TV Reception

[Sam Gangee]

Winston is partially correct about the SatCure method of fitting plugs.

"This method can leave a bad connection when used with cheap or thin or "low-loss" cable. However, good quality cable is thicker and so is the braid. This means that the braid is tightly pressed against the claws. In addition, if the copper braid is lightly coated with silicone grease, no oxidation will occur to compromise the contact in the future."

Also, SatCure advocates soldering the braid to the brass claw if you want to be absolutely certain of a good connection for life.

VHF TV is used in Ireland - unless it has been switched off recently. VHF is still used for radio, of course.

 

[winston1]

Vietnam... fine. Other countries.... fine. But the UK?.......now... today ...in 2014....not 20 years ago?????????????

The UK may not use VHF for TV but poor screen connections also affect UHF digital letting interference in.

 

[Lucid]

Well guys, I'm going to say that for me the jury is out on this one.

I have just done some tests with 2m of some cheap WF63. I hooked up from a Sky box RF out to have both DVB-T and analogue signals available. I then did three tests by changing the shield termination at the TV end. These were:

1) standard pigtail coiled around inside the claw

2) braiding cut down to a 5mm frill

3) braiding completely insulated from plug

Now, before we go any further I'll hold my hands up and say that I have very good DVB-T signal here from an unamplified Log Periodic. Also, I can't ask the world to provide an interference source, so I'll have to make do with the PC gear I have under the TV (a media server) and a couple of devices using wallwart style switchmode power supplies. The other thing to add is that I calibrate TVs and projectors as part of what I do. So my TV is set to be as neutral as possible regarding image processing and sharpness enhancement. Within the abilities of the TV, it neither underplays nor overplays the picture to make it better or worse than it actually is.

Okay, so what happened.

With DVB-T there was no difference with any combination. What surprised me was that the DVB-T picture remained stable - no blocking, no freezing - even with no shield braid connected at the TV plug end.

With analogue, 1 and 2 produced no observable difference on the TV. They may have done if I'd metered the signal with my Horizon meter, but since all the discussion has been about visible patterning onscreen then I felt that it wasn't appropriate to go to that level.

The only time that there started to be any evidence of an issue was when the screening braid was completely insulated from the coax plug. The picture developed a granular noise pattern along with six ghost bars reminiscent of grounding issues, except they were static rather than rolling.


As two further tests,

4) I shorted the centre conductor to shield using a single filament of braid. The braid was still insulated from the plug. As expected, DVB-T fell over. Analogue became exceptionally noisy.

5) with the centre conductor still shorted to the braid I then introduced a link from the cable braid to the plug. DVB-T returned, but blocky and freezing, so no surprise there. Analogue improved but was subject to random interference in the form of horizontal lines and the occasional glimpse of some 45 degree pattern lines reminiscent of PAL cross colour distortion, but the patterning was larger and not confined to colour boundaries.



Disclaimer:
I'm not suggesting that the methodology was rigorous, nor am I claiming that the results would be the same for everyone else given different source and display equipment. But within the limits of what is possible with a 15 minute test I am happy that the above is a true and accurate representation of my observations.


Conclusion:
I agree that shield integrity is important in all wired video systems. I even carry compression fit 75 ohm coax plugs for the very reason that they maintain shield integrity right to the end of the plug. What's at debate here is how far one has to go to provide an adequate connection.

Whilst I accept that soldering or different methods of terminating the braiding in to a Belling-Lee plug may well provide a superior physical connection, the question is does it make a difference in the real world of analogue and digital UHF TV transmissions for the average UK TV viewer. Based on the observations I've outlined above, and all else in the system being to standard, then I'd have to conclude that it's at best questionable. I see no evidence that the pigtail method is inadequate if correctly executed.

It's also worth mentioning that the Belling-Lee connector has issues of its own. The design is the better part of a century old and was never intended as a connection for UHF nor for 75 Ohm balanced systems.

Under exceptional circumstances interference could prove to be a problem. Perhaps where signal is marginal, or the aerial system is installed poorly, or there is an unusual level of local interference. But I think it's just that.... exceptional circumstances.


Epilogue:
The exercise has proved surprisingly useful in one aspect, and that's just how well the DVB-T signal stands up to having no screen connection. If you'd have asked me before to make a prediction then I'd have said that the signal would probably go blocky. I'm aware of the phenomenon where UHF signals can bridge a gap in the connection cable the thickness of a piece of paper. But I wouldn't have expected that the same would happen with the shield connection, if indeed that is what's happening.

 

[Sam Gangee]

Chris, you obviously have too much time on your hands!

