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Next Steps in Surround
Interchannel crosstalk can occur in in-the-band recording scenarios as well, when multiple microphones pick up one sound source and are mixed into the surround field in different locations. However, this is far rarer as the intensity and time of arrival of each sound to each microphone is far greater than in an audience perspective recording situation.
To reduce interchannel crosstalk, carefully choose the surround microphone technique. The microphone arrangement of the front three microphones should be positioned such that intensity of sound to each microphone or pairing or microphones is far greater than the adjacent microphone or pairing of microphones. Test results have indicated that microphone techniques such as the Decca-Tree, Fukada Tree and Optimized Cardioid Triangle (OCT) (see Figure 2) have lower amounts interchannel crosstalk and are also more desirable to listen to.
Taking this information one step further, the microphone techniques used to capture the surround channels are important as well to reduce crosstalk between the front array of microphones and the rears. Microphones that are assigned to the rear surround channels should pick up as little of the direct sound from the stage as possible.
Therefore, it is suggested that directional microphones are used for the rear surround channels. These can be direction microphones that have their null pointed at the primary sound-source such that the primary pick up is pointed elsewhere. Omnidirectional microphones can be used, but it is important that they are immersed in acoustic reflections of the acoustic space and have very little direct sound from the sound-source.
There are other techniques to use as well, and careful study and research should be used to pay attention to the amount of cross-talk that occurs between microphones in either an in-the-band scenario or audience-perspective scenario when recording surround.
Examples of these techniques include techniques such as the Hamasaki-Square (invented by Kimio Hamasaki of the NHK Science and Research Lab), which utilizes four figure-eight microphones arranged in a square about six feet apart, placed beyond the main microphone array anywhere from 12 to 20 feet behind the front microphone array. The figure-eight microphones face the side walls of the acoustic space and have their least amount of sensitivity point (null) towards the sound-source, usually on a stage, as shown in Figure 3.
Quite a bit of time and attention will need to be spent in learning how to capture and produce surround content. It is not nearly as easy as throwing up a stereo pair of microphones on a stand and hitting record. Multiple microphones will have to be placed with care. Live audience perspective recording situations should also address concerns such as visual aesthetics. Using all-in-one solutions may not produce the most useful and desirable result, so buyer beware. There are plenty of other key factors in producing quality surround content. It is highly suggested that anyone wishing to produce surround content listen to a wide variety commercially available and discern quality differences. Becoming educated on many of the other basics of surround production is crucial.
Kosiorek is the director of recording services at the Cleveland Institute of Music.
|Reference Level = -20dBfs digital = +4dBu analog|
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|Unified Clip Level = 0dbfs digital = +24dBu analog|
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