Each channel is measured and the graphs updated on alternate sweeps. When testing
mono circuits the channel should be selected on the LA101 with
before running the
sequence (after pressing the
key).
Sequence 16 is a Selftest sequence which can be used to test the LA100 using XLR leads
to connect the two units back to back. This sequence automatically selects the Selftest
tolerance in the LA102, so a PASSED or FAILED display will appear as soon as it has
finished. See section 10.6 if it fails.
400Hz test level
Sequence 17 is intended for testing FM transmitter systems where a 400Hz reference tone
is used and where the high frequency distortion and crosstalk tones are attenuated using a
50µs pre-emphasis curve (see below). Sequence 18 provides a sequence compatible with
German standards which specify distortion measured at +6dBu, sweep 20dB below 0dBu,
noise referred to +6dBu, crosstalk at +6dBu, W&F at 0dBu and phase at 10dB below
+6dBu. Sequence 19 provides a PPM (Peak Programme Meter) test using tone bursts and
reference levels. The user should compare the target levels shown in table 3.1 (and on the
LA101 display) with the peak meter readings. Note that this test sequence does not
generate any FSK as it is not normally used with an LA102 (although it can be used to
test the LA102 PPM available on level option 5).
PPM (peak programme meter)
Careful consideration has been given to the level at which crosstalk should be tested. Too
high a level can cause more, or less, crosstalk through overload, especially at high
frequencies if pre-emphasis is in use. Too low a level makes it more difficult to read
crosstalk in the presence of noise. While -10dB was considered it was felt too low to
allow discrimination from noise, even with the narrow band filtering in use in the LA102,
and so 0dB was adopted. Segment A gets round the problem partially by reducing the
level at high frequencies according to a 50µs pre-emphasis curve (-10.36dB at 10kHz)
with corresponding correction factors applied in the LA102. This will probably be most
useful in testing FM transmitter systems, where 50µs pre-emphasis and limiting are
applied. See section 4.1 for details of defining user sequences and section 4.5 for details
of altering segment levels.
Sequences need not be limited to the applications listed in their headings of course. Many
people will prefer to test cassette machines using sequence 11, and with type II or IV tape
the different high frequency saturation levels can be readily demonstrated by running
sweeps at different levels. See section 4.18 for details on running multiple sweeps.
The Lindos Default sequences were the only ones provided in early LA100 units. In
response to demand from users, the number of inbuilt sequences now exceeds 100, and
for ease of access they are arranged in sequence banks.
While the inbuilt sequences provide a variety of useful sequences, many users will want
to define their own sequences and this is readily achieved using the sequence editor built
49
3. Sequence Testing
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