2.11 Rumble Measurement Measuring rumble Gramophone systems
Turntable rumble may be measured by selecting Noise Option 5 (weighted) or 4
(unweighted). Both rumble measurements have the same HF roll-off above 315Hz but
the weighted measurement also rolls off below 315Hz to give a measure of the subjective
intrusion of rumble (see appendix G). The unweighted measurement is flat down to very
low frequencies. To measure rumble, take a reading while playing the quiet grooves of a
special test disc, and then take a normal level measurement using a track cut to one of the
following reference levels: 315Hz 2.71cms
-1
rms one channel, 1kHz 5cms
-1
rms one
channel, or 1kHz 10cms
-1
peak lateral. These are referred to in the various standards but
are in fact equivalent. The difference between the two readings is then the relative rumble
level. While it is also possible to set a test level and then read the relative rumble, visual
averaging may be needed, and this is easier using the bar graph. The LA102 uses VU
meter characteristics (with slow software averaging) for rumble measurement, as required
by IEC98, but later software may implement the very slow BS4852 characteristic which
gives exactly the same result but without the need to average visually. The unweighted
measurement differs slightly from all the standards which require a rather odd 10Hz
triangular roll-off. As the various standards then go on to require various additional roll-
offs to be applied for the measurement (20Hz unspecified IEC, 31.5Hz 6dB/octave DIN,
none BS) the precise 10Hz specification seems to be pointless. The LA102 has been left
flat down to 2Hz as it is felt that in practice the LF limit for this measurement is set
entirely by the rumble filtering invariably included in the cartridge pre-amplifier.
2.12 Crosstalk Measurement Crosstalk Automatic frequency selector
Pressing on the LA102 provides a narrow bandpass-filtered level measurement
which reduces the contribution from wideband noise by 10dB typically even when the
measured signal is close to or below the noise level (fig. 2.11). Measurement at six spot
frequencies is possible by selecting options 1 to 6, giving 100Hz, 315Hz, 1kHz, 6.3kHz,
10kHz and 40Hz respectively. Although other applications may arise where such filtered
measurement is useful, it is intended primarily for crosstalk measurement, either between
channels on a stereo signal or between unrelated channels in a mixer etc. Such
measurements are made by sending a tone on one channel and measuring on the other
channel, and on a stereo circuit this is simply achieved by appropriate use of the
keys on the two units. The reading obtained is absolute in dBu unless a ‘test level’ has
been set. It is NOT referred automatically to the level on the other channel as this may
not be the interfering channel in the case of mixer measurements. To refer the crosstalk
measurement to the level of the tone on the other channel, simply select level
measurement on the other channel and set a test level (eg press
.
Crosstalk options 7 and 8 permit some degree of noise reduction on crosstalk
measurement at a variety of frequencies. Option 7 introduces a 150Hz to 300Hz bandpass
23
2. Manual Operation
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