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Log Squared and Cumulative Energy Plot
 
 
 

Application note # 19
Sample Champion

 

Real Time Analyzer in Sample Champion - Features and use

 

 

In Sample Champion is possible to analyze simultaneously in real-time the 2 oscilloscope input channels and perform some DSP functions on them.

The RTA functions display can be opened by pressing the button:

placed on the remote bar.


In order to operate properly, the RTA window requires that:

- Number of input channels = 2
- INPUT BUFFER LENGTH = FFT LENGTH

For example, for a buffer of 16K samples, the settings are the following:


All the frequency windows in the main program must be closed when RTA functions are used.

In the SIGNAL ANALYSIS FUNCTIONS window, the FFTs of one or both input channels can be plotted simultaneously, using different graph options. Note that, in some cases, when the FFTs of both channels are plotted, it could be necessary to use the line plotting option, to prevent the masking of one channel by the other.

Also 1/3 Octave and 1/1 Octave plot of both channels or of a single one are available:

 


It is possible to optionally write the Octave or 1/3 Octave value and frequency directly on the plot.

 

From the FFT data of the 2 input channels it's possible to perform in real-time and plot the following DSP functions:

- Transfer function [L(1)] / [R(2)] (Frequency Domain)
- Transfer function [R(2)] / [L(1)]
(Frequency Domain)
- Cross-Spectrum [L(1)] * [R(2)]
(Frequency Domain)
- Cross-Spectrum [R(2)] * [L(1)]
(Frequency Domain)
- Coherence
(Frequency Domain)
- Auto-Correlation [L(1)]
(Time Domain)
- Auto-Correlation [R(2)]
(Time Domain)
- Cross-Correlation
(Time Domain)
- Correlation Meter
- Stereo Check

 

Transfer function

This function performs the complex FFT operation of channel 1 divided by the FFT of channel 2 (L/R) or the FFT of channel 2 divided by the FFT of channel 1 (R/L).

 



When measuring a transfer function, it's recommended to use a wide band signal (MLS) to obtain information on the entire audio band up to half the sample rate. In some cases it could be useful to make some averages to improve the measures.

IMPORTANT NOTE: when using the RTA functions, ONLY the complex average is available and NOT the power average. In other words, the average is done on the complex FFT data and when average option is enabled IT'S RECOMMENDED to use ALWAYS the internal signal generator to maintain the synchronism between each block of time data sampled and perform thus correctly the average.

 

Cross-Spectrum

This is computed by multiplying one complex spectrum by the complex conjugate of a second spectrum. It gives information about the power common to 2 signals.



 

In the figure above a Cross-Spectrum example is shown. The spectrum of channel 1 is plotted in green, the spectrum of channel 2 in red and the Cross-Spectrum in yellow. The frequency axis is linear. Also the MAX values of the spectrums are shown on the plot .

Note that the Cross-Spectrum of ch.1*ch.2 can be different from the Cross-Spectrum of ch.2*ch.1!

The spectra of the 2 input channels can be plotted on the same plot as the the selected function (in the frequency domain). The input spectrum can be plotted also in 1/3 octave mode, like in the example below (frequency axis logarithmic):

 

 

Coherence

This function is the ratio of the squared magnitude of the Cross-Spectrum and the spectrum of channel 1 multiplied by the spectrum of channel 2. It gives information about the mutual linearity of the channels.
The example below shows the coherence of a signal before (ch.1) and after (ch.2) a digital reverb effect.

 

Cross-Correlation and Auto-Correlation

These Time domain functions give information about the mutual correlation of 2 signals (or the same signal in case of Auto-Correlation). It is very useful for finding, for example, echo or delays in time data. In the example below a signal has been passed through a digital echo effect; the peaks correspond to the delays (with feedback) set on the effect machine.


To perform this test, a pulse signal has been used. It can be downloaded here and loaded in Sample Champion as shown below:


 

Correlation Meter

This function offers a quick and easy way to check the correlation between the 2 input channels. This measurement can be performed by pressing only REC (red) button (no signal generator) for measuring, for example, a stereo musical signal. In other cases it is possible to use the signal generator.


