Metering with DigiRack PhaseScope
This is an excerpt from my column “The Pro Tools Corner” at audioMIDI.com
Metering with DigiRack PhaseScope
SignalTools provides a set of useful metering utilities in both TDM and RTAS formats, and come automatically installed as part of the free DigiRack plug-ins that ship with all Pro Tools systems. Consisting of SurroundScope and PhaseScope, these tools provide access to critical information regarding a signal’s level and phase coherency, both paramount in any mix workflow. This week I will walk you through the PhaseScope plug-in and hopefully shed some light on the frequently misunderstood concept of metering in Pro Tools.
Why meters matter
A nasty side effect of the ease and accessibility of DAW recording, I often find that many aspiring engineers and producers know very little about topics such as metering, headroom, gain-stages, and other basic audio engineering concepts. As modern, virtually unclip-able mixers and plug-ins become the norm in the native DAW world, many simply ignore metering all together. The truth is, meters can actually be an engineers best friend, providing vital information about a signal’s level and phase as it relates to a specific system’s output capabilities. For example, meters allow us to make sure that our signals don’t exceed the maximum level allowed by a given system or likewise, dip below the noise floor. In a digital system like Pro Tools this is extremely important, as these systems have no headroom beyond the maximum quantization level of 0dBFS, often referred to as “full scale” or “full code.” Metering is also very important in post-production and broadcast, as specific program requirements are often defined for peak and average levels. If you think about it, mixing in a system without meters would be a bit like playing a sport without the boundaries of the field marked off.
Pro Tools metering basics
The track metering in Pro Tools can be a bit convoluted depending on whether you are recording or playing back audio, and whether or not you have enabled “pre-fader metering” from the options menu. As a general rule, whenever a track is record enabled, the track’s meters display pre-fader and pre-insert input levels in dBFS, regardless of the meter option selected. This means you can rely on the track’s meters when determining the optimum recording level of a signal, regardless of the volume fader’s position and any other gain-stages added by inserts. When a track is not record enabled, the metering is governed by the option “Pre-Fader Metering” found under the Options menu. When pre-fader metering is enabled, a track’s meters display the signal level after any plug-in inserts but before the track’s volume fader is able to add or subtract any gain (hence the term “pre-fader”). With pre-fader metering enabled, a signal that came in peaking at -5dBFS with no inserts, would read the same no matter how much you push or pull the fader.
Hint: in Pro Tools HD, when a track is input enabled (but not record enabled), a track’s meter will follow the same rules as any other non-record enabled track, taking into account any plug-in inserts regardless of pre-fader metering mode.
Using the PhaseScope plug-in
The DigiRack PhaseScope plug-in is found under the multi-channel “sound field” category and can be inserted on any stereo track. The PhaseScope provides level metering with 8 different meter types (Peak, RMS, Peak+RMS, VU, BBC, Nordic, DIN, and Venue), a Lissajous meter display, and a combo phase/Leq(A) display. The combo phase/Leq(A) can be selected under the options section in the lower left hand corner of the plug-in. I generally place the PhaseScope on my master fader, as master fader inserts are the only track inserts in Pro Tools that are post-fader. In this case, by placing the PhaseScope as the last insert in the chain you are able to meter right before the signal hits the D/A at the interface, this can be useful for checking the difference in peak and/or average level a buss compressor or brick wall limiter is adding to your mix or for checking final output levels when complying with post/broadcast standards.
Setting up the level meter:
The level meter defaults to “peak” metering in dBFS, where 0 dBFS represents full scale, or the loudest signal Pro Tools can send out to the D/A converters without clipping. See the DigiRack plug-ins guide for more information on the different metering types and reference calibrations. You can set the reference mark wherever you’d like, all it does is change the color of the meter when the signal exceeds the marker (which can be very useful in post production applications where peak and average values are more scrutinized, beyond just the defacto “clipping/not clipping”). Remember, how the dBFS scale relates to the analog world is far from standardized and entirely dependent on your converter’s calibration. For example, the 192IO is factory calibrated for 18dBs of headroom at +4dBu, therefore a sinewave playing out at -18dBFS in Pro Tools would read 0 VU on an analog meter attached to the 192s +4 dBu outputs. While the complex nuances of the dB scale and all of its variations are way outside the scope of this article, if you feel up to it and want to learn more, there are some great articles just a google search away.
How to read the Lissajous and phase meters:
The goal of a phase meter is to determine how similar the left and right hand sides of a stereo signal are in relation to each other. The way the two signals relate can greatly affect the mono compatibility of a mix (as is the case where the left and right hand sides are summed into a single mono channel). While it is becoming less common for people to digest music and film on mono playback systems, phase coherency is still an important consideration in finalizing a mix. In a worse case scenario, the left and right sides of a stereo signal would be identical but have opposite polarities, resulting in a complete cancellation when summed into mono. While this rarely occurs, the phase meter can easily identify even subtle phase issues by comparing the relationship between two signals. Generally, positive values above 0 indicate acceptable mono compatibility (a value of +1 would indicate a duplicate signal in the left and right channels completely in phase), whereas values from 0 to -1 indicate potential problems.
To experiment, take two identical mono signals on two separate tracks. Pan one signal hard right and the other hard left and look at the PhaseScope plug-in on the master fader, it should read +1. Now apply the Audiosuite>Other>Invert plug-in to just one of the signals (effectively flipping its phase 180 degrees) and look at the PhaseScope again, it should now read -1. If your monitoring system allows you to sum the main output to mono, engage that now. Pretty crazy huh? Now while it is unlikely for your mix to exhibit perfect inverse phase correlation between the right and left hand sides, this extreme example can help you appreciate what is at stake.
As opposed to reading the phase meter, reading the vectorscope (or lissajous figure) in PhaseScope can take a little more practice. The goal of the graph is to visually represent the relationship between the amplitude and phase of a signal in real time. Sound complex? Well to simplify this, you can generally relate vertical lines (or lines living in the top and bottom quadrants) as in-phase, where as horizontal lines (left and right quadrants) represent out of phase material. With practice, one can even recognize different stereo recording techniques such as X/Y coincident, spaced mic, etc simply by looking at the graph.
Using the Leq(A) Meter Display
The Leq(A) display is designed to show a true weighted average of the power level in a stereo (or multichannel) signal. This meter displays a “floating” average for the level over the chosen interval (1s,2s,10s,etc). This can be very useful when trying to compare the average level vs peak level of a mix as it relates to other mixes. Experiment by comparing the average level of different mastered music in your collection (hint: try comparing something from the 70s to something from the 00s). I usually start with the default interval setting of 2 seconds. As always, remember to use your ears in addition to any metering tools, as perceived loudness can vary greatly even with two signals sharing the same average level.