How to measure the THD (Distortion) given input

I have few message tones which combination of frequency signal. I want to measure the THD of that signal. How to measure it? I tried added the plugin of “Distortion Analyser” It shows the error of “Selection to short”. I tired with normal 1k sine wave its showing the THD values. But when it comes to combination of frequency with message tones. it shows the above error.

How to measure THD of the combined frequency input sound {WAV} file. ? Any other method or threat please help me out. !

combination of frequency signal.

I don’t think you do. In the hardware world, a distortion analyzer sets a total sound reference point at 100% and then adjusts frequency and phase controls to carefully eliminate one reference tone from the total. It measures what’s left in percent. You have to know what that one reference tone is and it has to be pure.

When you distort a pure tone, it makes other tones. You are measuring those other tones.

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Someone else on the form wanted to measure distortion at the end of a transmission system without knowing anything about the beginning transmission. Not as far as I know.

Let me describe the process. You Set Level until the meter is at 100%. Switch to Distortion. You set that big knob to the approximate pitch of the reference tone and then you rock it back and forth as the needle on the meter gets lower and lower. Then you go back and forth between phase and frequency until the needle is as low as you can make it. Then you throw the Automatic switch and the analyzer does the final adjustment. Read the meter which is frequently bouncing. Google and watch how they do it.

All that and the readings are usually posted as THD+Noise, since any noise is automatically added to the total whether you want it there or not.

I suppose you could use multiple tones with Analyze > Contrast and Effect Notch Filters if you could figure out a way to move notch filter frequencies one at a time and in super tiny amounts. Take the Before and After Contrast values and do the math.

Maybe notch out one tone at a time and use advanced math to figure out the combined percentage values? I don’t know about that.

It might be a career move to get accurate numbers. I haven’t tried to run two notch filters at the same time. Let us know how it goes.


It shows the error of “Selection to short”.

What version of the plug-in are you using?

How long is your selection and what’s the sample rate? Just looking quickly at the code (for version 3) it looks like you need at least 44,000 samples (about 1 second at 44.1kHz).

But when it comes to combination of frequency with message tones. it shows the above error.

If you are trying to measure the actual distortion you won’t get accurate results because with this plug-in anything in addition to the pure tone is considered distortion.

To answer the question directly:

To calculate “Distortion + Noise”, you need to know:

  1. The input signal “i”
  2. The output signal (“o”)
  3. The overall system gain (“g”)

Distortion + Noise = i - (o * g)

There’s a video describing how it is measured here: