LeCroy DDA 7 Zi series, Operator'S Manual

Page: 504 / 544

WavePro 7Zi
503
WP700Zi-OM-E-RevA
slightly above the +1 level. For E2PR4 the DDA uses 1.06045 for this value.
possible
error
messages:
"top level must be > 0.5 and <= 1.50 x sync peak"
keyword: TRELLIS_LENGTH
arguments: <value 11 to 81>
required?:
no
Notes:
Because of internal implementation constraints, the trellis cannot be 28 to 32 long, or 60 to
64 long. An attempt to set it within those ranges will be accepted but will actually set the
next larger permitted value, that is, 33 or 65. If this keyword is not seen its value remains
unchanged. If this value is never set the default is 81. Longer trellis size does not noticeably
slow down processing. Longer trellis size approaches true maximum likelihood to greater
certainty. Real disk channels tend to have short trellises and rely on encoding to avoid
sequences that would require a longer trellis to resolve. On a good signal all possible paths
being maintained in the trellis are the same beyond a few bits in, the vast majority of the
time.
possible error
messages:
"TRELLIS_LENGTH must be >= 11 and <= 81"
keyword: ZERO_CROSS_TO_SAMPLE
arguments: <value 0 to 0.5>
required?:
yes
Notes:
This tells the DDA how the correct sampling times relate to zero crossings during the VCO
synchronization preamble. The value is a fraction of the spacing between zero crossings.
Example: The VCO synch is normally a 2T pattern. For PR4 the samples are 1, 1, -1, -1;
there are two samples between zero crossings, and the first sample after a zero crossing is
one quarter of the distance to the next zero crossing. Therefore
ZERO_CROSS_TO_SAMPLE is 0.25 for PR4. For EPR4, a 2T pattern gives samples
0,1,0,-1; the first sample is at the zero crossing; therefore, ZERO_CROSS_TO_SAMPLE is
0 for EPR4. For E2PR4, a 2T pattern gives 2/3, 2/3, -2/3, -2/3; there are two samples
between zero crossings, and the first sample after a zero crossing is one quarter of the
distance to the next zero cross, just as for PR4. Therefore ZERO_CROSS_TO_SAMPLE is
0.25 for E2PR4.
possible error
messages:
"ZERO_CROSS_TO_SAMPLE must be >= 0 and <= 0.5"
Example File
An example of a user definition equivalent to E2PR4 (d=0) follows. This file was named E2PR4.UDF:
USER DEFINED VITERBI TRELLIS FOR LECROY DDA // a comment can go here
// The above line is required exactly as shown or the file will be rejected!
KEYWORDS_VERSION 1
STATES 16