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Topic: How DAWs draw Waveforms (Read 6644 times) previous topic - next topic
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How DAWs draw Waveforms

Hey Guys...

I hope I´m doing this in the right place. I have a question and to some of you this question may be idiotic but for me right now it feels somehow important for my understanding of digital technology. My question resolves around the drawing of waveforms in a DAW like SoundForge or iZotope. I don´t know which DAW tells me the truth by drawing this waveform. The reason for my question is that I read on some ridiculous analogue audiophile forum that digital signals are linear and therefore are not able to reproduce higher frequencies and that it gets worse the higher the frequency is. And as proof I´m shown some horrible looking waveforms.

I couldn´t believe my eyes so I tried to recreate it myself. I generated a sine wave at 10 kHz at 24/192 which was resampled to 24/44.1 afterwards. This is what I get:


This is the waveform drawing of the original in SoundForge


This is the downsampled version in SoundForge - looks horrible.


The downsampled version again, only this time in iZotope RX. Looks fine again.

I just want to know which version shows the 'true' signal. If I understood the Nyquist theorem correctly, the waveform will be perfectly reconstructed at the analogue output. But what does SoundForge show me then??
marlene-d.blogspot.com

How DAWs draw Waveforms

Reply #1 –
SoundForge looks like it's connecting the samples with straight lines (in both images) which is clearly not anything like what a real DAC does. iZotope RX appears to be correct, though it is probably assuming perfect reconstruction though is far closer to what a DAC does than what SourceForge is showing.

With perfect reconstruction a single sample should look like be a sinc pulse.  My guess is the person showing you these wave forms (I'm assuming only the SourceForge wave forms) doesn't know what a sinc pulse is.

Anyway, you can always play back one of these files and look at your DAC's output on an oscilloscope.  You might even find something interesting.  A while ago DVDdoug said that his sound card in his computer at work showed more of a stair-step look (like sample and hold, but on the output) indicating that the output of his DAC had no low-pass filter.  Again this is nothing like what SourceForge is showing, though IIRC, GoldWave represents digital signals this way.

How DAWs draw Waveforms

Reply #2 –
Quote from: greynol on
SoundForge looks like it's connecting the samples with straight lines (in both images) which is clearly not anything like what a real DAC does. iZotope RX appears to be correct, though it is probably assuming perfect reconstruction though is far closer to what a DAC does than what SourceForge is showing.

With perfect reconstruction a single sample should look like a sinc pulse.  My guess is the person showing you these wave forms (I'm assuming only the SourceForge wave forms) doesn't know what a sinc pulse is.
Phew, thanks. I was beginning to worry. And I encountered this kind of sine on a website by someone coming from Holland, who clearly was a vinyl fan and deemed everything digital inferior. At first the site was funny to read because he wrote so many interesting things about the history of digital recording (Soundstream, Denon PCM etc. etc.) and then in the end he revealed his true agenda: the reason why digital was inferior - by showing these sines. For the life of me, I can´t remember the adress of the site! Damn!

But thank you! 
marlene-d.blogspot.com

How DAWs draw Waveforms

Reply #3 –
Quote from: Cavaille on
I just want to know which version shows the 'true' signal. If I understood the Nyquist theorem correctly, the waveform will be perfectly reconstructed at the analogue output. But what does SoundForge show me then??


The visual representation is sort of arbitrary, so I wouldn't assume any of them are necessarily the same as what a good scope would show you.  Obviously your last example though is trying much harder, and doing some kind of sinc or spline fitting to the points while the first is just drawing lines between points.

How DAWs draw Waveforms

Reply #5 –
Spline fitting?  Please elaborate.

I don´t have Adobe Audition, can´t help out, sorry.
marlene-d.blogspot.com

How DAWs draw Waveforms

Reply #6 –
Quote from: greynol on
I am pretty certain CoolEdit/Adobe Audition graphically reconstructs based on the response of a sinc pulse.  With a single sample, what would spline fitting look like?

Is this what you want to see?
Ceterum censeo, there should be an "%is_stop_after_current%".

