Pro Tools Mastering Limiter Shootout: Part 1

If you want to get maximum level out of your Pro Tools mixes, you need a mastering limiter. But which one? We put the leading contenders to the test.

Limiters are audio devices that prevent a signal from going above a defined level, irrespective of the input level. When the input is below the predetermined level, usually called the threshold, the audio dynamics are not changed at all; any input signal that exceeds the threshold level is 'limited' to that level.

Limiters are often used to protect transmission chains, for instance in broadcast transmitters or CD mastering, to prevent peak distortion that would occur if the audio were allowed to pass unchecked. Because using a limiter allows us to control the maximum signal level, we don't need to leave any headroom in the system after the limiter in case the audio gets louder than we had expected. In digital systems, as opposed to analogue tape, this is important as there is no warning of distortion: the signal is OK until 0dBFS is reached, and then it clips in an ugly fashion.

In addition, using a limiter allows us to increase the loudness of the audio whilst maintaining the same peak level, if we are prepared to permit more frequent occurrences of limiting and therefore gain reduction of the peaks. These processes are being used more and more, especially in radio where everyone wants their stations to be the loudest on the dial, and we are seeing the same choices more and more in CD mastering where clients want their CD to be louder than anybody else's!

Single-band & Multi-band

There are two basic types of limiters: single-band and multi-band. Single-band limiters work on the full-bandwidth audio, meaning that a peak in any frequency band will cause the level of the entire audio signal to be reduced. As a consequence, you can end up with an effect called 'pumping' where a low-frequency instrument such as a kick drum forces the limiter to reduce the gain of all the elements in the mix. Hearing everything else go up and down in volume in sympathy with the kick drum normally isn't ideal!

Multi-band limiters, by contrast, divide the audio up into different bands — say low, low-mid, high-mid and high-frequency bands — before applying a separate limiter to each band and combining their outputs. With a multi-band limiter, a kick drum will trigger the limiter in the low-frequency band, so its level is contained without any effect on other material in higher-frequency bands.

You can also use multi-band dynamics units as dynamic equalisers. If you want to increase the apparent amount of bass, for example, a multi-band limiter can be used to increase the loudness of the low-frequency band only, say from below 150Hz. If you use gain boost on a conventional equaliser, you will eat into your headroom, but with a limiter you know precisely where your headroom will be, and by limiting the peaks and increasing the average level in that band you are, in effect, increasing the loudness by bringing up the volume of the lower-level sounds. Similarly, you can increase the 'warmth' of a track by increasing the loudness of the low mids, say between 150 and 500 Hz, whereas EQ boost in this band can easily make a track sound very 'muddy'.

In this shootout, I'm going to use a range of material in a variety of styles to compare the performance of a variety of Pro Tools limiting plug-ins. It wouldn't be fair to compare single-band and multi-band limiters directly, so I will do separate tests on a range of single-band limiters and another range of multi-band ones.

Single-band Limiters

There are a number of common features shared between all the single-band limiters in this test. They all have Input and Output level controls and Meters as well as Dither and Noise Shaping options. The latter are present because limiting is usually done as the very last procedure in the mastering process before the audio is truncated to 16-bit for release formats such as CD.

* Digidesign Maxim

Perhaps the best feature of Maxim is the visual display of a colour histogram, which plots input peak dB history during playback. The totals are indicated horizontally by distance and colour. This allows you to determine where the energy resides in a song, and provides visual feedback. Maxim 's histogram time scale varies so you see the entire history, whether the playback period is 10 seconds or 10 minutes long.

* Sony Oxford Limiter

Like many of the Sony plug-ins, Oxford Limiter (reviewed in SOS December 2005) has a number of features that are unusual for a single-band limiter. The first control that is a little different is the Auto Gain button in the Pre-process section. This enables it to handle wider input levels by modifying the attack and release times based on audio content. Unusually, the Attack time can also be modified manually.

The ability to set a longer Attack time allows the user to pass transients through to Oxford Limiter 's unique Enhancement function, which is operated by a slider that goes from zero all the way up to 125 percent, and is claimed to preserve the audio quality of transients whilst still preventing them from exceeding 0dBFS. The enhancement section can be employed on its own by using minimal gain reduction and slow timing settings in the Pre-process section, or it can be used to enhance highly processed content from the Pre-process section to achieve even greater perceived loudness. Sony are not afraid to point out that this section adds distortion but also go on to explain the potential benefits when used on appropriate programme content.

