Total Harmonic Distortion?

R

Ryan Teasnob

Member
Looking at stats for the RODE NT-USB+, it says "Dynamic Range @10% THD - 97dB." It also says "Input SPL @10% THD - 118.0dB (as per IEC 60268)."

What does that mean? I know THD is "total harmonic distortion." And in general that's not a good thing. But I don't really understand what "Dynamic Range @10% THD - 97dB" and " Input SPL @10% THD - 118.0dB (as per IEC 60268)" is trying to tell me.

Can someone translate this for me?

Thanks!

Ryan
 
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It's a sort of combination of tech specs and marketing.

Harmonic distortion is a bit complex - but is a comparison of the fundamental, against the harmonoc components that get unintentionally generated. If you have say, a tone - one frequency of 100Hz - then the circuit design will introduce harmonics, especially on the peaks when the circuit voltages approach their maximums in each section - and they accumulate. We have the dynamic range the system can generate - i.e. the range between being gone at the quietest, and maximum output on the loudest. On one hand we have the maximum range possible - in your example, 97dB from max to min - but at the max, then the 100Hz tone will have some 200, 400, 800, 1600 and so on - components superimposed on it, and this extra stuff is distortion. This distortion unlike guitar type distortion - which is similar, is unwanted and we hear it. Often caused by flat-topping where the signal hits max and just cannot go any higher.

Total Harmonic Distortion is the ratio of 'good' vs 'bad' components. 97dB with 10%THD could have been 95dB with 5%, or 90 with 0dB! It makes it very difficult to really say is good - because if it was something like a piano, or a voice, you'd hear distortion easier than if it was a rock guitar?

The best you can do is compare the spec with similar other items and look at the differences. 10% is pretty high.
 
Master of the understatement is Rob "harmonic distortion is a BIT complex"! His explanation his good however and Total Harmonic Distortion is used as a fairly blunt tool to describe the performance of any audio component or system. A loudspeaker (monitor) for example might have an SPL rating of "110dB at 1 mtr at 2% THD" (good for a nearfield monitor. Not much cop for a quality PA speaker)

As Rob says "10% THD at 97dB SPL" is pretty ropey. Capacitor mics usually have their SPL specified at the 1% THD point, and a figure over 100dB is common, but that is a USB mic so the converter might not be too good. Probably 16 bit anyway and a DR of 90dB is about all you are going to get.

However the raw THD figure can be misleading. There are two basic ways that electronic circuits 'distort'. AIs, mixers and just about all 'quality' audio circuits have very low distortion up to a maximum output, say +20dBu. Then at +21dBu it shoots up and gets very nasty indeed. A few devices, mostly using valves, have a THD characteristic that is proportional to output voltage. Thus a simple triode pre might give you 0.1% at 1V out (~+4dBu) but 0.2% at 2V and so on. This 'progressive' distortion is probably why people like valve kit?
The reason for this proportional distortion is because such circuits do not use Negative FeedBack. Valve, transistor or op amp. Don't matter, as soon as you use NFB you get less THD but that "woah ***k!" characteristic at the limit.


One very well known guitar amp, the Vox AC30 uses no NFB and has a characteristic sound because of it. Most of the big Fenders DO use feedback and are noted to be "clean and chimey" !

But, as I said, "THD" is a blunt tool and it is the TYPE of harmonics and their relative levels that denote a "tone". A very hotly debated subject enmmired in considerable BS.

Dave.
 
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