Specifications
2.26
Applications Manual
© 2003-2005 Prism Media Products Ltd
Revision 1.11Prism Sound dScope Series III
3.3 Glossary
The following definitions are used in the AES17 standard and in the dScope implementation of the
AES17 tests:
Aliasing components
- Aliasing can take place when a frequency above the
folding frequency
of
an
EUT
is applied to its analogue input (or where internal down-sampling occurs). Aliasing is
manifest by the appearance of a spurious frequency component at the EUT's output, below the
folding frequency by the same frequency as the input stimulus is above the folding frequency.
Thus the stimulus has been 'folded' about the folding frequency. See also
imaging components
.
D/A line-up
- A setting of the dScope's Inputs and Outputs which relates the maximum analogue
amplitude to the maximum digital amplitude. It is expressed as an analogue amplitude which
corresponds to 0dBFS (full-scale digital). This feature of the dScope is especially useful in
measuring
EUT
s with both analogue and digital ports (as is common in applying the AES17
standard) since it allows settings or measurements in one domain to be expressed in the units of
the other. For example, an A/D converter can be stimulated with a signal 1dB below that which will
produce full-scale output by setting the dScope's analogue generator amplitude to -1dBFS,
provided that the dScope has been set to the appropriate D/A line-up for the converter.
dBFS
- A logarithmic unit used to express signal amplitude in digital and mixed-domain systems in
terms of the maximum amplitude which can be accommodated by an
EUT
. A 0dBFS signal has
the same RMS amplitude as a sine whose peaks just touch the maximum level of the system. A
–6dBFS signal has half this amplitude, for example. Note that for non-sine signals such as square
waves or DC it is possible for the system to accommodate amplitudes greater than 0dBFS. In the
dScope it is possible to specify or measure analogue signal amplitudes in dBFS, in which case the
D/A line-up
is used as a reference.
EUT
- 'Equipment under test'.
Folding frequency
- Half the sample rate of the
EUT
. Input frequencies above the folding
frequency are subject to
aliasing
. Where an EUT applies internal down-sampling, the folding
frequency is half of the lowest internal sample rate employed.
Full-scale amplitude
- A signal whose amplitude is the maximum which can be accommodated
by the
EUT
. In the case of a sine, this amplitude is 0
dBFS
. For systems where the output is
accessible in the digital domain, full scale is defined as the RMS voltage of a 997Hz sine wave
whose positive peak value reaches the positive digital full scale. For systems where the digital
signal is not accessible, or where digital full scale cannot be reached, full scale is defined as the
level 0.5dB below that where 1% THD+N or 0.3dB compression occurs (whichever comes first) at
the EUT output for a 997Hz sine wave.
Imaging components
- Imaging can take place in an
EUT
with a D/A converter or which performs
digital up-sampling. It is manifest by the appearance at the EUT's output of a spurious frequency
component above the
folding frequency
by the same frequency as the stimulus is below it. It is
caused by insufficient stop-band attenuation in the EUT, and is most noticeable close to the folding
frequency. See also
aliasing components
.
Jitter susceptibility
- The effect on an
EUT
of jitter present on its reference sync input (which
may also be its data input). Such effects may include jitter passed to the EUT's digital output or, if
the EUT includes A/D, D/A or SR conversion, distortion of the audio signal owing to sampling jitter.
In severe cases, loss of digital data at the EUT's input can also occur.
Standard high-pass filter -
A high-pass filter with a stop-band beginning at the
upper band-
edge frequency
of the
EUT
. This filter is used to measure 'out of band' components.
Standard low-pass filter -
A low-pass filter with a pass-band up to the
upper band-
edge frequency
of the
EUT
. This filter is used to restrict measurements to 'in band' components.