Keep getting weird tones while recording

Amey,

I know its a pain - but until I get my new (old) PC and install Windows 8 on that, I cannot access Sound Cloud. I am not willing to risk installing 8 over my vital XP PC, just have too many vital programs that I cannot risk.

If you want me to listen to your sample, you will need to host it somewhere sensible, LOL ;-) - or email it to me.

Fred.

"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???" - Amey

If its an aliasing problem, then the recording should (will) have the aliasing.. There is no way that RF can be recorded and interfere with the playback D/A..

It works like this.. Signal with HF goes into A/D, A/D produces unwanted audio frequencies as a result of aliasing, and this unwanted audio IS RECORDED!

Playback reads the samples of whats been recorded, and the D/A converts this to analogue. The whole process is synchronous, and the playback side CAN ONLY reproduce what has been recorded - There can be no "embedded" RF in the samples, there can only be what was sampled - So if you are hearing aliasing in the playback, this aliasing WAS RECORDED!

So I have absolutely no idea whats going on - If it was just you who had heard it, I would suggest a problem with your laptops audio output.

Fred.

ps - does your laptop play back CD's MP3's etc ok? - might be worth playing other theremin tracks through it and see how they sound..

I hear them on my first generation iPad, and on my iMac.

Which theremin, Amé? I mostly record direct rather than miking the amp, and haven't had this issue.

Gordon, I am trying to record my Subscope. :(

i hope I didn't knock anything inside while I traveled for the show in NYC.

and like you, I'd rather record direct than with an amp. We add so many effects on, that we rather start with a very clean recording before hand...

Hi Gordon,

Are you the person who has "supersonic hearing" that RS was talking about?

;-)

No, I generally hear things at the speed of sound, not faster.

THank you all for the advice!

I am in the pocess to finish Silent Night on the theremin. I 'll let you guys know How it comes out :)

Ok - I have a cold at the moment and am really bunged up - But thanks to Christopher providing a working link, I have been able to listen to, and more importantly view the file.

I do not hear a typical aliasing tone - but something sounds wrong.. Doing a FFT on a few single cycles of the waveform (using zero-crossing to isolate these) I do not see any ghosts - what I get from the analysis of isolated waveforms is:

First column is the frequency devided by the fundamental, second is the frequency (this sample had 277Hz Fundamental), third is the amplitude (db).

One can see that the detected peaks (with possible exception of the 2237 Hz signal - but this is at such a low level I doubt that it could be the problem) all correspond to integer harmonics.

It is quite a different matter however if I take a FFT "snapshot" of a few samples - there is all sorts of anomaly - now this does occur naturally (due to vibrato and the fact that theremin pitch is NEVER constant - even over two samples) - But what I see in this waveform looks like some AF intermodulation - a sort of FM.

I have tried to do a FM demodulation on the signal, but had no luck so far.. I think I may need to import the waveform into my simulator and construct a circuit to decode it.

So -

1.) My hearing is not even covering the normal 16kHz bandwidth at present, so my ears cannot verify anything - the "problem" I hear may well not exist - it could be rattling mucus!

2.) As far as I can see there is no aliasing.

3.) I do see some unusual features when I do FFT on a "real" sample of about 100ms - and I think there may be some form of modulation occurring.

Fred.

ps - The fact that you dont hear the problem until it is played back, is a bit of a 'fly' in my evaluation - Aliasing would explain this - and it may still be aliasing - I may be doing something wrong when I look at individual waveforms..  but perhaps there is some other problem with your sound card..

To be honest, I do not trust my evaluation at all - It doesnt make sense, and is not "solid" to me. Perhaps the aliasing moves the problem behind legitimate harmonics at some frequencies, and I have just picked those frequencies to analyse (Yes, this certainly can happen! ;-)

"I have a cold at the moment and am really bunged up..."  - FredM

Hey, I had dibs on the cold!  OK, this is getting weird!

Amethyste, I just did an external S/PDIF loop with my soundcard and captured your recording in Adobe Audition.  The noise floor is around -66dB, peak around -17dB, which gives 50dB dynamic range (not super good, equivalent to ~8 bits).  In the noise floor after the end of your performance I hear a ~5kHz whine and some whiter noise, kind of like an AM radio set between stations.  All through your performance I hear a gritty intermodulation sound that seems correlated to how loudly you are playing the Theremin. 

So, I'd say get an output filter of some type. 

You might also try recording at a higher sampling rate, and then down converting (which, if done correctly by your recording software, would apply a sharp low pass digital filter right at the edge of audibility). 

What level are you recording at?  If low, boosting it somewhat might help SNR here too.

Thank you all so much for your input!

I am recording Mono ~16 bits ~ 44.1kHz on my soundcard and the same levels in my software. I know my software is outdated too (Cool Edit Pro)

Output filter? Like a ground loop eliminator? I am so sorry, I am totally clueless when it comes to this kind of thing, I am not the brightest!