Understanding Advanced Metering In Pro Tools

In this article we take a detailed look at audio metering in general and the advanced metering available in Pro Tools Ultimate and Studio and show you what they look like side-by-side.

Pro Tools Ultimate and Studio users have 17 different metering options, Pro Tools Intro and Artist users have only 4. In the Metering tab of the Pro Tools Preferences window, you can adjust parameters including decay rate and depth, the reference point, integration time, and the breakpoints, where the colours will change on the meter display. This enables you to personalise the metering in Pro Tools to suit your preferences exactly.

But before we dig into the 17 different metering options available in Pro Tools, we need to take a brief look at the history of audio metering so help understand how we have got to this point, and a glossary to help understand the different terms used in audio metering. Then we can better understand the 17 options and how to customise them to suit our workflows.

Why Do We Need Audio Metering?

A Bit Of History

In the late 1930s as radio broadcasting was being established around the world there was a real need for a reliable meter to display the signal being sent to the transmitter. In the UK the BBC developed their peak program meter (PPM). At around the same time, the German broadcasters were undertaking similar research, but because of the timing on the run-up to the second world war, could not collaborate with the BBC but both developed a similar peak reading meter, with a specially designed meter movement and an electronic circuit board required to produce a logarithmic scale.

In the US, radio broadcasters were looking at the same issue but realized that PPMs would be very expensive to roll out across all the broadcasters so looked for a passive low-cost solution and settled on what we know today as the VU meter, which required a simpler meter movement and a single resistor, and so was much cheaper to implement. The VU meter turned out to be a very primitive loudness meter, which approximates momentary loudness changes in program material but with the disadvantage of not being a peak reading meter.

One of the 'features' of a VU meter is that the top 50% of the physical scale displays the top 6 dB of dynamic range, but the meter’s useable dynamic range is only about 13 dB. Because of this small window, any low-level material will hardly move the meter, but maybe within the acceptable limits for the recording medium and the intended listening environment. What tends to happen with this relatively small window on the audio signal, is that users tend to push up their audio levels and/or compress the audio so that the level stays within this small window, which isn't always necessary.

As audio recorders were developed the need for some kind of audio recording level meter grew and the low-cost VU meter was an obvious prime candidate in the analog days of audio recording. However, there are large numbers of cheaply constructed meters which have been labelled “VU”. These different meters often don't actually conform to the official VU meter standard and have contributed to disagreements among program producers reading different instruments and what to set them to.

Peak Vs Averaged Meters

The principal meter used in DAW environments has historically been the sample peak meter. While this fulfils its role of monitoring headroom in a digital system it has never offered any meaningful information about the perceived loudness of the signal. In the days of analogue studios and tape machines, peak levels were less important and the VU meter was a commonly used device. While there are differences between perceived level and the indicated level as shown on a VU meter, it was still a useful device as it is essentially an averaging meter.

However as technology developed and digital technology was introduced, the use of peak reading meters grew, initially LED & LCD. Analogue tape recording is forgiving when it comes to distortion because tape distorts gently; however digital audio is not as forgiving when peaks occur. When it comes to digital recordings, it is either very good or very bad as a change of just 0.1dB can make all the difference. As a result, peak level metering became much more important and LED and LCD displays made peak level metering much more cost effective and we saw the growth of peak level sample peak metering across digital audio recorders including budget machines like DAT and Minidisc machines, but all with varying and often unknown ballistics, which made it difficult to interpret all these meters.

When it came to DAWs based on computers, metering became part of the software and was displayed on the screen, although using a lot of them tended to put an increased load on the graphics processing in computers.

Metering with peak level ballistics on very dynamic sources can encourage users to set levels too low, whereas RMS gives a more realistic VU style metering that can tell you what the 'real-world' gain structure is like. But remember that peaks remain peaks. If you get overs in peak metering, that is exactly the same as overs on RMS meters. The consequence of overload at any point in the audio signal path is distortion, RMS or peak has no effect on this.

Glossary Of Audio Metering Terms

We have already started to use terms like RMS, Peak, VU and ballistics and as the Advanced Metering in Pro Tools has 17 different meters with different features, parameters and design criteria as well as allowing the adjustment of a number of different parameters it is important to understand what the different parameters do and how they affect how an audio signal level meter works.

