FabFilter

Hi,

Pro-L 2 is absolutely a true peak limiter!

When comparing true peak level readings from different plug-ins, keep in mind that true peak metering is -not- an exact science; there is not one single method to implement true peak metering.

Therefore, different true peak meters may give slightly different readings. This is not a problem, and it does not mean one meter is wrong while the other is correct.

The EBU organization offers a set of test audio (tech.ebu.ch/docs/tech/tech3341.pdf), with which you can test metering for compliance with the EBU R128 metering standard. The true peak level tests actually have a tolerance of -0.4/+0.2 dBTP! This means that it's possible to have two meters that have a 0.6 difference in true peak reading, while they still both comply to the EBU R128 standard.

So, seeing differences of 0.1 or 0.2 dBTP is not unusual or wrong at all. But of course, if you see very large differences (e.g. > 0.6 dBTP), we'de surely like to know and like to investigate...

Feel free to re-post this on KVR of course.

Thanks! I've posted it over at KVR.

Hi guys,

After reading the full KVR thread (see link above), we decided to dig as deep as possible, and explain possible differences between the AAC RoundTrip tool, Pro-L 2 and other plug-ins with TP metering. What we've found, was actually quite interesting:

Apple's AAC RoundTrip as used for Mastered for iTunes etc), simply seems to use the example detection filtering (4x oversampling) as described in the ITU-R BS.1770-4 specs. This method is generally considered to be sub-optimal, and can give quite significant under- and overreads. As the spec already implies, "Higher sampling rates and over-sampling ratios are preferred".

In Pro-L 2, we've chosen to use 8x oversampling and steeper filtering for our true peak detection, resulting in much more accurate true peak readings.

Some plug-ins seem to have implemented that same, simple but sub-optimal example filtering from the ITU spec, which explains why their readings would match Apple RoundTrip's levels. However, most plug-ins, like Pro-L 2, implement a better true peak detection, and because of this, you might see 'overs' in Apple's RoundTrip AAC tools.

Having said this, we did find a small issue with Pro-L 2's true peak detection when running at higher samples rates (96 kHz and up), which could cause the readings to be a bit less accurate than we intended. Of course, we'll fix that in an minor update soon!

Cheers,

Like a bauss!
Thumbs up! Fabfilter

Awesome. Thank you! I shared this message over at KVR as well.

I read the KVR post in full. The KVR user bases his entire argument on three flaws. It is important to understand that the AAC RoundTrip tool is NOT:

1. for measuring whether another limiter is True Peak or not. .
2. a standard for compliance to ITU-R BS.1770-n specs and
3. definitely not a standard on how to catch Inter Sample Peaks.

Out of curiosity, Riture, do you know any tools that are not flawed in the manner you enumerated?

Hey sorry, my point was not that AAC RoundTrip tool is flawed in any way. Rather, it cannot be used to test whether any limiter is really True Peak or not. It is a highly useful tool to help compare an AAC encode against the original source, as well as to show clips that can occur during an encode. Nugen Audio's MasterCheck Pro serves somewhat similar functionality for other encodes.

Got it, thanks!

Pro-L >>> Pro-L2

Sorry guys...

Just wondering how much of a difference is there between Pro-L and Pro-L 2 when it comes to true peak limiting? Is it really that significant?

In simple words, is ISP metering and detection offered at 4x oversampling by Pro-L sufficient?

Thanks.

Hi,

The ISP metering in Pro-L v1 is sufficient to detect true peaks. In combination with 4x oversampling, you probably need to lower the output level to -0.1 or -0.2 dB and then verify that there are no true peak overshoots anymore. If there are overshoots, lower the output level further, etc.

What we've improved in Pro-L 2:

- True peak metering is even more precise with 8x internal oversampling (instead of 4x as used by Pro-L v1's ISP metering).
- The true peak limiting feature lets you set a desired output level in dBTP (a true peak value) directly, so there's no trial and error anymore. No need to verify that there are no true peak overshoots, because the limiter takes care of that automatically.
- Because of this, Pro-L 2 can also take advantage of the headroom in different parts of the audio material so it's more efficient in maximizing the level without introducing true peak overshoots.

Cheers,

What I don’t get is why Pro-L2 failing to limit my stems correctly when using the external side chain?

If I use L2 on my master bus, true peak readout is just fine, but it is next to impossible to limit multiple stems without exceeding the set true peak output.

The external side chain was THE feature for me that set it apart.

Hi Michael,

Without having access to your project, it's difficult to say what's going on. We've extensively tested the external side chain input with stem limiting and we are quite sure it's working correctly (but of course willing to look into this if you believe this is a problem in Pro-L 2).

What we found is that it's not that easy to set up stem limiting in many hosts: just one small mistake with a level setting somewhere and the whole system doesn't work anymore. I would recommend that you double-check your project to see if everything is set up correctly.

Some more info here:
www.fabfilter.com/forum/3090/pro-l2-external-side-chaining-nuendo-8

Cheers,