EQ Magazine - February 2008 - (Page 62) MIX BUS HOW TO PREVENT DIGITITIS by Bruce Bartlett You’re recording and mixing in a DAW set to 24-bit resolution, and you think you’re doing everything right, but the final product sounds vaguely distorted. The tracks were recorded at just under 0dB, and your meters don’t show any clipping, but the sound of your mixes is harsh and fatiguing. What’s going on? Chances are the signal is clipping, but the meters don’t indicate it. Here are three ways to fix the problem. But first, let’s go over some digital-audio basics. The A/D converter in your audio interface measures or samples the incoming analog signal many thousand times a second, and it converts those samples to the binary numbers that you record on your hard drive. Figure 1 shows an analog sine wave that is sampled periodically. The digital meters in your DAW show those sample levels—not necessarily the peak signal levels that appear between samples. In your D/A converter, a lowpass filter (or reconstruction filter) creates occasional peaks between the samples that can be up to 3dB higher than the measured level at certain high frequencies. If your recorded levels are near 0dB, this can cause clipping that the meters don’t display. Similarly, some plugins can create larger peaks than those that went in—without any increase in volume, or audible change in the program. Even an EQ cut or roll-off can increase signal levels by producing intersample peaks due to ringing at the cut frequency. LOVE THE PEAKS Fig.1. Analog signal levels can exceed digital sample levels. Fig. 2. Average levels and peak levels in a musical signal. As shown in Figure 2, a musical signal changes in level continuously as it plays. Imagine a musical passage with a lowlevel synth pad, but with high-level drum hits.The average level or volume of the passage is low, but the transient peak levels are high. Peak levels may be up to 24dB above average levels, depending on the type of signal. Percussive sounds have much higher peaks than continuous sounds (synth pads, organ, flute)—even if the two signals have similar average levels. The meters in your DAW can show signal levels in two modes: RMS and peak. RMS readings correspond to the average levels, and peak readings show the level of peaks, or short transients. The average or RMS level indicates approximately how loud the sound is, and the peak level shows how close the signal is to clipping. So, as you don’t want to clip or distort the signal while recording or mixing, use peak metering—not RMS. REDUCE RECORDING LEVELS In general, reduce your recording levels by turning down the gain in your mic preamps. Record at about –6dB maximum in peak-meter mode. One benefit of lower recording levels is that you won’t overdrive your mic preamp. The distortion of most analog gear increases as you approach maximum gain. Another benefit of reduced levels is that it creates some headroom for your plug-ins. Going for 6dB of headroom should eliminate any invisible overs. Then, you can set up your mix balances without having to adjust levels every time you insert a plug-in. Remember, the recorded level on each track drives the plug-ins—not the track fader, which comes after the plugs. If a track was recorded at 0dB, use your DAW’s trim control (or insert a –6dB trim plug-in) CONTINUED ON PAGE 65 62 EQ FEBRUARY 2008 www.eqmag.com http://www.eqmag.com
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