How DAWs draw Waveforms

Reply #7 –
CoolEdit/Adudition shows what the waveform SHOULD look like at the output of a DAC. Decent DACS will produce a waveform that will look very similiar, but only testing will tell you. You might find RightMark interesting for lookin gat your own hardware
http://audio.rightmark.org/index_new.shtml

How DAWs draw Waveforms

Reply #8 –
Quote from: greynol on
With a single sample, what would spline fitting look like?



Splines are just piecewise polynomials, so something like 1-x^2 near the impulse and zero elsewhere.  Looks like thats not the type of fitting being used though.

How DAWs draw Waveforms

Reply #9 –
Quote from: Cavaille on
This is the downsampled version in SoundForge - looks horrible.

The Soundforge display, which just connects the samples with straight lines, isn't representative of what would come out of any DAC. The iZotope and Audition plots emulate the signal you'd get from a DAC with a correctly functioning reconstruction filter (the component that removes the ultrasonic aliasing).

Here's another possible display representation of a 10kHz sine sampled at 44.1kHz (this time from Wave Repair):
[attachment=5797:New_1.png]
This one is representative of the signal that would come out of a NOS DAC without a reconstruction filter. Hopefully no such DAC actually exists.

As saratoga says, the display choice is fairly arbitrary. Programs like Audition and iZotope go to the trouble (and consume the necessary CPU cycles) to show you a nice clean display. Others, like Sound Forge and Wave Repair are lazy and don't bother. Provided you know your tools and what they are showing you, it's no big deal.

How DAWs draw Waveforms

Reply #10 –
Quote from: Cavaille on
I read on some ridiculous analogue audiophile forum that digital signals are linear and therefore are not able to reproduce higher frequencies and that it gets worse the higher the frequency is. And as proof I´m shown some horrible looking waveforms.

I think most DAW programs draw waveforms at 8 bits of resolution, even when the audio files are 16 or 24 bits. For example, Sony Vegas lets you choose between 8 and 16 bit graphics. I think of the waveform drawings as a 1-color "GIF file," and it doesn't have to be as accurate as the audio itself. If it were, the graphic file would be as large as the audio file!

--Ethan
I believe in Truth, Justice, and the Scientific Method

How DAWs draw Waveforms

Reply #11 –
Quote from: Ethan Winer on
I think most DAW programs draw waveforms at 8 bits of resolution, even when the audio files are 16 or 24 bits. For example, Sony Vegas lets you choose between 8 and 16 bit graphics. I think of the waveform drawings as a 1-color "GIF file," and it doesn't have to be as accurate as the audio itself. If it were, the graphic file would be as large as the audio file!

When in the context of graphics "8 and 16 bit" commonly refers to the color depth.  I'm not sure what you are trying to say - but it sounds like you're mixing apples and oranges, as reduced color information would in no way affect the "accuracy" of the drawn waveform, and there is no reason "the graphics file would be as large as the audio file".

Creature of habit.

How DAWs draw Waveforms

Reply #12 –
Quote from: pawelq on
Is this what you want to see?

I wanted to know if what you've shown could have been created using splines, since I've always assumed that Audition was doing it correctly and have never been given a reason to think otherwise.  Thankfully saratoga provided some more insight.

How DAWs draw Waveforms

Reply #13 –
Quote from: cliveb on
Here's another possible display representation of a 10kHz sine sampled at 44.1kHz (this time from Wave Repair):

This one is representative of the signal that would come out of a NOS DAC without a reconstruction filter. Hopefully no such DAC actually exists.

http://www.hydrogenaudio.org/forums/index....st&p=652658

How DAWs draw Waveforms

Reply #14 –
Quote from: Soap on
When in the context of graphics "8 and 16 bit" commonly refers to the color depth.

That's why I specified 1-color and put "GIF file" in quotes. What I imagine is a single value is stored in the file having 8 bits of vertical resolution. Again, I'm not really sure, but that makes sense based on the description in the Sony Vegas help plus the size of the peak files Vegas and Sound Forge create.

--Ethan
I believe in Truth, Justice, and the Scientific Method

How DAWs draw Waveforms

Reply #17 –
In RX, you have the selection of waveform interpolation order in Edit -> Preferences -> Misc.
It can be set to "Hold" (staircase), "Linear" (connect the dots), "Bandlimited" (sinc pulses), or anywhere in between.