# TV Modulators



## chausman (Oct 11, 2011)

My school has a small tv studio, with a few cameras and a video switcher, that used to be able to broadcast to the school wide network. They used to do weekly broadcasts as a class, but recently there hang been funding for it. I'm trying to get it used again, but we are having a problem with getting signal from the switcher to the rest of the building. When we have a camera setup, we can see on the preview monitor that it has a very clear signal (clear for being 15 year old technology), but a tv setup after the modulator in a classroom has a very low quality signal. We are currently using a Blonder Tongue MAVM modulator broadcasting on ch. 4. Would adding an amplifier help the quality? What are some that you would suggest for a signal being sent to every room in the building? I would say more, but thats about all I know. Other then the building is 15 years old and things are very hard to get to. We might start using it again, if we can get something usable from it. 


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## Nelson (Oct 11, 2011)

Do the classrooms have cable TV as well? If so, how is the picture quality? If that is poor as well, is there a distribution amplifier between the modulator and the splitters feeding the classroom televisions? If you are feeding more than two or three TVs, you will need some kind of distribution amplifier to overcome splitter and cable losses. The more TVs you have, the bigger amp you will need. Others here can probably suggest specific amps with a given number of television sets. If there aren't too many TVs, an inexpensive hardware store or resale shop amp would help. Otherwise, you might have to look at a commercial amp. We use a Blonder Tongue amp, but I don't know what model.

Has this system worked well before? If so, what has changed?

Its a lot of fun getting this stuff to work! I've always had an interest in video applications, so I'm glad to see you trying to get this studio back on the "air"!


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## jbrem003 (Oct 11, 2011)

For longer video runs you need to amplify your signal. How long is the able run between the video source and the video feed that is giving poor quality? Also, what type of cable are you running along?


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## venuetech (Oct 11, 2011)

There may be a switch at the central distribution point, Do classrooms get a exterior cable signal at this time?


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## TheatreImage (Oct 11, 2011)

There are several factors that could cause bad signal in a CATV setup. Higher end Modulator usually output 40 to 60 DBmV and will loose all there quality when they are turned down all the way, to properly balance the output you should use RF Pads. On the distribution side (the cable in the building) there will be either distribution TAPS or splitters. Both of these devices have Port to Port isolation which stops signals from interfering and transferring between output ports... The best way to broadcast from on room to the whole building is to send the signal to the main distribution point in the building thru a sub-carrier channel. Then Demodulate the signal and re-modulate to a regular channel like 4 for example. If the building distribution is designed properly the broadcast picture from the room will be as clear as the rest of the channel... Let me know if that helps... I can explain more in-depth if it would help...


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## danielp (Oct 11, 2011)

Back at my High school we had a broadcast systems, but sadly no broadcast room. Well, in my junior year they decided they wanted to broadcast an event taking place in the auditorium and after the state employees came in and set it up (incorrectly) they called me and had me troubleshoot the system.

Glossing over the copious errors to get to the point I discovered that, like you, we were using Blonder Tongue hardware except I believe we were broadcasting on channel 13. What this information doesn't tell you is that in reality where we had the video feed plugged into the cable network we were reverse broadcasting on channel F1, if I remember correctly. When I found this out I was very confused, how on earth do you get a letter channel? Well, quite simple actually, the letter channels, as I found, are broadcast on frequencies out of range of the standard CAT TV channels that your TV receives. This signal is then broadcast back to the CAT TV rack back in the server room at the school where it is received by a F1 channel receiver then injected back into the cable network on a "real" channel where the cable first enters the building, right after channel 13 is removed from the building cable feed.

Figuring out this routing took a while but once complete I was able to adjust signal gain and other settings on all the units to stabilize the broadcast as best I could on a Frankensteined high school television network with passive splits and burned out distribution amplifiers, which may be another issue you may have to look into.

Good luck, it can be quite an adventure working through those systems after so many people without a clue about broadcasting "try their hand" at it.


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## DrPinto (Oct 11, 2011)

A 15 year old system isn't that old at all. Also, Blonder Tongue equipment is pretty good stuff. Is there any way to find out who the AV teacher was back then and where he/she is now? That person may have some input into how to get it going again. You could also ask AV people for help from other schools in your district. I'm sure someone in your school system may have some ideas. One last resort may be asking for help from your local community access TV station. That's usually where AV geeks end up after graduation from high school.


