IPB

Welcome Guest ( Log In | Register )

What's the most commonly accepted definition of THD?, This is something that's bugged me for ages
uart
post Dec 21 2006, 16:46
Post #1





Group: Members
Posts: 757
Joined: 23-November 04
Member No.: 18295



Hi, something that's bothered me for a long time is that some authors define THD in terms of a power ratio (power of the higher harmonics divided by the power of the fundemental) and other authors define THD in terms of an amplitude ratio (square root of the above definition).

Of course if you express it in dB then it makes no difference, but if you express it as a percentage then it's makes a huge difference depending on what definition is used? So what's considered the most correct definition when expressing as a percentage, is it a power ratio (as in "sum of V_n^2 divide V_1^2") or is it the square root of this?

BTW, by comparison of the dB and the percentage figures reported by RMAA (rightmark audio analyser) I'm nearly certain that RMAA uses the "amplitude ratio" definition rather than the "power ratio" definition. Is this pretty much universal for audio related THD% figures or are there people using power-ratio for THD% figured as well?
Go to the top of the page
+Quote Post
 
Start new topic
Replies
uart
post Jan 3 2007, 02:55
Post #2





Group: Members
Posts: 757
Joined: 23-November 04
Member No.: 18295



QUOTE
Due purely to history, it's either expressed in dB, just like SNR usually is, or it's expressed as the amplitude ratio, which can be calculated via 10^(db/20) just like you'd expect.

If it's percent, you multiply the amplitude ratio by 100, to make it percentage.

So, 1% -> 40dB, and so on.


Look I've got no problems with that, it's how I've always calcuated THD. But I do have a problem with people referring to it as a power ratio (and not including the square-root in the formula) when, as you've pointed out, it (the sqrt) clearly is required to give correct perentage figures.

So the Wikipedia definition is wrong as are numerous other texts and sources -YOU NEED THE SQUARE ROOT SIGN IN THE DEFINITION OTHERWISE 1% IS NOT 40dB IT'S ONLY 20dB. CAN'T ANYBODY BUT MYSELF SEE THAT!!!!!


Whoops my capslock seems stuck. tongue.gif
Go to the top of the page
+Quote Post

Posts in this topic
- uart   What's the most commonly accepted definition of THD?   Dec 21 2006, 16:46
- - uart   For a good example of the conflicting definitions ...   Dec 21 2006, 16:59
|- - caligae   QUOTE (uart @ Dec 21 2006, 16:59) For a g...   Dec 23 2006, 14:49
|- - uart   QUOTE (caligae @ Dec 23 2006, 05:49) Appa...   Dec 24 2006, 13:51
- - uart   No takers? OK, here's a simple question. Say...   Dec 23 2006, 14:12
- - Woodinville   The question here is definition. THD in % is repo...   Dec 26 2006, 01:03
- - woody_woodward   It's been rather awhile since I had any engine...   Dec 26 2006, 04:30
|- - uart   QUOTE (woody_woodward @ Dec 25 2006, 19:3...   Dec 29 2006, 12:09
|- - Gigapod   QUOTE (uart @ Dec 29 2006, 12:09) ... Any...   Dec 29 2006, 13:12
|- - Kees de Visser   Probably the most widely used and respected audio ...   Dec 29 2006, 15:31
|- - Gigapod   QUOTE (Kees de Visser @ Dec 29 2006, 15:3...   Dec 29 2006, 15:51
- - Mercurio   I don't know how it is used in the audio field...   Dec 31 2006, 10:12
|- - uart   QUOTE (Mercurio @ Dec 31 2006, 01:12) I d...   Jan 1 2007, 12:13
|- - Mercurio   QUOTE (uart @ Jan 1 2007, 03:13) Can you ...   Jan 1 2007, 16:08
- - musicinmybrain   QUOTE (Mercurio @ Jan 1 2007, 11:08) QUOT...   Jan 1 2007, 17:43
|- - Mercurio   QUOTE A "resistive load" is a linear sys...   Jan 1 2007, 20:40
- - Mercurio   Ok, I think I have reached something. All detaile...   Jan 1 2007, 21:49
- - musicinmybrain   QUOTE (Mercurio @ Jan 1 2007, 15:49) What...   Jan 1 2007, 23:50
|- - Mercurio   QUOTE (musicinmybrain @ Jan 1 2007, 14:50...   Jan 2 2007, 09:01
- - uart   QUOTE btw, when you talk about "resistive loa...   Jan 2 2007, 14:45
|- - Woodinville   Goodness. We're still having this debate? ThD...   Jan 2 2007, 22:41
- - uart   QUOTE Due purely to history, it's either expre...   Jan 3 2007, 02:55
|- - Mercurio   QUOTE (uart @ Jan 2 2007, 17:55) Look I...   Jan 3 2007, 09:59
|- - Woodinville   QUOTE (uart @ Jan 2 2007, 17:55) So the W...   Jan 3 2007, 22:08
|- - Gigapod   QUOTE (Woodinville @ Jan 3 2007, 22:08) Q...   Jan 3 2007, 23:00
- - pdq   I thought of this thread recently when I came acro...   Jan 8 2007, 18:55
- - Serge Smirnoff   According to physical meaning THD should compare e...   Oct 7 2011, 16:02
- - Woodinville   QUOTE (Serge Smirnoff @ Oct 7 2011, 08:02...   Oct 7 2011, 22:09


Reply to this topicStart new topic
1 User(s) are reading this topic (1 Guests and 0 Anonymous Users)
0 Members:

 



RSS Lo-Fi Version Time is now: 24th April 2014 - 02:41