QAM Modulator Question

[IntelPennny4]

Hello i hope that im in the right spot to ask this question. i need help with a qam modulator. i want to combine my cable which is also qam signal to a HDMI source. i have already notch out the MHz i want to use which is channel 97 (102-108mhz) channel 30 here for my cable. i think i have a power issue or something. when i screw in the modulator into the combiner it wipes out my cable. i have try to stack splitters to knock down the power and it works but my cable channels are not all working like i lose power on the cable side. if someone could help or push me in the right place or tell me if im doing something wrong or give their two cents



thanks

 

[pacificrim]

There may be system traffic in that frequency range that is required by your cable STB to operate. Just a thought

 

[harshness]

I suspect that you outsmarted yourself when you assumed that channel numbers were wed to certain radio frequencies. The whole point of QAM in a cable TV system is to put multiple "subchannels" on a "physical channel". QAM is perhaps twice as efficient at getting data across so there are typically around 12 SD subchannels per physical channel and two HD subchannels per physical channel. PSIP is typically used at the physical channel level to determine what subchannels are present.

It is possible that you've managed to notch out the communications channel that the cable box (or equivalent) needs to authenticate itself as pacificrim suggests. Passive (or even active) notch-in devices aren't perfect so they often take a toll on the physical channels to either side of their notch frequency.

QAM modulation is perhaps only practical if you have more or less complete control over the physical channels and don't have to worry about interfering with data streams and multiplexed TV programming that might already be there.

 

[IntelPennny4]

so to be able to do what i want i need to find a freq that is not being used and use that?

 

[harshness]

so to be able to do what i want i need to find a freq that is not being used and use that?

You probably need to find three unused channels in a row and use the middle one. The danger is that there's no connection between frequency and channels anymore and the provider can change the layout any time they want. They can also annex anything in the DOCSIS ranges (5-200MHz upstream and 252-1,200MHz downstream -- up to 1.7GHz if/when they foresake MoCA) at will and they don't have to clear it with or warn anyone because it is their pipe.

The provider may or may not be willing to commit to what frequencies are unused. I wouldn't because I'd want to retain all the flexibility afforded by the technology -- not to chop it up because someone wants to permanently borrow a big chunk of my increasingly precious bandwidth for personal use.

 

[KF4YLM]

i have try to stack splitters to knock down the power and it works but my cable channels are not all working like i lose power on the cable side.

Filter design can be critical with amplitude digital mod schemes.... Group delay and other distortions on the desired pass band sometimes require consideration. Specific loading can affect a filter's characteristic, too.

For the other items mentioned previously, too, the project here is probably best accomplished in the digital domain, without the complexities of analog componentry. QAM RX 1 data can be muxed with QAM RX 2, 3....N, and remodulated to a totally new QAM waveform..

 

[harshness]

Alas, remodulating the entirety of the cable offering isn't practical if all you want to do is insert a few digital streams not to mention the complexity of authorization of all channels at once.

The technology of the day has pretty much closed the door on modulators for this specific purpose. One may find a combination that works for a time but it could stop working at any time and it could conceivably interfere with one's broadband Internet as more and more bandwidth is used for DOCSIS.

 

[IntelPennny4]

i have figured out what i had to do and everything is working now. i had to use a deletion filter aka brickwall type filter. everything is working good now

 

[harshness]

everything is working good now

If your cable TV service mysteriously fails in the future, you'll need to select a different frequency and install the corresponding notch filter.