Analogue distribution of signal from Sky box

[JohnW2]

@Lucid - reporting the results of my experiments in the order in which I did them .... I recently wrote:

... I was about to write a note to confess that I may not have given the filter a fair trial. Looking at the spec of the (Labgear) filter, I see that it's cut-off frequency is about 700 MHz, so that it is meant to 'pass' up to ch48 and 'reject' everything from ch50 upwards. However, although I had intended to change back to ch68 before I tried the filter, I forgot to do that - so what I recently reported with the filter was whilst still using ch48. Hence, once 'er indoors has stopped watching Sky, I'll change it back to c68 and see how the filter then performs.

Reverting to ch68 made no discernible difference to what the filter achieved - i.e. the picture quality with it was still clearly inferior to what I had been seeing with my in-line capacitor. However, in order to restore the ('near perfect', to my eyes) picture I had previously been seeing with the capacitor, I had to appreciably reduce its capacitance - to the extent of almost completely 'unmeshing' its plates. That seems to suggest that the 'interfering' signal was close to the RF channel being used (but with both ch48 and ch68, which seems odd).

What would be interesting is taking the 4G filter out of the equation, and then comparing the capacitor to a standard wideband signal attenuator. In fact, I'd go one step further and get hold of a 0-20dB variable attenuator .... Try this before and after the Sky box adds its signal.

I've so far only tried it before (upstream of) the Sky box. The instructions which came with it are 'long lost' but with the adjuster fully clockwise it doesn't seem to do anything noticeable, so I presume that is the "0dB" end of the attenuation range. With it adjusted to anywhere between the anti-clockwise end and the middle of its travel (so maybe 10dB-20dB if the adjuster is anything like linear in relation to dB), the result is similar to the ('near-perfect') picture I see with the (reduced value) capacitor.

I find that rather confusing, and difficult to interpret/understand. As above, the fact that I had to reduce the capacitance when I moved back up to ch68 seems to suggest that the interfering signal was fairly close in frequency to the channel being used. However, if that were the case then (a) as noted above, why apparently very similar amounts of interference with both channels? and (b) in the case of ch68, why did, say, 10-20dB from the attenuator have more beneficial effect than ~40dB from the filter? and (c) in the case of ch48, why did the filter have any effect (actually a marked effect, albeit not as much as my capacitor)? I'm still thinking, but the perception of different pair of eyes, and a new brain,' would be very welcome!

Do you think there is still something to be gained by trying the attenuator (or, indeed, my capacitor) downstream of the Sky box?

Kind Regards, John

 

[Lucid]

Do you think there is still something to be gained by trying the attenuator (or, indeed, my capacitor) downstream of the Sky box?

With no aerial connection in to the modulator (that's important), then testing the capacitor then the attenuator is about seeing whether the analogue RF output is too strong for the TVs' tuners.

Analogue is from before my time installing aerials. I'm far more comfortable with digital TV transmissions. Someone such as @winston1 would have a better grasp of the factors that would help or hinder analogue reception. My limited knowledge extends about as far as "analogue likes a lot of signal. It's hard to oversaturate the tuner" . But joking aside, I haven't enough experience with analogue to have a good grasp of its limits. With digital I can put on a meter and have a look at the bit error rates and such to see what's happening with the signal. I don't have the equivalent level of diagnostic tools for analogue. Is it based purely on signal strength?

My gut feeling is that it would be difficult to oversaturate the analogue tuners, but is it really possible to just keep piling up the amplifier power, and how does amplifier noise affect the results?

 

[ericmark]

In the early days, I fitted a coax to loft from living room, and from loft to each bedroom, and used a signal booster/splitter in the loft to combine the terrestrial TV and Sky plus the DVD recorder/player so I could continue to watch what we were watching down stairs on the 14" TV in the bedroom, not best picture, but good enough at the distance to TV at foot of the bed.

But for some time now been using 32" TV's in the bedroom, and with that size even the difference between SCART and HDMI can be seen, I compared same satellite unit (Icecrypt) using all three with very short leads between TV and box, and it was clear analogue TV is not really good enough for a 32" TV.

I still remember 9" TV monochrome with a large magnifying glass in front of it, with only 405 lines, but times have moved on.

Even digital I would send pictures to my son, very low baud rate, 1200 VHF not HF, and had to split the picture and recombine using 7 plus, never did get into slow scan TV, but know people who did, today hardly worth the effort.

I look at the pictures today sent by my daughter using whatsapp, and think does anyone still use packet radio?

