HDMI Modulator and distribution

[GoodDIYjob]

Hi.

At the inlaws house. I set up a newly purchased Techmate HDMI modulator with 4k pass through.

I also ran coax feeds using webro wf100 to 2 other rooms. Previously the aerial, a Fracarro Periodic Log fed one point only. I introduces a labgear LDL204 for this purpose as one run was nearly 30Metres, 1 was to TV so 2 metres another was about 8 metres.


Anyway, long story short. When ever I looped the aerial through the TM HDMI modulator it destroyed the TV reception. Quality dropped from 100 to 10 and 15 and no HD freeview channels could be received. The Aerial is aligned to Crystal Palace and the HDMI modulator is set to channel 38 which is 3 higher than the nearest low ouput broadcast mux the local london one.

So, further isolation testing. Aerial without amp just with F barrel connector to remote tv 40M plus run including aerial. OK. Quality 100, HD channels received.

Aerial input into UHF input of 4 way dist amplifier and running 3 x sets. All work fine, quality 100, all HD channels received.

TM HDMI modulator into Labgear 4 way dist amplifier, All sets pick up modulated HDMI signal well. Quality 100.

As soon as I looped the aerial input through the TM HDMI Modulator, it destroyed the free to air recepition. The HDMI modulated channel 38 was ok. No HD free to air channels at all and others pixilating....

Scratched head for a while, thought about going and buying a 2 way combiner to mix the aerial and RF out from the modulator outboard of the TM device. But then thought, I'll just try the VHF in on the labgear 4 way distribution box which accomplished same thing.

Problem solved. Free to air and HDMI mod output all good at all 2 edit 3 tv sets.

It all works but I did not expect the problem with looping the aerial through killing the free to air channels.
Any Ideas for explanation?

 

[winston1]

Overloading. The TM has an output of around 85dBµV and putting that through the amp as well is just too much.

Using the VHF input is pointless, it will just attenuate the signal by an unknown amount, you might as well not use the amp.

 

[GoodDIYjob]

Overloading. The TM has an output of around 85dBµV and putting that through the amp as well is just too much.

Using the VHF input is pointless, it will just attenuate the signal by an unknown amount, you might as well not use the amp.

Hi @winston1 . Thanks so much for reply.

Ok. So that sounds high, right? Why is it so high? Why is it not made with a more moderate level modulated signal output that would align better with terrestrial digital signals?

If it is overloading, why does it not appear to cause any issues when used as described without aerial feed loop through but still passing the modulated output through the amplifier on uhf input and then distributed on its own? The modulated frequency is outside the range of broadcasts being sought for free to air.

Given your response, what would you suggest? The thing is the amplifier does negate the calculated cable loss on the long TV point run, about 8 or 9db and I need to combine the mod output with the aerial feed somehow, otherwise I'd be running 2x cables to each TV point?

Thanks a lot for the advice.

 

[winston1]

Hi @winston1 . Thanks so much for reply.

Ok. So that sounds high, right? Why is it so high? Why is it not made with a more moderate level modulated signal output that would align better with terrestrial digital signals?

These modulators are often used RF distribution headends feeding many sets. The high output is required.
It is possible you may be ably to reduce the output in the menus. Look in the manual.

If it is overloading, why does it not appear to cause any issues when used as described without aerial feed loop through but still passing the modulated output through the amplifier on uhf input and then distributed on its own? The modulated frequency is outside the range of broadcasts being sought for free to air.

Overloading whether it be in an amplifier or TV tuner causes a problem called cross modulation due to the inherent non linearity of the amplifier or tuner. This causes signal to be repeated on nearby frequencies. With analogue you would see the strong signal floating under the wanted one. With digital it blocks it out.

Given your response, what would you suggest? The thing is the amplifier does negate the calculated cable loss on the long TV point run, about 8 or 9db and I need to combine the mod output with the aerial feed somehow, otherwise I'd be running 2x cables to each TV point?

Thanks a lot for the advice.

You probably don't need the amp, a good system should tolerate 8dB loss. You have already said " Aerial without amp just with F barrel connector to remote tv 40M plus run including aerial. OK. Quality 100, HD channels received."

But if you want to keep it combine the modulator output with the aerial after the amp using a 2 way splitter in reverse.

 

[GoodDIYjob]

You probably don't need the amp, a good system should tolerate 8dB loss. You have already said " Aerial without amp just with F barrel connector to remote tv 40M plus run including aerial. OK. Quality 100, HD channels received."

But if you want to keep it combine the modulator output with the aerial after the amp using a 2 way splitter in reverse.

