Weiss Exciter

Introduction

Is there a thing such as a "transparent exciter"?

Well, that depends on how you look at it. Traditional exciters typically add a lot of "normal" EQ-ing along with a just a teeny-weeny bit of distortion, so that the overall effect becomes simply that of an equalizer. Which indeed is very transparent, but you would be better off using a dedicated EQ, such as the Weiss EQ1, for that. On the other hand, if you overdo the distortion in a traditional exciter, you usually end up with too much audible distortion that doesn't sound very pleasing. Again, you would be better off using a dedicated saturator for those purposes.

So, you either have a lot of eq:ing, or too much distortion.

With Weiss Exciter, we use several measures to overcome these effects, to add sonically pleasing harmonics with minimal EQ effect.

The use of an adaptive non-linearity (Adaptive Mode) ensures that the harmonics are relevant and consistent regardless of signal level, while the Mastering switch reduces the amount of the fundamental frequency in the distortion to reduce the "EQ effect". Precise pre- and post filtering can further be used to target specific areas of your mix. Four different Harmonics Modes were mathematically designed to fit this exact purpose; sounding elegant while still producing enough harmonics to create the exciter effect. Additionally, the filters are phase-compensated to avoid phase-cancellation effects between the added harmonics and the original audio.

You will hear the result as a "high-end sparkle", like someone just pulled the lens in focus, and it will sound very different from regular EQs, saturators, or other tricks.

We are very excited (pun intended!) about this novel algorithm, and hope that you will find it useful when you need to add some extra shine to your mix!

First Use

Regardless of whether you use Weiss Exciter on a mix bus, in mastering or on an instrument track, the first steps are the same:

1. Increase Harmonics until the LED above the knob occasionally blinks red. That means that you get lots of harmonics but is avoiding hard clipping.

2. Use the filter sliders, especially the blue-green one on the top (Pre HP and Post HP), to set the targeted area. They might already be in a good place.

3. Adjust Exciter Amount to decide how much of the wet (excited) signal you want to add to your audio.

4. If you hear too much distortion, back off on the Harmonics knob.

Exciter Amount is a gain control at the end of the "wet" signal path, so if it's set at "off", you'll only hear the dry audio.

The next step would be to try different filter types and adjusting the stereo width.

1. Switch between the different filter types (green buttons at the bottom panel). The wide filters give you a gentler effect, but with less frequency discrimination. The "linear phase" mode preserves the phase, but with additional latency.

2. If you're exciting a stereo track, adjust the Width control to set the M/S gain of the wet signal. It will only affect the added effect, not the dry audio.

User Interface

The user interface consists of three main parts:

1. Top: Frequency visualizer with filter frequency sliders

2. Left: "Cog wheel" menu with advanced settings

3. Bottom: Main panel with the general functionality.

Most of the parameters can be set differently for different channels (L/R or M/S). If Ganged is turned on, controls will affect both channels.

Frequency Window

Blue Top Slider (Pre HP, Pre LP): Sets the high and low pass frequencies for the input filter, which determines which frequency range that is targeted by the harmonics. You typically want to set this at around 2-6 kHz.

If you double click the slider you can type in frequency value for the HP filter. Press the TAB key to set the LP filter's value.

Red Bottom Slider (Post HP, Post LP): Sets the high and low pass frequencies for the output filter, where you can filter out harmonics that you don't want. You often want to place the high pass filter higher than the Pre HP, since this will also make sure that you get less of an eq effect.

Tip: Since harmonics are at least one octave above the fundamental, it makes sense to keep the Post HP frequency set at least one octave above the Pre HP frequency. That means that the Post HP frequency should be at least twice that of the Pre HP frequency.

Red Line Graph: indicates where the harmonics have been added by measuring the difference between the input and output audio.

Blue-green Filled Graph: The frequency analyzer of the output audio.

Main Panel

Channel Controls

Bypass

Bypasses the effect. This is a soft bypass with latency compensation, designed for A/B testing the effect. If you wish to completely bypass the effect and save CPU, please use the bypass or on/off button in your DAW.

Ganged

Links the controls of channel 1 and 2.

M/S

Sets the internal channel configuration to mid/side, instead of left/right. This will affect the channel processing even when Ganged is off, since the harmonics will be generated from the mid and side channels, rather than from left and right.

Ch 1, Ch 2

Sets which parameter set that should be controlled. Ch 1 controls the left or mid channel, Ch 2 controls the right or side channel.

Harmonics Generation

Harmonics

Sets the amount of harmonics generated. This is similar to a "Drive" or "Saturation" knob. Use the Harmonics LED to guide you (see below).

Harmonics LED

If the LED is yellow, you are generating harmonics, but without hard clipping. If the LED is red, you are in the area of hard clipping. Typically, you would like to have occasional red flashes, but not constantly in the reds.

Subtle

Lowers the gain into the harmonics generator with the amount set by Subtle Gain (cog wheel menu).

2nd

Generates the even harmonics. The amount of even harmonics can be set by 2nd Harmonic Level in the cog wheel menu.

