Distortion is an ambiguous term and mainly the result of non-linear behavior when processing audio, but there is more to it. In addition to frequency response distortion, there is phase-frequency distortion, amplitude distortion and harmonic distortion. 
Furthermore, frequency response distortion can be desirable like the type of distortion induced by analog tape machines and vacuum tubes to a sinusoidal alternating current and which also produce as result, circuitry oscillation noise. In system measurements, all the harmonic components that have their ratio summed are represented as the 'THD' value, if you add N value you get total harmonic distortion plus noise. When you consider that noise such as tape hiss, RFI and hum can also be added to the signal, the term "distortion" becomes somewhat blurred or lost in semantics. Ironically, that's what some people regard as "analog warmth".
In this frequency context, "harmonic enhancement" tries to realign the harmonic distortion induced by non-linearity systems issues and can be described as the excitement of those periodic 'normal modes' or modal frequencies above their fundamentals {also known as their overtones}.
When applied, the perception of a "better sound" comes from exciting these overtones, but the problem is that at some point the medium that delivered the sound {even the environment} changes and so does the translation.
Finally, you can say that distortion is referred to non-flat frequency response, compression, modulation, aliasing, quantization noise, even wow and flutter. Add derivative terms like attenuation distortion, clipping distortion, music distortion {electric guitar) and you can end up very confused about its meaning.
Non Linearity and Response Non-linearity issues, although a general term, are pretty much what create the effects of distortion to sound. 'Non linear' meaning that output is not an accurate flat response or proportional to its input, where 'distortion' accumulates at every step of the processing stage right up to the sound delivery system {speaker/amplification interface}. When you add it all up, you get the sound you hear in the environment being monitored. Things are further "distorted" when the room itself has severe acoustic problems. That's why when the recording one hears it doesn't translate to other environments, one can say that there has been some sort of a change in response due to phase frequency distortion. This 'phenomenon' can then be addressed by adding phase and amplitude changes {a realignment of sorts} to certain, or even all frequency bands across the spectrum. "Harmonic enhancement" like the Aphex Aural Exciter which applies frequency dependent phase shifting with band filtering and compression is one type of enhancement, while another and different is the one patented by BBE Maxie which attempts to correct phase/amplitude issues when sound is being delivered .
While Aural "excitement" was the rage in the late 70's - and throughout the 80's, the need to use that type of enhancement in professional recording situations changed with the advent of professional mastering and all the great improvements in speaker technology, room design and of course, the digital revolution. The Ambiguity Of The Term Enhancement On a very busy mastering forum Quote: | How about removing harmonics? Do you think it can be done with notch filters, yes or no? | The answer:
You certainly can.
No one ever can create new harmonic content with a single narrow bandpass or a peak filter. However, these filters can alter by means of boost or cut the harmonic content of static materials consisting of steady notes. +
F is the fundamental frequency and f1 is f*1, f2 is f*2 etc.
An equalizer can not make a f*2, f*3 etc frequency appear. Filtering works on f*1 only.
Distortion is F*2, F*3 and so on. With a variable frequency input, the F*2 for example, it will also vary because of how fundamental frequencies can modulate and the incredible complexity of their harmonic structures.
"Distortion" can also be thought of as any output amplitude that is nonlinear to its input. When you use RTA to locate the harmonic content of say, a snare track, and you boost with narrow band pass filters its harmonics, you are "distorting" the sound. This could also be seen as "enhancing" it because you have virtually "excited" the upper harmonics making the overall sound a little brighter.
In the context of mastering however, this would not be too effective of course, but I find a significant improvement to the presence of a snare sound in mixes by inserting a single narrow peak filter at around the 4.5 - 5K range. If you need more information, go to our general discussion forum to sign up and to post questions or create a new topic. |
