Choosing & Using Enhancement Plug-ins

In this month's Pro Tools workshop, we concentrate on the top and bottom of the frequency spectrum, looking at plug-ins that claim to enhance treble and bass. How do they work, how should you use them, and most importantly, which ones are the best?

What is our fascination with enhancing audio by processing the bass and top ends of the audio spectrum? It is down to what we hear versus the way our brain actually interprets that data, an area of study that is often called 'psychoacoustics'. Nobody fully understands how the process actually works; as we well know, the human ear can be very easily fooled, and differences in 'perceived loudness' can most easily confuse that ear-to-brain mechanism. However, there are a number of tried and tested tricks that can be proven and are repeatable.

Waves Maxx Bass.Our brain adjusts its own internal equaliser depending on the volume of what we are hearing. This has been measured and quantified into what are called Fletcher-Munson Curves. At low volumes, we lose our sensitivity to the lowest and highest frequencies, and sound appears mid-range-heavy. As you increase the playback volume, the sound appears to increase in bass and treble content as well. At around 80 to 85 dB SPL, we actually hear the signal as near to flat as it comes, which is why 85dB SPL is often considered to be the ideal monitoring volume for mixing and mastering. Have a listen — it is surprisingly quiet! As the volume increases above this level, our ear/brain interface actually boosts the bottom and top end, so that at loud listening levels, we hear more bottom and top than there actually is.

The idea behind bass and treble enhancers, then, is to boost these areas of the frequency spectrum to compensate both for our lack of sensitivity to these areas at low listening levels, and for the fact that small speakers in TVs and radios can't reproduce these frequencies properly.

How Enhancers Work

Most of these devices work in the same way, whether they target the low or high frequencies: by creating harmonics of selected frequency bands of the audio being processed, and mixing those harmonics in with the original audio. Most of the bass processors we will be looking at depend on the principle that the ear/brain interface can reconstruct a missing fundamental frequency from just the harmonics the ear is hearing. For example, a bass guitar cannot be faithfully reproduced on a transistor radio with a small speaker, as the fundamental pitch is too low for that speaker to produce. However, the harmonics can be reproduced by the speaker, and the ear/brain interface recognises that these harmonics are related to each other, and reconstructs the missing fundamental. This phenomenon has been well known for centuries — pipe-organ builders use exactly this technique to emulate very long pipes that might have been too expensive to build.

Waves Renaissance Bass.Other elements used in some bass processors are low-frequency compression, which can improve the perceived loudness of the LF content without increasing the actual signal level, and time alignment, which can compensate for time or phase changes in the playback chain.

At the other end of the frequency spectrum, treble enhancers can reconstruct harmonics that have been lost through the imperfections of the recording process, or simply add sparkle to sounds that are too dull. As well as normal 'brightening' duties, this effect is very useful for treating audio that has been through a noise-reduction process and may have lost some of its high-end detail. It can also be used on a solo vocal to create a more intimate sound, as the process simulates the way sound is perceived when the source is close to the listener. The advantages of using an enhancer over an EQ are twofold: first, enhancement won't boost high-frequency noise such as tape hiss in the same way, and second, EQ will only work if there are some high frequencies there to begin with!

It is generally accepted that Aphex's Aural Exciter was the first treble enhancer, and the story goes that it was discovered by accident. A stereo valve amplifier was wrongly assembled so that one channel worked perfectly but the other channel produced a thin, distorted sound. Somehow, the outputs of both sides got combined and the result sounded cleaner and brighter than the original! There followed a lot of research and development to refine and control this 'accident' into the Aphex process we know today. So, what happens in an Aphex-type processor is that some of the input signal is tapped off and routed through a high-pass filter, to remove unwanted low frequencies that would ultimately produce a muddy or discordant sound, and then into a distortion process that generates harmonics. Small quantities of these harmonics are then mixed back in with the original signal to produce the effect of brightening and emphasising the transient detail.