A small amount of even harmonics can make the Exciter sound smoother and fuller, but too much will make the distortion sound grainy and broken.

Exciter Amount

Sets the total amount of the effect. This is a gain control on the wet path, similar to a Dry/Wet control, except that it never affects the dry path.

Why no Dry/Wet knob? It is not needed. Some processing, such as a normal distortion or EQ, is affecting the audio directly, whereas the Weiss Exciter is adding harmonics to a completely dry audio path. The Exciter Amount control is effectively a Dry/Wet control, except that you can't turn down the dry audio.

Mastering

Engaging this control will suppress the fundamental frequencies of the harmonics generation. This is one of the core features of Weiss Exciter, and enables you to create harmonics without the normal "EQ effect" that other exciters have. An interesting side effect is that the harmonics on their own can sound a bit strange when soloed, but together with the dry signal they will blend very nicely. There's a lot of math behind that...

Monitor

Turn on to listen to the wet signal only.

Filters

Filter Slope

Sets the filter slope (Gentle or Steep). A steeper slope will have a better frequency discrimination, but can be more audible.

Linear Phase

Turns on the linear phase mode.

By default, Weiss Exciter uses "minimum phase" mode filters which introduces phase rotation. The dry path is compensated to have the same phase rotation, but without the EQ gains, so that the internal effect behaves like a "linear phase" and we avoid phase cancellation issues. However, the output will have a phase rotation. If you want to avoid that, switch over to Linear Phase. That will instead introduce additional latency.

Output

Width

Sets the M/S width of the wet effect by adjusting the gain of the "side" audio. Does not affect the dry path. Will only work on stereo audio.

Cog Wheel Menu

Harmonics Mode

Selects the type of harmonic generation function used.

• Soft: Very soft knee with many harmonics. Best all-round method.
• Hard: Slightly harder knee with many harmonics.
• Poly 3: Polynomial method to generate only 3rd harmonics (when inside the non-clipping range).
• Poly 5: Polynomial method to generate 3rd and 5th harmonics (when inside the non-clipping range).

2nd Harmonic Level

Sets the level of the even harmonics when the 2nd switch is turned on.

Subtle Gain

The amount of which the Harmonics knob is reduced when the Subtle switch is turned on.

Adaptive Mode

Adapts the harmonics generator to the incoming audio. The use of an adaptive non-linearity ensures that the harmonics are relevant and consistent regardless of signal level. You would typically leave this on, unless you only want to generate harmonics on loud sounds, for instance on a snare drum.

Graph Range

Sets the zoom level of the red line, from +/- 3 dB to +/- 18 dB.

Analyzer Tilt

Sets the tilt level on the output frequency analyzer, to better show the response for high frequencies.

Copy Ch. 1 to 2, Copy Ch 2. to 1

Copies the settings from one channel to the other.

Non Channel Specific Controls

While most controls can be different for channel 1 and 2, for example the Harmonics level and filter frequencies can be set differently for left and right channels, not all controls are separate. Here's a list of controls that always controls both channels, regardless of whether Ganged is turned on or not.

• Ganged
• Mid/Side
• Channel Select
• Filter Slope
• Linear Phase
• Monitor
• Width
• Bypass

Technical Details

As mentioned earlier, Weiss Exciter uses a couple of different technologies to achieve a highly transparent and mastering ready audio processing. Some key concepts used in Weiss Exciter are explained below.

Additive Processing

In regular distortion, the effect is applied directly to the audio, and the representation of the sound wave is in itself being shaped by the distortion. That means that it is very difficult to guarantee the integrity of the audio and avoid unwanted side-effects. Even with the most cleanest setting you will typically get some sort of coloration ("box tone").

To avoid this, Weiss Exciter uses a parallel signal path. The "dry" audio is completely unaffected in Linear Phase mode, not even the oversampling filters are touching it, and the excited audio is added to the dry audio. That means that if you set Exciter Amount at its minimum position, the output will be identical to the input.

(In minimum phase mode, the phase of the dry audio is adjusted to achieve a clean addition of the excited audio.)

Linear Phase and Phase Compensated Audio Paths

To get the most transparent effect, you need to run Weiss Exciter in Linear Phase mode, because that's the only way to guarantee that the dry audio is completely unaffected by the processing.

However, when you need a low latency output, you should turn off Linear Phase and use the "minimum phase" filters. These have zero latency, but with the cost of a natural phase shift due to the Pre and Post filters.

To avoid phase issues between the dry audio and excited audio, we apply the same phase shift to the dry signal, so that the blend between these two happens without unintended phase cancellations or general "phasy-ness".

• If original audio integrity is important: use Linear Phase mode.
• If you use parallel processing outside of Weiss Exciter, use Linear Phase mode.
• If you need low latency or low CPU, use Minimum Phase mode.

Adaptive Non-Linearities

Many exciters will only excite the audio above a certain threshold, or worse, will sound very different depending on the input audio level. This often leads to a soft section of a song sounding dull, because it will get very little excitement, while the loud parts become too harsh. You could even get the opposite effect, depending on whether the distortion amplifies or attenuates the signal. All in all, you might make it to sound good in one part of the song, but it will be difficult to apply the same setting on the whole song or project.

