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Live Digital Mixers and Distortion

As more digital mixers appear in live sound applications, some questions have come in from people accustomed to analog audio. One very common example is:

Q: “Why can’t I let my meters run a little into the red like I could with my old (analog) console?”

A: In the world of digital mixing, you’ll find you have to treat levels with a little more respect than in analog systems in order to avoid distortion. Analog and digital systems work very differently in this area.

Distortion is simply defined as an unwanted change in the audio waveform, and is often caused by clipping. As you crank up input levels, you reach a point where the system cannot recreate at the output the extreme peaks and troughs at the input; the result is distortion. In analog circuits distortion tends to come on gradually, increasing in severity as input levels are increased. Because it can sound good to the human ear, some types of harmonic distortion have long been exploited to “warm up” certain sounds. So, although the point at which distortion begins can usually be quite well identified, you can generally push this level a little without ruining your mix.

Digital systems, on the other hand, deal with a precisely defined range of dynamic levels. As you increase the level going into the mixer’s analog-to-digital converter, the numbers coming out get bigger and bigger (more ones than zeroes) until you run out of bit resolution at the converter (maximum input level). Instead of the gradually developing harmonic distortion of an analog board, digital distortion occurs suddenly and always sounds awful.

Metering on a digital mixer also works differently. In analog mixers and recorders, equipment is designed to operate at various operating levels, such as +4dBu (most professional equipment) or -10dBV (much project and consumer gear), and the relationship between 0dB on the meter and the point at which distortion starts is not fixed. Also, because analog distortion develops gradually, you can almost always get away with pushing your meters into the red. In a digital system, metering is relative to 0dBFS, which is the point at which the system has run out of numbers and you cannot go any higher without creating distortion. Because distortion occurs as soon as you exceed 0dBFS, when you set levels on your board you need to make sure that even the peaks of your input signal don’t light the red LEDs at the top of the meters. This might feel “wrong” for a while if you’re used to working with analog mixers, but a few blasts of digital distortion will help you learn to change your ways.

These days many digital systems have a specified level below 0dBFS that they effectively call zero, or the “nominal level.” This level is the point at which the voltage on the I/O would correspond to a standardized operating level such as +4dBu (1.23 volts) or -10dBV (.316 volts). (Please feel free to peruse our inSync Summit on +4 and -10 levels for much more detail on this.) Typically digital systems will use something between -12dBFS and -18dBFS as this nominal level point, and usually there is some notation in the metering system accordingly. Quite often this nominal point can be adjusted to the user’s desired level, which is beneficial for keeping metering consistent throughout a signal path.

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