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>@9
Multitone Audio Test System User Manual
V 3.32 29 / 71
V
V
n
with n number of signal components
RMS
i
i n
= =
=
∑
( )
..
2
1
Equation 8 RMS Calculation
Unfortunately, when applying this formula on the discrete spectrum of a FFT analysis, the
result correlates in inverse proportion to the blocklength / number of bins in the respective
band. Therefore, to calculate the TD+N result out of a digitized signal, the
RSS value (Root
Sum Square) has to be used.
V V with i counter over all bins n m
RSS i
i n m
= =
=
∑
( ) ( .. )
..
2
Equation 9 RSS Calculation
The accuracy of this approach can be proved with any spectral analyzer. The better the
resolution (i.e. the higher the blocklength), the lower is the amplitude of a single bin, since
the total energy of the band is constant. Consequently, the summing-up of the bins always
result in the same value, regardless of the chosen resolution (blocklength).
Interpretation of TD+N
To interpret the TD+N value correctly, it has to be considered that this result slightly differs
from a conventionally measured
THD+N value. With conventional THD+N analysis, a single
tone stimulates the DUT. This frequency component is subtracted from the received signal
after transmission. The ratio of the remaining level to the total input level gives the THD+N
and
SINAD result respectively.
On the other hand, the transmission of a multitone stimulus will result in the appearance of
many harmonics and intermodulation products. However, it is neither possible to relate any
of these signal components to a certain signal bin of the original multitone signal, nor to
differentiate the received signal between harmonics and intermodulation products.
Distortion Plot
Fig. 20 shows a typical distortion plot, derived from the returned distortion results of
>@9
.
•
The first value in the plot D1 equals the RSS result (TD+N value) of the band between the
first bin
≥
20Hz up to the last bin < SignalBin
#
1.
•
All further results Dn represent the bands between the first bin > SignalBin
#
n up to the
last bin < SignalBin
#
n+1.
•
The last distortion result represents the band between the first bin above the highest
signal bin up to the last bin
≤
20kHz.
Please notice, that both the even & odd bins of the received signal are considered for the
TD+N calculation.