音频编辑

发布日期: 2000-03-11
作者: Sascha Eversmeier, Berlin, Germany
插件名称: THD Soft Saturation Simulator
版本号: 0.92


WHAT IS THD? ------------------------------------------------------------

- An analog-style soft saturation simulator

Typically, THD is the abbreviation for 'total harmonic distortion', a common measurement for the amount of - often unwanted - harmonics in a complex signal. Typically, engineers are aiming at a THD level as low as possible, the processed signal should be as clean as the incoming one, without any noisy artefacts due to bad circuit design (here: saturation or clipping).
However, sometimes we beg for additional harmonics. Guitar amps, tube equipment, tape recorders driven to their limits, warming individual tracks or complete mixdowns... the list is long.

Talking about terms like 'analog sound', 'warmth', or 'punch', we mostly describe things going on in the analog domain. Mechanical and electrical units, the flow of currents, heated parts, circuits influencing each other, even by air, by random, magic or whatever... a well-balanced chaos. An engineers' work, but of finite control.

But the digital world is different. We can do everything at a click of a button. Our recordings sound clean, noiseless, perfect... but somehow cold.
I sometimes miss my analog gear when doing a mixdown. A have a limited amount of good sounding outboard gear, I simply can't afford buying multiple instances of tube preamps or such just to have each track processed individually on the mixing desk.
That's why I developed THD.

This tiny device is some sort of saturator. On good days, it behaves similar to a tube input stage, adds some gentle warmth to the input signal, brings up the overall level and just sounds as if it's consisting of analog circuits.
On bad days it creates muddiness, sounds confusing, crappy and digital as hell.
Well, this is truely not a matter of good and bad, it is just a matter of beeing the right tool for the right job and a pair of sane ears to measure this, though this is pretty subjective.


HOW DOES THD WORK? -------------------------------------------------------

This comes first: This is NO tube amp. It cannot be one without a built-in tube. I'm sorry I didn't manage to beam one inside. As long as there are no such techniques to do so, we have to rely on digits and calculus and simply do some approaching simulation of the outside world.
But this is not as easy as it sounds.
a) The analog world is a complex one. Don't expect every habit of analog gear to be successfully simulated. A lot of research and much of processing power is needed.
b) I admit, I am still a rather unexperienced programmer. Expect only a few aspects covered by this work.

Here's the concept:

The signal you feed THD with is beeing run through several stages of different processing stages. I'd like to explain things now like we were in the analog domain:
The 'heart' of this unit is a nonlinear gain stage, a saturator. If you know about tubes or transistors, you certainly would have heard about clipping. All electronic devices clip when driven beyond certain specifications (here: the amplifying region).
Though this is so elemantary and really obvious, there are huge differences: A tube usually will saturate with less harshness than a transistor would do. The tube's transition from 'no clipping' to 'full clipping' is quite smooth, which results in a more early increase of harmonics, but of pretty low level at first.
A transistor acts totally different: Its amplifying is quite ideal until a certain threshold is reached (the clipping point). It suddenly produces harmonics as well, but they hit you unexpected and are of very high level. Why? Imagine a transistor clipping a sine wave (the fundamental of every sound among us). You'll notice the sine becoming a square wave when the clipping point is reached (a graph would reveal very sharp edges). A square wave is - technically speaking - a sine wave with infinite amount of harmonics. The production of harmonics even continues beyond our hearing perception, with a very slow decrease in volume: That's the harshness, the highs are very present.

Digital clipping is hard clipping at its best. An algorithm has a definite region of numbers to calculate with. That's its amplifying region, so to speak. Apart from round-off and D-A conversion errors, this is very linear.

So, if you want to model tube-like behaviour, you have to transfer a typical transition curve ('no clip' to 'full clip') to the available range of numbers. That's the so-called waveshaping process: You simply map incoming volume to outgoing volume. Doing this, you'll get a first approach to 'analog style'.
But this is just some part of the story.

