From: elmquist@pez.ssesco.com (Chris Elmquist)
Subject: Re: Cable reception
Organization: SSESCO
Date: Tue, 6 Oct 1992 18:27:51 GMT

In article <1992Oct5.000218.2291@ultb.isc.rit.edu> rxv9839@ritvax.isc.rit.edu writes:
>In article <BvI9Ey.1qF@ssesco.com>, elmquist@pez.ssesco.com (Chris Elmquist) writes:
>>In article <petec.718042450@splat> petec@splat.dell.com (Pete Cruz) writes:
>>>I just bought a 46" TV and have now noticed fairly bad reception from my
>>
>>For me, I get herringbone interference on channels 37, 38, 39, 40, 41, and 42.
>>The herringbone is very visible on larger screened monitors like 27" and less
>>noticeable on a 19" and not at all objectionable on a 13".
>>
>>When the Cable Co. comes out to make measurements... they bring a portable 9"
>>TV so that they run it on batteries in the bright sunlight out in the yard.
>>For some reason (?? what could it be ??) they have never seen the herringbone
>>on the 9" but admit to seeing it on my 27".  
>.........
>
>To elucidate this matter just a bit--
>Grainess, snow that the original poster was having is a very different problem
>in cable than herringbone.  Herringbone can come from loose fittings, impedance
>mis-match and other causes, both in your TV and in the cable companies headend
>and distribution plant.  You are correct that it is most likely in the headend,
>however your TV set is not positively ruled out by the test you describe so
>far.
I didn't want to get too far off the other posters topic...  you are correct
that the problem I am seeing is different than what he is seeing.  In the
general sense though it is a reception problem.

I believe I have eliminated my TV from having a problem by the following
actions (which I did not mention in the previous post).  I have taken the TV
to another part of the city which is served by a different headend.  There, the
picture is near perfect with no sign of herring bone.  I have tried other sets
(as well as VCRs and cable tuner boxes) at my location and all of them produce
the herring bone on channels 37, 38, 39, 40, 41, and 42.  The actual frequencies
of these channels (300+ MHz) have no nearby high power transmitters operating
anywhere in the Minneapolis/St. Paul area (I have checked... and I have
listened on a communications receiver) so I have ruled out ingress. 

My current theory is that they are transmitting the audio subcarriers on these
channels off frequency enough so that it spills into the video portion of
the carrier.  Although I don't have equipment accurate enough to measure
the *actual* frequency of their carriers, I did measure the step between
each video and audio carrier as I moved from channel 36 to channel 43.
Most of the audio carriers are seperated by exactly 6.0003 MHz but when I step
from channel 36 to 37, there is only a 5.9993 MHz difference and then
when I step from 42 to 43 there is a 6.0013 MHz difference.  This tells
me that the audio subcarriers of my problem channels are all shifted down
in frequency.  The video carriers all stepped exactly 6.0003 MHz.

I don't know all of the details of how the modulators work in this
GI/Jerrold monster.. but I'm guessing that they have seperate oscillators
for the video and audio carriers and that some adjustment is provided
to keep them on frequency.  Until proven otherwise, I will keep pestering
them to prove that these channels are on frequency.  If they choose to
replace all of their line components between me and the plant, well, that's
OK too.  *That's* their nickle.

>If the cable company has other problems on your line it may be a logical
>troubleshooting procedure, rather than easter-egging, that they are using to
>correct the problem.  (Not intended as a defense of them, simply an observation
>of another possibility.  I'd probably defer to your on-site evaluation if you
>have a sound basis for it.)  They probably use battery operated TV's and small
>at that for in the field (outside of homes) troubleshooting.  Its a trade-off. 
>Most mobile power sources don't treat TV's kindly and can induce more problems
>than the systems really have.  Its hard to troubleshoot a noise problem if your
>generator is putting glitches all across the TV screen.  And after you have
>blown up a TV or two on a poorly regulated power invertor, one opts for a
>battery TV.
Sure...  that all makes very good sense except that their position is that
because they don't see the problem on this little TV that there is *no*
problem.  I don't think it's reasonable to limit the scope of their
*quality* service to TVs with screens 9" or smaller.

>And here's a helpful tip.  After they're alll done and if you still have
>herringbone ask if they have a "sweep" program and can they sweep out to your
>house.  It is a process where they insure the flatness of their amplifier
>response and can make a big difference in picture quality if it is radically
>off.

Thanks.. that is a good tip.  I will ask them about the flatness of the
amps.

Chris
Chris Elmquist, N0JCF
elmquist@SSESCO.com
73267.2711@Compuserve.com
(612)342-0003@work (8am-5pm CST6CDT)
Newsgroups: rec.video.cable-tv,misc.consumers,alt.cable-tv.re-regulate