DSP Engine: Headroom Management

Introduction

Headroom Management is part of the DSP Engine. For more information on accessing the DSP Engine, see here.

Digital signal processing can result in "clipping", or audio samples that exceed the allowed range. These samples are "clipped" to a maximum or minimum value. In some cases this can produce audible distortion. This is easily avoided by slightly attenuating the signal – to make "headroom" for sample values that may increase as a result of signal processing.

Headroom Management is at the top of the list because it's one of the most important things to pay attention to in Roon's DSP Engine. If things don't sound right, one of the first things you should do is make sure that the signal isn't clipping.

How to Manage Headroom

First, you'll need to access the DSP Engine. Click the volume icon in the footer, then select DSP. You can also right-click or long-press the Zone Icon in the footer, then select DSP from there. 

Locate and enable Headroom Management. 

Activate the Clipping Indicator.

If the Clipping indicator won't toggle on, check to ensure that Headroom Management is enabled. Start by setting the Headroom adjustment to -3dB. If Headroom adjustment is left at 0dB the audio stream will not be modified

Watch out for Clipping

When clipping occurs, the signal path light will momentarily turn red like this:

Roon's clipping indicator is always at maximum sensitivity: even a single sample can set it off.

If you see clipping, try creating a little bit more headroom.

You add headroom by increasing the amount of reduction. So if you are getting clipping at -3dB, try -3.5dB or -4dB next. 

Why Clipping occurs

There are several reasons why clipping may occur, and not all of them are immediately intuitive.

Because the DSP is actually making things louder

This is a "simple" case. It's possible to configure all of these filters in a way that increases the overall playback volume.

1. Parametric Equalizer
   
2. Procedural Equalizer
   
3. Convolution
   
4. Speaker Setup

Sometimes the easiest solution is to reconfigure these elements to be quieter, and other times, it's simpler to compensate using headroom management. Up to you.

Because of "inter-sample overs"

It's possible for a totally "in-range" piece of source material to have "out-of-range" samples after upsampling. The very simple explanation is: when the interpolator "connects the dots" to form a signal at a higher sampling rate, sometimes the new "dots" fall out of range - causing clipping.

This is essentially a defect introduced in the mastering process, and is not a problem with the upsampler. Unfortunately, most software silently ignores these defects, and provide no means to track or manage them.

Because of minimum phase processing

Because of the phase shift associated with minimum phase processing, it's possible for DSP that doesn't make things louder to nonetheless cause individual samples to clip. Roon's equalizer features are all minimum phase, and its sample rate converters can be configured to be minimum phase. Convolution filters may be minimum phase depending on how they were constructed.

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