Lindos LA100 - Subtracting Reference Curves; Tests Which Always Subtract a Reference Curve; Oscillator Weightings

3.17 Subtracting Reference Curves
The LA102 can subtract a reference response curve from the frequency response results.
This has many applications: loudspeaker and microphone measurements often need to be
corrected to allow for deficiencies in the system; similarly, record frequency response
measurements on a tape machine may be corrected to allow for errors in the replay
response.
The reference response should be measured either by using a normal Lindos test sequence
containing a frequency sweep, or by using the test tape or frequency sweep mode (see
chapter 6). The reference results should then be stored in one of the memories (by
pressing
and the memory number when the results are displayed). The results to be
corrected should be obtained in the same way and the reference results may then be
subtracted by pressing
when the frequency response is displayed and then entering
the memory number for the reference results. The frequency response results in memory
0 will be modified by subtracting the frequency response results in the reference memory,
but none of the other results will be affected.
The subtraction works by subtracting each sample in the reference memory from each
sample in memory 0. The left channel is subtracted from the left channel, and the right
channel is subtracted from the right channel.
The subtraction can only be used when configuration C1 is set, otherwise an error
message will be displayed (because if more than one frequency response curve was in
each memory it would not be clear which should be subtracted from which). The memory
to be subtracted must contain valid frequency response results covering the same
frequency range, otherwise it will produce meaningless results (eg do not subtract a 10Hz-
30kHz response from a 20Hz-20kHz response).
3.18 Tests Which Always Subtract a Reference Curve
Some measurements will always require the subtraction of a reference curve, and this can
be achieved by using the \n control segment which instructs the LA102 to subtract the
reference curve in memory n. The \n segment must appear after the frequency response
segment, otherwise the subtraction would occur before the frequency response had been
measured (the LA101 only sends this segment once, and if sending a two channel
sequence it will only send it when both frequency sweeps have been run). Some care
should be taken when using this segment to ensure that the correct reference curve is
stored in memory n before the sequence is received, and that LA102 configuration C1 is
set.
3.19 Oscillator Weightings Weighting curves Filter testing Equaliser testing Testing filters
An oscillator weighting can be used when running sequences (see section 2.27). The
weighting is selected on the LA101 by pressing
in manual mode before running the
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3. Sequence Testing