Let's look at how the various processors function that are available as plug-ins on the Pro Tools platform, starting with the low-frequency processors.

Low-frequency Processors

Waves Maxx Bass

This is the classic harmonics-generating LF processor plug-in. The graphical user interface gives a very clear explanation of what is going on: the sky-blue section presents the original bass signal that is fed into the side-chain of the processor, and the yellow section represents the harmonics that Maxx Bass is adding to the signal. The relative levels of original sound and harmonics are controlled by the appropriate faders to the right of the graphical display.

As well as enhancing the low end of a wide range of audio content, Maxx Bass also has a special feature for compensating for small speakers. In this application, you would replace most if not all the original bass with the harmonics created by Maxx Bass, so taking advantage of the way our ear/brain interface can put back in the fundamental frequencies even though they aren't there. The advantage of filtering out the original bass is that you won't be throwing large amounts of LF audio at speakers that can't handle it and so then distort. This trick is widely used in all sorts of exhibition-type applications, where small speakers have been used for a variety of reasons but the client still wants 'quality audio'. In the screenshot (left) I have taken out most of the original bass and left just the harmonics and it is amazing — the LF sounds as if it is still there!

Maxx Bass has more controls to enable the process to be fine-tuned. The Frequency control is roughly similar to a crossover control, adjusting both the cutoff frequency of the side-chain filter and the depth of the replacement harmonics. The Harmonics section has a high-pass filter which controls the slope of the low-frequency end of the harmonics path, and a Decay parameter which controls the relative levels of the harmonics in the series; higher values mean that each harmonic is only slightly lower in level than its predecessor. Higher values will make for a richer sound, but also can become blurred as well. Lower values will sound more natural on full-range systems, but may not be audible enough on small speakers. The Dynamics section enables a compressor to be inserted in the side-chain, 'fattening up' the harmonics sound. If you use the compressor, you will need to reduce the level of the harmonics coming back into the signal.

Waves Renaissance Bass

This plug-in is an improved and refined version of Maxx Bass. It has a simplified interface, but the underlying principles are the same, in that a side-chain process creates harmonics from the LF content of the signal and these get mixed back in with the original for an 'enhanced' effect. You can use Renaissance Bass for the 'small speaker' effect — set the Frequency control to the LF cutoff point of the speaker and bring up the Intensity till it sounds right.

Aphex Big Bottom Pro

Aphex first introduced the Big Bottom Pro process in their Model 104 hardware unit. They have since released a Pro Tools plug-in version of this bass enhancement process, which is now distributed through Digidesign. Aphex are understandably coy about exactly how their processor works, but reading between the lines of the sales talk, it seems that the process filters off the LF part of the signal, compresses it and mixes it back in — there doesn't seem to be any harmonics generation going on. The side-chain is fed from the input signal after the input level control, then through a Drive control, before going into a low-pass filter with a range of 40 to 400 Hz. The output of the low-pass filter is then fed into the dynamics section before finally going through the Mix control, which isn't a wet/dry control — it adjusts only the level of the processed signal being mixed back in with the original unprocessed audio.

The Dynamics section also has an auto threshold option, which dynamically adjusts the threshold of the compressor to the level of the audio going through it. This is useful if you have some audio that varies in level, as it helps to match the degree of the processing with the dynamics of the original. The dynamics section has a compressor gain-reduction meter display, which is very useful to show you how hard you are driving it. In addition, the Mix stage has a phase switch so you can invert the phase of the processed signal being mixed back in with the original. Big Bottom Pro is really an low-frequency dynamic EQ processor, then, as it doesn't seem to add any harmonics into the signal path.

PSP Mix Bass

PSP Audioware are a small but prolific plug-in manufacturer based in Poland, and Mix Bass comes as part of their Mix Pack. It has three elements: a dynamics section like that featured in Big Bottom Pro, a harmonics generator like the one in Maxx Bass, and a soft-clipping algorithm which PSP say is to prevent digital distortion when going over 0dBFS; they claim it allows an increase in level of around 3dB before noticeable artifacts appear.