The result gives a correlation index between the 2 channels. Specifically:


+1 means that LEFT and RIGHT signals are identical

0 means that LEFT and RIGHT signals are not correlated

- 1 means that LEFT and RIGHT signals are identical and
exactly out of phase (180º)

 

 

Stereo Check

This function plots the LEFT channel versus the RIGHT channel. It's easy, in this way, to obtain information about the 2 channels.

- Example of a stereo musical signal:


- Example of a mono musical signal. The amplitude of the 2 signals are exactly the same since the slope of the line is exactly 45 degrees:


- Example of a signal present only on the right channel:

 

 

Audio Quality functions

In an early version of Sample Champion the computation of SNR, THD, THD+N and IMD had to be done in the Audio Quality Plugin. Now the computation of SNR, THD, THD+N and IMD can be performed in the RTA window.

 



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 (Settings/General/Custom Signal) a single pure tone must be selected. The following figure shows an example (1 kHz tone).

 

Then a SYNC REC/PLAY () measurement cycle can be started (for example with a loopback connection).

 

 

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


A data box containing all details about the parameters computation can be shown inside the data box (View details option).


Different methods can be selected for the evaluation of the Noise Floor value in the SNR computation.



1 - computation of the Noise Floor level as logarithmic sum of narrow band power levels in a frequency range selected by the user. This option can be used for estimating the Noise Floor WITHOUT the harmonics due to distortion; it is, for instance, possible to select the band 100..800 Hz when a 1 kHz tone is used as signal generator.

2 - manual: this option allows to set manually the Noise Floor value. It can be measured, for example, recording the background noise in absence of input signals.

3 - computation of the Noise Floor level as logarithmic sum of narrow band power levels on the whole bandwidth from 20 to 20000 Hz.

 

 


A yellow line corresponding to the Noise Floor level and a blue line under the frequency range selected by the user for the computation can be optionally visualized on the plot.

 

 

If the pure tone used for the measure falls inside the frequency range selected by the user, it is automatically excluded from the computation.

The Noise Floor value used for THD+N computation is not influenced by this option and is computed on the whole bandwidth; all harmonics generated by the 2 pure tones are also automatically excluded.

IMD computation is not influenced by this option.

 

THD (Total Harmonic Distortion) and THD+N
(Total Harmonic Distortion + Noise) measurement

THD and THD+N can be computed using in two different ways.




If the option THD Mode is unchecked, the following formulas are used:




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



where term n is the noise power level.

 

If the option THD Mode is checked, the following formulas are used:


and

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

 

Measurement procedure:

In the Custom Signal Window a single pure tone must be selected.

A SYNC REC/PLAY () measurement cycle can be started.

 

 

THD and THD+N will be computed and shown.


The View details window can show additional information:

 

 

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.

A SYNC REC/PLAY () measurement cycle can be started.

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

 

SNR, THD, THD+N AND IMD MUST BE MEASURED
ONLY USING THE INTERNAL SIGNAL GENERATOR

 

- Load and Save Spectra in RTA window

Spectra measured in RTA window (narrow band, 1/1 or 1/3 octave) can be loaded and saved. The functions (Transfer function, Cross-Spectrum, Coherence, Auto-Correlation, Cross-Correlation) can be computed in post-processing mode on loaded data.


Left channel and right channel FFT banks can be saved and loaded independently.

Data of scope spectrum or functions can be exported as TXT files (with options set in Settings/File Options) and the graph can be saved as BMP image.

 

POINTS TO REMEMBER WHEN USING RTA FUNCTIONS:

  • THE AVERAGE IS COMPUTED ONLY IN COMPLEX MODE

  • ONLY THE INTERNAL SIGNAL GENERATOR MUST BE USED

  • IF THE YELLOW LABEL "BUFFER OK" ON THE REMOTE BAR IS NOT VISIBLE EVERY SAMPLING CYCLE, THE STEP VALUE MUST BE INCREASED TO AVOID LOSS OF DATA



 

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