>@9
Multitone Audio Test System
User Manual
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frequency shifts, generated by the DUT (modulators / demodulators or tape machines with
speed differences) or slightly different clock frequencies of a separated generator / analyzer
pair are eliminated automatically.
Multitone Signal
Right after the header information follows the
multitone signal itself, i.e. transmission of the
signal bins. The duration of the multitone signal depends on the block length as defined in
Table 1
. The multitone signal is transmitted at least 3 times and may be repeated by applying
the command
OUTPut:MTONe:MTONelength
. For instance, several multitone blocks may be
transmitted before the analysis starts, in order to stabilize the DUT and to let transients
disappear. The analysis itself requires a minimum of two blocks.
Data Acquisition
Wake-up Sequence
The
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analyzer continuously executes a FFT of the input signal. As long as there is no
signal or any non-correlated audio information, no action is started. But as soon as the
analyzer detects a header, i.e. the
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-specific frequency / level pattern in the input signal,
it wakes up and records the incoming multitone signal.
Please notice that the trigger signal can be detected up to -20dB below the set range. That
means for example with an input range set to -6dBu the trigger can be detected from levels
as low as -26dBu.
Synchronization Mode
Normally, the analyzer of
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uses the internal sample frequency clock of the generator.
This mode should be used for all applications where no frequency shifts occur on the signal
transmission path.
However, in cases where the device under test (DUT) changes the frequency of the
transmitted signal, the analyzer has to synchronize itself onto the incoming signal itself. For
this purpose, each header of a multitone burst contains a SYNC block, providing a fixed
frequency, onto which the analyzing DSP may synchronize its sampling clock. This feature
and the choice, whether a header shall be transmitted at all, may be activated with command
INPut:SYNC [INTernal¦INTNoheader¦EXTernal¦EXTNoheader]
, offering the four following
modes.
INTernal
The analyzer is linked to the generator clock of the same unit and the multitone signal is
preceded by a header (trigger & SYNC block). This mode may be chosen if the multitone
burst is generated in the same unit where it is analyzed and if no major frequency shifts
occur in the DUT. The burst is initiated with command
OUTPut:MTONe:STARt
and
transmission must not show a delay of more than 1s.
INTNoheader
Again, the analyzer is linked to the generator clock of the same unit, but no header is
transmitted. The benefit of this mode appear in noisy environments, where the trigger cannot
be detected, and for analysis of signals being generated by the DUT itself. However, the
max. allowable transmission duration is 50ms, i.e. the multitone burst must ’arrive’ at the
analyzer at latest after this time from the moment of its initiation.
