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C H A P T E R
4 :
G E T T I N G
F R A M E W O R K
be used. The shortest possible delay is in the rage of one to two seconds,
depending on the load of the system and the length of the files. A –Repxx will
be appended to names of the recorded files, where xx stands for the index of
the repetition.
All of the above settings may be freely combined and can be used for audio as
well as for telephony interfaces and a combination of both. On audio interfaces
call control is skipped and the call is immediately connected.
Another difference compared to the Telephony Standard View is related to the
selection of Origin and Termination. In the Expert View it is now possible to
choose different reference files for both sides of the call. This is especially
interesting if tests are being performed under double talk conditions. In this case
it is advisable to use different files in both directions in order to avoid, that echo-
cancelers cancel one of the two signals in the system. It is also possible to
amplify the recorded signals before the files are written. This is mainly required
for recordings made with Opera's audio interfaces, if the device under test
operates with very low levels, like e.g. head- or hand set interfaces on
telephones / mobile phones. In this case OptiCall™ makes the recording with a
bit resolution of 24bits. On this data the gain factor is applied, and afterwards
the signals are scaled to 16bit resolution and stored on the disk. App. 70dB gain
can be applied without loss compared to an analog amplifier and a subsequent
G.711 coding.
Note:
The gain factors are effective for recordings made on the audio
interfaces only. For telephony interfaces they are ignored.
OptiCall uses a mechanism called DDLC™ to compensate for the unpredictable
latencies of the Windows NT operating system. During this process some
internal delays may be adjusted. If OptiCall™ detects that it can not compensate
for the system latency, it will repeat the call automatically with adjusted
parameters. This may happen up to four times. DDLC™ enables OPERA to
measure delays with very high accuracy. Experiments showed that 99.5% of the
results had a delay accuracy within a range of ±3ms (using the POTS interfaces
at 8kHz sample rate). Nevertheless, the accuracy of the results will follow a
statistical distribution. For most accurate results we recommend performing a
number of measurements and to take the average after brushing off 5% of the
results as outliers.
T O
K N O W
T H E
O P E R A ™
51
Delay Accuracy /
DDLC
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