Hi again @winston1

all really helpful and practical.

what i havent given you is the location, signal strengths etc for the muxes from crystal palace at the site.

But they are in Farnham and 35 miles from crystal Palace transmitter as the crow flies. Coverage map suggests inside good coverage area, but only just not much of farnham is. They,re on the south side of farnham up a hill to some extent.

Most other houses on the road point to crystal Palace. Guildford transmitter would be another option as it is virtually the same alignment but vertically polarised. Lower in output but quite a lot closer, say 8 miles or so at a guess..

A lot of other properties run mast amifiers, you can see them but that could be a hangover from analogue days...

But, to get to the point eventually, I don't think his signal strength is that high to start with on free to air. So although the single run I tested to the most remote point worked at 100pc quality, signal did say 30 or 40%. But percent of what value. I don't have a proper signal meter device.

That why I thought with 8db loss, maybe a little more, from a signal that maybe the low side of good would be wise to amplify, especially when splitting amongst another 2 x sets, 3 in total used.

On the dB gain adj (in the mod settings), it seems there is adjustment but it starts from zero, and I don't know what the max negative adj is. I could need to knock off what 20 or 25db? I wonder if it will go that low. I couldn't even find the output level anywhere in the specs. You managed to somehow

If I was to combine the mod signal after the amp, I'd have to do that 3 times! It would be more practical but probably more flawed to combine with aerial pre amplifier but with a greatly reduced mod output.. if it doesn't adjust I'll have to use a inline attenuator on the output.

Will all this trouble be any better than the work around I've got that seems to be working reliably for last 3 days?....

Thanks again. Really knowledgeable...

 

[winston1]

But, to get to the point eventually, I don't think his signal strength is that high to start with on free to air. So although the single run I tested to the most remote point worked at 100pc quality, signal did say 30 or 40%. But percent of what value. I don't have a proper signal meter device.

Quality is much more important than strength. 100% quality means there are no errors. I presume you are getting the figures from a TV. 30% strength is meaningless because 30% of what. Different manufactures TVs give different values.

That why I thought with 8db loss, maybe a little more, from a signal that maybe the low side of good would be wise to amplify, especially when splitting amongst another 2 x sets, 3 in total used.

OK.

On the dB gain adj (in the mod settings), it seems there is adjustment but it starts from zero, and I don't know what the max negative adj is. I could need to knock off what 20 or 25db? I wonder if it will go that low. I couldn't even find the output level anywhere in the specs. You managed to somehow

I don’t have the specs but I did play with one a while back for someone. I would turn it right down and see what happens.

If I was to combine the mod signal after the amp, I'd have to do that 3 times! It would be more practical but probably more flawed to combine with aerial pre amplifier but with a greatly reduced mod output.. if it doesn't adjust I'll have to use a inline attenuator on the output.

I hadn’t realised your amp was one with multiple outputs. I had stupidly assumed one output followed by a splitter.

Will all this trouble be any better than the work around I've got that seems to be working reliably for last 3 days?....

Using the VHF input will probably introduce big and unknown losses which could be different for each channel.

Thanks again. Really knowledgeable...

You’re welcome.

 

[GoodDIYjob]

Quality is much more important than strength. 100% quality means there are no errors. I presume you are getting the figures from a TV. 30% strength is meaningless because 30% of what. Different manufactures TVs give different values.

Yes, I am referring to the tv metering. I have no idea what their baseline for 100% is so 30 or 40% remains a bit meaningless.

As mentioned the loss on the 30M run is around 8db (calculated) not metered. and a 4 way passive splitter (if I did not use amplified) would be 7.5db each output. So call it 16db total. On their free to air muxes, off Crystal Palace 35miles away, off a periodic log, I was not feeling that confident the level would be robust. But only testing in situ with a passive splitter to prove it or proper analysis with a meter analyser would tell us / me / you.

The labgear 4way amp splitter is listed as 8db gain per port.

Using the VHF input will probably introduce big and unknown losses which could be different for each channel.

Yes the vhf input must be adding some unkown attenuation enough to resolve the situation as without it the free to air signal was swamped per your initial responses.

The losses to each channel frequency I suppose could only affect the hdmi mod signal from the TM4k Mod that is feeding into the 4 way amplified splitter this way via VHF input port, i.e. it should have no bearing on the free to air channels fed into the amplified splitter via the UHF input port so if it was going to create a problem it seems to me it would manifest with a problem on the HDMI modulated signal? or am I off track again?

Thanks again