The other two different functions of the Oxford Limiter are the Recon Meter and Auto Comp buttons, which are designed to deal with the fact that the reconstructed analogue audio from the output of a D-A converter can peak over the maximum level even though there were no individual samples that measured above peak level. This is often called 'inter-sample peaking'. Enabling the Recon Meter function allows the output meters to show the equivalent analogue levels after reconstruction and they can be up to 3dB higher, albeit for the duration of the peak of one audio cycle at high frequency. You can then choose to reduce the level of the output section to compensate or use the Auto Comp function to help manage this automatically.

* Waves L1, L2 & L3 Ultramaximisers

In terms of the parameters they present to the user, Waves' three Ultramaximiser plug-ins are very similar. L1 and L2 are both full-band limiters, but L2 doesn't have L1 's Analogue Domain option, which, like Oxford Limiter 's Recon meter, will show the peak levels in the reconstructed analogue signal. L3, meanwhile, is very different 'under the hood' as it uses a multi-band engine with a single overall gain control section. L3 introduces a drop-down Profiles option providing a range of different algorithms to enable you to select one that best suits your programme content. I've included L3 Ultramaximiser in the single-band category because although it has a multi-band engine, unlike L3 Multimaximiser, it doesn't offer any individual control over the separate bands.

* Izotope Ozone Loudness Maximiser

The Loudness Maximiser is one small part of this very comprehensive mastering suite from Izotope, but in the interests of fair play, I'll only consider the Loudness Maximiser section when making comparisons with the other single-band limiters in the test.

Multi-band Limiters

* Izotope Ozone Multi-band Dynamics

Multi-band Dynamics is another part of this mastering suite from Izotope. Again, in the interests of fair play, I'm going to consider the Multi-band Dynamics section in isolation when making comparisons with the other multi-band limiters in the shootout. Izotope's unconventional user interface (overleaf) takes a little getting used to, in particular because you can't click on the values on the controls when you want to adjust them with a mouse. When you click on the numbers Ozone only allows you to change the value with the keyboard, and the only way to clear it is to hit the Enter key on the keyboard. To adjust using the mouse you need to click on the 'gas gauge' itself.

However, once you understand how Ozone presents the settings, there is a lot of data available to you from the one window. In the top section you can set the crossover frequencies for the multi-band sections and click on 'M' or 'B' to mute or bypass individual bands. Clicking on a band switches the bottom section to present the controls and settings for that band. Ozone 's Multi-band Dynamics section has a number of special features. Like Maxim, it has a level histogram (a level meter with memory), but unlike most histograms which display level history against time, the histogram in Ozone displays wider lines where the signal level most frequently occurs, so you can very quickly see where the average levels are and so can set the threshold accordingly.

* IK Multimedia T-Racks Multi-band Limiter

Like Ozone, IK's T-Racks is a mastering suite containing multiple tools, but as with Ozone I am only going to consider an individual section in isolation for the purposes of this comparison. The Multi-band Limiter section is a three-band limiter with a fairly conventional set of controls. Controls worth highlighting are an Overload parameter, which determines the ratio of clipping to limiting of the peaks, and a control described as Input Drive, which is, in essence, an input gain control enabling the limiter section to be driven more or less hard. In addition there is an Output level control which has a range that goes above 0dB, so there is potential to get the level architecture all over the place in this section!

T-Racks has a 'retro' look and uses knobs throughout. I find adjusting knobs with mouse not very intuitive — I never know which way to move the mouse to adjust the knob — but after a while I got the hang of clicking on a knob and then moving the mouse left or right to adjust the setting. Knobs are great on real bits of kit but I prefer software manufacturers to use linear controls on their user interface which, in my opinion, are much more intuitive to use with a mouse.

