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Study of the effects of Nonlinear Distortion on the Perceived Sound Quality

0bs3rv3r

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If adding distortion makes music more pleasing, I wonder why the music mixers have not found this. Some guitar effects are distortion, so it should not be too hard for them to relate to mixing. There must be some experiment or maybe accident that make people find out adding distortion is better. Of course those are just guesses.


But they have. Plugins for recording and mixing software, that add "pleasing" distortion, already exist, and I have to presume, are being used. Edit: sorry, I see others have already replied about this. Maybe I should add, that I have used just such a plugin and I quite like the effect on the sound. Definitely more enjoyable/pleasing!
 
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0bs3rv3r

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I've noticed songs are much dryer and sterile on headphones, that is defiantly one place where tubes can.

With headphones you get no room interractions. A better way, than just adding tubes, would be a room simulation algo - maybe simple light reverb even
 

audio2design

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Music is art. Intentional distortion (tube guitar amps) has been used for ages. Ditto the compression of tape as it enters saturation (was always a process variable). Now distortion and noise is added during post as noted above. Some of use have even made money on it :) Again, music is art. There is no right or wrong, but the artist(s) get to choose what they release.

We know that people prefer in many cases added distortion or noise, but the big question is, does this (always) mean less information reaches the conscious brain from the original pure recording or are we in some cases increasing the information content from the original signal that reaches the brain?

We know that added noise can increase the ability to pick up low level signals (in machines and the brain). And while white noise can mask other noises, it can also keep us focussed on important sounds in an otherwise too quiet environment where everything becomes a distraction. Intentional cross talk can help center an image in a poor acoustic environment.

So what does distortion do ....
 

RHO

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but the artist(s) get to choose what they release.
I think most record companies make that decision. I think these days many artists have to make compromises because of requirements put on them by the record company they signed with.
 

audio2design

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I think most record companies make that decision. I think these days many artists have to make compromises because of requirements put on them by the record company they signed with.

I am not sure that is a bad thing :)
 

RHO

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The real one, or the perceived one from use of dynamic range testing software that does not work properly with vinyl?
The real one.
You don't even need to measure it. You can hear it. And artists complain about it.
 

GimeDsp

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I think I posted about all the rack gear and plug ins designed for distortion a few pages back, its been popular since the 80s and the"distressor" is a very common piece of gear now.

Heres the thing though. In a treated room with boss hog speakers a mastering engineer can apply distortion carefully to the mix. With side chains from mid/side possessors/eq they can also treat the center and sides seperatly.

In a non treated room by the time you hear the distortion you are likely a great deal beyond hearing threshold in a treated room.

I am not saying all mastering engineers use subtly just that that can with high resolution systems.

We know from the loudness wars and from famed engineer Bob Katz that the artist/label gets what they want!
If its high distortion and low dynamic range they get it!
 
OP
Pinox67

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Some friends have pointed out to me that some aspects of my previous post are described too “hermetically”.

One of the aspects on which I struggled the most in providing explanations is that relating to the sentence where I affirm that "the amount of distortion digitally injected is that actually audible only at a certain level of listening", set as a parameter in the model. Therefore, even the listening tests are only valid for this listening level.

To better explain this point, it is necessary to spend few words on how a preamp, or the preamplication section of an integrated amplifier, "arrives" at the output signal with the desired level, based on the volume control.

In short, the amount of the amplification of the input signal applied by the preamp stage is always the same: it is the one reported in the technical characteristics under the item Gain (G below). For a preamp it is typically between 10dB and 20dB. For example, if we have a sinusoidal signal with a peak-to-peak voltage value Vin = 1Vrms in input to an amplifier with G = 20dB, we will have at output the same sinusoid with Vout = 10Vrms. Since this value determines also the listening level, it must be controlled by the user. This naturally happens with the volume knob, which attenuates the signal. Based on the point of the preamp circuit where this attenuation is performed we can distinguish two design approaches:

A. Attenuation is performed on the input signal and then amplified.
B. The input signal is first amplified and then attenuated at the output.

