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WM-OM-E Rev I
Note:
Unless a drive is in a diagnostic mode (direct write), data is scrambled and encoded when written. In the drive, after
the Viterbi detector finds the bit pattern that was written, the bit pattern is decoded and de-scrambled to recover the user
data. The DDA does not do the decoding or de-scrambling; it does not need to recover the user data in order to find
problems in the quality of the head signal or differences from a reference. This means, however, that each bit in error in the
head signal may turn into multiple bits in error in the user data (for example, if it resulted in a different valid 9 bit pattern in 8/9
encoding).
In addition, a normally operating drive tries to correct short bursts of errors in the user data using an Error Correcting Code
syndrome, so an error in the head signal may result in no bits after Error Correction if it was within the capability of the ECC
to correct it. Usually the bit error rate is measured without Error Correction to check drive operation. The ECC is only capable
of correcting a limited number of errors in a sector, so keeping the number of raw errors low is a requirement.
The SAM values shown by Channel Emulation with a reference signal directly reflect where the
DDA’s channel emulation believes an error (without Error Correction) would occur. The SAM
threshold determines the value of SAM below which a bit position is recorded on the Channel
Emulation’s "error" list. For Channel Emulation with a reference, you can set the SAM down to -1.
This allows the use of either inclusion or exclusion of positions with close to zero SAM, which might
or might not actually be an error. If the threshold is set near -1, only places where the decision to
reject the reference bit sequence was made with considerable certainty will appear in the DDA error
list. If the threshold is greater than zero, all the places where Channel Emulation calculates the bit
sequences will differ, and some places close to making an error (small positive margin) will be
included in the error list.
If Read Gate is present, it does not necessarily go false immediately after the last byte of valid
information usually error-correcting code (ECC). The delay is due to the propagation time through
the channel chip and any delays from the controller. Therefore, you can specify the amount of
"garbage" data to ignore after the end of the written data and before Read Gate goes false. Errors
detected in this area will be ignored.
When Channel Emulation with a reference signal runs, the ML markers ("+" signs) are
automatically displayed. These show the Maximum Likelihood sample sequence that the DDA’s
channel emulation chose, based on the head signal and the possible sequences. They are drawn
at the expected level at the time the channel sampled the head signal. Since Channel with
reference requires a stored reference signal, the reference trace will be overlapped with the head
signal.
The ML markers do not appear if more than 500 are needed on the display; if they are on but not
visible, zoom in on the head signal.
Channel Emulation with reference uses the same channel emulation as Channel Emulation without
reference signal.
Channel Emulation with reference requires the following setup, including a stored reference signal:
•
Head Signal:
The VCO Synchronization field is needed at the beginning of the area to
be analyzed. If the VCO Synch field is not found, Channel Emulation with reference will
not be able to analyze the signal.
•
Reference:
A reference waveform must be acquired and processed by selecting
"STORE REFERENCE" --- subsequent acquisitions will be processed and then
compared to this.
•
Signal Type:
Specifies the type of PRML channel.
Содержание DDA-3000A
Страница 1: ...LECROY X STREAM OSCILLOSCOPES O P E R AT O R S M A N U A L SEPTEMBER 2006 ...
Страница 18: ...16 WM OM E Rev I BLANK PAGE ...
Страница 64: ...62 WM OM E Rev I icon in Channels dialog zooms one trace zooms all displayed traces ...
Страница 65: ...X Stream Operator s Manual WM OM E Rev I 63 then Math Setup then Measure Setup then Utilities Setup ...
Страница 80: ...78 WM OM E Rev I 4 Select a language 5 Click Next when the Wizard appears ...
Страница 81: ...X Stream Operator s Manual WM OM E Rev I 79 6 Select Complete setup ...
Страница 157: ...X Stream Operator s Manual WM OM E Rev I 155 ...
Страница 170: ...168 WM OM E Rev I 5 Read the status information in line StatusDescription ...
Страница 214: ...212 WM OM E Rev I Delay Parameter Find Range selected ...
Страница 263: ...X Stream Operator s Manual WM OM E Rev I 261 ...
Страница 284: ...282 WM OM E Rev I ...
Страница 285: ...X Stream Operator s Manual WM OM E Rev I 283 Using a Surface Plot Excel Example 5 ...
Страница 321: ...X Stream Operator s Manual WM OM E Rev I 319 Further Examples of MATLAB Waveform Functions Negate the input signal ...
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