* Waves L3 Multimaximiser

L3 Multimaximiser uses the same five-band limiting algorithm as L3 Ultramaxiser, but this time it offers complete control over each band, with individual Gain, Priority and Release controls. The Gain control is before the Threshold so increasing the Gain doesn't change the overall output level — that remains set by the Threshold. The Priority control enables you to adjust the Threshold of each band, so enabling you to be able to 'prioritise' one band over the others — if one band is being over-compressed, you can increase its Priority and L3 will share the difference between the other bands so that the sum of all the bands is always what the master Threshold is set to. If you need to increase the output level of a band, peaks and all, you can do so by using the Gain and Priority controls together.

The Tests

For the tests I took five clips covering a wide range of different programme material: a drum & bass track, a rock opera with solo vocal and choir, a jazz quartet piece, a solo vocal ballad and a classical orchestra piece. First, I measured the loudness (Leq A-weighted) of the unmastered version of each track to get a reference. Then I used each limiter to increase the loudness as far as I could without experiencing unacceptable side-effects, measured the loudness of the processed version and came up with a 'loudness improvement' score for each track processed by each limiter. The loudness is a definite measurement, and although I acknowledge the point that what count as unacceptable side-effects are subjective, the fact I did all the tests during the same session, then reviewed the results and redid any that I wasn't happy with should give as fair a test as I could achieve. See the 'Test Results' tables for more information.

Single-band Limiters

* Drum & bass

L3 Ultramaximiser stood out on this, perhaps not surprisingly, as its multi-band engine handled the strong bass element of this clip much better than the others, although Oxford Limiter and Ozone also did very well. With L3 I tried a number of Profiles, and there were definite tonal differences between them, showing the different settings of the multi-band engine. I ended up using the Loud and Proud Profile as it didn't change the tonal balance too much but was able to handle the strong bass without pumping or distortion.

* Rock opera

All the limiters handled this clip well, but L3, Oxford Limiter and Ozone stood out above the rest. The multi-band engine of Waves' L3 Ultramaximiser enabled the vocal to retain a 'cut' through the backing without having the sense of being 'sat out on top'. Oxford Limiter produced the smoothest sound and the Ozone Loudness Maximiser did an excellent job at producing the loudest version of this clip.

* Jazz quartet

None of the Waves limiters performed particularly well on this clip, although the multi-band engine in L3 gave it a significant edge. The problem in this clip was that the piano under the vibes was easily distorted. Oxford Limiter performed well and produced a natural sound, but Ozone just pipped it again, producing that extra loud sound.

* Classical

On this clip I was extremely surprised at how well Digidesign's Maxim did. To further improve the sound with Maxi m, I used the Mix option set to 50 percent so that the result was a combination of the direct signal and the limited signal, which was even better. This process is called parallel compression and is ideal for classical music. Maxim is the only plug-in with this feature as standard; you can always create it yourself by using routing and extra channels on the mixer, but be aware that in Pro Tools LE you will need to compensate manually for the delay incurred by inserting Maxim in the 'wet' channel. Oxford Limiter just made it for me over Ozone, and perhaps this shouldn't be a surprise as Sony have worked very hard to produce a limiter that works transparently on classical music, and have been responsive to comments coming back from users.

* Vocal ballad

On this clip I could initially only manage 2 to 3 dB of limiting before I could hear L1 working too hard, but by increasing the Release time to maximum I managed another couple of dBs. With L2 I could manage another 3dB of limiting over the L1 as long as I turned off its auto release control. As with the rock opera clip, L3 produced a vocal sound that still cut through without being too forward in the mix very successfully. Oxford Limiter and Ozone came a very close joint second but in this case L3 just did it for me.

Conclusions

* Waves L1 Ultramaximiser

In general, I could only manage to get the Threshold down to -6dB below peaks before there was noticeable distortion, which came as no surprise to me. I have always found L1 to be very good for up to 6dBs of gain reduction, but after that, distortion starts to rear its ugly head. The jazz piano easily distorted and I could only manage 3dB or so of gain reduction on that clip.

That said, however, L1 Ultramaximiser is still a good limiter and excellent value for money as part of the Native Power Pack. When I need more gain reduction with L1, I will often use Waves' C1 compressor before it, with a low compression ratio of say 3:1 or even 2:1 to gently contain the programme content and then allow L1 to work with around 3 to 5dB of gain reduction. The two work together very well when working on a budget.

