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C H A P T E R
4 :
•
If a POTS interface is selected for either the terminating or the originating
side, you must use an 8kHz, 8Bit G.711 mono reference file.
•
The recorded files will always be of the same format as the reference file
was.
•
Never directly feed back a digital AES/EBU output signal to the input of the
same board. (also not indirectly by feeding it through the second audio
interface). If the digital input is used, the digital output is synchronized to
the clock of the digital input signal. The result of feeding back the output to
the input is than a deadlock since no clock is available in this case.
•
When using both analog audio interfaces simultaneously, make sure that
both boards are synchronized, as explained earlier in this chapter.
•
Please note that synchronizing two LynxONE boards works for sample rates
above 24kHz only. At lower sample rates the drift between the clock
oscillators of the two boards will generally be low enough to perform
measurements without additional synchronisation.
•
On the LynxONE boards each board may be operated fully independent
from the other.
•
On the Digigram audio boards, all signals must be fully synchronous.
•
Take care to not overload the analog inputs. Using the mixer controls on
Windows based PCs will in general degrade the signal quality, since the
attenuation/amplification is applied to the digital sources, and not to the
analog signals. This will in general result in a decrease of the signal to noise
ratio. If possible you should leave the mixers in their factory default settings.
•
If you assess PC based equipment, we recommend using the best sound
cards available (e.g. the LynxONE boards which are used in OPERA as
well).
•
Recordings made from the audio interfaces will always be stored as 16bit
linear stereo wave files.
4.3 The OPERA™ Framework
4.3.1 The Underlying Generic Algorithm Model
OPERA™ is based on a generic algorithm model, as outlined by Figure 4.21
below. It consists of two inputs, one for the (unprocessed) reference signal and
one for the signal under test. The latter may be, for example, the output signal
of a codec that is stimulated by the reference signal. In a first signal processing
step the peripheral ear is modelled ("perceptual model", or "ear model"). In a
consecutive step, the algorithm models the audible distortion present in the
signal under test by comparing the outputs of the ear models. The information
obtained by this process results into several values, so called MOVs ("Model
Output Variables") and may be useful for a detailed analysis of the signal.
G E T T I N G
T O
K N O W
F R A M E W O R K
54
T H E
O P E R A ™
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