Analog gear consists of a large amount of passive parts surrounding active devices, for example a tube with lots of capacitors. A typical circuit design does not only lead to some static waveshaping, but also to some dynamic action of the whole unit.

I tried to approach this behaviour inside THD as well as static waveshaping. You can compare dynamic action (charging and discharging of capacities) to a compressor: The signal peaks are affected while lower signal portions stay untouched.
The result may be a more punchy sound. Why? Because the peaks shift the whole signal along an imaginary y-axis. Keeping a waveshaping transition curve in mind, the signal now becomes amplified in a more asymmetrical way. This results in a different creating of harmonics (more even). Electrically speaking, this is a DC shift (an offset).

Furthermore, THD uses some sort of pre-emphasis and de-emphasis circuit. You may know such techniques from tape recorders that alter the frequency band before recording and set back the original state when playing back. For example, the Dolby system acts in such a way, but the recording circuit itself does also similarly.
We may need to apply this 'trick' to saturators as well to achieve a greater level of loudness (A very little amount of distortion is acceptable on bass frequencies, but you can easily aim at higher levels on the treble without making distortion really audible. In fact, a lot of 'exciters' work this way).



THE INTERFACE AND ITS CONTROLS ------------------------------------------

- GAIN:
This sets the overall operating level. The higher you set this level, the more it will affect the so-called waveshaping process. When you increase the level, you should notice a LED light up (next to the gain slider). It is usually best to let the LED flash occasionally (and thereby intensively) to achieve maximum 'punch'.


- PUNCH:
The more constant the LED is lit, the less dynamic behaviour you can expect from the 'punch' section. In this case, the signal will be distorted asymmetrically all the time, which creates more even harmonics, but of quite a constant high level. The resulting sound can be muddy and unpleasent. Please listen closely!


- CLIPSHAPE:
Low settings produce the least audible distortion (so it will affect 'punch' perception as well), but more hard clipping. The more you move up the slider, the more loudness you may perceive. But as stated above, you prvoke early creating of harmonics, so occasional comparing with the original source (-> 'bypass') is recommended.


- OUTPUT: Nothing spectacular here. Just to keep levels balanced if needed or set it to 0dB to use THD as a 'maximizer'.


- +6dB-Switch: Press this one and you double the input level. Please note that this affects the whole processing, so if you raise the input here, you may have to readjust the other settings.


- BYPASS-Switch. The MOST IMPORTANT control on THD. Press it from time to time. Please.



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Known bugs and unintended behaviours in the current version (v0.9) (this should not worry or even shock you. I'm shure this device IS operational and I use it myself without much care...):

- Trying to run multiple instances may crash the PlugIn as well as the host. This may especially occur when one THD editor (the interface) is already opened and another editor is beeing opened as well.
- The 'punch' LED does not light up in sync to the sound output (it runs a bit forward). This is due to the audio driver's output latency and may depend on your particular audio hardware and your current settings. I will most certainly fix this lack soon, so that the display is properly synced to the output as long as the host reports valid latency values.
- THD may block the hosts updating of graphics when a mouse button stays pressed on a slider. Until this is not fixed, I recommend not to provoke such temporary display 'freezing'. But it should not be a major problem if you operate it normally.
- The device momentarily assumes a sampling rate of 44kHz. It operates at other rates, but the sonic results may differ. Some stages already adjust themselves accordingly, but others would require valuable CPU time to adapt. So I was better of with letting them expect a fixed rate. This may be a topic for future versions because it would be a serious performance matter.


Anyway. Have fun!

Sascha Eversmeier
Berlin, March 11, 2001
http://www.eversmeier.f2s.com
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This program was written using Microsoft Visual C++ 5 and the Steinberg VST PlugIns software development kit (SDK).
VST is a registered trade mark of Steinberg Media Technologies AG. All copy rights acknowledged.
All other copyrighted trade marks belong to their respective owners.


   
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下载：

http://www.digitalfishphones.com/binaries/THD.zip