Work Arounds for Panning with 2-Channel Oriented Equipment

Even when the simple "hard" assignment of input to output channels needs some expansion to panning of one input source "in between" two output channels, still only a 2-channef panner is necessary, with the outputs of the panner routed to the correct 2 channels. So interestingly, a console designed for multitrack music production may well have enough facilities for straightforward multichannel mixes, since panning for many kinds of program material is limited to a hard channel assignment or assignment "in between" just 2 channels. For such cases, what the console must have is an adequate number of busses and means of routing them to outputs for recording and monitoring.These could be in the form of main busses or of auxiliary busses. Thus, a console with a stereo 2-channel mixdown bus and with 4 aux busses can be pressed into 5.1-channel service (although you have to keep your wits about you).

Clearly, it is simpler to use purpose-built multichannel equipment than pressing 2-channel equipment into multichannel use, but it is worth pointing out that multichannel mixes can be done on a 2-channel console with adequate aux sends. With this feature an adequate number of pairs of channels can be represented and thus input channels mapped into pairs of output channels, and "pair-wise" panning performed. Here is how this is done:

• Use output channel assignments for the medium in use. For 8-track multichannel mixes for television, this is seen later as: (1) left, (2) right, (3) center, (4) LFE, (5) left surround, (6) right surround.

• Assign an input track such as 1, to a bus pair, such as bus 1-2.

• Assign the bus pair to output channel pairs, bus 1 and 2 to output channels 1 and 3, respectively. (This requires that you build aux input tracks.)

• Now the stereo panner on input track 1 pans between left and center.

• If you have to continue a moving pan from left through center to right, then you will have to split the track in two at the point where it goes through center, using the first track for the first part of the pan, and the second track for the second part. This is because there are only 2-channel panners and no dynamic bussing during a session. Although this is clumsy, it does work.

Panning Law

The "law" of a control is the change of its parameters with respect to mechanical input. For a volume control, this is represented by the scale next to or around the control knob that shows you the attenuation in decibels for different settings of the control. For a panner, at least two things are going on at once: one channel is being attenuated while another is fading up as the control is moved. The law of a panner is usually stated in terms of how many decibels down the control is at its midpoint between 2 channels. Very early work at Disney in the 1930s determined that a "power law" was best for panners, wherein the attenuation of each of the channels is 3dB at the crossover point. This works perfectly in large reverberant environments like the original Disney dubbing stages because sound adds as sound power in the reverberant field, and two sources 3dB down from one source will produce the same sound pressure level as the single source. However, things get more complicated when the sound field is mixed among direct, reflected, and reverberant. The BBC found in the 1970s that about 4.5dB down in each of 2 channels produced equal level perception as a single source when sound was panned between channels in an environment much more like studio control rooms and home listening ones. The 3dB down law is also called a "sin-cos" function, because the attenuation of 1 channel, and the increasing level of another, follow the relationship between the sine and cosine mathematical functions as the knob is turned.

In fact, panning based simply on level variation among channels greatly simplifies the actual psychoacoustics of what is going on with a real source. Amplitude panning works best across the front, and again across the back, of a 5.1-channel setup, but works poorly on the sides, for reasons explained in Chapter 6,

An additional knob on some panners is called divergence. Divergence controls progressively "turn up" the JeveJ in the channels other than the one being panned to (which is at full level), in order to provide a "bigger" source sound. With full divergence, the same signal is sent to all of the output channels. Unfortunately, sounds panned with divergence are subject to the precedence effect, and putting the same

sound into all of the loudspeaker channels causes the listener to locate the sound to the closest loudspeaker to their listening position, and the production of highly audible timbre and image shift effects.

It is an interesting historical note that the invention of divergence came in the early 1950s in Hollywood, about the same time that Helmut Haas was finding the summing localization effect called Law of the First Wavefront described in Chapter 6. It seems highly unlikely that the inventors in Hollywood were reading German journals, so they had probably never heard of this effect, and did not know the consequences of their actions. The motivation was also not only due to the idea of getting a bigger sounding source, but also to reduce the obvious timbre variations as an actor would enter screen left and exit screen right panning across up to 5 channels, with a different timbre produced by each channel due to the lack of room equalization and tolerances of loudspeaker drivers in these early systems. Divergence helped to conceal the major timbre shifts.

It should be said however that the curiosity of sounding comb filtered and of localizing to the nearest loudspeaker has been put to good use at least once. In the voice overs of the main character in Apocalypse Now, the intimate sounding narration is piped to all three front channels— the divergence control would be fully up at least with respect to the front channels on the console. This helps, along with the close-miked recording method, to distinguish the "inside the head" effect of voice over, and lends maximum intimacy to all parts of the cinema because as one moves left and right across a row one finds the voice over to stay continuously more or less in front of one. That is, as you move off the centerline the narrator's image shifts until you are in front of the left channel loudspeaker, and straight in front of you now is the direction you hear. There is certainly some comb filtering involved in driving all three front loudspeakers, but in this case it is not seen as a defect because the more different the voice over sounds from production dialogue the better. Walter Murch, sound designer of the picture, has talked about the fact that the narration was recorded three times—for script reasons, for performance reasons, and for recording method reasons—with improvements each time. He has also said that he took as a model the voice over in the earlier film Shane to distinguish voice over from on-screen dialogue.

A further development of the divergence concept is the focus control. Focus is basically "divergence with shoulders." In other words, when sound is panned center and the focus control is advanced off zero, first sound is added to left and right, and then at a lower level, to the surrounds. As the sound is panned, the focus control maintains

the relationship; panned hard right, the sound is attenuated by one amount in center and right surround, and by a greater amount in left and left surround. Focus in this way can be seen as a way to eliminate the worst offenses of the divergence control. If a source needs to sound larger, however, there are other methods described below.

Дата добавления: 2015-10-30