determine the nominal level (used for frequency sweeps) and is also used to normalise
crosstalk and noise measurements. If run as a single segment the tone is held after the
segment has finished.
4.7 Frequency Sweep Segments (OPQRSUXorux) Segments O,P,Q,R,S
Various frequency sweep segments are available, providing a range of sweep speeds
(1½s, 5s or 20s), different frequency ranges (20Hz-20kHz, 10Hz-30kHz, 300Hz-18kHz
and 300Hz-8kHz), different amounts of headroom (see section 3.14) and different
printout frequencies. All of the frequency sweeps take 256 level measurements per
channel over their frequency range and the LA102 sampling rate is varied accordingly.
Segment U is a standard 5s frequency sweep segment covering the frequency range 20Hz
to 20kHz. Segment S is a slow sweep, 20 to 20kHz in 20 seconds, which is better able to
follow steep filters. Segment X is a fast 1.5s sweep but care is needed when interpreting
the results as it cannot follow fast changes in the level. However it is very useful for the
setting up of tape head alignment, especially if it is repeated (‘X<’). To achieve the fast
speed the resolution of this fast sweep is less than the slower sweeps (causing obvious
steps at low levels, especially if the graph scale is increased).
Segment U Segment X Segment o Segment r
The spot frequencies used in the printout and for tolerance testing are shown in fig. 3.2.
Segment O is a 5s sweep with special frequency listing on printout to comply with British
Telecom (BT) lines tests EPS81/EPS84.
Segment u Segment x
4.8 Noise Segments (L,M,N,n) Noise segments 22Hz-22kHz bandwidth A weighted noise CCIR weighted noise
Segments N and M measure peak (PK) noise as required by the CCIR standard as well as
peak unweighted noise and mean (MN) weighted noise. ‘Peak weighted noise’ is the
peak reading of the quasi-peak rectifier taken over the measurement period, which will
normally be slightly worse than the mean, but some circumstances, such as telephone
dialling pulses on lines, may result in a larger peak to mean difference. Segment N (8s) is
usually used, but segment M (30s) can be used where a longer sampling window is
required (again where the noise is intermittent). Segment L measures ‘A’ weighted noise,
and 22-22kHz noise using the rms rectifier. Segment n measures CCITT O.41 weighted
and unweighted psophometric noise but is only recognized by LA102 sets equipped with
a suitable filter board (see appendix I.5).
Slow noise measurement
All noise measurements are relative to the measured test level. For example, if ‘TL OUT’
is +2dBu, and the noise level is displayed as -95dB, then the absolute level of the noise is
-93dBu. It is therefore important that the noise segment is preceded by a test level
segment (T or V) otherwise a
SEQ ERROR 13 will be generated. Sequence error 13
73
4. User Defined Sequences
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