Ryan Avery - Audio DSP Software Engineer, Digidesign - has responded to our comments on the new AAX Pro Limiter plug-in.

In regard to Russ’s observations about how the Pro Limiter AudoSuite Loudness analyser appears to work Bryan has had this to say..

Russ, when you are selecting multiple tracks and processing things in Audio Suite, Pro Tools treats them as if they were stems in a surround file. So, for instance, selecting and analyzing a group of 6 tracks with the Loudness Analyzer will treat them as if it was a 5.1 surround file. Thus the true and sample peak will display the max values of all of the tracks - not exactly the result you describe, but still very useful.

So my understanding is that if you pick up 3 stereo files, or 6 mono files, it will treat them as if they were one 5.1 stem. This is helpful if you are working in surround but if you are trying to analyse several mono or stereo files, it is not so helpful because the different channels in the surround format are weighted slightly differently when producing a single loudness figure for a 5.1 file. So my recommendation would be to analyse stereo files one at a time, if they are are multiple tracks. If they are all on the same track then you should be OK to analyse mutliple files together to get a single loudness figure for them all.

On the DUC Ryan has responded to observations which I voiced both in my video here and also on the DUC. Here is his full post on the DUC…

Hey all,

I’m a bit late to the party, but I wanted to try to clear up some confusion regarding the Pro Limiter support of the various loudness standards. 

The best simple explanation on the differences between R128, A/85, and BS.1770-3 I have found online is a video by Andrew Mason, chairman of EBU:

One point that was raised was the idea that we don’t support the A/85 standard as required by the CALM-Act, which is misleading. R-128 and A/85 are both a series of recommendations about what loudness values (integrated LKFS, true-peak max, LU Range, etc.) are acceptable for broadcast. There are some key differences between them (-24 LUFS reference point for A/85 vs. -23 LUFS for R128, etc.), but in general the differences are fairly minor. But it is important to note that by themselves, the standards don’t give you a way to actually measure the loudness. Instead, they make reference to BS.1770-3 for that.

BS.1770-3 is the loudness metering standard that can be used to measure the values that are needed by either R-128 or A/85. So although Pro Limiter doesn’t directly integrate the recommendations of A/85 (or R-128 for that matter) into its GUI, the metering values produced by the plug-in can be used to make your audio content R-128 or A/85 compliant. And Pro Limiter is fully compliant with BS.1770-3, including loudness gating and K-weighting.

Some might say that we’re not able to fully support A/85 because we don’t do automatic dialog detection to help define the anchor element. But I would counter this by saying that since there is no industry-standard way to automatically detect dialog, these algorithms can’t necessarily be trusted to provide accuracy for all content, and thus are not foolproof. And A/85 is fairly ambiguous when it comes to whether dialog detection or gated measurements are preferred. I found this guide to be a helpful read.

Finally, the K-weighting stuff has generated some confusion. Indeed, there is some confusion between a few different “K” terms in the manual, which we hope to clean up. K-Metering, K-Scale Metering, and Bob Katz should not be referenced in regarding Pro Limiter (so whoever brought up “K-20” was mistaken). We don’t do anything related to those in this plug-in.

However, Pro Limiter *does* use what are called K-Weighted loudness meters as defined in the BS.1770-3 loudness standard. K-weighting helps compensate for some of the variations in perceived loudness of low and high frequencies.

A bit of a long post, but I hope it helps clear some things up.

__________________
Ryan Avery
Audio DSP Software Engineer, Digidesign

Thank you for clarifying what is happening under the hood of the metering section of the new Pro Limiter.  The confusion has arisen because of what is in the manual, which as users is all we can go on. 

One point that was raised was the idea that we don’t support the A/85 standard as required by the CALM-Act, which is misleading. R-128 and A/85 are both a series of recommendations about what loudness values (integrated LKFS, true-peak max, LU Range, etc.) are acceptable for broadcast. There are some key differences between them (-24 LUFS reference point for A/85 vs. -23 LUFS for R128, etc.), but in general the differences are fairly minor. But it is important to note that by themselves, the standards don’t give you a way to actually measure the loudness. Instead, they make reference to BS.1770-3 for that.

I raised it because of what it is in the manual. On page 11 it says…

Pro Limiter provides numeric displays that show the current loudness or peak level of the processed signal. Note that Pro Limiter conforms to the “EBU R128” and “ITU-R BS.1770-3” standards for loudness metering.

The only statement in the manual about which version of the loudness recommendations it conforms to is R128. No mention of A85. I accept the differences are minor. However we make programmes that have to comply to these different recommendations because they are what are specified in the delivery specs as laid out by our clients, the broadcasters, and we have to confident that the meter displays those parameters. On the loudness figure as you say there is a 1dB difference between R128 & A85, but with a 1dB tolerance and R128 and a 2dB tolerance on A85, as laid out in the video you refer to, it would be possible that a programme mixed to A85 would fail the R128 spec. I understand it isn’t as simple as that, for example, the gating specs are different between R128 and A85, all of which means that yes in reality they do work out to be very close, BUT we cannot afford to get it wrong. If a programme is rejected by the broadcaster because it fails their delivery spec then it costs us in time and money to fix it and more importantly it doesn’t do our reputation any good. :(

Finally, the K-weighting stuff has generated some confusion. Indeed, there is some confusion between a few different “K” terms in the manual, which we hope to clean up. K-Metering, K-Scale Metering, and Bob Katz should not be referenced in regarding Pro Limiter (so whoever brought up “K-20” was mistaken). We don’t do anything related to those in this plug-in.

As you say, the confusion has been created by what is in the manual. I quote from page 12 of the manual…

K-scale meters are RMS-based scales with an integrated sample peak meter as a secondary value. For more information about K-scale metering, see the paper “An Integrated Approach to Metering, Monitoring, and Levelling Practices” by Bob Katz, Digital Domain, Inc.

Again from page 12, I quote…

Integrated Loudness K-Meter (I) Graphically displays the current integrated level of the processed signal using K-scale metering.

I think we agree that the manual needs an urgent rewrite to resolve these confusions.

All of these points should not detract from the fact that the Pro Limiter is a great sounding limiter. None of these issues relate to either the sound or the way the limiter works. These relate to the loudness metering which as I say was an inspired idea to integrate into a true peak limiter plug-in. We just need to be confident it is showing us what we need to know.