In my previous sentence I was implicitly referring to the first approach, which is by far the most used. Let's briefly see both below, in relation to the static nonlinear distortion model described previously.
In the following, please remember that the masking effect of our ear:

  • It is higher for frequencies close to the fundamental; in particular it is more extended towards frequencies with a higher value than the fundamental.
  • It is higher when the level of the fundamental is higher. In other words, at low volumes the masking effect is much more modest, for intensity and extension.

Type A: Input Attenuation + Amplification
For the first type of preamp the signal management is outlined in the following figure.

Figure 1 - Input attenuation + Amplification
Pasted Graphic 6.png
Basically, the input signal x(t), which we assume of fixed intensity Vin for simplicity of exposure, is attenuated by a certain amount controlled by the volume knob, before being subjected to the fixed amplification part of G value. The volume control then determines the portion of f(x) curve (responsible for the distortion) around the origin that is actually used and therefore the maximum extension to avoid clipping.
From the point of view of output distortion this implies that:
  • Given that the deviation of the linearity of f(x) increases with the rise of the value of x, then the distortion will also follow the same trend: low (in dB) at low volumes and higher at higher ones. Considering the masking effect of our ear, if the distortion is more concentrated on low orders, this behaviour is a positive aspect.
  • If the linearity deviation of f(x) is already present in the neighborhood of 0, such as crossover distortion (actually, unlikely for a preamp stage) then we will have high distortion even at low volumes and, given the harmonics of high order generated for this situation, and listening will likely be unpleasant. As the volume increases, these distortions decreases in dB and the masking effect makes it less perceptible.
For this type of preamp the digital distortion simulation is valid only for a certain volume level. On the simulator it is possible to configure how much of the f(x) curve to use, and therefore how much distortion to inject, which cannot then be modified once the distorted digital signal has been generated, as happens by modifying the volume of the real amplifier.
What has been described can also be applied to power amplifiers, as the gain G is fixed: it is the preamp that determines the part of f(x) that is used via Vin.


Type B: Amplification + Output Attenuation
For the second type, used less frequently, the signal management is outlined in the following figure.

Figure 2 - Amplification + output attenuation
Pasted Graphic 5.png
Here the input signal x(t) is amplified first and then attenuated before the output to the desired level. In this case, a much wider part of the curve than the previous type is used. The amplifier circuit must have a very high voltage swing here, which requires more demanding circuitry and components. The amount (in dB) of the output distortion is practically independent of the set volume. In theory, amplifiers with crossover distortion could benefit from this design, since this is then attenuated at the output and can first be “covered up” by distortions due to the more distant parts of the curve.

For this type of preamp the digital simulation of distortions is correct for any volume level, since the amount of curve f(x) to be used in the simulation is always the same, like for the real amplifier, regardless of the volume.


Final remarks
To understand which of the two types of situations we are in, it is necessary to measure the distortion of the harmonics for different volume settings. Assuming that the crossover distortion is absent, if the distortions (in dB) are practically independent of the volume then we have the second type; if the dB values rise as the volume increases, we have the first type.

What about the differences in sound? In my limited experience on the comparison of the two types, in the second warmth, body and dynamic contrast of the sound change little with the volume; with the second, these characteristics have a higher variability. How, depends on the type of circuit. Nelson Pass, in a rather famous article wrote in 2008 (here), states about the 2nd and 3rd order distortions of a power stage by varying the input level:

"The sound of 2nd order type circuits is often praised as “warm” and by comparison 3rd order type circuits are often noted for “dynamic contrast”. 2nd order type amplifiers seem to do particularly well with simple musical material, and 3rd order types generally seem to be better at more complex music. Figure 4 shows a distortion curve of two power stages operated without feedback – the blue is single-ended Class A, the red is a push-pull Class A.
fig_4_distortion_in_single-ended_vs_push-pull.png
In Figure 4 we see that the 2nd order type declines inversely to the output voltage (the square root of power), and the 3rd order type declines inversely to the square of the voltage (inversely proportional to power). There may be also a relation between this and the perception of “warmth” versus “dynamics”, but it is not clear to me at this time."

I hope the explanation has been helpful in understanding one of the reasons that cause changes in sound characteristics with volume.
 
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