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Question relating to Nyquist frequency

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Re: Question relating to Nyquist frequency

Postby Tim Gillett » Mon Nov 11, 2019 11:19 am

Folderol wrote:
Tim Gillett wrote:Nah, bats 200kHz, dolphins only 160 kHz.

Bats rule!
So they were were illegally receiving Droitwich in the days of the domestic radio license.
Who knew? :lol:

Yes definitely tax dodging freeloaders. But only young bats could hear the programmes as it was at their extreme upper frequency limit. The older bats couldn't understand what all the fuss was about...
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Re: Question relating to Nyquist frequency

Postby Hugh Robjohns » Mon Nov 11, 2019 11:57 am

:bouncy:

Cetaceous and chiroptereous amusements aside, it's enormously concerning (to me at least), that well established music artists like Peter Kriek (although he is far from alone in this regard) -- who inherently have significant influence over their followers -- state their gross technical misunderstandings as scientific fact. :frown: And these kinds of nonsense myths always seem to spread far quicker than the truth...
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Re: Question relating to Nyquist frequency

Postby Tim Gillett » Mon Nov 11, 2019 1:24 pm

True but some record companies don't always set a good example on this either with their now up to 192/24 releases. There's a promo for a historic Callas box set which tells us in all seriousness, or at least strongly implies how much better these old recordings now sound in HD compared to poor old CD quality. But listen how the promo only plays excerpts that were originally well captured and so also sounded fine on CD. The originally seriously distorted forte sections we don't get to sample...
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Re: Question relating to Nyquist frequency

Postby merlyn » Wed Nov 13, 2019 7:00 pm

I've found this thread interesting. I'm surprised that aliasing hasn't come up, which is the reason that the input has to be band-limited, or filtered in less technical speak.

Much as making fun of audiophiles is amusing you don't have to be a bat to hear filter artefacts in the passband. One legitimate reason for a higher sampling rate is nothing to do with our perception of ultrasonic frequencies. It's to make the design of the anti-aliasing filter easier, cheaper and with less likelihood of it affecting the passband.

Folderol wrote:For example, I have a KA6 here that performs very well, however hit it with a really clean square wave from a signal generator and the ringing is quite obvious..

This interested me. Did you hear ringing or did you see it on a scope? I fed my soundcard a square wave and got this :

Image
I'm not sure I heard it as I don't have an easy way of listening to it without going through the computer at the moment.
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Re: Question relating to Nyquist frequency

Postby Hugh Robjohns » Thu Nov 14, 2019 12:12 am

merlyn wrote:One legitimate reason for a higher sampling rate is nothing to do with our perception of ultrasonic frequencies. It's to make the design of the anti-aliasing filter easier, cheaper and with less likelihood of it affecting the passband.

Fair point -- although the benefit of double sample rates is not so much about making the filter design easier as accommodating the inherent flaws in standard filter designs.

It's certainly true that typical 'half-band' filters operating at base sample rates have the potential to introduce aliasing, although these unwanted artefacts usually only become apparent when recording very high levels with very strong harmonics or HF content.... Which is not that common a situation, to be fair.

Did you hear ringing or did you see it on a scope?

I believe he was talking about seeing it on a scope. There really isn't anything to hear. As explained earlier, it is a natural and benign side-effect of some very high order and inherently inaudible harmonics being removed.

But it's worth noting that this is a different kind of 'ringing' to the often memtioned pre- and post-ringing associated with the linear phase filter's impulse response.
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