* Waves L2 Ultramaximiser

When L2 came along, much more severe amounts of limiting were possible and the loudness improvements confirm this, although sometimes they were not as impressive as I would have expected. It was interesting to note that on the jazz piece, L2 was only just a little better than L1.

* Waves L3 Ultramaximiser

The multi-band engine and the choice of Profiles are reflected in L3 's results, which are consistently better than those for L1 or L2. However, I found the range of Profiles somewhat limited, and would have valued some more 'transparent' ones.

* Digidesign Maxim

This plug-in didn't handle jazz very well; as with L1, the piano was easily distorted. I liked the histogram, and especially the way it works in conjunction with the Threshold to make setting up very clear and easy. Remember to click on the histogram to reset it, as it retains the display from the previous region used.

Izotope's comprehensive Ozone mastering suite includes both a single-band limiter and multi-band dynamics module.

* Sony Oxford Limiter

This has the most controls to set and so there are lots of options, but the wide range of presets enables you to get going very quickly, and then it is a matter of making fine adjustments to suit each particular track. The Audiosuite version seems to work slower than most others when processing a file. Overall, Oxford Limiter produced the most 'natural' sound of all the single-band limiters.

* Izotope Ozone Loudness Maximiser

It was interesting that on the jazz clip, I found I needed to use the 'Soft' mode instead of 'Intelligent' to get a reasonable amount of transparent limiting. I was able to use Intelligent mode on the classical clip, but I found Transparent Character even better. Ozone tended to produce the loudest results except for the drum & bass and the classical clips, where L3 won through. The histogram becomes very intuitive when you see it in action. I was, however, a little surprised to see the output meters registering over 0dB when all the other sections were in bypass — it was showing +0.4dB at times. So I reduced the Margin down to -0.4dB and all was OK. For some reason, this only happened on the drum & bass clip.

Multi-band Limiters

* Izotope Ozone Multi-band Limiter

I spent a lot of time working with the Multi-band Dynamics section of the Ozone plug-in but was unable to significantly increase the loudness of a track using this section alone, especially on the drum & bass clip. Even with significant amounts of limiting in each band, once the output level had been adjusted so there were no peaks over 0dBFS on the output, there was only a small increase in loudness in the output. Reading Izotope's excellent guide to mastering it became apparent that they have not designed this section to increase the loudness. It is very much a dynamic EQ processor, which it does very well as I found with all the clips with the possible exception of the drum & bass clip. The reason this plug-in got lower scores is due to its poor loudness performance and should not be taken as any indication that it is a poor-quality plug-in.

* IK Multimedia T-Racks Multi-band Master Limiter

When I used the same preset on the Audiosuite version the rendered file was 6dB lower than with the RTAS plug-in used on an insert on the same track, so to get the same output level I had to turn up the Output level on the Audiosuite version by 6dB. I don't know if this is a bug or some quirk on my system. Other than this, this module performed remarkably well on most of the example clips, and did especially well on the drum & bass clip.

* Waves L3 Multimaximiser

Initially I found the lack of presets or Profiles on this version meant I had to work harder to get a good sound. However, as I used it more I started to get used to interpreting the display, and when I could see and hear one band working too hard, adjusting the Priority spread the load onto the other bands which resolved the problem very nicely.

Overall Comments

There isn't really a clear winner in the single-band limiter section: it is definitely a case of 'horses for courses'. The Izotope Ozone Loudness Maximiser definitely wins on the loudness scale, but for sonic quality it is really too close to call between Waves' L3 Ultramaximiser, Sony's Oxford Limiter and Ozone. If pushed I would probably go for the Sony Oxford Limiter for classical work and the Izotope Ozone Loudness Maximiser for the rest, but L3 is a good general performer.

The Waves L3 Multimaximser is the clear winner in the multi-band limiters section for sonic quality, but the very affordable T-Racks is definitely a 'best buy' in this category. As already mentioned, Ozone 's Multi-band Dynamics is an excellent dynamic EQ module; on its own, it's not really meant for loudness improvement, but when used in conjunction with the Loudness Maximiser, you have a very powerful processor package in the Ozone 3 plug-in.