Not to outdo anyone or nothin' but I just wanted to show off my latest project. (Sorry it is a bit long, but I wanted to get this all on here for others.)

First, a bit of background. You may recall that last year I posted a thread on adjacent channel interference occuring when I tried to use vintage TVs on analog cable. http://antiqueradios.com/forums/viewtopic.php?t=134661&highlight=adjacent+channel+interference. In the end of that thread I concluded that the IF filters in (most?) vintage TVs do not cut off sharply enough at to allow for adjacent VHF channels to be receiver without adjacent channel interference causing a 'herringbone' type pattern on the screen. Indeed, even Wikipedia mentions this issue http://en.wikipedia.org/wiki/North_American_broadcast_television_frequencies Modern 'cable ready' TVs use IF filters which are designed to receive adjacent channels to prevent this problem from occurring.

Since one of my favorite parts of working on vintage TVs is getting the tuners and IFs aligned and working correctly, I really wanted a way of using more than just channel 2 or 3 on my vintage sets - hence my trying to use them on analog cable.

With this in mind, I created the list below. Each number represents a channel. A blank line represents a frequency gap between channels. An 'x' beside a channel represents one which will not be used to prevent adjacent channel interference. From this table it is obvious that the maximum number of VHS channels which can coexist without interference is seven. They are 2, 4, 6, 7, 9, 11, 13 or equally 2, 4, 5, 7, 9, 11, 13.

2
3x
4

5x
6

7
8x
9
10x
11
12x
13

Creating a signal containing only these seven channels proved harder. I first tried buying channel elimination filters to filter out the offending channels from analog cable. However, they are too pricey to buy new and when I found them on eBay they were never tuned for the channels I needed. I tried retuning them but this proved impractical due to the number of coils I had to wind and tune - 6 per filter. I also had trouble getting the Q of the re-tuned filters high enough to work on the high VHF channels. I eventually had to give up.

After some more thought, I realized that analog cable would not be around forever and that I was going to get much better results using agile modulators to create the channels that I want rather than eliminating those which I don't want. Hence, I bought seven agile modulators off eBay for ~$25 each - an amazing deal considering that these things sold new for ~$1000 each.

The problem then became "How to I feed signal to seven agile modulators simultaneously?" I initially wanted to use DTV boxes to do this. After all, DTV is the direct replacement for analog broadcasting and the idea of creating 'The Ultimate DTV Converter' had a lot of appeal. However, I discovered that I can't receive DTV from my apartment here in Cambridge and that DTV converters still run ~$40-$50 each for the good ones. Thus, I decided to use the analog cable TV that we get free with our (rather high IMO) rent and I bought seven agile demodulators to go with the seven agile modulators. To complete the system I bought a passive head-end combiner to combine the outputs of the seven agile modulators together into one cable and a distribution amplifier to amplify the input cable signal sufficiently to provide enough signal strength to each of the demodulators. A schematic of the completed project is below.



And last but not least a couple of pictures:





Note that it is built on a couple of 16U rack rails so it holds itself together as a unit. Eventually I am going to have to move this monster, perhaps cross-country, so that was a must. To support the components in the back so that they are not cantilevered by the rack rails, I installed rubber feet of the correct height on the rear corners of each. I also custom-made each cable to assure that they were exactly the right length and looked as neat as possible. The power-strip and splitter on the back are held in place with mounting tape so that I did not have to damage anything and could still mount them securely. Finally, the power cord from the power strip is strain-relived by being clamped to one of the case-screws on the top agile demodulator.

For reference, the equipment used is as follows:
Modulators: DX Antenna DSM-140B
Demodulators: blonder Tongue AD-1B
Passive Headend Combiner: Pico Macom PHC-12G
Distribution Amplifier: Pico Macom CA-30RK500
Splitter: blonder Tongue DGS-8

Although I do not yet have an antenna set up to try actually put analog TV back on the air (in my apartment anyway) the cable TV output works VERY nicely and produces a better picture on my little 1980's portable Panasonic TV than I have ever seen before. It is clear that the set was made to receive only seven channels!

Although I did not tally up the exact cost of the project, it was only a few hundred dollars worth of parts, well within the range of other hard-core vintage TV collectors. It is also future-proof in that I can easily replace the agile demodulators with cable-TV boxes, DTV converters, or even just DVD players depending on what the source of TV is in the future. For now though, I am just looking forward to getting to watch TV over the air and in analog black-and-white - just like it was meant to be enjoyed!

-Matthew D'Asaro

When you say 'demodulator', do you mean analog receiver for each channel with audio and video output?

Johnnysan -
Yes. Exactly. They are Blonder Tongue AD-1B model agile demodulators. Each one takes cable TV in and produces component audio and video signals for a single channel. They are effectively just digital tuners like are found in VCRs but packaged in a rack-mount cases.

Blunder-Tongue?

Looks like you custom-made all those cables; must have been fun.

So tell us what you find is the best antenna. On mine I discovered simple rabbit ears worked the best, but I didn't try anything in the way of proper loading

Typo is fixed. I meant Blonder Tongue of course.
And yes, I made every cable by hand. The RG-6 cables terminated in F-connectors were easy since I have a compression tool for those. The RCA cables for the audio were harder since I needed to solder on the ends. I used RG174 for those since I have a whole roll of it, but I kept having problems with the shield melting through the dielectric when soldering them. You have to solder real fast...

