DSD to PCM Settings

Overview

Roon supports a couple of settings related to DSD to PCM conversion. This is a technical explanation of what they do and why they are needed.

You can find the DSD to PCM settings at the bottom of the Audio tab in Roon's settings window.

DSD to PCM Gain Boost

Most DSD content is generated from a DXD master. DXD is PCM at 32bits/384kHz or 32bits/352.8kHz. This PCM signal is converted into a DSD signal via a process called Sigma-Delta Modulation.

One of the design tradeoffs in Sigma-Delta modulator design is the management of mathematical instability. Sigma-Delta modulators utilize feedback (or feed-forward) loops that can become unstable if the input signal is too loud.

One of the most common practical measures used to prevent instability is limiting the amplitude of the input signal. In fact, the Scarlett Book (SACD) specifications require that the input signal be limited to -6 dBFS (meaning, 6dB less dynamic range than the original signal).

This has a side effect: if you were to play a CD and SACD side by side on the same transport, assuming the transport does nothing to compensate for the -6dB adjustment that happened during authoring, the SACD would come out 6dB quieter.

Roon's default is to reverse that -6dB adjustment when converting DSD to PCM. This should compensate for loudness differences introduced during the authoring process, and allows DSD content to take advantage of the full dynamic range of the resulting PCM samples.

This is great for all content that was properly adjusted during the authoring process. Unfortunately, there is content in the world that was not authored properly. If you hear clipping with some of your content, you may want to turn the 6dB adjustment down to 3dB or 0dB and see if it gets better.

DSD to PCM Filter

Most of the energy in a DSD signal is comprised of high-frequency noise (mostly above 30kHz). This noise is an implicit consequence of encoding the signal into a 1-bit stream (meaning: the file isn't wasting bits representing it, rather it is inherent to the DSD representation). this requires some changes in how we think about the signals.

DSD DACs are designed to do the "right thing" with DSD and output a reasonable analog signal that is not filled with high-frequency noise.

When a DSD signal is down-sampled to PCM, some of the high-frequency noise is retained in the PCM representation. While this PCM representation mathematically accurate, it is not a nice thing to send to a PCM DAC or to your analog gear.

In many cases, the DAC or some downstream component will filter out the noise anyway. In other cases, SQ might be compromised or audible artifacts could result. In other cases, noise might reach your amp/speakers which aren't meant to deal with that kind of noise. In extreme cases hardware damage could result.

Which brings us to the filter settings:

These settings allow you to apply a slow-roll-off IIR low-pass filter to the signal. They are gentle filters, and have very nice impulse response characteristics so as to avoid compromising the DSD signal. These filters attenuate that high-frequency noise.

The 24kHz filter results in frequency response characteristics that are nearly guaranteed not to offend your PCM gear, but this involves placing a filter fairly near to the limits of human frequency perception. It's a conservative/safe setting.

The 30kHz filter is the best compromise, it's the default in Roon, and what we recommend for most users. This setting nicely removes nearly all of the noise spectrum in DSD, while leaving some space between stuff we can hear and the filter itself.

The 50kHz filter preserves significantly more noise (especially during DSD64 playback), but is even less likely than the 30kHz filter to muck with frequency response in an audible way. This doesn't mean it's "better". It's possible that with some gear, this filter will sound worse than 30k depending on how the gear reacts to the HF noise.

Using DSD to PCM conversion without a filter is NOT RECOMMENDED unless you are 100% sure that there is another low-pass filter in your setup that will serve the same purpose.

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