#512112 - 04/13/04 10:18 AM
96k problem (?)
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flaky
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Registered: 04/13/04
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Having finally managed to get a computer that enables me to run my 896 at 96k I thhough I'd have a look at the noise of my desk using a freq. analysis program. I found quite an enhanced peak at abou 33K. I've eliminated all I can think of - removed desk, shorted input to MOTU,switched off my monitor whilst recording etc perhaps its even my frq.analysis program - is there a reliable downloadable one? perhaps it's a very resonant anti-aliasing filter?? perhaps its's not a problem as obviously I can't hear it(!) but still thought I'd ask some opinions.
many thanks
(also e0mailing motu obviously)
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#512113 - 04/13/04 11:31 AM
Re: 96k problem (?)
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Bill@Welcome Home Studios
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Registered: 08/23/03
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What, exactly, is the problem? (And while we're asking, what is the desk?)
Are you hearing something disturbimg at 96k that you did not hear at 48 or 44.1?
Very oftewn upgrading a piece of a system will reveal holes in other parts of the system, but I would not expect a bump in that range to be much of an issue.
If it is in the desk and it is a quality desk, then perhaps there is an errant part... or perhaps the designer did this on purpose. Without knowing more, we'd all just be guessing.
Bill
_________________________
"...it's easier than hitting the kids, and almost as much fun..."
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#512114 - 04/13/04 12:14 PM
Re: 96k problem (?)
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flaky
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Registered: 04/13/04
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Thanks for replying Bill
[What, exactly, is the problem? (And while we're asking, what is the desk?)
Are you hearing something disturbimg at 96k that you did not hear at 48 or 44.1?]
The desk has been eliminated from the problem - sorry if I was unclear - it was just curiosity that led me to 'look' at it's outputs. I've basically got the 896 connected to a G4 and I've grounded the XLR pins to get the nearest I could to no-input. I expected to get noise from the analogue stage of the input all of the way across the spectrum but found a noticeable peak at about 33K. I am not hearing anything wrong so mauybe that suggests I shouldn't worry about it ? But if we're recording at 96k - implying a bandwidth of circa 40K then we're accepting that what happens above 20K is an issue? I was just really trying to find out if this sort of thing was common, and also trying to avoid any problems that might occur further down the line, for example, with some DSP, or SR conversion for CD production.
I've spent many hours watching musicians edit timing that they argued was out on a waveform display, even though they couldn't hear anything wrong! and was a bit concerned that I was falling into a similar trap with recording. It's probably just taking time to aquire confidence with a new system etc
Thanks again for replying
flaky
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#512115 - 04/13/04 12:35 PM
Re: 96k problem (?)
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Nika
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Registered: 03/22/01
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The increased noise is likely the results of the delta sigma modulator's noise shaping in the converter chip. All 1 bit or multi-bit based converters use noiseshaping to decrease the noise in the audible range and push it up into the non-audible range. The noisefloor in these converters rises sharply above 22KHz such that up around 70KHz or 100KHz the noise exists at a level of something like -12dBFS. Since you are only working at 96KS/s the anti-aliasing filter is rolling off everything above, say, 40KHz, which makes the noise look like it has a bump below 40KHz. If you went to 192KS/s sample rates you'd see this noise floor get very loud up to 70KHz or so.
Nika.
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#512116 - 04/13/04 12:36 PM
Re: 96k problem (?)
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Nika
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Oops, I forgot. To answer Bill Park's question, the "problem" with this has to do with this material bleeding into the audible range through certain types of processing.
Nika.
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#512117 - 04/13/04 04:01 PM
Re: 96k problem (?)
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flaky
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Registered: 04/13/04
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is rolling off everything above, say, 40KHz, which makes the noise look like it has a bump below 40KHz. If you went to 192KS/s sample rates
Thanks Nika
Ity's more of a peak than a bump! - infact if I normalize the file and play it back a few octaves lower on a sampler it's audible as such but not really when there's any programme.
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