In the last post, I talked about why a THD+N measurement is useless if you don’t know about the type of distortion that you’re measuring. Let’s now talk about another reason why it’s useless in isolation.
Once again, let’s assume that we’re doing a THD+N measurement the old-fahsioned way where we put a sine wave into a device, and apply a notch filter to the output at the same frequency of the sine wave and find the ratio of the level of the sine wave to the output of the notch filter.
This time, instead of taking a signal and distorting it, I’ll do some additive synthesis. In other words, I’ll build a final signal that contains four components (although they’re not entirely independent…):
- a “signal” consisting of a 100 Hz sine wave which we’ll call “the fundamental”
- sine tones at frequencies that are multiples of the fundamental frequency (in other words, they are harmonically related to the fundamental).
- sine tones at frequencies that are not multiples of the fundamental frequency (in other words, they are not harmonically related to the fundamental).
- wide-band noise
Let’s start by listening to the original 100 Hz sine wave with a level of -10 dB FS without any other additional components. If you hear any distortion or noise, then this is a problem in your playback system (unless your system is so good that you can hear the quantisation error caused by the fact that I didn’t dither the signal).
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Now let’s add noise. I’ve added noise with a white spectrum and a level such that a THD+N measurement will tell us that we have 10% THD+N (relative to the level of the 100 Hz sine tone signal). In other words, I have a sine wave with a level of -10 dB FS and I have added white noise with a long-term RMS level of -30 dB FS.
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To play the media you will need to either update your browser to a recent version or update your Flash plugin.
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The punch line!
Each of the six signals I’ve presented above in Versions 2 through 7 (inclusive) is a “distorted” version of the original 100 Hz sine tone in Version 1. Each of those six signals will have a measurable THD+N of 10%. However, it is quite obvious that they have very different spectral patterns, and therefore they sound quite different.
This isn’t really revolutionary – it’s jut another reminder that a THD value, in the absence of any other information, isn’t terribly useful – or at least, it doesn’t tell you much about how the signal sounds.
Andreas Cygnus says:
Brother, you have cleared up the bull and saved me much anxiety. When I read your info, I realized I was worried for nothing. I am very new to sound technology; still, I have a natural aptitude toward technology and some physics, that helps me to catch up fast and comprehend simple logic, as you so clearly explained. I was going through the specs of a “Marantz SR7007” av receiver and the power amplifier’s rated output says:
Front:
125 W + 125 W (8 ohm, 20 Hz – 20 kHz with 0.08 % T.H.D.)
195 W + 195 W (6 ohm, 1 kHz with 10 % T.H.D.)
You see, I would like to mate this receiver with a pair of AT1 speakers from “Atlantic Technology”. The AT1 is a 6 ohm speaker at 200 watt max. So, my kind friend, if you could possibly take a look at the pdf manual/specs of the Marantz SR7007 and then see the spec sheet of the AT1 from Atlantic Technology – I would appreciate your feed back as to weather this would be an amazing or unwise setup.
Receiver link: SR7007
http://us.marantz.com/us/products/pages/ProductDetails.aspx?CatId=avreceivers&ProductId=SR7007
Speaker link: AT1
http://www.atlantictechnology.com/default.asp?NodeId=159
Thank you for your smart research and very useful knowledge.
Chris says:
Good day!
I know this may never be read however, I have a similar question.
Same sr7007 as above….6ohms 10%thd at 1 K H z which would be 1000hz. Based on your article which was working at 100 Hz I don’t know if there’s any difference or if I even have enough information to determine there’s a difference in the amount of distortional get 195 W Now I do I generally trust marantz, sure. But I do like clean sound- Ion the other hand was planning to drive some PSP’s T2’s
geoff says:
Hi,
Your guess that you don’t have enough information to make a decision is correct. Basically, what I hope to have shown in the posting above is that a THD+N measurement is basically useless – easy to measure, but fairly devoid of useful information… Of course, it gives you some arbitrary limit to look at, but without giving a detailed description of the exact characteristics of the type of distortion, a number isn’t enough information.
Cheers
– geoff
Chris says:
Thank you! Guess I’ll just have to email Marantz~ only to satisfy my curiosity lol. You’d think there would have been more responses in this since this as this AVR has been successful for years….and in stereo it does sound good…..Its either a misprint or a negligible “value” to consumers ears. Either way I’m getting the PSBs. Cheers!