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## thesigma (Oct 11, 2011)

Until last year I worked as an AV tech for the local school board (I am now a radio Engineer). Iwas responsible for the CATV distribution system for over 21 buildings and worked on nearly all of them during my 5 years there. There are dozens of things that can cause a "low Quality Signal". If you had access to a CATV signal meter (or spectrum analyser) your job would be much easier, without one you are working blind. Who in your school district is responsible for the CATV systems within the schools? In MY state (VA) the Public television station would work on them, for a fee of course, but very reasonable, they were doing this before I got there and for my first year I would work along with the field engineer they sent out. eventually my skills grew, and the budget at said station grew smaller, the field engineer became the chief, and only engineer (downsizing) and thought they still did the work, the backlog was great. 

the cable company can also be your friend, or a school officials friend. In our county the cable company depended on a franchise agreement with the county which got us things like access to their fiber network for our inter/intra net, and some repairs would be done as a courtesy. some buildings even had thier hardline run inside, and they pretty much had to work on those because the tools are very expensive.

some questions: 
Does it look bad everywhere? this can help isolate problems if some rooms are good and some bad, you may have a failed cable or device on one leg of the distribution.

does the Cable TV signal look bad also? this will help you know if the problem is with the in house signal chain. If everything else looks good, look ath the chain from the modulator to the combiner. If it is all bad you should start looking post combiner.

did it ever work right? I worked on systems that were very old and ones newly installed that did not work right and could not ever work right because they were improperly designed. these can be a nightmare, it helps greatly to know that the system is capable of working correctly. if it did, di some thing change since it was working?

Are there channel elimination filters on the incoming CATV feed to eliminate the channel you are broadcasting on? A well designed system will use these even if the channel is/was vacant. I have had interference coming in on an empty channel cause problems with in house broadcasting. I have also had cable company channel reassignments cause problems when a previously empty and unfiltered channel was populated by the cable company. these filters are often left out or eliminated from proposals because good ones are very expensive.



Barring that some things that may help:

Splitters and taps can fail.

amps can fail

The cable itself can fail and allow interference in or make a poor connection.

A signal that is too hot (high) will look mostly like a signal that is too low, so do not assume the signal is too low off the bat.

The modulator should have a gain knob on it's front panel, you can play with this and see if it improves the signal, though they typically only attenuate 20 dB and you will have 40-60dB gain out of that modulator. there will also be other settings but you really can not properly set any of them, even gain, without a signal level meter.

i'm sure there are other things I am forgetting, but basically you can start from the hints here, but your job will be very difficult without the signal level meter, the one I had on that job was over $800, and the cable co. guys all said it was a toy.....theirs were about $1600 or so I think...

good luck and don't be afraid to get some help.


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## museav (Oct 11, 2011)

I agree that there could be many factors and one would have to know more about the current signal flow and equipment to make an informed assessment.

Do not jump to the conclusion that an amp is necessary or will help, in fact it could actually make things worse. The goal is getting a proper level at the end points and having excessive level causes its own problems. Over the years I've probably pulled as many RF amps out of systems to fix problems as I've added to them, especially cheap, fixed gain, non-tilt adjustable RF amps.

A backfeed system like those that TheatreImage and danielp described might be common for supporting signals back to a headend from multiple locations but is not the approach I've typically seen used for inserting local programming from a studio. In the school RF distribution systems I designed around 15 years ago I often accounted for backfeeds from auditoriums, large classrooms, etc. but where a studio was involved I usually used direct feed from the studio to a modulator. I would take the main cable feed, insert a channel filter to clear out an unused or unwanted channel and run that into a combiner with the modulated studio signal in order to add the studio signal onto the main cable feed, which then fed the distribution system.

If the system worked in the past and still works for some other signals then the problem is probably not the distribution system itself but more likely in how the signal from the studio interfaces with that system. An example of such a problem would be if the distribution system was updated to support digital signal compatible ATSC modulators and demods/tuners but how the studio ties into that system was not similarly updated.

Of course another potential factor is that for a generation brought up on digital HD viewing, a composite video signal distributed over an analog RF distribution system and possibly viewed on a digital display may result in what is perceived as a poor quality image even if everything is working properly.


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## thesigma (Oct 11, 2011)

Hah, tilt, knew I was forgetting something, though I doubt it is his problem. Basically, as a signal travels along a length of cable the higher frequencies attenuate faster than the low ones, eventually you get to a point where low frequency channels look good, and higher ones get grainy and even higher ones may go out alltogether.

also should mention that if you can get ahold of the signal level meter, analog NTSC signals should be between 0 and 10 dB, although most sets don't mind 15dB and some can pick up a clean signal in the negative.

All the systems I worked on or designed myself had the studio feed directly to the modulator. High schools had the headend in the control room, middle and elementary mostly had the headend/controlroom/studio all in one. we had one or two schools that had the sub band modulator system on a cart, htey were never used because no one knew how to work them. I trained them how to do it and they still didn't want to hassle with it. We ended up just using a direct Composite video/Balance audio line from 4 locations (Gym, Cafeteria, Principals office, Library) to the headend, and later Cat 5e and Muxlab Baluns. Both worked well, muxlab easier to install if I was doing the work myself, but cost a little more.

but I digress from the topic at hand.

OP, feel free to ask more questions, if you can answe some of the other questions asked of you we can help you more.

or find a friend at the cable company....


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## Repeater (Oct 11, 2011)

Is there a DTV station on ch 4 in the area? (I mean, actually using ch 4 to transmit on-it may be branded as another channel). Ingress from that station to your system will cause noise on your desired signal that looks just like a weak signal.


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## chausman (Oct 12, 2011)

Sorry for the delay, I had a hard time getting time with a real keyboard to answer your (many) questions.

The classrooms do have outside channels, they don't look great...worse than the signal from our modulator. (It is also on a different line ("Utility" instead of "School").