Weiss Exciter uses an adaptive algorithm (Adaptive Mode) to preserve the amount of harmonics, regardless of audio level. It knows the audio level and can adjust the harmonics generation. It might seem superfluous, but the effect is that you can dial in a sweet spot with Harmonics, with the "nice sounding" first couple of harmonics, and that sweet spot will be preserved throughout the song.

Technically, it is easy to create a "compander" style circuit to get similar results, but that compander will create harmonics and have a "sound in itself", especially if you make it as fast as it needs to be. The adaptive mode calculations in Weiss Exciter are guaranteed to have minimal amount of distortion, while keeping the harmonics generation musical.

To get a transparent excitement you need control over the generated harmonics. Too little harmonics, and there is no effect at all. Too many, and the audio will start to sound grainy, distorted or harsh. Adaptive Mode keeps the harmonics in check.

Red curve: Adaptive Mode On, blue curve: Adaptive Mode: Off.

The graph above shows the difference between Adaptive Mode On and Off. With Adaptive Mode On (red curve) you will keep your audio within the sweetspot, creating harmonics without hard clipping. With Adaptive Mode turned off the audio will start to clip when you increase the signal level.

Similarily, if you look at the harmonic spectrum from the Weiss Exciter, you will see that with Adaptive Mode turned off (blue lines), a lot of harmonics are created when the audio reaches high audio levels (when the Harmonics LED turns red), while with Adaptive Mode On (red lines) the harmonics are kept in their sweetspot. For these graphs, we gain staged the levels so that the harmonics would be equal at -30 dBFS (black lines).

Reducing EQ Effect

Weiss Exciter avoids the EQ effect most exciters have. Many exciters are not much more than an equalizer in a fancy package. You either get too much distortion, or too much eq, and not much in between. Factory presets are often 99% EQ and 1% distortion. This makes it difficult to hear what these exciters really add, is it just EQ or can I actually hear a tiny bit of saturation?

Engaging the Mastering switch will dramatically reduce the amount of "EQ" you get from Weiss Exciter. The fundamental frequencies have been mathematically reduced from the harmonics generation so when you add the excited audio to the dry audio, you will only add the newly generated harmonics.

This EQ effect can be further reduced by the use of the Pre and Post filters: if you target the area between 2-4 kHz, all harmonics will be generated at frequencies above 4 kHz. By setting the Post HP filter to twice the Pre HP frequency, you can further filter out the fundamental frequencies from the excited audio.

Different Harmonics Modes

The core of the algorithm is the harmonics generator. We use two different styles of harmonics generation.

First style consists of Soft and Hard, which are generating a smooth spectra of overtones. The transition between subtle harmonics and clipping is also smooth and seamless. The advantage is that these work for most material, and are easy to dial in. The disadvantage is that it can't do mathematically limited harmonic spectras, which the polynomial methods below can.

The second group consists of Poly 3 and Poly 5. These are polynomial non-linearities of order 3 and 5, and will not generate higher order harmonics unless the Harmonics LED turns RED. The sound is very transparent, but the transition between the subtle third or fifth order harmonics to clipping (infinite harmonics) is not as smooth as for Soft and Hard (which would have made it really difficult to dial in unless you have an adaptive non-linearity!)

Analyzing Plugin with External Tools

It is possible to measure the performance of Weiss Exciter using external tools, such as Plugin Doctor. However, linear analysis is not a valid test method, since the algorithm is non-linear and dynamic. (The first tab in Plugin Doctor, "LinearAnalysis", is not valid analysis method for this type of algorithm. The curve you see is not the frequency response, since it's not mathematically possible to measure the frequency response on this class of non-linear algorithm using Dirac pulses.)

Generally, you would need multiple methods where each covers a small part of the algorithm to get insight into its functionality. For example, harmonic analysis and Volterra series based analysis (such as "Hammerstein") gives you a snap shot of steady-state behaviour of the non-linearities, but doesn't capture the transient responses. Regardless of method, the results you get are from an idealized use-case, and it is important to know the theory behind the method before drawing conclusions.

Since Weiss Exciter is "additive", it adds the effect to the dry audio, the best analysis method would be to engage the Monitor switch and listen (or measure) to the added audio using a real world input signal. Be prepared though, the sum is greater than the parts (no, not really, the addition is purely linear), and the monitored audio can sound quite weird even if it sounds super-smooth when blended with the dry audio.

Credits

Thomas Andersson -- lead DSP developer, signal processing
Cameron Clark, Anton Eriksson, Fredrik Jansson, Kim Larsson -- signal processing
Niklas Odelholm -- product design, signal processing, user manual
Maxus Widarsson -- product design
Joschka Weiss -- product design
Patrik Holmström, Anna Pohl Lundgren -- framework programming
Johan Bremin -- quality assurance, presets
Ulf Ekelöf -- graphics rendering