the presence of the double tone, and used to re-normalise the reading if the level changes
(as for THD measurement).
Another possible signal source is a test CD such as the the HiFi News test disc III. This
disc contains double tones with centre frequencies of 1kHz, 5kHz, 10kHz and 15kHz
(nominally), as well as a digitally generated Lindos test sequence. Residual readings of
below -90dB can generally be expected. The Philips test sample 3 disc (410 055-2,
SBC421) also contains suitable double tones, together with a double tone sweep.
Centre frequency
Difference frequency distortion tests are normally carried out using a test signal having a
peak to peak amplitude equivalent to that of a sine wave of the specified test level, and on
the test CDs mentioned the level of the double tones has been so adjusted. Any gain
adjustments to the system under test should therefore be made using a sine wave from the
disc, not the twin tone. Measurements will be valid above about 1kHz, below this the
measuring set will not adequately reject the double tone signal itself.
Note that only second order difference frequency distortion, resulting from an
asymmetrical transfer characteristic, is detected by this method. Many devices, such as
tape recorders, give rise predominantly to third order components. These are close to the
signal and more difficult to measure.
2.17 Quantising Distortion (QD) Measurement
The LA100 system measures quantising distortion on a 40Hz tone by notching out the
fundamental, and rejecting low harmonics with a 400Hz high-pass filter to leave only
high frequency components, which are then measured in one of two ways. W&F option 4
reads rms with 22kHz bandwidth, option 3 uses CCIR weighting and quasi-peak
detection. 40Hz was chosen to permit use with the Philips Test CD mentioned above,
which has 41Hz tones at 0, -24 and -30dB. Distortion components on the LA101 produce
a residual reading of -74dB, but this need not be a limitation as QD measurements are
most relevant at low signal level where they will usually be much worse than this. Like
THD measurement, the LA102 measures the level of the fundamental when this
measurement is selected. The numerical readout indicates the level of the distortion,
relative to the level of the fundamental, while the bar graph indicates the absolute level
being measured. Remember to press the
key (or the key) to re-measure
the level of the fundamental if it changes. Note that this measurement need not be limited
to digital systems and has been used to measure modulation noise on tape. It is also a
useful measure of low frequency rattles and turbulence on loudspeakers.
Loudspeaker testing
2.18 Frequency Intermodulation (FIM) Measurement
FIM or Frequency Intermodulation is a measurement of the degree of frequency
modulation of a 3kHz tone by a 300Hz tone, and is normally relevant to gramophone
systems where variations in the vertical tracking angle of cartridges results in a
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2. Manual Operation
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