True Peak Metering

Robert Katz

Dear Bob Katz,

I hope this email find’s you well. My name is Jens Gerlach.

First I’d like to thank you for your book „Mastering Audio“. This book is really interesting and the reason for this mail. I haven’t read all topics yet and there are some things I partly don’t understand and some I don’t understand at all but I guess that’s because I’m not deep enough inside. I’ve been using WaveLab for several years as a hobby. In the past I liked to cut and combine/mix live sets (Techno/Electro) and create CDs. That was the reason to go for WaveLab Pro instead of Elements because of the CD markers and unlimited file size. Nowerdays there is very few time for this but from time to time I’d like to play some music with WaveLab.

So far so good. Why I really write this mail is one thing that has always been interesting to me: Metering. Metering has been always important to me since my childhood/youth. Beginning with simple analogue ones on tape decks up to what’s possible today. Of course, I noticed there is a variety of different meters that behave and operate in different ways. To get a really good understanding your book came right in the way. I heard about the K-System Metering before but I didn’t know it was developed by you. I think I mostly understood what you explained within the book. But there’re still some things I’d like to talk to/ask you.

Some time ago I got a digital DK Audio meter for a good price. You wrote those meters almost follow the K-System standards. But I’m unsure which level is really shown on the meter. The scale goes from -50 to +5dB. The meter strips can show PPM and digital peak at the same time. I thought PPM and digital peak is the same but it looks like it is not. My output where the meter is connected to is limited to -20dB. The meter shows the digital peak at -20dB. The PPM level is ~ 10dB above digital peak. So what level is PPM really? If I got you right, the ideal reference values are: 0db VU | -20dB FS | (83dB SPL) | 0,775V. How does the PPM level fit to that?

Another thing is the metering within WaveLab. Since the last versions the meter can be switched between digital peak and exact peak level. All the stuff I have is already mastered but I noticed if I change to exact peak several music indicates over between 0 and +1db. Is that a result of peak normalizing? I think this should not happen while mastering but as you said it became a bad behavior (my opinion) to just try to knock down the 0db FS. I really don’t like that.

One more thing is when I choose the meter as K-System I’m unsure if it applies to the peak or VU meter or both of them.

Since my English is not perfect I apologise for any mistakes.

Thank you in advance for a helpful answer.

Sincerely

Jens Gerlach

Dear Jens:

Thanks for your kind words. The DK meter is not good enough anymore and out of date. 

PPM is a quasi-peak measure. It’s out of date and should not be used anymore. It does not reflect actual peaks nor averages, so it’s somewhere in between. 

Digital Peak is also inadequate. You should look at true peak on an ITU or R-128-compatible meter. 

I’m not sure what Wavelab means by “exact peak” but I suspect it is the ITU “True peak” because it can read over 0 dBFS. It’s not the result of peak normalizing, it’s what will happen in the analog domain or after requantization due to sample rate conversion or other processes. Start here and read this: https://pdfs.semanticscholar.org/5ef1/92d3abd1b8e28037a03ba287a048d177a3c0.pdf

Hopefully it will help. 

The K-System applies to the averaging portion of the meter. Really I consider the metering portion of the K-system to be superceded by a good LUFS meter. I would suggest you use an LUFS meter with 0 LU set to your goal, for example, -23 LUFS, -20 LUFS, -16 LUFS, -14 LUFS, or even -12 LUFS depending on your application. 

Hope this helps,

Bob

Dear Bob,

Sorry for my late reply. Thank you for your information. I don’t know if I got everything within the document but I had a research for up-to-date loudness meters and how they work.

„It’s not the result of peak normalizing, it’s what will happen in the analog domain or after requantization due to sample rate conversion or other processes.“

I’m wondering about that because after the DAC the signal enters anaog area, does it? So I guess the quality of ADC and DAC is very important to avoid over or distortion, isn’t it?

What I’m also wonding about is why True Peak can exceed 0 dB without creating any distortion or over (at least for a few dBs) in the digital domain. I guess there is a limit anyway.

Thank you in advance,

Jens

Dear Jens:

Over 0 dBFS can certainly create distortion. Yes, the quality of the ADC and DAC are very important. It doesn’t create an overload in the digital domain is only if the sample peak prior to the output does not exceed 0 dBFS, If it does, and it’s for a long enough duration, then distortion will be audible even if the true peak does not exceed 0 dB TP. 

Here’s a good essay:

https://pdfs.semanticscholar.org/5ef1/92d3abd1b8e28037a03ba287a048d177a3c0.pdf

Hope this helps,

Bob