Sky HD system - illogical problem?

[dennishoy]

Folks,

I'm fairly 'logical thinking' but this one has me scratching my head.

Ok, I've got a Sky HD box connected to a single dish and fed by a dual LNB through an identical pair of coax cables. I try to think of it like an audio stereo system (L + R) where the pair of channels can be swapped over for test purposes, but with a pair of identical video feeds instead of audio signals.

Now here's the problem..... think logically !!

Just a few of the (CBS etc.) channels are cutting in and out tonight.
They are on the same satellite transpoder.
'CBS Reality+1' is so poor, there is no picture or sound.

The on-screen digital signal strength meter shows 'signal channel 1' to be of low strength and 'zero' quality on these channels. The same display for 'signal channel 2' is over 90%, signal quality is 80%. Logical deduction - either half the LNB has failed, the corresponding coax cable is faulty or one of the pair of tuners in the Sky HD box has failed. The satellite dish cannot be misaligned.

I then changed Sky channels and the on-screen digital signal strength meter showed 'signal channel 1' to be of 90% and signal quality to be 80% quality and the same strengths on 'signal channel 2'. Logical deduction - there is nothing wrong with the system.

Conclusions....
If the coax cable, half the LNB or SKY HD box tuner were faulty, then all channels would exhibit the same problem, not just a few channels on the same satellite transponder.

If the satellite transponder was faulty, then both signal channels 1 & 2 would be missing, not just one of them.

My theory is that the hot weather may have affected one half of the LNB and/or one tuner in the SKY HD box. If the CBS signals were from a weaker transponder, then the combined combination of original weak satellite signal + poor half of the LNB + weak Sky HD Box tuner may perhaps result in a 'no signal' condition'.

If so, then re-aligning the dish for the absolute strongest signal may improve the reception of the poor channels. The problem is that an external satellite signal strength meter is not tunable to any particular frequency so I cannot use it for this purpose. I think it uses the whole received frequency spectrum for an indication of signal strength.

I could however use the Sky HD box on-screen graphics display as a 'tunable' signal strength meter provided I set the Sky box to the worst of the affected channels first.
Any thoughts?
Suggestions please?

It's amazing how quickly the evening goes once you get bogged down in this sort nonsense.

[boiss]

I presume from channel 1 you mean LNB output 1 etc. I had something similar a few years ago and I fixed it by cutting a few cms (3cm) off the cable. I guess you are changing the feeds/LNB outputs by just swapping the cables. Strange question but Why?? If both cables go to the same place or do you intend to connect another Rx?
Depending on which transponder the Rx is set to, then the levels will differ quite a bit, not 90% though.
I have 2 dishes with 1 Quad LNB, and 2 dual LNBs.
one quad output and an output from each of the LNBs are switched using Diseqc switch units for the main TV and with2 old Rxs for the other room.
Good luck
Mike

[camallison]

How well protected is the F connector joint that goes into the LNB? Could it have allowed water ingress under the insulation? I ask this because I had a similar problem where water had "wicked" under the insulation and, as Boiss suggests, cut the cable back a little and remade the connection. This time, I taped it with a turn of self-amalgamating tape and then put a boot on with silicone to seal it. Worked a treat.

Colin

[dennishoy]

Thanks for your thoughts, guys.

I solved the problem this morning as I couldn't see much last night in the dark.
I hope this may be useful to others who may experience a similar problem.

Satellite channels are normally 'polarised'; some horizontal and other vertically to avoid interference between adjacent frequencies. Polarisation is perfomed within the LNB by software in the receiver changing the supply voltage down the coax cable. However, Sky HD boxes are made cheaply for a mass market aimed at the Sky package only, and as far as I know, this feature has been omitted. (I think) all the channels are transmitted with horizontal polarisation.

When I have set up 'proper' non-Sky satellite systems, there is an extra adjustment called 'skew' where the LNB can be physically rotated in it's clamp on the dish to fine tune the polarisation so that the V and H channels are separated as much as possible. This is not nnecessary with a Sky HD box and most LNBs are fixed vertically in the clamp so any rain runs down the cable and not into the LNB connector.

The hot weather had expanded the cheap, plastic LNB clamp and the LNB itself had rotated slightly under the weight of the cables. Had there been any vertically polarised channels, they would all have been lost. Only the weakest horizontal channels were lost on one half of the LNB only.

I rotated the LNB for the strongest signal strength using the on-screen Sky HD Box digital display tuned to the weakest channel (CBS Reality+1). All channels are now working perfectly.

The signal strength of the stronger channels will not increase above 90% and signal quality remains at 80% so it is a waste of time to try and get more (for me anyway).
Like many digital TV systems, you don't get a reduction in picture quality, just complete loss of picture when the signal drops below a certain threshold.

