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  #131  
Old 02-06-2002, 11:22 AM
F Umminger F Umminger is offline
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Default Re: Protools 192k

Doug-

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:<HR>
I realise that a 15k square wave is not a "real-world" signal, but it's still a little scary to see this amount of aliasing happening at 44.1kHz sampling. Some real-world signals must be doing this to the convertors - a piano for example.
<HR></BLOCKQUOTE>

The square wave created by the signal generator is not going through the input converters. If you sent an analog square wave in through the converters (say from a synth or a function generator) then the converters would filter out the harmonics above 22.05k and you would not hear this aliasing. The old signal generator was instead doing the equivalent of sampling without filtering out the high frequencies, which is why it generates aliasing noise.

The latest version of the signal generator that is now shipping with HD uses a new algorithm that does perform some filtering and it therefore has much lower aliasing noise. The aliasing noise is still significantly higher than you would get sending an analog square wave through the converters, though.

Please keep in mind that the signal generator plugin was really only meant for very basic testing purposes (is audio getting from point A to point B?), and as a pitch reference. No real effort was made to generate high-quality alias-free square, saw, and triangle waves. This has unfortunately caused some confusion.
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  #132  
Old 02-06-2002, 12:22 PM
Doug Ring Doug Ring is offline
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Default Re: Protools 192k

John - it's a /discussion/, nothing more. we don't expect to arrive at hard and fast conclusions here, we just enjoy the chat and the speculation. If you don't find any of it relevant, that's okay. Imagine you were in a room with a load of car nuts talking about carburettors and compression ratios. You'd still be able to get into your car and drive away afterwards, whether you understood a word or not, right? Well that's all we're doing here - talking about what's under the hood!

Nika, I don't know if you and I agree 100%, because you seem to see no benefit in 192 whereas I do. That's okay though. I'm enjoying the discussion.

Frederick: you make some good points which I should have realised myself. Obviously the input convertors aren't involved with the Sig Gen, and there will also be some gentle filtering on those convertors. I've never had the chance to use a steep LPF when recording piano into a digital system, though, and I was wondering if its high overtones being aliased could account for some of the less-natural sound of recorded pianos in general.

So if I understand correctly, the square wave from the Sig Gen is the only thing within the digital chain inside Pro Tools which is capable of causing aliasing because even with a low fundamental frequency signal, the harmonics are way beyond the D to A convertor's limit, whereas everything else has been filtered on the way in? Out of curiosity, how high does the Sig Gen go?
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