From: rxv9839@ritvax.isc.rit.edu
Subject: Re: Cable reception
Reply-To: rxv9839@ritvax.isc.rit.edu
Organization: Rochester Institute of Technology
Date: Fri, 9 Oct 1992 05:00:30 GMT

>>>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".  
>>.........
>>
>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
**details deleted **
>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.

You are correct.  This can cause the problem you describe. However be aware
that many cable companies are required to offset certain channels in the
aeronautical frequency bands by about 5Khz.  Where these mesh with non-offset
channels you will see a variance.  In any event a 1.3 Khz difference should
generally not cause a problem.

>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.  

You are absloutely correct.  Aeronauticals have frequency stability
requirements which the operator must meet.

>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.

And will improve  your neighbors' cable service.  No doubt they will invite
you over for popcorn and a pay-per-view event.

>>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.

With that you and all 9" and larger TV owners will agree.  And I do too.  Their
argument is not supportable.  NOBODY watches a 9" TV these days.

Also congratulation on being in the 0.5% technical minority.  NOBODY carries a
27" TV cross town to another headend to check out reception.  I applaud you. 
The cable company might value your resume.
Bob Vogel

Newsgroups: rec.video.cable-tv