The ObjectiveThe objective on using a loudness limiter, ideally, would be a transparent increase in loudness of the mix. Although this may not be entirely possible, we would like to approach this ideal. In previous tips I have specified a maximum limiting of 6 dB on the peaks, simulating the loudness advantage that analog media has over digital media.
Maintaining transparency in loudness limiting depends on the time parameters that we use in the limiting - the attack and release times of the gain reduction. Actually, it is 99% dependent on the release time - the attack time can be set as fast as possible in almost every case.
ADSRThe volume envelope of the waveforms holds the key to setting release times. The envelope is often called the ADSR, for the different areas of the envelope (attack, decay, sustain & release). The "transient" in the waveform is caused by the first part of the envelope (the attack and decay), forming a spike at the beginning of the waveform.
The loudness advantage of analog media has to do with "control" of the transients due to tape saturation (sometimes erroneously referred to as "tape compression"). There is no control in a non-overloaded digital recording. We, instead, put that control in by using a loudness limiter. The bottom line is that we are trying to control the transients of the percussive instruments in the mix.
Maximum loudness with the minimum alteration of the sound to the ear is obtained by applying the limiting only during the transient as shown in the illustration. The time of this transient, however, varies with different frequency percussive instruments. My measurements of the transient time on example files were about 12 ms for a high hat and 32 ms. for a foot drum.
To just control transients, the release time should be set somewhere between 50% and 100% of the transient's time. Thus, the release time needs to be set between 6 and 30 ms. (a 5:1 difference).
Longer than ideal release times tends to bring down the body of the instrument's sound, reducing the amount of loudness obtained with the limiter. Release times set to be ideal for the kick tends to bring down the body of cymbals tending to cause a dulling of the cymbal sound (and other midrange and high-frequency instruments) in the mix on every kick drum beat (called "pumping"). Release times set to be optimum for the cymbals tend to alter the waveform of the kick drum (as well as other low-frequency instruments), causing distortion. Many loudness limters start with a 10 ms. release time which has to be varied by ear, depending on how loud the various instruments are in the mix.
Intelligent (Variable Release) Loudness MaximizingThe above guidelines have to do with how to set the release time on a brickwall limiter. The Ozone Loudness Maximizer also has an "Intelligent" setting which varies the release time of the limiter dependant on the amount of time that the envelope exceeds the threshold. Thus for low-frequency waves the release is made much longer and for high-frequency transients the release time is made much shorter. The release time indicated is the fastest release that the maximizer will apply for the quickest high frequency transients. Ozone doesn't specify what the longer release time (on bass waves) would be, but it sounds like it is a medium release time (close to 100 ms.) at the mid point.
Ozone has added a "characteristic" designation for different release times as follows:
0.0 - 0.1 ms. = "Clipping"
0.2 - 0.9 ms. = "Very Fast"
1.0 - 2.9 ms. = "Very Fast & Loud"
3.0 - 4.9 ms. = "Smooth"
5.0 - 10.0 ms. = "Transparent"
In general, more release time should sound more transparent, and shorter release times should sound louder, but more distorted.
Listening to just a bass wave, and the limiting set to 9 dB (much higher than normal), the bass has little distortion until you drop the release time below the 5 ms. level. The distortion you hear, however has somewhat of a pleasing edge to it (for pop recordings), all the way down through the "Very Fast And Loud" range. The bass is somewhat louder if the release time is lower than 3.0 ms.
Listening to just a high hat wave (with the same limiting) gives similar result with a bit more loudness difference overall. Overall both instruments sound more evenly louder and less distorted than with the "Brickwall" setting.
In using the loudness maximizer intelligent mode, I would recommend putting the release time at the mid point of 3 ms. and increasing or decreasing it by ear.
Soft Knee LimitingWhen you apply soft-knee limiting, you will tend to get overshoot of the transients. In the samples I used for analysis, I got a 4 dB overshoot when I tried to apply soft knee limiting of 9 dB.
The difficulty I have with Ozone's Soft Knee mode on the loudness maximizer is that it has a very deep, fixed knee. Soft knee limiting tends to let the initial attack of percussive instruments though; almost similar to how reducing attack time will let them through. With the soft knee limiting, however, these transients are somewhat controlled (and are not controlled at all with increasing attack time).
Host ProgramsOzone is a plug-in program that requires a host program. I use it as an insert in Stienberg's Wavelab program. Wavelab has a much more sophisticated soft knee limter that has 10 different knee settings. As a result I use the Wavelab software for any soft-knee limiting I use.
Bob Dennis
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