Ballistics - This is how the meter responds to the incoming audio. For example, with the VU Meter, it takes the needle about 300 milliseconds from the time a signal hits the meter for it to overcome the inertia and ramp up to a reading position. Similarly, it takes another 300 ms for the needle to recover once a signal stops. Whereas the BBC PPM has a rise time of 10 milliseconds, thirty times faster than a VU meter. To allow the user to see transient peaks the BBC PPM has an extremely slow fall (or decay) time of 24 dB in 2.8 seconds.
Crest Factor - This describes the difference in dB between the averaged level of the signal (the kind of level displayed by a VU type meter) and the peak level (as displayed by a sample peak meter). A steady state sine wave will have a Crest Factor of 3dB and music often will have a Crest Factor of around 10dB with the most dynamic music typically having a crest factor of up to 20dB.
Decay Time - This is the time it takes for the meter display to drop back from a displayed peak. These are often specified for a number of dBs in a finite amount of time. For example, the BBC PPM has a decay time of 24 dB in 2.8 seconds.
Integration Time - This is the time it takes for the meter to display an audio level from the time the audio signal is applied to the meter. For example, the VU meter has an integration time of 300 ms. The BBC PPM has an integration time of 10 ms whereas the DIN PPM has an integration time of 5 ms. When it comes to BS1770 loudness meters the Momentary loudness measurement has an integration of 400ms and the short term measurement has an integration, or more simply is averaged over 3 seconds.
K Scale Metering - These are RMS-based scales with an integrated sample peak meter as a secondary value designed and developed by renowned mastering engineer Bob Katz. The K system sought to address the over-use of dynamic compression and the resulting loudness war in music mixing by standardising metering with known monitoring levels. K-Scales are popular with music mixers that are looking for an indication of overall loudness. You can learn more about the K Scale metering system in our article in the A to Z Of Pro Tools series - A-Z Of Pro Tools - K Is For K-System. It should be noted that the K-system predates EBU R128 and ATSC A/85 which are both based on the BS1770 standard and that the K-system was always designed for music production rather than broadcast production. It is also important to understand that Bob Katz's K Scale has nothing to do with K weighting, which is the EQ curve which is part of the BS1770 standards.
PPM - There are various different kinds of Peak Program Meters (PPM). They fall into several categories.
Quasi-peak programme meter (QPPM). This only shows the true level of the peak if it exceeds a certain duration, typically a few milliseconds. On peaks of shorter duration, it indicates less than the true peak level. The extent of the shortfall is determined by the 'integration time'. There are essentially two different quasi-PPM designs that have roots in meters originally developed in the 1930s for the AM radio broadcasting networks of Germany (Type I - DIN) and the United Kingdom (Type II - BBC). Quasi-PPMs use a short integration time so they can register peaks longer than a few milliseconds in duration. In the original context of AM radio broadcasting in the 1930s, overloads due to shorter peaks were considered unimportant on the grounds that the human ear could not detect distortion due to momentary clipping. Ignoring momentary clipping made it possible to increase average modulation levels. In modern digital audio practice, where quality standards are hopefully much higher than AM radio in the 1930s, clipping of even short peaks is usually regarded as something to avoid. On typical, real-world audio signals, a quasi-PPM under-reads the true peak level by 6 to 8 dB.
Sample peak programme meter (SPPM). This is a PPM for digital audio—which shows only peak sample values, not the true waveform peaks (which may fall between samples and be up to 3 dB higher in amplitude). It may have either a 'true' or a 'quasi' integration characteristic.
Over-sampling peak programme meter (True Peak meter). This is a sample PPM in which the signal has first been over-sampled, typically by a factor of four, to alleviate the problem with a basic sample PPM. You need this oversampling to establish what is happening to the signal level between the samples.
VU Meter - A volume unit (VU) meter or standard volume indicator (SVI) is a device displaying a representation of the signal level in audio equipment. The original VU meters were a mechanical meter movement and the mass of the needle causes a relatively slow response, which in effect integrates the signal, with a rise time of 300 ms. 0 VU is equal to +4 dBu, or 1.228 volts RMS. The meter was designed not to measure the signal, but to let users aim the signal level to a target level of 0 VU (sometimes labelled 100%), so it is not important that the device is non-linear and imprecise for low levels. In effect, the scale ranges from −20 VU to +3 VU, with −3 VU right in the middle. The VU-meter intentionally "slows" measurement, averaging out peaks and troughs of short duration, and reflects more the perceived loudness of the material than the more modern and initially more expensive PPM meters. The VU meter’s ballistics were designed to work well with the spoken word. Its 300 ms integration time means you can see the syllables in the spoken word and so it looks very comfortable with speech but be aware that this doesn’t make it accurate.