High-frequency Processors

Aphex Aural Exciter

Aphex started it all off back in 1975 with their first Aural Exciter, and this Pro Tools plug-in is modelled on the Type III Aural Exciter. The input signal is split off to feed the side-chain after attenuation and a section called a Spectral Phrase Refracter (SPR). The side-chain includes a tuneable high-pass filter, harmonics and waveform sections and a mix control, before the added harmonics are mixed back with the unprocessed path to produce the effect.

Let's take a more detailed look at the controls, as the naming doesn't always make clear what they actually do. The first control is Level, which affects the level of both the processed and unprocessed paths, but as you would expect, doesn't affect the signal when the plug-in is in bypass. The next three controls all relate to the tuneable high-pass filter. The Tune control sets the cutoff frequency of the filter, and can be set from 700Hz to 7kHz. The Peaking control sets the slope of the filter at its turnover point, and goes from a smooth transition when set to minimum to a very much steeper transition which has the expected side-effect of any steep-sloped filter: a significant peak in the response at the top of the slope. In most applications for a high-pass filter, this would be considered undesirable, but here it seems to be used as an advantage; if you do want to smooth out the response whilst maintaining the steeper slope of the filter, you can use the Null Fill control to do so. All these controls interact with each other to a greater or lesser extent as you adjust them.

The Harmonics control sets the amount of harmonics generated, and the Timbre control determines the ratio of odd to even harmonics that are produced. The ear/brain interface tends to find even harmonics more pleasant to listen to, but the odd harmonics in a square wave-type sound can 'cut' through a mix more easily. The final control is Mix, which controls the amount of processed signal mixed back in with the original.

Then we move onto the buttons above. The first is the Drive button, which optionally adds 12dB gain at the start of the side-chain path. It is there because there will be times when you will need more signal to drive the side-chain. Next is the Density switch, which switches between two different types of harmonic generators to give you a choice of harmonic 'colours' to choose from. The Ax switch enables you to turn off the side-chain path whilst leaving the Level and SPR controls in circuit. The Solo switch I found very useful, as it disables the direct path so all you hear is the processed signal, enabling you to hear exactly what the plug-in is adding to the signal. Then we come to the SPR (Spectral Phrase Refracter) switch. This switch enables a phase adjustment which advances frequencies below 150Hz passing through the plug-in, both in the processed and unprocessed paths. This, Aphex claim, corrects a bass delay anomaly which is inherent in audio circuits, restoring clarity and openness and increasing the apparent bass level without using EQ. The Link switch enables you to link or unlink both sides of the process path, so together with the LR switches, you can unlink the left and right channels in the side-chain and have different settings for each.

PSP Mix Treble

PSP Mix Treble comes as part of the Mix Pack, like its sister plug-in Mix Bass. There are four sections to the Mix Treble plug-in. The first, a hiss remover section, is a dynamically tuned low-pass filter designed to filter out the high frequencies when there is relatively little HF content in the signal going through it, so using the 'masking effect' to give a perceived noise reduction. The second section is called Transients; based on a high-frequency compander, it is claimed to enhance transients damaged by poor-quality equipment, improving definition in the mid- and high-frequency bands without increasing the noise floor.

The third section is the Enhancer, which works by converting a stereo signal from left/right to Middle + Sides, using a high-pass filter on the Sides signal, and then converting the output back to LR. The final section, Harmonics, has a tuneable bell filter capable of distorting a band of the signal, which then feeds a harmonic generator producing both odd and even harmonics. Finally, a high-pass filter removes the fundamental component from the added harmonics.

Izotope Ozone Harmonic Exciter

This is part of the Ozone 3 suite, which we first saw in our shootout of limiter plug-ins (May 2006). The Harmonic Exciter section can be configured to cover up to four bands, so giving you the option to excite any frequencies on your track. Another feature of the Harmonic Exciter is that the way it creates the harmonics is by modelling tape or tube saturation. This, Izotope say, produces a more musical sound, with the tube saturation generating mainly even harmonics for a warmer sound and tape saturation producing both odd and even harmonics for a more aggressive result.