-Matthew

I use a system a bit like yours, except that I don't have the DTV reception problem. To recreate our original analog OTH channels here, I have six DTV boxes tuning in digital channels 2, 4, 5, 7, 9, and 11, and these feed six B-T agile modulators creating analog signals for those channels. A B-T passive combiner combines them all into a single cable. Right now I don't broadcast, but have this thing feeding a bunch of TVs in one room by cable. I may plumb the analog signal into my house's TV antenna cabling, but don't have that up and running right now.

Anyway, this all works very nicely. I too like to run my vintage TVs on multiple channels. That's part of the fun.

Wow, this is a great setup, quite cool that you could use it to turn on an old TV and be able to tune in the whole basic array of local stations with the original tuner!
I would like to set up something like this, but I wonder if there is a way that cheaper RF modulators could be modified to transmit VHF channels other than the channel 3 or 4 that they typically output? I have also seen a transmitter kit in a kit catalog (Ramsey Electronics) that I think has a tunable output, but thought I also heard that someone built that kit, and it had poor performance. Let us know your thoughts, especially if you might have seen or built such a TV transmitter or modulator kit.

7JP4,

Great work and a very nice broadcasting rack! Your work is very neat and compact.

I haven't gone the demodulator route, but I do have 5 different agile modulators with 5 different video sources including a B&K 1077B so I can broadcast a test pattern on one channel. Fun projects for old TV's.

At about $25 for an agile modulator, is there really anything cheaper?

I built the Ramsey kit, which was absolutely horrible. Transmitting distance was measured in inches and the signal was awful. You can get the same performance by sticking a bent coathanger into the RF Out jack of an old VCR.

Phil Nelson

It would be cool to find an agile modulator capable of producing sub CATV channel T13. T13 covers frequencies only slightly different from channel 1 (43 MHz - 47.5 MHz vs. 44 MHz to 50 MHz) and so would probably work on channel 1 sets!

-Matthew D'Asaro

Well this is annoying! I tried a couple of antennas and got dismal results. The range was inches! Earlier in development I had tried a random piece of wire connected to the output of one of the demodulators and had gotten passable results, so I started suspecting the passive head-end combiner. Sure enough, my passive headend combiner is loosing about 10db of signal strength for each of the channels per the spectrum analyzer. I guess I will have to switch to an active head-end combiner... For now I have reduced the output level and will use it to pipe the signal directly to the sets. I will keep you all posted.

-Matthew

What loading is the combiner expecting to see? And does it have any kind of input/output gain controls?

The whole system (combiner included) is designed for 75-Ohm impedance. My spectrum analyzer is 50ohm impedance input, so that might explain part of it. The headed combiner is passive and so has no gain control. Indeed the specification sheet http://www.picomacom.com/specs/PHC.pdf states that the insertion loss (max) is 18db!! If I can find one cheap I will get a Blonder Tongue ZHCA-16C such as this one http://cgi.ebay.com/BLONDER-TONGUE-16x-ACTIVE-HEADEND-COMBINER-ZHCA-16C-750-/150511868625?pt=LH_DefaultDomain_0&hash=item230b34dad1. It claims to put out a maximum of +48dbmV or about 1mW. Hardly the 100mW I thought I was going to get, but much better than what I have now.

-Matthew D'Asaro

Find a Cable TV amplifier with a gain of 30 db or so designed for apartment house distribution and connect it to the output of your combiner. This will allow an output of close to 60 dBmv. With only 7 channels, the amplifier will be able to handle that with no problem.

I have at least one agile modulator that should be able to be set for channel 1 with its DIP switches. I don't (yet) have a TV with channel 1 working to test it, though.

Awhile back I decoded the workings of the setup switches on a Blonder Tongue agile modulator and figured out how to set it up for channel 1. One of the other forum members (I think it was Adam2) tried it on one of his sets and found that it worked. I can't find the thread right now. If I find it I will post a link.

Well, it is finally done. I bought an active headend combiner from eBay (PICO MACOM CHC16U/550) and am using it to drive a pair of vintage rabbit ears. The CHC16U is effectively just a cable TV splitter/combiner in series with a power amplifier and it works great. I was able to get almost 100mW out of the system before I adjusted the carrier levels on the spectrum analyzer to make them all equal and afterwards it is a little over 30mW. One of the modulators is putting out significantly less power than the others, so I might want to investigate that one, but for now I am happy with the results. I can receive a crystal-clear picture all over our apartment and in the apartments most of the way down the hall as well.

I must say it is quite an experience to see TV over the air. This is the first time in my life I have been able to do so as my parents lived in a valley on the outskirts of Seattle and by the time I left home analog TV was no longer being broadcast.

Below is a picture of my antenna (Thanks to smokestuffer and Randy Bassham for the cable needed to fix it!) and an updated version of the schematic of my system.


(Yes, that is my Radiola 18 that the antenna is sitting on. I need to restore it, but I have no way of safely dealing with the asbestos under the power supply chassis while living in a tiny apartment.)



-Matthew D'Asaro

Congratulations! This project was quite an undertaking.

Kevin

Looks like I spoke too soon. There is some ghosting on channel 4 that looks as if I have another channel on top of it, and also for some reason a weaker version of channel 4 is appearing on channel 2. I think I might be getting some leakage into the system somewhere so I ordered some terminators to terminate all of the unused ports on the channel combiner and the test ports coming out of the modulators. I will see if this helps. If not, I will try reducing the output power - perhaps I am getting intermodulation?

-Matthew