Application
note # 19 
Sample
Champion

Real Time Analyzer in Sample Champion  Features and use
In
Sample Champion is possible to analyze simultaneously in realtime
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 realtime and plot the following DSP functions:

Transfer function [L(1)] / [R(2)] (Frequency
Domain)
 Transfer function [R(2)] / [L(1)] (Frequency
Domain)
 CrossSpectrum [L(1)] * [R(2)] (Frequency
Domain)
 CrossSpectrum [R(2)] * [L(1)] (Frequency
Domain)
 Coherence (Frequency Domain)
 AutoCorrelation [L(1)] (Time Domain)
 AutoCorrelation [R(2)] (Time Domain)
 CrossCorrelation (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.
CrossSpectrum
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 CrossSpectrum example is shown. The spectrum
of channel 1 is plotted in green, the spectrum of channel 2
in red and the CrossSpectrum in yellow. The frequency axis
is linear. Also the MAX values of the spectrums are shown on
the plot .
Note
that the CrossSpectrum of ch.1*ch.2 can be different from the
CrossSpectrum 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 CrossSpectrum
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.
CrossCorrelation
and AutoCorrelation
These
Time domain functions give information about the mutual correlation
of 2 signals (or the same signal in case of AutoCorrelation).
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)
(f2f1)
(f12*(f2f1))
(f1(f2f1))
(f1+2*(f2f1))
(f1+3*(f2f1))
(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, CrossSpectrum,
Coherence, AutoCorrelation, CrossCorrelation) can be computed
in postprocessing 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