It's also worth mentioning that the Belling-Lee connector has issues of its own. The design is the better part of a century old and was never intended as a connection for UHF nor for 75 Ohm balanced systems.

Balanced? I thought coax was unbalanced.
Anyway, you are correct in principle. The "Belling Lee" TV plug was originally designed for Medium Wave radio reception. Its performance at UHF is acknowledged to be "sh;te".

But that didn't prevent Nokia from using it for the LNB inputs on its MK1 SAT1700 analogue receiver (with disastrous results). They changed it to 'F' connectors on the MK2.

 

[Lucid]

Chris, you obviously have too much time on your hands!

It's also worth mentioning that the Belling-Lee connector has issues of its own. The design is the better part of a century old and was never intended as a connection for UHF nor for 75 Ohm balanced systems.

Balanced? I thought coax was unbalanced.
Anyway, you are correct in principle. The "Belling Lee" TV plug was originally designed for Medium Wave radio reception. Its performance at UHF is acknowledged to be "sh;te".

But that didn't prevent Nokia from using it for the LNB inputs on its MK1 SAT1700 analogue receiver (with disastrous results). They changed it to 'F' connectors on the MK2.

Yeah, I'll give you that. It should have been 75 Ohm Balun systems. Having said that though, it only requires two connections for a balanced interconnect. Normally they're twisted pair, but I wonder if the same could be achieved with coax?.... Probably not.

You're right though, Belling-Lees do suck. They're not even 75 Ohm, so there's an impedance mismatch straight away with the aerial and coax cable. Are TV tuners 75 Ohm? But anyway, I guess that's what we're stuck with until the industry (if ever) moves to F connectors.

 

[winston1]

Chris, you obviously have too much time on your hands!

Indeed. But at least test 3 prooves that an open circuit screen affects analogue pictures. The pigtail method, though not shown here, will have intermittent contact eventually and cause the same effect.
As far as DVB-T is concerned, it is immune to reflections etc and as he had a very high signal level any losses are hidden by the digital cliff. It would have been more relavent with a weaker signal as experienced by many. And more scientific to use some proper test gear to measure signal level and BER.

I agree B/L connectors are poor, nearer 50 ohms than 75. Even more reason than not to make it worse by not fitting them properly. It is no harder to do it properly than improperly.

 

[Sam Gangee]

[The pigtail method, though not shown here, will have intermittent contact eventually and cause the same effect.

Nope. As I already quoted:-
"This method can leave a bad connection when used with cheap or thin or "low-loss" cable. However, good quality cable is thicker and so is the braid. This means that the braid is tightly pressed against the claws. In addition, if the copper braid is lightly coated with silicone grease, no oxidation will occur to compromise the contact in the future."

 

[Lucid]

Indeed. But at least test 3 prooves that an open circuit screen affects analogue pictures.

Well that was never really in doubt. The question wasn't "Will no shield connection cause a problem?" But rather it was, "Is there justification for abandoning the pig tail method?" That question is posed because you said that the pig tail method doesn't provide adequate contact. Well, I disagree. I think if it's done correctly that there's no issue. I have enough customers in widely enough varying installations to form a reasonable sample group. In addition I have my own cables which have been wired this way for enough years that any progressive deterioration through oxidation or metal fatigue would start to become evident.

Badly made cables will cause a problem, I'm sure. But on present and past evidence I can see no justification for abandoning a tried, tested and proved method.

As far as DVB-T is concerned, it is immune to reflections etc and as he had a very high signal level any losses are hidden by the digital cliff. It would have been more relavent with a weaker signal as experienced by many. And more scientific to use some proper test gear to measure signal level and BER.

No, hang on. You stated very clearly that an intermittent shield connection would cause blocking and freezing. The way you stated it made it clear this would be instantly recognisable from the screen. Well it didn't. Not even when there was no shield connection at all!!

I already said about metering the signal... and yes, I can measure and display signal level and BER. But your point was clear. To paraphrase: "A bad shield connection will let in interference and cause blocking and freezing" No meter is required to see blocking and freezing on screen. I stated in my conclusion that it may be an issue when the system was poorly installed or the signal is marginal. But not on it's own if my experience and these tests are anything to go by. Sorry, but that's that.

 

[Sam Gangee]

Advantages of the method:

1. Quicker.
2. Doesn't require copper braid to be cut and disposed of.
3. Maintains connection when cap is slackened.
4. Requires no cutters - can be achieved with a blunt penknife.

 

[winston1]

Indeed. But at least test 3 prooves that an open circuit screen affects analogue pictures.

