Surround sound & Tracking

This is a text about how to imply surround sound in your tracked music. Although it's only based upon two simple tricks, I consider this advanced tracking, since the real need will only show when you're going (semi-) professional. This results in the fact that the methods are mentioned in short directions of where to find the desired option, that require thorough knowledge of the tracker program. I explain the system for Impulse Tracker, since it's the only tracker I know that has an explicit surround-function. People that use another tracker must be aware that the letters of the effects can differ or that there is no "surround-effect" at all (That doesn't make it impossible to use surround, but it's going to cost you diskspace and channels...). People that don't use a tracker program at all and just want to learn about surround, should just ignore the "So many ways of doing this:"-sections. The pictures say most.

Those of you who use Impulse Tracker might think using surround is just this easy: tap 's' on the channel's panning slider or use the 'S91' effect. And indeed, playing it through a set of stereospeakers, that will do to have a nice effect. Using a surround decoder things are slightly different, though.
Since Pro-Logic Surround decoders are getting cheaper and the difference between stereo and surround can be compared to the difference between mono and stereo, you could as well do it well.
I first noticed the fact just setting the channel in surround mode wasn't enough, when I went to a local radiostation with one of my tunes. In Belgium local radiostations broadcast in mono only, so when my music was played, my lead instument and the scary vocals I had picked out carefully just disappeared!
First I tried to fix this just switching all surrounded channels to central panning, but the seductive surround-option kept calling, so I tried to find out more about it. It ended up with me buying a surround decoder!

The a-pictures show an example of a sinewave, using yellow for the left channel and red for the right channel. The middle white line separates both channels (it's not an X-axis).
The b-pictures show a set of stereospeakers (same colourcode applies here). The arrows show the direction the sound comes or seems to come from. The relative size of the arrow gives an indication of the loudness of the sound coming from that direction. The arrow's colour tells about the signal that's coming: same colour means same signal, inverted colour means inverted signal.
The c-pictures show the same situation as the b-pictures, but now a Dolby Surround Pro-Logic decoder with 5 speakers is used instead of the 2 stereospeakers. The mid-front speaker is orange. The rear speakers are blue.
Note that the Pro-Logic system is a 4-channel system, despite the 5 speakers! The rear channel is spread over two speakers to make the spatial illusion more realistic. Real 5-channel (and more) surround systems do exist (just think of Dolby's "5.1" and "AC3" systems or the "THX" system), but are not as widespread as the Pro-Logic System.
The Pro-Logic system mathematically puts 4-channel information into 2 channels. (Don't get scared; implying surround doesn't require any calculation of yourself: computer does it all for you. And it isn't that complicated anyway.)

Also note that, since this text is about tracking in surround, I always start from a mono (1 channel) source (namely a single sample) that has to be put somewhere in the stereo (2 channel) or the surround (4 channel) soundfield.
1. Central

This is the same signal put on both left and right channel. It's the dullest way of placing your sound, but it might work fine for basses and kick/bassdrums, but try to avoid this for anything else.

Four ways of doing this in IT:
1. Make the whole song mono (F12, select mono) ...duh!
2. Put a central pan on whole the channel (F11, press 'm' on the channel you want central or use the X40-effect in your music)
3. Give the sample or instrument a proper default panningvalue (F3, set default pan to 32 or F4, select panning and set default pan to 32)
4. Use two channels: one on the uttermost left position, the other on the uttermost right position. Play the same thing in both channels, at the same volume-level (namely each the half of what you wanted totally). (Don't actually *do* this! It takes more processing time and lowers the quality!)

2. Panned

Panning is actually playing the same signal on both channels, at different volume levels (ok, the picture gives an extreme example of that) . Use it at a fixed position or pitch related for leads (maybe with a little bit pan swing: F4, panning: give the maximal percentage of random deviation of the current pan-value), in sweeps for strings and such. Use your imagination!
5 ways of doing this in IT:
Mainly the same as ways 2 to 4 from the central position-methods, but use the sliders to set the desired value. Make sure mode is stereo (F12, select stereo). You've also got the panning envelope (F4, panning) and the panbrello effect (Yxx in your music) for sweeps. (Very well explained in the IT manual...)

3. Surrounded

Here's where the tricky things appear: as you can see in fig.3a, surrounded sound is nothing more than the normal sound through one channel and the inverted version through the other. That's why, on stereo equipment, these channels seem to come from both speakers at the same time, though not from the middle. Just make the sum: (y) + (-y) is always zero! Where both channels come together (in the middle) sound just disappears! Funky effect on a stereoset, but this is also exactly the way the sound for the rear speakers is encoded into the 2 normal channels' data. The decoder filters it out of the signal that goes to the other speakers and plays it only through the rear speakers. Since those typical surround rear speakers can't handle too much bass and since they are only to spatialize, most bass is filtered out too! So if you put some nice bassline into a surrounded channel, it gives a quite weird effect on Dolby Pro-Logic equipment.
Now just adding a channel with the same data in the front center (and, off course, dividing the volumes by two) to normalize this wouldn't be a bad idea, if the next problem didn't come up: just try adding up the right channel of a central panned source to that one of a surrounded source... result: nothing... add up the left channels to each other and we get the sound twice as loud. You get the same effect as if you would have just panned the stuff all to the left!
So we have to use another trick: The SDx command. With this command you can delay one of the channels (almost) inaudible and the wave will be shifted (phased) enough to make the sum more (or less) than zero.
Off course you can play with the panning of the non-surrounded channel and volume slides in both channels to make real great stuff: sounds flying from behind you to front-left, then to front-right and back to the rear, just around the clock!

The effect can be even more interesting using similar sounding but not identical samples (e.g. different string-samples).

So we have two major rules:
One: don't let a surrounded channel play alone since the effect is too explicit on about any real surround equipment.
Two: don't let a surrounded and an identical but normally panned channel play together if one of them isn't shifted a bit because the effect is quite similar to just panning the whole thing to the left.

That's all you need to know about Pro-Logic encoding! You are now ready to track some rocking Dobly Surround music!
PS: People using a tracker not supporting surround functions can use a wave editor to invert a copy of the sample they want in surround. Playing the original on the left and the inverted sample on the right has the same effect as the S91 command in Impulse Tracker.
Written by Cantaloup. Contact me if you have questions or remarks.