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17th June 2014, 05:22 PM  #11 
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Join Date: Feb 2013
Location: Scot[ch]land

Thought so, I understand the recording side more than the playback regarding folding etc.
Alot of lectures on the web don't actually state recording or playback, it's confusing at first and hard to tell which side is which. But regarding hash on the playback side, are there any examples of the nature of it? If there's something beyond 22.05, i'm sure any fairly decent amp can manage. Was it perhaps more to related to the effect on passive components and tweeter combinations? It would be interesting to test one of these NOS CD players for this "ultrasonic content", that they were keen on filtering out.
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20th June 2014, 10:36 AM  #12 
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Something that really remians unclear, probably a really stupid question but hey ho,
Take a 44.1k CD, is each stereo channel comprised of information up to 44.1khz?
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20th June 2014, 11:01 AM  #13 
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44.1kHz is the rate at which the incoming audio is sampled. 16bits (the other part of it all) sets the dynamic range.
Draw a 1kHz sinewave on a piece of paper and put the timescales in for 1 cycle. Now divide that x axis time scale equally so that there are 44100 points. draw a line up from each point and where it intersects the audio is where a sample (which corresponds to the level) is taken. Now try it for a 22.05kHz sinewave. Not many samples are there yet that sinewave can be reconstructed correctly from that data. So each channel could in theory have info up to 22.05kHz (the magic one half of the sampling frequency). In practice it will be limited to a little lower than that. The 16 bit part means there are 2 to the power 16 individual levels available (65536).So if you want 2 volts as a max level, then 2 divided by 65536 means each discrete level is "0.03millivolts". 65536 times 0.03 millivolts is 2.
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20th June 2014, 11:23 AM  #14 
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So I'm confusing rate with frequency and there definately isn't any frequecy content beyond 22.0 on a CD and if there is it's not useful.
That's plot explanation really did the trick, thanks.
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20th June 2014, 11:41 AM  #15 
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There is nothing on a CD above around 20kHz (22.05 theoretical limit).
I've just found this that might help explain it better. CD  The Inside Story  Part 8  Oversampling and Filtering
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20th June 2014, 12:06 PM  #16  
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Quote:
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20th June 2014, 02:32 PM  #17 
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Thanks for that link moo,
I was keen on gettin a NOS up and running without the analog filter after the TDA but, clearly this woul allow whatever this is in the below picture to get through What exactly are the additional four bars (I note they repeat infinitely)...
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20th June 2014, 06:32 PM  #18 
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The "four bars" represent the noise component from the DAC if the "music" were white noise. Everything from almost DC up to 22.05kHz. OK, that's a paradox, lets say white noise sharply cut off at 22.05kHz
Because the sampling is like a 44.1kHz squarewave, the harmonics of that extend outwards, so 44.1, 88.2 and so on to infinity. In practice the level does fall away pretty quickly the higher up you go because of the limitations of the circuitry in passing such signals. When a signal is sampled, you get an upper and lower sideband centred around the 44.1kHz sampling frequency. Its very important that that lower sideband doesn't "overlap" into the wanted audio below. I did some more searching and turned this up which looks pretty comprehensive. You'll see its quite a vast subject with a lot of number crunching but some parts of it might help you understand the basics The Sampling Theorem and the next bit, http://www.dspguide.com/ch3/3.htm
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20th June 2014, 06:55 PM  #19 
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Indeed. A very bad idea. The low pass filter is an essential part of the DAC.

21st June 2014, 09:40 AM  #20 
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I'm familiar with nyquist and sampling theory, it's just that alot of lectures don't really go in depth explaining "sample pulses" beyond 44 that much, where they are derived from, or why these pulses fold infinitely. I was after a better explanation on pulses... I got the impression they only repeated in os applications, but revisiting the lavry paper shows they exist even in nos. (though it is not clear seeing as it is the paper on "oversampling" as it implies in the title). So really, just to confirm they definately are present even in nos applications.
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Last edited by giro1991; 21st June 2014 at 09:50 AM. 
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