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Topic: CD artifacts (Read 4768 times) previous topic - next topic
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Re: CD artifacts

Reply #25 –

The analogy "a square ledge in a tank of water - a wave will reflect off the sharp edge" I'd assumed this was a good analogy
to start with for explaining "pre-ringing artifacts" you get from a low pass filter (cutting everything off at 20khz). But, in practice,
not sure how soundwaves generated in this way will bounce off something. The quote then refers to "the wave "bouncing" off the band pass filter" - don't get this as it sounds like something that is happening to the generated analogue signal inside the DAC rather the soundwaves in the room.

So if someone has a model/explaination for what the soundwaves leaving the loudspeaker are doing to generate pre-ring artifacts
please post.

For clarity this is of interest rather 'baiting' as suggested by ajinfla.







Re: CD artifacts

Reply #26 –
Quote from: Paul77 on
So if someone has a model/explaination for what the soundwaves leaving the loudspeaker are doing to generate pre-ring artifacts please post.

Soundwaves leaving the loudspeaker do not generate pre-ring artifacts.

Re: CD artifacts

Reply #27 –
@Paul77 you are mixing too many things.

* Ringing: Some information on wikipedia, although it might be a bit dense https://en.wikipedia.org/wiki/Ringing_artifacts
The basics: Ringing means to oscillate. Instead of having a steady signal, the signal oscillates at sharp changes.
pre-ringing and post-ringing: Ringing is seen before and after the cutoff place. Having both is just natural, but depending on the type of windowing and filter response type, one can avoid pre-ringing.

Aliasing: A generic article also on wikipedia. The textual explanations are a bit complicated, but the image can be understood: https://en.wikipedia.org/wiki/Aliasing
The basics: Aliasing means getting a modified signal that contains something mirrored from another place.
Here is where your example of a tank of water could work, but only if you understand what we are talking about:


Lets think about a tank of water ( or a swimming pool, if you're more famirialized) where the water is calm.
You drop something into it, and water waves get generated from that point moving away from this point.
When one of this waves reaches the wall, it cannot continue advancing and bounces back.

Now, with that idea in mind, let's take a frequency spectrum of an audio signal.
You would have some frequencies in there, let's think about a "two-mountains and a lake" graphic  =  ^o^
If we change the frequency of this, so that the second mountain is after the cutoff frequency, something like this: ^o|^ and no filter is done when doing this operation, the signal will actually become this:  ^ô (i.e. the second mountain will mirror and mix with the part of the signal that was there. in this case, our "lake").


Also, aliasing can happen on the opposite direction, which is what this "CD to analog" thread is about.
A sampled signal when converted from digital to analog, needs a lowpass filter (actually, this is called a reconstruction filter) in order to remove the mirroring caused by straight conversion from discrete values to continuous values.

This is what the image called "spectral folding, caused by sampling" tries to show in that Wikipedia Aliasing article.

As you were told, this might have been a problem with cheap computer equipment on the 90s, but even then, it was perfectly possible to not have such a problem.
Problem happens when the output jumps from one value to another, and stays at that value until the next sample come in. A reconstruction filter properly interpolates from the first value to the next, providing a continuous signal.

To think about this problem, one can think about a square wave.
A square wave is a signal that has a fundamental frequency, and then it has many harmonics that are multiples of that frequency. If you filter all harmonics, you get a sine wave. If you have just the fundamental and the first harmonic, then the sine wave starts to deformate, but still resembles a sine wave.
The more harmonics that you add, the more it changes from sine wave to square wave.

Converting a digitized (sampled) signal to an analog (continuous) signal resembles this sine and square problem.
If you don't use the reconstruction filter, the analog signal between the digitized samples resemble a square wave, and as such, has harmonics.
But in this case, you don't have a fundamental frequency, but instead a full band signal.


Oh... and indeed, this has nothing to do with speakers. Speakers might change the shape of the spectrum (have more bass or miss some mids or whatever..), but do not generate frequencies, although depending on the materials, they can ressonate and cause distortions.

Re: CD artifacts

Reply #28 –
I don’t know that lakes and mountains is a good analogy, in fact I consider it to be a very bad one, ignoring the fact that they are coming on the heels of a giant misconception.
 

Re: CD artifacts

Reply #29 –
Well, to be fair, if an audiophile sees a CD and soundwaves are generated, pass through the ear canal and into the cranial cavity, there will certainly be some wall bounce echo, which could then manifest as "ringing" artifacts.
So not too far fetched, nor any iffy analogies needed.
Loudspeaker manufacturer

Re: CD artifacts

Reply #31 –
I believe that would be the point.

Re: CD artifacts

Reply #32 –
@greynol : I just used the image of two mountains and a lake in order to easily write with text what I wanted to represent as a frequency plot. 
I am not sure if you mean that it is difficult to see it, or you mean that the whole "bounce back" or mirror concept is not applicable to what we are talking about.

Let's better get an image from our well-known resampler tests page (I've specifically chosen images that help see the effect. I'm not making audio judgements):
Here we can see aliasing: https://src.infinitewave.ca/images/Sweep/XRECODE3_swr.png
Here there isn't: https://src.infinitewave.ca/images/Sweep/XRECODE3_sox.png

On both cases, the signal would increment above the top if it could.
In the first image, we can see the bouncing (mirroring) of the signal that should have gone above, but instead goes down and even bounces again when reaching the bottom.

That's what I was talking about.

Re: CD artifacts

Reply #33 –
Sure, but there absolutely is no bouncing of the sort going on.

Remember this guy is talking about waves in water that meet the edge of a surface. This is nothing like ringing, aliasing or mirroring. Trying to shoe-horn in a water analogy is only going to reinforce the idea of there being a relationship between what are two very different phenomena.

The standing wave in an empty cranial cavity isn’t a good analogy, but the quip was never meant to be. ;)

Re: CD artifacts

Reply #34 –
If the user is confusing sound waves with the effects that ringing or aliasing are having on the frequency domain then yes, any kind of explanation about bouncing will be misleading.  I expected to correct that when I explicitly talked about frequency domain.

If we talk on the amplitude domain (i.e.. sound waves), then there's no good explanation to be told.

Re: CD artifacts

Reply #35 –
Ringing/resonance is pretty easy to understand.  A bell rings or a guitar string rings/resonates, or you'll hear the effects of  ringing/resonance if you speak into a drinking glass or into the sound hole of a guitar.   Those lousy-sounding "one note" subwoofers you sometimes hear in cars are resonating/ringing.    I guess you might get ringing if you "play around" with a parametric equalizer.    But of course you can't hear ultrasonic ringing. 

For aliasing I can't think of an analogy and you'd probably have to make an unfiltered down-sampler or build an unfiltered ADC to demonstrate it.     I've never actually heard aliasing so I can only imagine what it might sound like .

Re: CD artifacts

Reply #37 –
The example is a bit cherry picked here as the sound of wind chimes themselves end up sounding like something else entirely, so it's extreme in that regard.  It's very difficult to explain what aliasing is in an analogy but it's effects are quite demonstrable.  There isn't typically much outside of the audible spectrum other than noise and properly designed ADC will filter it all out.