Total Harmonic Distortion measurements

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Application note # 8
Sample Champion

 

NOTE: since Sample Champion Version 3.8, it is possible to measure SNR, THD, THD+N and IMD in RTA Window
(more information here).

So, the Audio Quality Plugin is no more
required and supported (but still available)

 

Measurement of SNR, THD, THD+N and IMD


First of all, select appropriate measurement settings in the "Settings" window. For example use following ones:

- Sampling Rate: 48000 Hz
- Block: 16
- Mode: Average
- Step: 4
- Custom Signal
- FFT Length 4K
- Weighting Window: Full Blackman-Harris

and connect the audio device under test to the sound card.
In the following examples, the test has been performed using a "loop-back" configuration.

 

SNR (Signal to Noise Ratio) measurement

The SNR value is the ratio of the peak power level to the remainig noise power.

Measurement procedure:
In the Custom Signal Window a single pure tone must be selected. The following figure shows an example (1 kHz tone).

Figure 1 - Single pure tone selection

Open the Scope Window, press the "Select All" button and pull down completely the X Zoom slider. Press the "Syncro Start (REC & PLAY)" button and check the input and output levels. The wave resulting from the measurement should look like the following one (note that all data must be selected by means of the weighting window to achieve an high degree of accuracy):

Figure 2 - Sampled 1 Khz pure tone

Now the Audio Quality plugin can be opened (from the "New Measurement" Window).

Figure 3 - Audio Quality Plugin

The first page of the plugin shows Custom Generator settings (the frequencies of the signal generator pure tones can be set or modified only from the main program).

In the Audio Quality plugin, select the SNR page and start a new measurement cycle.

Signal to Noise Ratio measurement

Figure 4 - SNR measurement

The space on the right shows the detected pure tone, represented by a green bar. The space on the left reports the measured value and the computed SNR. In the registered version of the plugin, this text can be copied to the Clipboard.

If the Averaging Mode has been selected, at each cycle the SNR value will decrease until the minimum value is reached.

 

THD (Total Harmonic Distortion) measurement

The THD is defined by the following formula:

where terms 2..N are the power levels of the harmonics and term 1 is the power level of the fundamental (the pure tone).

The THD+N is defined by the following formula:

where term n is the noise power level.

NOTE: since Sample Champion Version 3.8 and above, it is possible to select a different computation formula of THD and THD+N.

THD and THD+N can now be computed in RTA window (see here), so there is no need to use the Audio Quality Plugin.

In RTA window is available the THD Mode option.

Enabling the THD Mode option, THD will be computed by means of the following formula:

and THD+N will be computed by means of the following formula:

In normal measurements (with low THD and THD+N) the 2 methods will give identical results, since more than 99% of the measured energy is always contained in the fundamental harmonic (H1).

In RTA window is implemented also a new method for the evaluation of the Noise Floor (see here a detailed explanation).


Measurement procedure:
In the Custom Signal Window a single pure tone must be selected.
Keep the same selection in Scope Window as in the previous case.
In the Audio Quality plugin, select the THD page and start a new measurement cycle.

Total Harmonic Distortion measurement

Figure 5 - THD measurement

The space on the right shows the detected pure tone, represented by a green bar and the first 9 harmonics, represented by red bars. The space on the left reports the measured values and the computed THD and THD+N. In the registered version of the plugin, this text can be copied to the Clipboard.

 

IMD (InterModulation Distortion) measurement

This parameter gives a measure of the distortion caused in the device under test by two pure tones (cross modulated power). The following harmonics are considered:

(Tone 1 = f1)
(Tone 2 = f2)
(f2-f1)
(f1-2*(f2-f1))
(f1-(f2-f1))
(f1+2*(f2-f1))
(f1+3*(f2-f1))
(2*f1)
(f1+f2)
(2*f2)
(3*f1)
(2*f1+f2)
(2*f2+f1)
(3*f2)


The IMD value is computed as the ratio of the sum of the power levels of the intermodulation harmonics to the sum of the power level of the two strongest tones.

Measurement procedure:
In the Custom Signal Window two pure tones must be selected.

Keep the same selection in Scope Window as in previous cases.
In the Audio Quality plugin, select the IMD page and start a new measurement cycle.

Intermodulation Distortion measurement

Figure 6- IMD measurement (19 and 20 kHz tones)

 

Intermodulation Distortion measurement

Figure 7- IMD measurement (5 and 6kHz tones)

The space on the right shows the two detected pure tones, represented by green bars and the considered harmonics, represented by red bars. The space on the left reports the measured values and the computed IMD. In the registered version of the plugin, this text can be copied to the Clipboard.

Common choices of the two fundamental frequencies are:

  • SMPTE: 60 Hz and 7 kHz (4:1 ratio)
  • DIN: 250 Hz and 8 kHz
  • CCIF: 19 kHz and 20 kHz

The Audio Quality plugin shows also the narrow-band version of the computed spectrum.

Figure 8- Narrow band spectrum

 

NOTE: since Sample Champion Version 3.8, it is possible to measure SNR, THD, THD+N and IMD in RTA Window
(more information here).

So, the Audio Quality Plugin is no more
required and supported (but still available)

Back to Application Notes

 

 

 

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