Tales of VCF 5.0? - test-drb@ccmp.vtda.org

30 Oct 2002

...
 From: "Jim Battle"
&lt;frustum(a)pacbell.net&gt; 
...
 At 06:09 PM 10/28/02 -0800, dwight elvey wrote:
 Hi
  Actually, I was bummed. ...
  I had a Poly-88 setup but I think only one in 10 noticed
and even then, I think only 2 in those ten knew what it was. 
I spotted it, and I knew what it was.  But how could you tell 
that?  Probably other people were the same.

   Of course, my display was all about tape
recovery and not
the Poly-88. I don't think, maybe, 5 people all together
realized that. 
That, I didn't spot.  I noticed the waveforms on the wall, but amidst the 
din (auditory and visual) it didn't register.  I wish I had noticed, 
though, since it is an area I want to spend some time working on one of 
these years.  Rather than letting it be lost to the moment, is there any 
chance of you writing up your work and putting it online somewhere (even 
posting it to this list and letting it get archived would work).

I've read a half dozen very terse summaries of emulators and such that do 
this, but most work only on really clean tapes.  They simply do some DC 
correction, perhaps a bit of filtering, and then time zero crossings and 
decode from there.  Have you done something more sophisticated than 
this?  Some of the Sol tapes I've looked at are way too corrupted for such 
a simple technique to work.

There are a lot of communication theory ideas that could be applied to the 
problem directly; in addition, the file format is going to have some known 
structure that could be used to help train the receiver and disambiguate 
some cases.

I'd be interested to hear more.
-----
Jim Battle == frustum(a)pacbell.net

Hi Jim
 I will try to put something together. I did have an outline
posted at the exhibit but hoped many would ask questions
( quite a few did but only about 1/3 of those knew what
a Poly-88 was ). First, I didn't use any of the high tech
methods. I know a reasonable amount about DSP, having taken
a couple of courses and tinkered some as well. There is
little in DSP that will fix a dropout caused by folded tapes.
 DSP can be used for such problems as high frequency rolloff
or print-thru. These problems are often over stated and
are not the most common problems.
 Understanding how the data is recorded and how your machine
reads these back can be a large help. One also needs an alternate
method of reading the tape because the canned software usually
won't continue after an error. You also need to know the
method used to validate the replay. If you are lucky, they
used a CRC or ECC. In most cases, it was a simple check sum.
 Recovery is more about bringing all the known stuff together
and recreating the missing parts. As long as the recorded
data is something like a program or text the problem can
often be solved. If it is something like raw data, ( strings
of meaningless numbers ) the chances are poor.
 One of the first things to look for is duplicate tapes. Not
all errors happen at the same place and same time.
 Like solving a mystery, it is all about putting all of the
pieces together and not just a single part. It is easier then
one would think but does take some time to develop the
tools needed to get all the information.
 I don't have a web site to publish such information but
I suspect someone will provide it.
 Anyway, I apologize for not having a bigger title. Even though
Sellam gave us plenty of extra time, I was still doing a lot
of last minute preparations.
Dwight