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Odd TV channel

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Shaun

Sun Sept 07 2008, 11:17PM

Registered Member #690 Joined: Tue May 08 2007, 03:47AM
Location: New Jersey, USA
Posts: 616

Hi everyone,

So I just started college a few weeks ago, and when I plugged the TV into the cable outlet in my dorm (co-ax), I noticed that channel 99 shows some kind of a weird waveform-type display:

It reminds me of my oscilloscope's display, and at the bottom right you can even see a trace selector. Keep in mind I have no control over this, I can just watch it. The waveform itself does jump around a lot, but maintains that general shape (downward slope w/ large spike on the right). I also see a "40.00 MHz" displayed.

The audio on this channel is white noise, and all other channels work fine. Just wondering, does anyone have a clue as to what the purpose of this channel is?

Thanks,
Shaun

Ken M.

Mon Sept 08 2008, 12:10AM

Registered Member #618 Joined: Sat Mar 31 2007, 04:15AM
Location: Us-Great Lakes
Posts: 628

Maybe a senior clasman was messing with the cable?

Backyard Skunkworks

Mon Sept 08 2008, 12:27AM

Registered Member #1262 Joined: Fri Jan 25 2008, 05:22AM
Location: Maryland, USA
Posts: 451

Ask around campus a bit, I'll bet it's some experiment that they want people to be able to monitor at any TV. Just don't get fooled when they try and put one over on the freshman

ShawnLG

Mon Sept 08 2008, 04:54AM

Registered Member #286 Joined: Mon Mar 06 2006, 04:52AM
Location:
Posts: 399

When I used to have cable, it also had that channel with the spectrum displayed. I do not know what it is for.

Coronafix

Mon Sept 08 2008, 05:00AM

Registered Member #160 Joined: Mon Feb 13 2006, 02:07AM
Location: Melbourne, Australia
Posts: 938

I saw a similar channel when I was in Manila (Philippines) recently on the hotel cable.

Seoul_lasers

Mon Sept 08 2008, 05:45AM

Registered Member #1630 Joined: Sat Aug 09 2008, 11:36AM
Location: Seoul Korea
Posts: 115

I have seen this all over the world.
England, Japan, Germany, Canada, USA, South and North Korea, France...
This channel is used to test the line signal quality.
You will notice mV on one side of the graph and Frequency on the other.

dbmV is the signal gain/strength.

Ultra7

Mon Sept 08 2008, 12:10PM

Registered Member #1157 Joined: Thu Dec 06 2007, 12:11PM
Location: Houston, TX
Posts: 307

5 - 42 Mhz is the cable industries' return data frequency area.
We set the spectrum analyzer to a center frequency to monitor the entire return path.
The large spike on the far right are actual modems ranging with the system and transmitting data.
the smaller spike on the left is the return data carrier for Video On Demand.
The spike in the center could either be another VOD area, or it could be someone injecting a carrier.

What you do if you don't have enough DSAM or SDA 5000 meters for all of your cables guys, is you broadcast a shot of the spectrum analyzer of the node onto a channel that anyone with a test TV can monitor. Each node in the system will have a different shot of the upstream of their particular node.
That way, if there is noise on the line somewhere in the node, you can have one guy watch the Carrier to Niose floor on that channel, while 2 other guys go and narrow down where the noise is coming from and systematically remove it from the system.

I just happen to work for the cable company, so for once I feel like the Dr.

Shaun

Mon Sept 08 2008, 01:31PM

Registered Member #690 Joined: Tue May 08 2007, 03:47AM
Location: New Jersey, USA
Posts: 616

Ahh, thanks guys (especially Ultra7). It did seem like it had something to do with on-site testing by a cable repairman, or similar.

So is this something only used on smaller cable networks? I've never seen it at my house or in any normal residential area, but CoronaFix saw it in a hotel, and I am living in a dormitory.

Ultra7

Mon Sept 08 2008, 01:48PM

Registered Member #1157 Joined: Thu Dec 06 2007, 12:11PM
Location: Houston, TX
Posts: 307

Shaun wrote ...

So is this something only used on smaller cable networks?

Exactly.
See, a spectrum analyzer is a pretty expensive piece of equipment, and the Signal Level Meters that have them are as well. So if you are having intermittent noise issues in a nice small plant area or if you have a mainly contractor maintained area, you broadcast the output of your Spectrum analyzer onto an unused or test channel. That way, everyone can monitor the noise or traffic of the return path. It's also time for the annual FCC Proof of Performance tests, so this might just be a temporary thing.

Here is a nifty thing to do.
Set your TV to this channel during a low traffic time, and then jump on the internet and start pinging yahoo or some other host that will accept a ping and watch the spike on the TV. Every time you transmit, you'll see the carrier spike out.

Although, you being on a campus, I'd imagine there are not too many "Low-traffic" times.

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