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This is a quick rundown of the analog test. The test is basically measuring the filtering that happens within the analog electronics of the drive. Showing what the low pass filter discards as happening “too quickly” and when the gaps get so large that the drive starts spitting out noise (due to automatic gain control or some other similar function) which I refer to as “bit injection”.

The noise test on the left side gives us a base line for the density of noise that is generated by the drive when in “full panic” mode. A track is wiped clean of any data and the number of flux transitions per rotation are recorded and graphed out. This is a worst-case kind of scenario.

The primary analog test uses a test track that is comprised of a whole bunch of smaller tests. A test has a header, gap, payload, and settle. The header contains information about the specific test (like the gap size). Following that is the gap (no flux area) whose duration ranges from 1µs to 48µs in 250ns increments. Then there is a payload which is a special bit sequence that is used to be able to detect bit slip, desynchronization, and other conditions. And then the settle which is a bit pattern that lets the analog amplifier/gain control/whatever cool down before the next test.

So, the track where the test is going to go is first wiped (WRREQ on to engage head erase coil and writing no data). Then the tests are generated and written. Read it 50-ish times. What it is checking for is the integrity of the gap and payload. If the gap is clean (no bits injected) and the payload is as well, then that is a success and it adds to the gray lines in the graph. In the case of a failure, the payload is checked for integrity and if the payload is intact, then we check the gap. If the gap has a spurious transition (injected bit), then the time offset from the last flux transition of the header to the first spurious transition is recorded onto the graph at the bottom. The gold line is then calculated based on the successes. When the line is all the way at the top, then it indicates that the gap and payload were read 100% reliably.