Well that was never really in doubt. The question wasn't "Will no shield connection cause a problem?" But rather it was, "Is there justification for abandoning the pig tail method?" That question is posed because you said that the pig tail method doesn't provide adequate contact. Well, I disagree. I think if it's done correctly that there's no issue. I have enough customers in widely enough varying installations to form a reasonable sample group. In addition I have my own cables which have been wired this way for enough years that any progressive deterioration through oxidation or metal fatigue would start to become evident.

Badly made cables will cause a problem, I'm sure. But on present and past evidence I can see no justification for abandoning a tried, tested and proved method.

As far as DVB-T is concerned, it is immune to reflections etc and as he had a very high signal level any losses are hidden by the digital cliff. It would have been more relavent with a weaker signal as experienced by many. And more scientific to use some proper test gear to measure signal level and BER.

No, hang on. You stated very clearly that an intermittent shield connection would cause blocking and freezing. The way you stated it made it clear this would be instantly recognisable from the screen. Well it didn't. Not even when there was no shield connection at all!!

I already said about metering the signal... and yes, I can measure and display signal level and BER. But your point was clear. To paraphrase: "A bad shield connection will let in interference and cause blocking and freezing" No meter is required to see blocking and freezing on screen. I stated in my conclusion that it may be an issue when the system was poorly installed or the signal is marginal. But not on it's own if my experience and these tests are anything to go by. Sorry, but that's that.

A bad shield connection will let interference in. Of course if you exceptionally have a very strong signal the interference may not show up. Not everyone does.
The vast majority of people's reception problems are caused by poor flyleads, often due to incorrectly wired plugs.

 

[Lucid]

A bad shield connection will let interference in.

Yes, but a pigtail isn't a bad shield connection. At least it isn't if done with reasonable competency. That's the point.

Of course if you exceptionally have a very strong signal the interference may not show up. Not everyone does.

That's not what you said originally. "With digital it lets interference in causing clicks and intermittent freezing" No mention of other conditions that had to be in play before poor shielding starts to become a problem.

And my signal isn't "exceptionally strong". There's nothing exceptional about my signal compared to that of my neighbours. It's simply at the upper end of the acceptable signal range for DVB-T/T2 because of line of sight, location, and a decently installed quality aerial system.

The vast majority of people's reception problems are caused by poor flyleads, often due to incorrectly wired plugs.

The vast majority of interference issues are due to crappy fly lead shielding. But it's very little to do with how the plug ends are terminated and far more to do with the fact that the braid cover percentage along the length of the cable is so ruddy pathetic. Unless the lead is broken, then the braid will be adequately connected inside the moulded plugs. The customer is just using the "comes free with" or cheap supermarket pre-made leads and it's the cable's poor shielding down its length which is the Achilles heel. So all this trying to maintain the coaxial braid shielding all the way in to the plug is okay in theory, but OTT in practise in a domestic setting.

The situation may well be different in a commercial head-end rack. Then again, what professional head-end gear uses Belling-Lee instead of crimp or compression F connectors or BNCs? As for "it may affect the picture in VHF systems"..... B&W 405 line TV is a service that we haven't used as a national transmission system in the UK for the better part of 3 decades, so who the hell cares?

If the problem was as bad as you're making out then Belling-Lee plugs would have been phased out long ago. There are alternatives in both crimp and compression fit that would do a damned sight better job. But the issue is cost.

 

[winston1]

B/L plugs have to be used because that is what TV input sockets are. Where does 405 line come into it? VHF is in use on distribution systems in the UK and broadcast overseas. Obviously you are not prepared to accept facts. But I say again why do it wrongly when it is so easy to do it correctly?

 

[Sam Gangee]

There's one other point worth noting: IF the DC connection to the braid is lost, the "pigtail" still provides a capacitive connection that is greater than that of the alternative method. At UHF frequencies the impedance is quite low so there will be no noticeable drop in signal level. So all the controversy regarding whether the DC connection is good or bad is irrelevant as regards the UHF signal.

 

[winston1]

There's one other point worth noting: IF the DC connection to the braid is lost, the "pigtail" still provides a capacitive connection that is greater than that of the alternative method. At UHF frequencies the impedance is quite low so there will be no noticeable drop in signal level. So all the controversy regarding whether the DC connection is good or bad is irrelevant as regards the UHF signal.

I'm glad you brought this up as yes it does provide a capacitive connection. But losing DC is very relevant. People using magic eyes or masthead amplifiers rely on a good DC connection to power these devices. The pigtail method does not give a reliable DC connection.

 

[Lucid]

VHF is used in the UK on distribution systems.

Where. Who is using it? Please be specific; how does it relate to receiving and watching DVB-T/T2 in the UK today?