Specifications

2.20
Applications Manual
© 2003-2005 Prism Media Products Ltd
Revision 1.11Prism Sound dScope Series III
3.2.6 Amplitude non-linearity
The Amplitude non-linearity tests are arranged in the following subsections, which correspond to the
equivalent subsections of the AES17 standard:
Level-dependent logarithmic gain
Intermodulation distortion (IMD)
Signal modulation noise
Low-level noise modulation
Total harmonic distortion and noise (THD+N)
3.2.6.1 Level-dependent logarithmic gain
Applies to:
EUTs with analogue or digital inputs and analogue or digital outputs.
Aim:
To measure the change in gain of the
EUT
with signal amplitude, sometimes called 'deviation
from level linearity'.
Method
A 997Hz sinusoidal stimulus is applied at –5
dBFS
. The output of the EUT is measured after passing
through a 1/3 octave 997Hz band-pass filter. The gain of the EUT is calculated as being the ratio of its
output amplitude to its input amplitude. The narrow-band noise floor of the EUT is also measured
using the same band-pass filter, but with the stimulus muted.
The stimulus is reapplied, and its amplitude is then reduced in 5dB steps, until it is within 5dB of the
measured narrow-band noise floor, noting the output amplitude at each step and calculating the
inferred EUT gain. The results are graphed for each channel, with relativity to the gain measured at
–5dBFS expressed in dB, as a function of the applied amplitude. At the end of the test, a worst-case
gain deviation (in dB) is also reported for each channel.
When the A/D and D/A portions are available separately, this test should be repeated for each section
independently.
3.2.6.2 Intermodulation distortion (IMD)
Applies to:
EUTs with analogue or digital inputs and analogue or digital outputs.
Aim:
To measure the intermodulation distortion of the
EUT
, using the two most common techniques.
Method
The first IMD test uses a twin-tone stimulus with one tone at the
upper band-edge frequency
and the
other tone 2kHz below that. The amplitudes of the tones are equal, with the combined peak amplitude
adjusted to equal the peak amplitude of an equivalent sine wave at
full-scale amplitude
. An FFT
technique is used to measure the second- and third-order difference components for each channel.
This type of test is often referred to as a 'CCIF' IMD test.
The second IMD test also uses a twin-tone stimulus, but in this case the tones are at 41Hz and
7993Hz. The amplitude of the higher frequency tone is a quarter of the amplitude of the lower one. As
before, the combined peak amplitude is adjusted to equal the peak amplitude of an equivalent sine
wave at
full-scale amplitude. The components caused by modulation of the high frequency by the low
frequency are measured and expressed relative to the amplitude of the high frequency tone, in dB, for
each channel.
This type of test is often referred to as a 'SMPTE/DIN' IMD test.