How to Prepare for the Coming Storm of 5.1-Surround Projects

How to Prepare for the Coming Storm of 5.1-Surround Projects
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Downmixing Basics
There’s one other bugaboo to watch out for when doing surround. All 5.1 mixes might be downmixed to stereo at some point. For instance, if the consumer listens to a Dolby Digital DVD or DTS CD of one of your mixes and selects the stereo option on the receiver or surround processor, the six channels of information are mixed down to a pair of stereo channels and sent out the main left and right outputs.

In a downmix, the center-channel information is added into the left and right front channels equally, while the left and right surround channels are added into the left and right front channels. Some systems add the LFE channel into the stereo pair, and other systems throw away the LFE information.

This would be fine in a perfect world, but lots of potential phase conflicts crop up. For instance, if you put some sort of delay between the left-front and left-surround channels, you could end up with a huge, phasey sound when those channels combine into one. In such cases, a mix that sounds great in 5.1 surround can be unlistenable when downmixed to stereo.

If you think your carefully crafted 5.1 mix will never be heard in stereo, think again. Just as we need to check stereo mixes for mono compatibility, we also need to check 5.1 mixes for stereo compatibility. At the very least, a stereo version of your songs might be needed for radio play, and Lady Luck will probably choose the downmix of the one song that sounds as if it were mixed in your washing machine. Some very high-end surround mixes done by the most famous engineers on the planet sound horrible when auditioned in stereo. Guess what? Those engineers didn’t understand the effects of downmixing.

What can you do about it? At the very least, you need a way to monitor the downmixing-cancellation effect. Many monitor controllers, such as the MultiMax, have a button that engages Downmix mode. Then you can easily hear phase problems.

Furthermore, always do a separate stereo mix of any 5.1-surround mix. This is particularly important for the new DVD-Audio (DVD-A) format, which has enough data-storage space to include both a 5.1-surround and stereo version of your mixes. That way, when consumers select stereo mode on their receiver or processor, they hear your stereo PCM version rather than a downmixed version of the 5.1 mix.

The proper place for this true-stereo version is on the same 8-track master tape that holds the 5.1 mix, on tracks 7 and 8 (see the table “Surround Track Assignments”). Although you have to do the mix in two passes—one for surround and the other for stereo—you’ll be way ahead of the game when someone requests a stereo version of the mix.

Encode Thyself
The final part of making a 5.1 mix is the encoding process. Fig. 5 shows where the encoder fits in the mixing chain in a hardware-based system.

(click image for larger view)
FIG. 5: If you are using a hardware-based system, as opposed to a computer-based system, you need to patch the output of your 8-track mixdown deck to a Dolby Digital or DTS encoder. With a computer-based DAW, of course, this is all done in software. The system shown here produces files that can be burned to CD-R.

Up until a few months ago, making a one-off DTS disc of a 5.1 mix required sending a six-track tape to DTS; the company would then encode the tracks and send you a DTS CD-ROM. Now you can get Minnetonka Audio’s SurCode DTS ($499), a software DTS encoder for Windows that lets you encode your six discrete surround tracks as a DTS file with a WAV extension. This DTS file can then be burned onto a standard CD-R disc with any CD-burning application, such as Adaptec’s Easy CD Creator. The file will play back through any home-theater system that includes a DTS decoder and a CD player with a digital output. (Don’t try to play the CD from the analog outputs; all you’ll hear is ugly noise.) Unfortunately, Minnetonka only develops software for Windows. I hope a Mac equivalent of Surcode DTS will be developed eventually.

Although a DTS disc looks like a Red Book CD-R (technically, it’s an Orange Book disc), it probably won’t play back in all DVD players. That’s because the color of the dye and reflective layer in the CD-R medium itself might be incompatible with the wavelength of the laser in the DVD player’s pickup. In addition, gold-colored CD-R media (such as the Kodak discs) seem to have a better chance of universal DVD playback than the dark-blue or green CD-R discs. Some of the newer Pioneer DVD players are advertised as having dual-laser pickups, and they seem to digest any color (and chemistry) CD-R I make. Interestingly, a CD-RW (rewritable) disc will play back on nearly every DVD player, even the old ones. So if you can’t get a CD-R disc to play in your DVD player, burn a CD-RW disc and give it a spin. Of course, you can always play a DTS disc in any CD player with an S/PDIF output and a DTS-equipped receiver.

Those who want to use Dolby Digital encoding for surround mixes can burn a 44.1 kHz version of an AC-3 file and put it on a CD-R disc. You can get Dolby Digital encoders from Minnetonka Audio (SurCode Dolby Digital, $995) and Sonic Foundry (SoftEncode 5.1 Channel, $995). The encoding procedure is the same: load the six discrete audio tracks into the computer and toss them into the encoding software. Selecting the AC-3 WAV output creates an AC-3 file that’s been padded out to fit in the exact same space as a stereo PCM file. Again, this file can be stored and burned on a CD-R or CD-RW for playback in many home-theater systems.

This method isn’t recommended by Dolby Labs, and it’s not 100 percent reliable, because some Dolby Digital decoders don’t expect an AC-3 file to come from an S/PDIF bitstream with the Audio/Data flag bit set to Audio. Nevertheless, it works perfectly on many decoders and receivers. Until Minnetonka licensed the DTS encoder algorithms a few months back, this was the only way to hear a one-off version of your surround mixes on a home-theater system without dragging around a multitrack deck and mixer.

Surrounded by Info
There is much more to say about surround-mixing basics, but I’ve given you enough to get started. In future articles, I’ll discuss subjects such as tracking for 5.1 mixdown and exotic surround microphones. But for now, it’s time to start exploring the new world of multichannel music. It’s a trip worth taking.

Mike Sokol is a live-sound and recording engineer with some 30 years of experience on both sides of the console. He conducts free surround-mixing seminars at recording schools around the country; see www.modernrecording.com for tour dates.

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