MAAT GmbH RSPhaseShifter

MAAT GmbH RSPhaseShifter

By Barry Rudolph

Precisely controlling phase between two correlated audio tracks is an age-old technical problem for recording and mixing engineers that goes back to stereo mastering and disk cutting. The producer and engineer had to make sure their stereo mix was mono-compatible and ready for mastering and disk cutting master acetates. In disk cutting, severe phase/polarity problems are manifested as vertical, upward motion of the cutter's head. This can cause the cutting head to jump out of the groove and spoil the entire disk.

The quick, foolproof check for phase problems is to monitor stereo mixes summed to mono. This will immediately ascertain what (if any) elements of the mix disappear. That super-wide stereo pad or wacky effect sounds great but it might not be heard summed to mono. Today, even though not as many vinyl records are produced and sold, phase and polarity issues are still with us--take a listen to streamed audio over the Internet. Some smart speaker systems have only one mono transducer for a subwoofer (LFE).

Fortunately, correcting this problem is now easily accomplished using the 64-bit precision, MAAT GmbH's RSPhaseShifter plug-in running in your DAW. This simple tool now is just like using an equalizer or compressor plug-in. Besides its utility, it also opens up a whole new realm of creative control and harmonic restructuring of stereo audio unlike any other processor.

Based on Roger Schult's/German Audio Labs active analog W2324 Phase Shifter module, RSPS is an accurate, frequency-dependent "phased rotator" that corrects differences in phase when combining (mixing) multiple microphones or recorded tracks of the same source that are delayed from each other by less than 15-ms.

This situation happens with room mics or multiple microphones recording a single guitar amp, mixing a DI direct signal with a microphone on a bass amp, or altering the phase between a subwoofer and your main monitors.

What Happens?

The resulted differences in phase causes comb filtering. An annoying frequency-dependent phenomenon where peaks (build ups) and nulls (dips) occur in the audio wave. On a spectrum analyzer, these many peaks and dips look like the teeth of a comb with the space between them getting smaller and smaller as the frequency goes higher and higher. Eventually, above about 4kHz, cancellation still occurs but the Q of the resulting peaks and dips are too narrow and close together to make any audible difference.

The MAAT RSPhaseShifter uses a single, all-pass filter, which is a phase manipulator in which the phase of the audio signal passing through it is adjustable but not the signal's amplitude.

It is important to know that setting up the MAAT RSPS requires identifying the phase/polarity problem by listening to the audio--there is no screen to tell you've set it correctly or not.

MAAT Digital also makes the 2BC multiCORR plug-in that shows correlation or L/R phase conflicts in individual, frequency 31-bands. But you may like your new phase shift changes you've made. 2BC multiCORR is a good check or way to hear phase problems straight away and it also offers monaural monitoring.

GUI

A mono instance of RSPS comes up as a familiar-looking, 2-Slot 500 module, just like the W2324, with a simple set of controls plus an adjacent blank panel. The default settings for the controls starting from the top are: Gain with a range of +/- 5dB; the default is 0dB. In practical terms, the Gain control allows for handy level adjustment when processing stereo or two mono tracks mixed together.

Below Gain is the Phase control knob for the all-pass filter with a phase shift range of 15 to 180-degrees (default is 90-degrees). Choices include: 15, 30, 45, 60, 75, 9, 105, 120, 135, 150, 165-degrees.

To the left of Phase control is the three-way Phase Mode toggle switch with: 0-degrees (no phase shift) for comparing to the track's original phase to the selected phase shift. The switch's top position is +180-degrees (or a range of +180 to 0), and when the toggle is switched down (default), it is -180-degrees (or a range of 0 to -180-degrees). This is a very small toggle that I found difficult to operate and hard to visually "see" which state it is in.

