Seeing is believeing? Another trick of Pseudo Science: Demo

Seeing is believeing? Another trick of Pseudo Science: Demo


TLDR:

Analog or Digital? Magic?

Source (at 4:33 D/A and A/D | Digital Show and Tell (Monty Montgomery @ xiph.org) (youtube.com) )

How often you see a "perfect" and "smooth" sine wave on an oscilloscope (as shown above) when someone was attempting to "prove" that CD-quality music can reconstruct the analog output like the original. 

To quote the exact phrase, "the analog ouput is still a perfect sine wave exactly like the original" (at 5:04 D/A and A/D | Digital Show and Tell (Monty Montgomery @ xiph.org) - YouTube)

Someone may said, "Yes, that's a perfect sine wave. Everyone can see a smooth sine wave is reconstructed from CD-quality digital input. Don't you see it? I can use an oscilloscope to reproduce the same thing myself to verify it."

Hmm.. that's the art of Pseudo Science. They attempt to show "factual evidence" in front of you and tell you that you could do the same test / experiment yourself to verify their saying.

The Magic Part

They won't tell you that they are using an analog or a digital oscilloscope with limited resolution. Most of these oscilloscope are analog.

For the above one shown in the "Monty's video", he used an analog oscilloscope (link) to show the "smooth" sine wave output. 

I am not sure if he selected analog oscilloscope intentionally or it was because digital oscilloscope was not that popular in 2012. 

Due to the architecture/design of analog oscilloscope, it has a tendency / property to smooth out the displayed waveform.

Based on Copilot, it said digital oscilloscopes were popular in 2012. Only Monty would know why he selected an analog oscilloscope when he took the video.

During my discussion in Head-Fi about Hi-Res music (in 2024), a fellow member did the similar test / experiment and showed the following  "perfect" sine wave using his analog oscilloscope (with digital storage):

Its manual can be found here. It is an analog oscilloscope with digital storage (8-bit). Is he tricked by the "Monty's video"?

Analog oscilloscope is a great device. It helps you to see what's happening but it is never a high-precision / high-accuracy measurement instrument. High precision / accuracy multi-meter would do a better job (but it cannot let you see the waveform) in terms of measurement. I think it should be highlighted in basic measurement course like electronic/electrical measurement 101 at college. No?

Digital oscilloscope is better in terms of precision / accuracy for measurements; however, it suffers from the resolution issue. To my understanding, they started with 8-bit then moved on to 10-bit. 12-bits are getting popular these days (of course, a higher bit ones would always be available for very special / professional usage).

For example, Rigol just released (in 2023) a new high resolution series of their digital oscilloscope.



The Truth

Let's have a look of the following waveform (source: How to Measure Oscilloscope ADC Dynamic Range (youtube.com)) using a 12-bit resolution digital oscilloscope (Rigol HDO4204)



Is it a "perfect", "smooth" trianglar wave? Well... looks like it is but... let's zoom in...

Here is what it looks like 


Is it a smooth line? 

You cannot see this with analog oscilloscope easily.

It is up to you to insist the above trigular waveform is a "perfect" and "smooth" waveform. But in my eyes, they are not. Period.

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