound. His technique, patented in 1933, was used in some of the earliest stereo recordings.

The reason Blumlein's M/S microphone technique is preferred by broadcasters is because M/S recordings are always mono-compatible. Unlike other stereo recording techniques, this makes M/S safe and protects the mono signal if anything goes wrong.

M/S stereo audio allows video crews to capture super-realistic sound images to match the video. Crews who record only mono audio in the field and depend on a post production mix to generate stereo are missing a unique opportunity to add significant excitement and production value to their work.

Whether recording directly to a multichannel video device or an outboard audio recorder, it is now very easy to record stereo ambience on a pair of channels while simultaneously using standard mono field techniques on the other channels.

Though "panned mono" stereo recording through a mixer is the most common way to produce stereo tracks, "true" stereo recording is a more viable technique for field video recording. True stereo uses two microphones to pick up ambient sounds.

Illustration by Izotope.

Illustration by Izotope.

With M/S, the basic concept is that the Mid microphone, with either a cardioid, hypercardioid or shotgun pickup pattern, acts as the center of the stereo image. The Side mic's figure-8 pattern, aimed at 90 degrees from the source, picks up ambient and reverberant sound coming from the sides.

Since it's a figure-8 pattern, the two sides are 180 degrees out of phase. The front of the mic, which represents the plus side, is usually pointed to the left, while the rear, or minus side, is aimed to the right.

Modern M/S microphones are coincident because a pair of microphones are sandwiched closely together in the same plane. M/S mics can be either a pair of mics or a single piece mic with two capsules inside. Videographers will find the single mic configuration more convenient, since it is less clumsy, offers ease of use and offers the most consistent results with video.

In the field, it is advisable that the sound operator simply treats the M/S microphone as two separate units, routing the signal from the M capsule to channel one and the signal from the S capsule to channel two of the portable recorder. Use of a matrix device with an M/S microphone in the field to create left-right stereo sound for direct feed to the video recorder should be avoided when the camera shots are not pre-determined. This would mean the sound operator could only guess whether the perspective of the stereo recording matches the visual image on tape. In video production, it is usually best to avoid use of the M/S matrix until post production, thus allowing the editor to "paint" the sound to match the corresponding video picture. 

By recording the M and S channels discretely, the editor has a range of choices in the edit bay. Perfect mono may be chosen by selecting the M channel only. By processing the M and S channels through the matrix, the ratio of side level and mid level can be controlled and the stereo perspective varied in the mix.

Sennheiser MKH-41 M/S Stereo Mic.

Sennheiser MKH-41 M/S Stereo Mic.

Since the stereo imaging is directly dependent on the amount of signal coming to the side channels, raising or lowering the ratio of Mid to Side channels will create a wider or narrower stereo field. The result is that you can change the sound of your stereo recording after it's already been recorded.

Control of this stereo perspective is essential to matching video picture with sound. For example, when the camera lens is on a wide shot of a crowd the broad pickup of environmental sound is desirable. But when the shot changes to a close-up, the sound pickup should narrow accordingly with the lens. It becomes highly distracting when the "size" of the sound does not match the picture. Think of the matrix device as an audio width control that works with a video zoom. 

Several M/S microphones designed especially for video field production are now available from a wide range of microphone vendors. Many portable mixers have M-S recording matrixes built in, as do DAW in post.

Just because many producers and videographers don't use or understand stereo is no reason for the more innovative creators of video programming to remain on the sidelines. Once you experience the difference, there will be no turning back. 

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