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

How to Prepare for the Coming Storm of 5.1-Surround Projects
You're Surrounded

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You Need More Power
You’ll probably need to upgrade your power amps, too. Many big studios can afford separate amplifiers and a bass-management controller to run them, but personal-studio owners should instead consider buying a large home-theater receiver. Priced between $500 and $1,000, such receivers provide more than 100 watts per channel and a single level control that adjusts all channels simultaneously.

All the main speakers should get the same amount of power, but the subwoofer probably needs as much juice as all the other speakers combined. Giving the subwoofer two to three times the power of one of the main speakers will probably suffice. It’s important to understand that many subwoofers include their own internal power amps matched to their particular drivers. As a result, virtually all modern home-theater receivers include five channels of amplification and a line-level output for the subwoofer.

Make sure the receiver sports Dolby Digital and DTS decoding as well as discrete analog inputs that bypass the decoders. Meant to accommodate future decoders, these inputs are ideal for monitoring the six discrete channels of the mix. With such a receiver, you can compare your mix with commercial mixes from DVDs or DTS CDs. (To hear commercial discs, you also need a DVD or CD player with a digital audio output that passes both DTS and Dolby Digital bitstreams to the receiver.) If you’re doing your own DTS or Dolby Digital encoding, the receiver provides the only way to listen to your final mix as the consumer will hear it. These receivers also offer bass management (more on that later) and speaker-calibration options.

I’ll Level With You
Properly setting the relative volume level of each speaker is extremely important. It’s very easy to tell when the left and right levels are wrong in a stereo mix; you can hear the sound leaning one way or the other. Hearing the balance with a surround system is not so simple. You’ll need to purchase an SPL (Sound Pressure Level) meter to do this properly, and you’d be amazed at how many people use $50 Radio Shack meters for the job.

FIG. 3: Calibrating your speakers to ensure that their levels match is absolutely critical to 5.1 mising. Feed pink noise through each speaker and check the level on an SPL meter. Full-bandwidth speakers should output 85 dB SPL, and the subwoofer should deliver 95.

I could write a book just about speaker calibration, but for now, here’s the quick and dirty. Grab some limited-band (100 Hz to 2 kHz) pink noise from the mixing-console noise generator, a home-theater receiver, or a test CD and patch it to an input channel on the console. Make sure to set the mixer’s output gains to unity. Set the console’s input strip so that the output level going to the mixing deck is at –20 dB (below 0) on the deck’s meter. This is the standard reference level for surround mixes.

Feed the pink noise through one speaker at a time and point the SPL meter in the speaker’s direction while holding the meter in the mixing location (see Fig. 3). Trim the gain of the corresponding amplifier channel so the meter reads 85 dB SPL. Repeat this process for each full-bandwidth speaker, one at a time, until you have the same output for each. (Most home-theater receivers include individual level controls for each channel, but they are often buried in a menu system that might require a video monitor for viewing and navigating.)

Next, run low-frequency pink noise (25 to 80 Hz) to the LFE channel. In theory, the gain of the LFE channel should be set 10 dB higher than that of the main channels (95 dB SPL), as read by a real-time analyzer (RTA). But a Radio Shack meter has a lot of low-frequency rolloff, and you’re feeding it less than two octaves of audio information in this case. These factors cause the meter to read lower than the true output level. As a result, when the LFE level is correct, the Radio Shack meter will show approximately 90 dB SPL—4 to 6 dB higher than the level for the five full-bandwidth speakers.

For diffuse surround speakers used in cinema mixes, the rear surround levels are set to 82 dB SPL (–3 dB relative to the other full-bandwidth speakers). For really small mixing rooms where you can literally reach out and touch the speakers, Dolby recommends setting the surround speakers 2 dB down; that is, at 83 dB SPL. It can be a bit confusing. For most music mixing, having all five speakers set at the same level is close enough.

If you don’t get these levels correct, all the mixes you do will have incorrect surround and center-channel levels, or the LFE level will be out of control. Those problems will force listeners to jump up and adjust the levels on their home systems.

Bottom Feeding
Bass management is probably the least understood part of surround mixing. It’s very important to comprehend how it works, lest you make mixes that sound great in your studio but prove unlistenable on a standard home-theater system. As noted earlier, in 5.1 surround, each of the main channels is rated at 20 Hz to 20 kHz, while the LFE channel is rated at 5 to 125 Hz.

Any of the main program channels can go as low as 20 Hz, but very few home-theater speakers can produce output that low. A clever circuit in the preamp/processor or receiver of a home-theater system removes any low-frequency information below a certain point (80 Hz is the THX standard) in the main channels and reroutes it to the subwoofer.

In 5.1 surround, the subwoofer does double duty: it handles all the bass below 80 Hz for all the main channels, as well as the “point one” LFE channel, which might be earthquakes or explosions.

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