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From: grr@cbmvax.UUCP (George Robbins)
Newsgroups: comp.sys.amiga
Subject: Re: Better video, in three easy steps.
Message-ID: <2416@cbmvax.UUCP>
Date: Tue, 29-Sep-87 02:16:54 EDT
Article-I.D.: cbmvax.2416
Posted: Tue Sep 29 02:16:54 1987
Date-Received: Thu, 1-Oct-87 04:36:33 EDT
References: <3795@zen.berkeley.edu> <169NETOPRHM@NCSUVM> <3822@zen.berkeley.edu>
Reply-To: grr@cbmvax.UUCP (George Robbins)
Organization: Commodore Technology, West Chester, PA
Lines: 48

In article <3822@zen.berkeley.edu> bryce@hoser.berkeley.edu.UUCP (Bryce Nesbitt) writes:
> In article <169NETOPRHM@NCSUVM> NETOPRHM@NCSUVM.BITNET (Hal Meeks) writes:
> >The big question is -- Why (and how) does it work?
> >I would like some idea of what I am doing before I go munging
> >around in the innards of my brand new beige box.
> 
> [read the referenced article to know what is going on]
> 
> Ferrite beads are commonly used on output signals to attenuate higher
> frequency noise that might otherwise be radiated exte rnally to the
> computer.  (Incurring the wrath of the Federal Crap-Cutters :-).
> 
> Why ferrite was used on the *VIDEO* output is beyond me.  I'll need to
> assume that for the A2000 it has something to do with the German version of
> the FCC, which has what I have heard are strict requirements. ( I don't
> know about that rumor... US companies always use the FCC as a scapegoat for
> late products. I've never heard this excuse used for a German design...
> so... )

Somehow, I don't think you've tried to get an entire system thru FCC lately.

> Why, again, I don't know.  Even without the ferrite that connector is not
> going to radiate... it's connected to a shielded cable.  My circa 1986 FCC
> docket is not clear if one must test with and without cables attached. If
> so, the solution is simple: add a 15 cent stamped metal end cap.  An empty
> connector would be covered, a full one would have a shielded cable.  Users
> would be instructed to use the cap or a cable.

Uh, what goes in one end comes out the other, not to mention leakage from the
less than 100% efficient shielding.  The video output consists of more or less
square edged pixels derived from the 7 MHz clock.  This decomposes into useful
video in the maybe 0-14 MHz range, and a bunch of higher order harmonics that
exceed the bandwidth of the monitor, but tend to sneak out of the monitor or
cable and into the FCC test receiver.  This effect is worse when the monitor
is separatly powered and thus has multiple ground paths.  Properly chosen the
ferrite beads work magic and chop the bad stuff without perceptible effect on
the video.

Current FCC test requirements specify that the device be tested in a system
configuration.  That is if you sell a monitor/modem/printer it must be tested
with these devices attached.  It must also pass with any arbitrary positioning
of components and cables.  Doens't matter that the user normally doesn't wrap
his mouse cable in a snake coil and leave it lying on the keyboard. 

-- 
George Robbins - now working for,	uucp: {ihnp4|rutgers|allegra}!cbmvax!grr
but no way officially representing	arpa: out to lunch...
Commodore, Engineering Department	fone: 215-431-9255 (only by moonlite)