Yes, the system has worked well before, and the teacher who originally used the system with a class, is still here. (and one of my teachers)

The cable runs are very long. I can't say exactly, but everywhere in the building. Probably close to a 1000' total.

I don't know what kind of cable is being used, only that is is terminated with coax.

I'm not sure if we are using ch 4 straight from the modulator, but I don't think we are. It (the modulator) has a sticker on the front that says ch. T18, which leads me to believe that it is being modulated, then sent to a distribution point, and then de-modulated, and re-modulated to the ch 4 that the TVs in each room are set to to watch whatever we broadcast.

All tvs that we had on at the time looked the same.

I'll look into all the channels that work or don't work on the outside cable signal.

It did look different this morning when I got a camera out then last week when we set it up. I don't know what was different between those times though...

I do know that it won't be as clear on here as an HDTV, but I'm not just basing all of this from what I've seen other places, but the preview monitor (pre-distro) is clear, and the tv in a classroom (post-distro) looks bad.

Thanks


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## Nelson (Oct 13, 2011)

Can you tell us more about how the classrooms switch from "utility" to "school"? Are there actually two different cables coming into every classroom?

Assuming there is just one distribution system, used for both cable TV and school studio broadcasts, I'd look at the distro system and see why cable TV looks bad. It is quite possible that fixing the bad cable TV signal issue will fix your broadcast issue. Assuming you are comfortable working on the school's cable TV distribution system (and allowed to work on it ), analyze each link in the system and see where the picture quality falls off. Does the signal look clear if you connect a monitor to the incoming cable TV feed? Is the amp on and working? Does it look clear after the first splitter? And so on.

If the teacher that used to use the system is still working, can you ask his/her assistance? It might be much easier to ask their help than to "reinvent the wheel"!


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## chausman (Oct 13, 2011)

Nelson said:


> Can you tell us more about how the classrooms switch from "utility" to "school"? Are there actually two different cables coming into every classroom?
> 
> Assuming there is just one distribution system, used for both cable TV and school studio broadcasts, I'd look at the distro system and see why cable TV looks bad. It is quite possible that fixing the bad cable TV signal issue will fix your broadcast issue. Assuming you are comfortable working on the school's cable TV distribution system (and allowed to work on it ), analyze each link in the system and see where the picture quality falls off. Does the signal look clear if you connect a monitor to the incoming cable TV feed? Is the amp on and working? Does it look clear after the first splitter? And so on.
> 
> If the teacher that used to use the system is still working, can you ask his/her assistance? It might be much easier to ask their help than to "reinvent the wheel"!


 
One jack for Utility, on for School. Yes, two lines run to every room. 

I've got some help looking through the runs to see where they are going to, to find the actual components. I'll work on trying to get to everything.

I'm comfortable working with it. It doesn't appear like anyone really has a problem with me and another student working on it. 

I would like to ask the teacher, but he doesn't know. He just knows that "this needs to be plugged in here, and then the TVs are on channel 4". 


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## venuetech (Oct 13, 2011)

I have a "T" channel modulator but it is only used when we need to have the cable company broadcast an event. we coordinate with them and during tech they send a technician over (with the $1600 tester) once he is happy that we are sending a solid signal on the T-modulator he goes out to the "headend" to check the signal at that point. when the time comes he goes back out to the headend and patches our signal to the proper cable channel.
In our case the T-modulator is not used for the daily announcement broadcast.

finding the distribution point will likely answer a lot of questions this is likely near the telephone punch blocks are or where all your network cables end at a switch rack.


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## museav (Oct 13, 2011)

T7 through T13 are CATV sub-band channels, the channels usually used for things like backfeeds, but I don't know what Channel T18 would be.

One thing to check might be to see if the quality is essentially the same everywhere or if it tends to vary. Something like it being better closer to the headend and getting worse as you get further away could suggest certain issues being involved. If it is the same everywhere then you probably have to start looking back at the system prior to where it starts to split or tap.


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## chausman (Oct 13, 2011)

museav said:


> T7 through T13 are CATV sub-band channels, the channels usually used for things like backfeeds, but I don't know what Channel T18 would be.


 
Oops... 

Its T8, not 18. I'll try sometime to get it running to multiple places at once to see what happens. 


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## chausman (Oct 14, 2011)

Just learned a little more. The "Utility" line has a good number of channels that work and are clear. The "School" line has most of the channels as in "Utility", but they look terrible. (IMO) What I _think_ is happening, is that the incoming signal is split, with one going to the classrooms, and one going to a combiner which has our broadcast. I don't know if this is possible, but that seems to be what's happening to me.


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## Blacksheep0317 (Oct 16, 2011)

If you want to PM me I think I may be able to help..just alot of trouble shooting to do via forum at the moment. I just recently overhauled my old HS in which we were using repurposed blonder tongue gear from a distance learning lab. We now have 6 modulators ganged together broadcasting locally, and out to a trunking station to feed the district and a trunk to the cable network for some public access stuff we do. Plus I am working on doing web simulcast with the new console we got. I think we can prob get you somewhere.


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