Advanced Metering Choices

Having covered the background of audio metering and the different terms used in audio metering, let's take a look at the 17 different metering options available in Pro Tools Ultimate and Studio.

Sample Peak has been the default Pro Tools metering since it was included in Pro Tools 11. The scale and the decay time is calculated in dB/second, which results in slower decay in metering compared to older versions of Pro Tools that used what is now called Pro Tools Classic. The Sample Peak option is the only one of the 17 Meter Types that has a zero sample integration time, and so it should display the full dynamic activity of the digital signal at every moment in time, which means it should display all the peaks from every audio sample, but as it is a sample peak meter it will not show what is happening to the peak level in between the samples, as this requires oversampling to establish what the intersample peaks or True Peak levels are. If you need to read the true peak level then you will need a 3rd party metering plug-in.
Pro Tools Classic is the legacy Pro Tools scale and metering ballistics. This is the metering that Pro Tools has had for a long time before Pro Tools 11 and I suspect is included for backwards compatibility and legacy support.
Linear provides direct one-to-one linear metering of sample peaks in the audio signal with a metering range down to -40dB. Avid recommends Linear for post-production and music mixing scenarios. Avid suggests because it uses a fast decay time, Linear provides direct one-to-one linear metering of sample peaks in the audio signal with a metering range down to –40 dB. This also apparently offers higher metering resolution closer to 0dB, which can be particularly useful for mixing and mastering.
Linear (Extended) provides the same ballistics as Linear, but the meter scaling extends down to -60dB to give more low-level display information.
RMS provides metering ballistics that displays the average level (root mean square of the signal) over a range of time. It is a kind of volume or loudness measurement providing an average signal level, unlike peak metering, on the other hand, which display the peak signal level at any given point in time.
VU(volume unit) was the metering system that is popular for music and dialogue mixing, the VU scale used in Pro Tools is extended on the low end from –23 dB to –40 dB to accommodate a wide range of material without the need for re-calibration.
Digital VU provides VU ballistics with a modern digital scale.
PPM Digital has a similar integration time to Sample Peak metering, but different scales and decay times. Popular in Europe and Asia with broadcasters, and also with US Film consoles, PPM Digital has a similar integration time to Sample Peak metering, but different scales and decay times. The PPM Digital option, like all other Meter Type options besides Sample Peak, does not report every transient (such as a series of very fast transients). However, even though it under-reports the actual digital dynamic range, Avid say that it more closely matches human hearing of loudness over time.
PPM BBC uses 4dB spacing between scale marks. Other organisations around the world, including the EBU, CBC and ABC, use the same dynamics but with slightly different scales.
PPM Nordic is a Scandinavian variant of the DIN PPM with the same integration and return times but a different scale. It has “TEST” corresponding to Alignment Level (0 dBu) and +9 corresponding to Permitted Maximum Level (+9 dBu). Compared to the DIN scale, the Nordic scale is more logarithmic and covers a somewhat smaller dynamic range.
PPM EBU is identical to the British PPM (BBC) except for the meter scale. It is a variant of the British PPM designed for the control of program levels in international program exchange (Type IIb PPM in IEC 60268- 10). The meter scale is calibrated in dB relative to the Alignment Level, which is marked “TEST.” There are ticks at 2 dB intervals and at +9 dB, which corresponds to the Permitted Maximum Level.
PPM DIN is used in German broadcasting, the nominal analogue signal corresponding to Permitted Maximum Level was standardised at 1.55 volts (+6 dBu), and this is the usual sensitivity of a DIN-type PPM for an indication of 0 dB. The Alignment Level (−3 dBu) is shown on the meter by a scale mark at −9.
K-12 - The K-scales are RMS based scales with an integrated sample peak meter as a secondary value. K-12 should be reserved strictly for audio to be dedicated to broadcast, though broadcast recording engineers may choose K-14 if they feel it fits their program material.
K-14 is for mastering when working in a calibrated mastering suite.
K-20 - Using K-20 during mix encourages a clean sounding mix that is advantageous to the mastering engineer. At that point, the producer and mastering engineer should discuss whether the program should be converted to K-14, or remain at K-20. If mixing to analogue tape, work at K-20, and make sure that the peak levels 'off-tape' will not exceed +14.
VENUE Peak provides the same ballistics as Sample Peak, but with VENUE meter scaling to +20dB. Note that VENUE is metering analog style for both inputs and outputs, so has 20dB of headroom above the 0dB reference point before the maximum operating level is reached.
VENUE RMS Provides the same ballistics as RMS, but with VENUE meter scaling to +20dB.