So today I will record a program on a hard drive and physically move it TV to TV or use Sky Q, and more and more using facebook and youtube to watch films. Even those I have on DVD and harddrive. Same with CD's and records, easier to say "Hey Google play David Carradine americana" than it is to find the record, OK vinyl maybe better, but used google even to find out how to spell vinyl. Be it google mini or Alexa we have got use to voice control, soon we will loose the use of out legs!

As a radio ham like I think yourself, yes still interested, but today only use of radio is to work out how the local heritage railway is running and when I need to get off the line. (I am permitted I volunteer so trained as a railway worker).

I learnt about Ceramic Filters and getting a high Q figure in University and the old RAE, but is it really worth the effort?

 

[winston1]

With analogue a minimum signal is about 1mV or 60dBuV before noise begins to show. Some sensitive tuners can go down to around 1/2 mV. A typical modulator in a set top box gives around 76dB uV so in theory could be split 8 ways. Overloading will occur in the early 80’s dBuV and initially shows up as cross mod. A decent 18 element outside aerial in a good signal area, say up to 25Km from a main station will cause overloading on analogue. I had it when I was in SE London and fixed it with an 18dB attenuator (later changed to an 8way splitter to different rooms).

Your situation is interesting and puzzling. I was wondering if you have a strong VHF signal breaking through or even Tetra just below the UHF TV band.

 

[JohnW2]

With no aerial connection in to the modulator (that's important), then testing the capacitor then the attenuator is about seeing whether the analogue RF output is too strong for the TVs' tuners.

I'm a bit confused there. I've already demonstrated that if I disconnect the aerial, then 'the problem' goes away (i.e. the distributed Sky RF seems to produce a near-perfect picture), so there isn't then really anything left for the capacitor (or an attenuator) to improve.

On the other had, I suppose there would be some academic (but really no practical) interest in trying capacitor and/or attenuator downstream of the Sky box, with the aerial connected (hence some degradation of the analogue RF signal) - since that would presumably clarify whether the problem was arising at the level of the modulator and/or the tuners of the remote TVs. Maybe I'll try that 'for completeness'!

Analogue is from before my time installing aerials. I'm far more comfortable with digital TV transmissions. Someone such as @winston1 would have a better grasp of the factors that would help or hinder analogue reception. My limited knowledge extends about as far as "analogue likes a lot of signal. It's hard to oversaturate the tuner" .

My experience, mainly from decades ago, is the opposite - relating entirely to broadcast and amateur radio analogue signals, at a time when the nearest to anything 'digital' was teleprinter ('RTTY') signals (and, I suppose Morse code!)!

But joking aside, I haven't enough experience with analogue to have a good grasp of its limits. With digital I can put on a meter and have a look at the bit error rates and such to see what's happening with the signal. I don't have the equivalent level of diagnostic tools for analogue. Is it based purely on signal strength? My gut feeling is that it would be difficult to oversaturate the analogue tuners, but is it really possible to just keep piling up the amplifier power, and how does amplifier noise affect the results?

As with anything else, it's obviously down to SNR and, under normal circumstances, I think you're right that it is probably difficult to 'saturate' an analogue tuner. However, it can happen, whether due to very high strengths of the 'wanted' signal or even very high off-frequency signals - I've certainly seen that when multiple amateur radio transmitters (often on different frequency bands) have been functioning 'in the same field', and messing up each other's receivers - but that's a pretty extreme situation.

Don't forget that my earliest recent experiments showed that inserting amplification anywhere, both upstream and downstream of the modulator, made the situation far worse.

Moving back to the present issue, I still do not have anything like a confident idea of exactly what has been happening, and why my various interventions have had the effect that they did. However, pragmatically, I have determined that either a capacitor or an attenuator will solve the problem to my satisfaction, and that either of those is superior to a 700 MHz low-pass filter.

Moving forward, remember that I only tried the capacitor because I could not initially find my attenuators so, moving forward, I'll probably go with an attenuator (probably a fixed-attenuation one), since I don't know what 'changes' I or someone else may implement in the future - the attenuation provided by the capacitor is not only obviously frequency dependent, but is also crucially dependent on what 'load' it is connected to.

For readers of this (if there are any left!), whether amateur or professional in relation to such matters, I suppose there is (if they haven't already learned it) a lesson to be learned from my experiences - namely that if one experiences problems such as I had and a 4G/5G filter does not provide the degree of improvement that one would hope for or expect then, for the sake of a few seconds, it's probably worth trying an attenuator in place of the filter!!

Kind Regards, John

 

[JohnW2]

With analogue a minimum signal is about 1mV or 60dBuV before noise begins to show. Some sensitive tuners can go down to around 1/2 mV. A typical modulator in a set top box gives around 76dB uV so in theory could be split 8 ways.

