| Application
note # 8 |
Sample
Champion
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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)
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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.
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).
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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.
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.
Figure
6- IMD measurement (19 and 20 kHz tones)
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
|
