Padding Analog Mixer's Digital I/O

I own a Mackie Onyx 1220i Mixer. Great device with excellent preamps. It supports discrete channel tracking into a DAW via Firewire. In order to do so the signal needs to be converted from analog to digital. I've read that Mackie uses high quality D/A Converters manufactured by Cirrus Logic.

The level meters on the mixer are not PPM meters with a 0dBFS ceiling. In fact there is additional headroom above 0dBu. We can take advantage of the additional analog headroom and mix above 0dBu. This will sort of maximize the signal and maintain impact. The key is to make sure your mix level is well below the point at which distortion becomes audible. 

Digital Audio Workstations run in the digital domain, where 0dBFS is the clip point. If your levels hit (or surpass) 0dBFS, you're clipped. There's no additional headroom above 0. The point is 0dBu on your analog mixer is not level aligned with 0dBFS in your DAW. If they were,  your use of headroom on the mixer (above 0dBu) would result in clipping during analog to digital conversion. This is the reason why mixers like the Mackie pad or attenuate the converted signal routed through firewire. It's all about headroom.

Here’s how to test the relationship between the meters on a Mackie and the meters in Pro Tools and/or Logic X:

Pass a +4dB 1kHz Sine Wave (Line Level) through a Line Input on the mixer. With channel in Solo, adjust the gain to read 0dBu on the mixer’s meter. Route the mixer’s output to a track in the DAW and note the input level.

In this scenario the input level will be about -18dBFS. That’s an 18dB pad …

Why so much?

It’s important to understand the difference between tracking something like spoken word vs. an aggressive percussive instrument where the padded signal would be much less of an issue. The effectiveness of the pad depends on how the mixer is used and what is passing through it. 

Some people feel the pad and even the amount of padding should be user defined options. As it stands in situations where a hotter incoming signal is necessary, simply insert a Gain Trim to boost the input level.
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