Disc reader project -- quick status update

Al Kossow aek at bitsavers.org
Wed Dec 9 12:16:26 CST 2009

On 12/9/09 8:40 AM, Keith M wrote:

> Thanks for the discussion.
> I guess I've had some misconceptions and am trying to work them out.
> When I think of sampling, I think of regularly recording the value of a
> pin every so many units of time.

There is no need to record every value if it is only a binary signal, you
can store the delta time to the next transition. This is the difference on
a logic analyzer between 'state' and 'timing' modes. In 'state' mode, you
sample and save every signal at each event. In 'timing' mode, a high resolution
clock is used to measure the durations of each signal.

The argument is how much accuracy is required to accurately describe the flux
transition points in time, given the positional jitter introduced by the recording
and playback mechanisms, and the distortion that occurs in the magnetic medium
depending upon the pulse rate of the recording.

Another problem is even though it is using saturation recording, the media may have
flaws, or the head may clog due to shed, causing pulses to drop out. Data recovery
has to cope with the fact there may be pulse dropouts, sometimes requiring the
recovery program to 'wiggle' the head across tracks to try to rub the clog off of
the heads.

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