Conversion To PCM

Introduction

Roon is capable of reading DSD files and playing DSD to audio devices, but there are many situations where something that started out as DSD will turn into PCM before reaching the audio device. This article should help give some insight into the underlying reasons why this happens. As with many aspects of understanding audio processing in Roon, the Signal Path feature is a great way to understand what's going on.

Devices must support DSD to play DSD

Roon supports DSD playback to Roon Ready devices, audio devices connected directly to Roon, Roon Server or Roon Bridge, HQPlayer, and the Logitech Squeezebox Touch (when the EDO plugin is used). Every audio device reflects a set of capabilities to Roon. If Roon doesn't think that the device can play DSD, then DSD content will be converted to PCM during playback. If you see a signal path like this, where the only thing going on is DSD->PCM conversion and (possibly) a subsequent sample rate conversion, it's almost certain that Roon is converting because it doesn't think your device can handle DSD directly:

Devices Must Be Configured to play DSD

For audio devices connected to Roon, Roon Server, or Roon Bridge, it's important to make sure that: 

1. If an exclusive mode setting is available, it is enabled.
2. "DSD Playback Strategy" is set to "Native", "DSD over PCM v1.0 (DoP)", or "Initial dCS Method", depending on what your hardware supports.
   
3. "DSP Volume" controls are disabled.
   

If you are using a Roon Ready USB or Digital Bridge, make sure that DSD support is enabled in that product's setup interface. Sometimes this is a web page, or an iOS/Android app. For Roon Ready bridge devices, note that in some cases enabling a software volume control disables DSD capabilities on that device.

DSD and DSP

DSD, as a representation of an audio signal, is not very friendly to digital signal processing. While it is technically possible to process audio at DSD rates, it is very processing-intensive and brings with it a set of tradeoffs and complexities. For a technical primer on some of what's involved, this paper is a good starting point. As such, Roon currently does not support processing DSD signals directly--if the processing is requested, Roon begins by converting DSD into an extremely high-resolution form of PCM called DXD. In Roon's processing engine, the 64-bit DXD stream has the same data rate as a DSD512 stream--so there is plenty of resolution to ensure impeccable quality. Roon is not alone in this choice--DSD content is very often mixed/mastered in a DXD environment for the same reasons, and most other software players do not process DSD directly. If you are interested in processing DSD content from Roon without converting the DSD to DXD first, it is possible to use Roon with HQPlayer--an external piece of software that specializes in this sort of processing.

DSP Features often require DSD to PCM conversion

Assuming that your device supports DSD, the most common reason for DSD to PCM conversion is because some processing is taking place. In all cases, it should be possible to see what is going on using Roon's signal path feature. The features that most frequently cause DSD to PCM conversion are:

Volume Leveling

Crossfade

DSP Engine

Downmixing

For example, in this case, DSD to PCM conversion is happening because Volume Leveling was enabled:

And in this case, DSD to PCM conversion is happening because the Parametric Equalizer is configured in this zone's DSP Engine:

If desired, Roon can process DSD content and output it as DSD

In this signal path, a DSD64 track is being processed with the parametric equalizer, before performing the final output in DSD256 format. This was accomplished by choosing "DSD" output in the "Sample Rate Conversion" section of "DSP Engine". Signal paths like this one require a powerful CPU.

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