Surround Sound Mixing – Part 3 of 5
This is a guest post by mix engineer, Unne Lilijeblad over at www.mix-engineer.com. This is the third article in a five part series about his experience with mixing in this still under utilized medium for listening to music. In the last article Unne talked about the being disappointed with DVD audio discs. This week he talks about recording and mixing in stereo.
Recording and Mixing in Stereo
There are many different ways of creating a stereo mix. For example, when mixing multiple mono sources, such as recordings of an electric guitar, an electric bass, a saxophone and a vocalist, done with one microphone each, they can be panned across the stereo image through the use of the pan knobs found on all mixing consoles and in all DAW:s. The pan knob divides the signal between the two speakers. Turn the knob to the left and it will send more of the signal to the left speaker and less to the right, resulting in our ears interpreting the sound as if it originated somewhere to the left of center.
Another way to achieve a similar result is through the use of delay. With a real acoustic sound, such as someone clapping their hands in front of you, but a little to the left, the sound waves reach your left ear slightly before they reach your right since the distance from the source to the right ear is greater than the distance to the left. To achieve a similar effect while mixing in stereo, you can delay the signal being sent to the right speaker slightly, and the result will be that the listener “hears” the sound coming more from the left. There are limits to how much delay can be used though, because at a certain point, we start distinguishing the two signals as separate signals instead. This technique also always causes phase issues to some degree
True stereo recordings, done with multiple microphones, take advantage of one or both of these effects during the recording. One example is placing a pair of microphones in front of an orchestra in a so called “ORTF” -configuration – French radio people at the “Office de Radiodiffusion Television Francaise” came up with this approach in the 1960’s, trying to mimic how human ears work. Two cardioid microphones are spaced apart about 7 inches, (approximating the distance between human ears) and aimed away from each other at a 110 degree angle, allowing engineers to take advantage of both arrival time differences and the volume differences between the microphones’ on and off axis response.
Another example, that only takes advantage of volume differences, is the X/Y stereo configuration, involving two microphones placed extremely close together (eliminating arrival time differences) but aimed away from each other at a 90 degree angle. This approach gives a fairly realistic stereo image while still maintaining good mono compatibility. (Summing a stereo image that includes arrival time differences, whether recorded that way or created with delay during mixing, can often lead to undesirable effects such as comb filtering and loss of sound at certain frequencies.) Additional stereo recording techniques include M/S and Blumlein.
Over all, I would say that stereo recording techniques definitely provide more realistic sounding results than stereo images created during mixing with the above mentioned techniques. However, what works best in any given situation is of course completely at the discretion of the engineer and will depend on his or her artistic goals.
In the next installment Unne will discuss Mixing in Surround.