Below the Phase control is a Frequency control with 13 fixed steps between 16Hz and 4kHz. They are: 16, 25, 40, 63, 100, 160, 250, 400, 630, 1000, 1600, 2500, and 4kHz. These are spaced 2/3 octaves apart but you can "get in the cracks" by holding down the shift key and mousing. Above 4kHz, cancellation does occur but the Q of the resulting combs or dips are so narrow that they are not audible.

Stereo Operation

A stereo instance of RSPS puts in play more functionality. With stereo tracks, generally the practice is to manipulate the phase of only one of the channels. When inserted on a stereo track, RSPS comes up again showing the Right channel of a two-slot module. The blank-panel module represents the unprocessed, Left channel. If desired, the Flip button transfers the right channel's settings over the left channel with the right channel now becoming unprocessed.

Clicking on the Mirror button links or copies the existing settings of the channel over to the other channel. Mirror mode also links (gangs) the controls of both modules. This is a reciprocal link; moving a control on either module changes the other's controls in sync. Holding the Control key and clicking on the Mirror button allows for individual changes on either module while the blue mirror button (annoyingly) flashes. But if you click again on the Mirror button, the Left controls resync to the right channel's settings. I also found this annoying so I would create a preset keeping the channel's disparate settings intact in preset memory. Lastly, there is the Delta or difference button, a monitoring feature useful for checking and modifying a stereo Mid/Side recording. You may alter the phase separately on Mid and Side tracks.

In the Studio Mono

I found RSPS to be an interesting sound design tool to restructure a synthesized octave double track of a bass guitar track--it was actually two sampled and quantized bass tracks. I liked the perfectly in sync'd octave but it disappeared sometimes in relation to the other bass playing in the normal octave. With some experimenting, I ended up with RSPS on the octave track set to +120-degrees and Phase Mode switch down. With Frequency set to 160Hz, the octave track cancelled out mostly except when the bass part went down the octave. The octave also had a couple of hot notes that were also taken down. Both tracks were panned dead center.

A bass guitar direct and amp recording was helped significantly. I used the +180-degree Phase Mode switch position and 75-degrees of shift. Frequency was 25Hz. While listening in mono, my process was to find long sustaining bass notes and hold down the Shift key to mouse around the Phase and Frequency controls to find the "sweet spot." This made the bass more solid and even sounding; I didn't have automate certain weak notes as before.

A popular recording technique for snare drums is to use both a top and bottom mic with usually the bottom mic's polarity flipped 180-degrees. The two mics are aimed at each other with a snare drum in between. I brought up a recent mix I was working with two snare drum tracks and (on the Pro Tools waveform page) saw the top mic's track was 5-ms ahead of the bottom. RSPS mostly changed the tonality a little. But it was the different approach that I liked.

In The Studio Stereo

My first stereo foray into the RSPhaseShifter was on a stereo piano track. I was doing a mix with a stereo grand piano that I didn't record and there were moments where certain chords and single notes sounded phasey and strange. You may split the stereo track into two mono tracks with a mono RSPS instance on both but you'll lose a couple of important features.

A stereo instance comes up as a "double-wide" slot with left channel a blank panel and the right channel with the same collection of controls as the mono instance. In stereo, there is the Mirror button to link the two channels controls together to rotate the phase of both channels together at the same time. This is for changing them relative to another track in your mix--such as distant piano mics. Sometimes it sounds better to shift the process to the right channel (with the left not processed) and the Flip button accomplishes that with the left's blank panel switching with the right's control panel.

For all my stereo processing call me old fashion, but I always listen in mono to both L/R tracks added together. Once I'm happy with the tone change or not, I split them back out. On my old Tektronix 465B oscilloscope connected to the L/R stereo bus, I take a look at how much and how often I see out-of-phase audio. MAAT GmbH makes the free GON Phase Scope for this purpose as well.

MAAT Digital RSPhaseShifter is an excellent tool for correcting inevitable phase problems. It is a winner and easy to use. It sells for $99 MSRP.

MAAT RSPhasshifter

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