components in the audible range ?
With headphones can two supersonic tones (say 30k and 33K) at high levels create IMD with products in the audible range ?
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The easiest way to find out is to actually try it with various headphones. In a quick test with random cheap headphones, I could only hear a 3 kHz IMD tone at higher than normal volume setting (and with a 0 dBFS ultrasonic signal that is unlikely to occur in music).
Which is pretty much what I got with Monty's test http://people.xiph.org/~xiphmont/demo/neil-young.html#toc_1ch but where did this spuriae come from?
Was it from the digital source (two different PCs with different sound chips IDT and Realtek), DAC sections, the amp sections or the headphones.
All 3 of my "good" headphones (DT880, HD589, AD700) seem to produce the same effect to about the same degree and the effect is the same on two different DAC/headamp units with two different PCs as sources, maybe I'll swap in the M^3 amp and see if that makes a difference but I doubt it.
EDIT: Nope swapping in the M^3 made no difference.
Edited by nick_charles - 12/6/13 at 2:31pm
I got similar results to you. Two headphones, one of which is also a DT880. Using a 192 capable soundcard though nothing special.
Reducing the level of the two tones in Audacity by 3 db dropped the audible IM result lots more than 3 db. I would estimate maybe 15 db or a bit more. Another 3 db drop killed it even more than that. At which point it was on the edge of even being heard with volume well up.
An additional 3 db drop (now 9 db below the original signal which 3 db from max anyway) made it disappear. So that would indicate likely overload distortion somewhere in the chain. Just guessing would think it in the analog portion. Have a better rig I will try it on later, let you know what happens.
Anything with nonlinear distortion will produce IMD.
Any nonlinear signal processing, the DAC, the amp, the headphones .. all will produce harmonic and intermodulation distortion. Since audio stuff is optimized for a ~20 kHz range you will usually see quite an increase in distortion above that range.
The problem with IMD is that a completely inaudible 30 and 33 kHz tone will produce a (theoretically audible) 3 kHz intermodulation product.
Edited by xnor - 12/6/13 at 5:31pm
That could be tested with some measurements on the DAC and amplifier outputs.
Indeed, a signal with very high ultrasonic content and silence in the audio band is rather unnatural.
Right, I took the analog out from the DAC and recorded it using the line-in on my PC set to 24/96 and using Auadacity. I tested the levels using music and then recorded the 30K/33K combination
a very different result. The audible component was a very low level hiss/whistle, the spectrum shows quite a lot of spreading, whether this is from the DAc or ADC I do not know - exporting the results to Excel I get
So the 3k component is there but is much less audible after going straight from the DAC and into an ADC and even cranking it up it is now barely audible. Not sure quite how to interpret this ?
It is probably from the relatively short Hann window used for creating the plots. It can be fixed by changing the window type to Blackman-Harris or Gaussian (a=4.5), which have much lower sidelobe levels. The second graph shows a lot of low level peaks, though, those could be some kind of DAC or ADC problem.
Edited by stv014 - 12/9/13 at 2:38am
I am using the onboard soundcard (IDT) on my HP desktop with the annoying "never get rid of it" Beats Audio. I cannot vouch for the quality of the ADC. The DAC is a 2009 Zero so nothing special
I'm hesitant to invest in another soundcard just to unequivocably find out how awful my DAC is, but i suppose I could stretch to something like the Asus Xonar DG
Edited by nick_charles - 12/9/13 at 10:50am