It also has the feature to add small amounts of delay to each band, so enabling you to mimic the phase-shifting effect of the BBE Sonic Maximiser (there is a BBE plug-in, but it is only available in VST format so I haven't covered it in this article). The BBE effect is based around the theory that loudspeakers have a tendency to add progressively longer delay to higher frequencies, so the process does the reverse by adding more delay to the lower bands, leaving the high frequencies undelayed.

Using Low-frequency Processors

If you are processing individual elements, always check how they sit in the mix. In the early days I overdid the Maxx Bass processing on the kick and bass guitar on a number of tracks. I ended up taking most of the effect off when it came to mastering them.

This sort of process is, however, great for strengthening speech. I use it on radio documentaries, especially for BBC Radio One and Two, as it helps to give presenters the 'Bill Murray' effect. (Bill Murray is a famous voiceover artist with a really deep voice.) It is also great for improving gunshots and explosions, giving them more depth and impact.

Of the low-frequency processors in the test, I prefer Renaissance Bass unless I need more control, in which case I go for Maxx Bass. The small-speaker process from Waves is a winner for me. It's great for treating audio that you know is going to be played on radios or TVs. I wasn't particularly impressed with Big Bottom Pro — it doesn't add harmonics, just compresses the bottom end, so it is no good for the small-speaker trick — and I didn't particularly like the graphic interface on PSP's Mix Bass; the window is too small, so I can't see all the information.

Treble All Round

I like the Unlink feature in Aphex's Aural Exciter. For example, I tried a range of different settings on the left and right channels of a solo vocal, and it definitely helped to give the track a sense of spread and space. I also tried it on an acoustic guitar track, where again, it opened out the sound when used in moderation. Like all these processes, it can make the result tiring to listen to, so remember to keep checking and comparing the sound to make sure you don't overdo it. The effect of SPR is very subtle to my ears and initially, I couldn't really decide whether I preferred it in or out; after some very careful listening on a variety of source material, though, I wasn't impressed and would tend to leave the SPR switch off.

The Mix Treble plug-in seems to be designed more as a restoration tool, as the hiss remover and transients sections are both designed to try to undo the damage from poor recording equipment or incompetent users! I found it very easy to overdo the Transient processing, which resulted in louder elements 'popping' out of the mix. The Enhance section was very nice at spreading and putting more space into sounds, but it was very easy to put the Harmonics section into distortion and end up with a fuzz guitar sound instead of a pleasant acoustic guitar, so as before, it is definitely a case of 'little is best'. Having 38 presets to start from is a very nice touch, but you definitely need to understand exactly how each of the sections works to get the best out of them. I didn't get very far with Mix Treble without reading the manual, and I still felt as though I was working blind, as there are no meters or any other indication of how much of anything you are using.

The Harmonic Exciter in Izotope's Ozone is my favourite of all the high-frequency processors by miles. It is intuitive, the metering and the interface tell you exactly what is going on, the interface works well when driven by a mouse (as you all know by now, I don't particularly get on with interfaces that use rotary controls) and the results from it sound very sweet and smooth, with more space and air. I could get very good sounds from it without having to refer to the manual to understand how it works. The addition of band delays to create a BBE-type effect is a bonus, as is the option to use 'tube' saturation. This creates a smooth sound, and with all the bands in use, you can create a valvey sound across the whole range.

Conclusions

To sum up this workshop on enhancers of all descriptions, the core tip is to use small doses of the chosen effect and constantly check back with the bypass button, listening to other tracks to make sure you don't overdo it. In the constant drive to create louder and brighter mixes, Waves' Maxx Bass or Renaissance Bass and Izotope's Ozone 3 Harmonic Exciter stand well out of the crowd as the tools to use from this bag.