Keep getting weird tones while recording

How can this be only in playback? I saved the wav rIght from my computer? I am lost here...

One other hypothesis..

Older sound cards had lower sampling rate - 48kHz for example, as opposed to 192kHz available on todays high-end cards.

I suspect (but am no expert on sound cards) that the roll-off frequency (-3db) of input filters to the card is set to 1/2 the sampling frequency.. This would mean that the cards with higher sampling frequencies will probably allow more "RF" through..

The quantity of higher frequencies which pass through the filter will be a function of that filters response - the steepness of its curve after the cut-off frequency, and whether this curve has any areas where attenuation is poor - but if we say that the roll-off is 24db per octave, it can be seen that a filter tuned to 24kHz will reject 250kHz to a  greater degree than a filter tuned to (say) 92kHz.

I fully accept that the above is a simplistic "analysis".. But I wonder, nonetheless, whether one might get better performance with a theremin when using a older high-end soundcard sampling at 48kHz, than from a soundcard sampling at 192kHz.

If the input filters are fixed to relate to the maximum sample rate, then reducing the sample rate on a modern high-end card will not solve the problem.

I may, of course, be talking complete BS - It may be that the input filters to all soundcards are set to roll-off a little above 20kHz.. This would seem sensible to me anyway, as nothing above 20kHz is useful.

Fred.

My card is 44khz

It is me again,

I apologize if I am silly.

Amey Test Wave 6.3 meg

I need someone to tell me at what point the noise becomes obvious using Audacity so I can find the flaw in my method of audio trouble shooting.

Ignore the skipping. )-'

Amethyst, IMO everyone here is giving you very good advice.  I couldn't hear anything wrong in your recording, but I've got a cold at the moment and the washer is running in the other room, so not the best for listening.  I assume people are hearing aliasing (faint tones that often run counter to the main tones i.e. rising when the main tone is falling).  (Also, I had to remove the '#' from the URL you posted to get to the sample.)

Modern A/D D/A converters are oversampled and can probably tolerate ultrasonic inputs easier, particularly when you record at a high sampling rate.  (But if you convert this to a lower sampling rate you can still get aliasing if you don't filter in the digital domain before doing the downsampling.)  Best to kill the ultrasonics in the analog domain (where they are still linear) before they reach the soundcard and alias.  The ground isolator RS Theremin pointed to is a transformer, which would likely help due to the low pass nature of transformers.  Better would be a 2 or 4 pole LCR (or similar) low pass filter at the Theremin output.

"My card is 44khz" - Amey.

ok ;-)

Perhaps the input filtering wasnt expecting to have large quantities of RF to deal with! - Lets be honest, most audio equipment doesnt put loads of RF rubbish down the cable or radiate a spectral mush! ... In fact, digital equipment designers generally take great care to ensure that the output filters on their equipment get rid of any HF artifacts, and there is a whole huge sub-industry in providing specific components to get rid of HF from digital audio.. In this respect, theremin designers really have no excuse - there are off-the-shelf drop-in components available to take care of the problem.

But anyway..

Proximity, Earthing, Screening - These are, in my view, the next things you could try..

An isolation transformer as Christopher suggested could also help - IF you have taken care of all the other issues - These can work because the transformer can act as an in-line filter.. IMO its not a reliable solution for every case, because capacitive coupling between the windings could pass RF easily.. but its probably worth a try if all else fails - If the mush is actually coming down the signal line, it might be the one thing which works!

The problem is that we cannot reach over the internet with a soldering iron and bag of bits!  - This is one of the kind of problems where hands-on messing about gets things done far quicker than any speculating - Checking grounding etc, and a few resistors, capacitors and inductors soldered in line in different configurations would establish the source of the problem and would likely lead to a simple solution!

Fred.

" I assume people are hearing aliasing " - Dewster

Just for the record, I am not able to access Soundcloud - so I havent heard the sample. I notice that Thierry was the first to give advice here, and presume this was based on listening to the sample - and I therefore trust his judgement with regard to the source of the problems.

Fred.

Fred please listen to the sample... If you can. Maybe you will hear something different?

Omg! I just listened to the sample from my iPad and I am not hearing the tones that I did hear on my laptop... WTH???

RS You might be onto something!!

ps: I did order a hum filter, would be a good thing to have anyways for recording vocals too... So! I'll let you know when I get it! Thank you :)

"How can this be only in playback? I saved the wav rIght from my computer? I am lost here..." - Amey

It all depends on how you are monitoring..

If you are monitoring the signal before it has been digitised, you wont hear the problem until you play back the recording..

If you are monitoring the signal from the "playback" D/A (which some cards allow you to do through the "advanced" options) then the digitised input (A/D) is routed to the output (playback) D/A, and you will hear the problem as it occurs.

Alas, I dont think the above monitoring method is common (may be wrong - but it wasnt common when I had problems) - One can find some apps which allow routing of signals, but I only messed about with these many years ago before I had my Hercules 16/12, which allows one to route whatever one wants wherever one wants, so I cannot advise about whats available now (I think I was using Windows 95 or 98 at the time, I got my Hercules when XP wouldnt talk to my previous Guillimot sound interface).

Fred.