I've never fully understood what is said about splitting TV signals, the implication seemingly being that (the voltage of) the signal is divided equally between all of the connected tuners. Is there perhaps an assumption being made about the input impedance of the tuners, because I would have thought that if, say, they all had very high input impedances, then the 'full' signal (voltage) would be presented to all of them?

Your situation is interesting and puzzling. I was wondering if you have a strong VHF signal breaking through or even Tetra just below the UHF TV band.

Yes, as you say, interesting and puzzling - I might even go as far as saying 'makes no sense'. What you speculate about is certainly one of the types of explanation I have contemplated but, if something like that were happening, why should one get considerable (albeit not 'complete') improvement by inserting a low-pass filter which provided minimal attenuation below about 700 MHz?

 

[JohnW2]

I still remember 9" TV monochrome with a large magnifying glass in front of it, with only 405 lines, but times have moved on.

Yep, like many other households, we got out first TV to watch (together with much of the neighbourhood!) the Coronation in 1953! Quite a nice piece of (mahogany) 'furniture' and I suppose 'amazing technology for its day, but only one channel, and that on-air for only a few hours per day!

Kind Regards, John

 

[ericmark]

Not quite that old, I was about 6 before I watched TV, so 1958 approx, not sure, but I was interested with muffin the mule.

Not sure when we first go a TV, I remember my dad removed the back, and wrote down every valve number, and bought one of each, and if the TV stopped working, would change valves one by one until it worked, then order a replacement.

I remember being given an old TV when we moved into our first house, the dropper resistor had failed, I found the value was nearly the same as the iron, so a 13 amp socket and the iron was used as a mains dropper, but if wife had been ironing some items requiring little heat, the thermostat would open at the most inappropriate time.

Do kids today even know what a valve was?

 

[JohnW2]

Not quite that old, I was about 6 before I watched TV, so 1958 approx, not sure, but I was interested with muffin the mule.

To be honest, I'm not sure that I actually remember watching the Coronation, since I was only about 3.5 years old at the time - but I certainly remember the TV (which we obviously had for a good few years thereafter), and well as knowing the story about when/why we got it.

Do kids today even know what a valve was?

I think a good few probably do, although they probably call them 'tubes', since it seems fashionable to believe that (usually very expensive) 'tube amplifiers' produce 'better quality' (music) than do the semi-conductor equivalents (just as many seem to believe the same about vinyl records!)!

Kind Regards, John'

 

[winston1]

I've never fully understood what is said about splitting TV signals, the implication seemingly being that (the voltage of) the signal is divided equally between all of the connected tuners. Is there perhaps an assumption being made about the input impedance of the tuners, because I would have thought that if, say, they all had very high input impedances, then the 'full' signal (voltage) would be presented to all of them?

The input impedance of a TV tuner is (well should be) 75Ω.

 

[JohnW2]

The input impedance of a TV tuner is (well should be) 75Ω.

As I implied, I imagined that was what was being assumed. However, at least in this day and age, I can think of no reason why is has to be anything like that low, even if/though it is fed by 75Ω coax..

 

[bernardgreen]

since it seems fashionable to believe that (usually very expensive) 'tube amplifiers' produce 'better quality' (music) than do the semi-conductor equivalents (just as many seem to believe the same about vinyl records!)!

Vinyl records and valve amplifiers do produce better quality ( as in more accurate reproduction ) sound than semiconductor amplifiers and digitized data converted from and then to "analogue" waveforms.

 

[JohnW2]

Vinyl records and valve amplifiers do produce better quality ( as in more accurate reproduction ) sound than semiconductor amplifiers and digitized data converted from and then to "analogue" waveforms.

I realise that's what some people believe. Digitizing is a different matter but, in terms of analogue amplifiers, I'm not convinced that one could not almost perfectly emulate the performance of a valve amplifier with semiconductors.

Kind Regards, John

 

[RandomGrinch]

For readers of this (if there are any left!), whether amateur or professional in relation to such matters, I suppose there is (if they haven't already learned it) a lesson to be learned from my experiences - namely that if one experiences problems such as I had and a 4G/5G filter does not provide the degree of improvement that one would hope for or expect then, for the sake of a few seconds, it's probably worth trying an attenuator in place of the filter!!

I'm still reading!
Although I have to say the lesson would probably have been lost on me; I would have swapped to the DVB-T modulator long ago!

 

[JohnW2]

I'm still reading! Although I have to say the lesson would probably have been lost on me; I would have swapped to the DVB-T modulator long ago!

You might not have done so if not all of the TVs you wanted to feed understood what 'digital TV' was

Kind Regards, John