Re: [fluid-dev] Purpose of dither?

[Z F]

--- Mihail Zenkov <address@hidden> wrote:

> 
> :) Where are you read this? I six years do amplifiers (mainly on
> tubes)
> and acoustics. All amplification strongly linear. Otherwise, we have
> big THD and IMD.

Well, what can I say, tubes afford huge dynamic range and so they go as
linear as possible providing good quality. Consumer electronics,
generally, does not have a huge dynamic range, so to solve the problem,
speaker output (in other words, the last stages of amplification) are
non-linear. This causes non-linear distortions in the signal, yes, but
it is less noticable than clipping.

Where did I learn that... when I was little I used to fix analog
consumer audio/video equivment, but I do not do that anymore. 

> 
> 2. Dynamic gain and gamma correction just do mezzo piano from my
> pianissimo and mezzo forte from fortissimo.

Dynamic gain controll will do that, gamma-correction will not.

> 
> I have equipment with self dynamic range more 100 dB. I want use
> fluidsynth in home recording studio and need maximum quality.

If you have that than I do not understand why you have a problem.
The problem is that input to it is 16bit digital? Than it is not 100dB.
It the input is more than 16 bit, use it, no problems.
If the input is analogue, use 24 bit card and no problem.
If the input is analogue and you use 16 bit card, (ideally change the
card). The DAC in the 16 bit converter should not be linear, but
I assume all consumer electronics have linear DACs, so it can not be
fixed. As the last resort, use dithering, as you probably do. But, you 
have to understand that this is not the problem of the synthesizer, it 
is the problem of your setup which you want to fix. In terms of
effort, introducing dithering is the easiest, but is not optimal.

As I said, I have a similar problem because my computer speakers
are connected to the line output of 16bit card. Cannot be fixed. wrong
setup, but I know that.. :)

> 
> Harmonics - any magnitude at any frequency _correlated_ with signal!
> Any other - just noise.

fixed amplitude at a fixed frequency will not be correlated with signal
but it will hurt your ears and it will be --- just noise.... So it is
not important to talk about harmonics but rather about spectral
distortions in general.

>  
> > As such, no matter what you do, with or without dithering, you
> damage
> > the spectrum. The difference is that with dithering, our brain is
> less
> > sensitive to it that is why you do not hear the spectral
> distrotion.
> 
> No. You can measure THD and IMD - without dithering it very bad. With
> dithering it _very_ good. All digital conversions need  dithering. It
> not psychoacoustics artifact. It just solution for fix one problem in
> digital theory. Mostly all digital converter in scientific devices
> applies it, it just can't right work without it.

It is all because of wrong setup -- an attempt to use 16 bit. Digital
theory suggests how to hide the spectral distortions so that they will
not produce that psychoacousic artefact. Spectral distortion is a
speectral distorsion. One can only change the distorsion, but one can
not avoid it. In other words, dithering is needed only for cheap
solutions since high dynamic range is expencive. But if one goes this
route, than one has to understand the consequences. On the other hand
we always want more for less...

On a personal note, I was very upset when I had to make a decision and
to by a digital piano with a built-in synthesizer because my family
members were afraid of the computer sound. The sound from that piano
is not bad when it is on the loudspeakers, but when I connect
head-phones, I hear a lot of distorsions. This is how ambient noise
hides the distortions, in other words, does the dithering for you.
I do not see how adding noise is a solution even though perceptually,
it sounds better.


ZF

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