6.2 Frequency Sweep Mode
Press to enter frequency sweep mode (fig. 6.4). The LA102 will
display
SEQ 3 FREQ SWEEP and range and channel indicators. The frequency sweep
should now be played from the test tape, CD, gramophone disc or other source. Press
to exit from this mode when the sweep has finished and the LA102 will display
the frequency response graph. Compact disc
The LA102 will measure and record the level whenever it thinks the input signal is valid,
but note that it expects a continuous frequency sweep and will not reject speech, music or
FSK which it will attempt to measure producing unpredictable results. Individual tones
are best used with sequence option 2 (above) which will reject voice announcements and
will also average for a longer period.
Only one channel is measured at a time but it is possible to record one sweep, change
channel and then record the second. The current input channel is indicated by an L or an
R in the top right corner of the display. If a tone is detected on one channel only then the
LA102 will automatically select that channel; this is useful when using test CDs where a
left channel sweep is often immediately followed by a right channel sweep. The
key
can be used to select the input channel explicitly and should be used when the LA102
cannot select the channel automatically (for example when the sweep is present on both
input channels).
The level range must be set with the range keys as the LA102 does not autorange in this
mode (it cannot know in advance how high the signal will be and cannot change range
during a sweep because that will cause samples to be lost). Unfortunately the LA102
does not have enough time to produce a bar graph display and the scale is displayed
simply to allow the user to set the range with the range keys. It is best to set the range too
high (resulting in reduced accuracy) rather than too low (which may cause an overload).
Only frequencies within the range 20Hz to 20kHz will be recorded and a 32 pixel wide
bar graph showing which samples in the 20Hz-20kHz frequency range have been received
is displayed to show the progress. If the frequency sweep does not cover the 20Hz to
20kHz range only the received frequencies will be plotted on the displayed and printed
graphs. Moving the cursor to a frequency which wasn't recorded will show
RANGE? in
place of the level as for the test tape mode described above.
Ideally the frequency sweep shouldn't be faster than about 3 octaves per second (ie 3.5s to
sweep from 20Hz to 20kHz). The LA102 attempts to take 256 samples over the
frequency range but will interpolate any missing samples before plotting the graph.
103
6. Using Test Tapes and Discs
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