Filter types and phase response
There are three filter types available for the DSP engine in both the DAW plugin and the standalone app of Reference 4. All three filter types differ audibly and have individual technical aspects: phase response, pre-ringing, latency figures, and processing power required.
Filter type can be selected at the bottom of the Reference 4 interface, with the latency figures showing next to it, on the left. Phase response can be viewed by ticking the Phase response checkbox in the FR curve selection view. Latency figures and phase response curves will change dynamically, depending on the selected filter type.
Linear Phase mode
In this mode, the DSP engine makes sure the phase of the signal remains untouched. There is no phase distortion introduced and there should be no change in the phase response of the system with calibration on/off. To ensure this, a latency of 20-50 ms is introduced to the signal. This mode requires the most processing power out of the three filter types.
Linear Phase mode is also subject to audible pre-ringing - this is normal and expected for the Linear Phase mode. Pre-ringing is a backward echo; it sounds like a strange sucking sound. It is most noticeably heard on transients, particularly audible on individual drum tracks.
Zero Latency mode
This mode ensures 0 ms of internal processing, there is no latency added to the signal. Zero Latency mode is the most efficient of the 3 filter types and requires the least amount of processing power.
This mode is best suited for live tracking/monitoring and other latency-sensitive mixing situations, however, it does introduce a phase shift. This can be viewed in detail in the Phase response curve. The exact phase shift depends on the applied calibration profile but usually stays within the ±30 degree band, so it’s barely audible.
IMPORTANT! True zero-latency monitoring can only be achieved using the Reference 4 DAW plugin format. The standalone app still has some latency introduced by the virtual device driver chain (latency is affected externally due to the Sonarworks virtual audio driver relationship with the operating system audio engine).
If you’re seeking to keep the phase response changes to a minimum while also retaining low latency figures, use this mode. The Mixed filter mode balances the Linear Phase and Zero Latency modes by introducing a minimal amount of latency and a very slight change in the phase response.
Recommended filter type settings
There is no definitive correct or incorrect way of using different filter types - settings will depend on your individual mixing and monitoring needs and preferences.
However, for a simple and stable workflow (at least while getting familiar with the software), we do recommend using Zero Latency or Mixed modes for the majority of the mixing process, then switching to Linear Phase towards the end of the mixing process, to check for any phase-related issues.
Feel free to submit a support request if you still have any questions!