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If the 3% distortion point does not occur in the 8dB covered by this segment the LA102
will not be able to measure the MOL and will indicate the failure by giving a result of
+99.9dB if the distortion was below 3% at all levels in the range, or -99.9dB if the
distortion exceeded 3% at the lowest level used. Should this occur the segment should be
given a measurement level which will allow the 8dB range to include the 3% MOL point.
If no measurement level is specified segment H will cover the range from 0dB to +8dB,
while H4 will cover the range from +4dB to +12dB (as with all segments these levels are
relative to the LA101 test level − see sections 3.7, 3.10 and 4.21).
When measuring MOL the LA101 holds the first level of the level sweep for ½s to allow
the LA102 to accurately record the starting level. The sweep then progresses at a rate of
1dB/s over an 8dB range and the LA102 continuously measures the absolute level of
THD until it is -30.45dB (3%) below the expected level of the signal. The LA102 then
measures the actual absolute level of its input signal. Note that the expected level of the
signal is based upon the initial level and the known rate of increase, but this may be
different from the actual level if compression is occurring in the system. This means that
the distortion may actually be greater than -30.45dB when the LA102 decides that it is
-30.45dB below the expected signal level (ie if the actual signal level is less than
expected). The LA102 attempts to correct for this by calculating the MOL from a
weighted average of the expected level and the actual measured level.
In practice the result is usually accurate to within ±0.2dB of the manually measured MOL
with reasonable quality tape and little compression. With poor quality tape the error may
be as much as ±0.5dB. For best results the segment should be run at a level just below the
anticipated 3% MOL level to minimise any errors due to compression effects. For
example if the 3% MOL is known to be around +6.5dB best results will be obtained by
running segment
H6 whereas segment H will give less accurate results because of the
output level error due to the compression effects over a 6dB range.
4.15 Difference Frequency Distortion (d) 2nd order difference frequency distortion
Segment d measures second order difference frequency distortion with a double tone
having a 1kHz centre frequency and 70Hz difference frequency (nominally), as explained
in section 2.16. The level of each tone is -6dB relative to Test Level, so the rms level of
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4. User Defined Sequences
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