This experience leads me to believe that the usual signal strength meter is useless for setting up a dish and LNB properly. It cannot be channel-tuned so must pick up the whole spectrum which is why you get all-or-nothing when you use one of these devices.

In my experience, once you have aligned your dish, there are a couple of extra taskes to complete. One is to rotate the LNB back-and-forth for the strongest signal and then tighten the clamp only enough to hold it. The second is to slide the LNB backwards-and-forwards in the clamp for the strongest signal to find the focal point for that particular dish. Once both adjustments have been made, and no further signal improvement can be made, the clamp can be tightened a bit more to prevent the LNB from moving on a hot day.

So in conclusion, the recent hot weather had allowed the cheap and nasty plastic clamp to expand which had allowed the LNB to rotate and to lose the weakest signal.

Hope that helps.

Sorry to be long-winded, but installers charge stupid money to fix these faults which everyone can do themselves if they know where to look.

[Mark Hennessy]

Satellite channels are normally 'polarised'; some horizontal and other vertically to avoid interference between adjacent frequencies. Polarisation is perfomed within the LNB by software in the receiver changing the supply voltage down the coax cable. However, Sky HD boxes are made cheaply for a mass market aimed at the Sky package only, and as far as I know, this feature has been omitted. (I think) all the channels are transmitted with horizontal polarisation.

No, this isn't the case. A couple of randomly chosen links:

http://forums.digitalspy.co.uk/showthread.php?t=1176626

http://free-tv.ie/astra-satellite-frequencies

With the normal mini-dishes, satellite reception is pretty marginal at the best of times, and is affected by all extremes of weather. Anything less than a 100% perfect installation will suffer from occasional channel loss, and obviously cheap plastics where once metal was used might well contribute to this.

I got $ky Digital back in 1999 or 2000 - and back in those days you had to pay a massive(!) £40 for the installation (the boxes had just become free at that point - before they were £200 IIRC). The chap who installed the dish was a local self-employed engineer who did jobs for Sky, and as he wasn't especially busy on the day, he ended up staying for about 90 minutes, drinking tea and chatting about technical stuff (he realised who I worked for, so as you can imagine, we had plenty to discuss!). Nice guy - who did a proper job. I never suffered reception problems in the 5 or 6 years that I lived in that house - but plenty of our neighbours did with their 20 minute free installations!

Haven't had Sky since then. Hardly watch TV these days

Cheers,

Mark

[ianhumph]

Just reading this thread (and in particular, looking at the previous post...) got me thinking a couple of things here.

I had Sky Digital installed around the same time as Mark did; which was around the turn of the millennium. I found one day, I'd lost a lot of channels ('No signal' message...) and the situation remained for days, so I contacted my landlord and got him to report it. The visiting engineer replaced the dish and told me he and his colleges had spent recent weeks installing replacements for many customers as a lot of the early dishes had LNBs that had proved faulty, in a short space of time. I'd had my installation around 6 months by then, which meant fortunately, I didn't have to fork out for a new dish or the visit, as it was still under the 12-month contract (free box & dish, as long as you stay for 1 year.)

A friend is having similar issues; she is saying that her Digibox is probably to blame, as channels 'come and go' (only very occasionally though) but I think that it's FAR more likely to be the LNB, or the dish needs re-alignment OR possibly the feeder cable... (?)

[dennishoy]

Satellite channels are normally 'polarised'; some horizontal and other vertically to avoid interference between adjacent frequencies. Polarisation is perfomed within the LNB by software in the receiver changing the supply voltage down the coax cable. However, Sky HD boxes are made cheaply for a mass market aimed at the Sky package only, and as far as I know, this feature has been omitted. (I think) all the channels are transmitted with horizontal polarisation.

No, this isn't the case. A couple of randomly chosen links:

http://forums.digitalspy.co.uk/showthread.php?t=1176626

http://free-tv.ie/astra-satellite-frequencies

With the normal mini-dishes, satellite reception is pretty marginal at the best of times, and is affected by all extremes of weather. Anything less than a 100% perfect installation will suffer from occasional channel loss, and obviously cheap plastics where once metal was used might well contribute to this.

I got $ky Digital back in 1999 or 2000 - and back in those days you had to pay a massive(!) £40 for the installation (the boxes had just become free at that point - before they were £200 IIRC). The chap who installed the dish was a local self-employed engineer who did jobs for Sky, and as he wasn't especially busy on the day, he ended up staying for about 90 minutes, drinking tea and chatting about technical stuff (he realised who I worked for, so as you can imagine, we had plenty to discuss!). Nice guy - who did a proper job. I never suffered reception problems in the 5 or 6 years that I lived in that house - but plenty of our neighbours did with their 20 minute free installations!

Haven't had Sky since then. Hardly watch TV these days

Cheers,

Mark

I stand corrected. Thanks, Mark.
Sky must have updated the system over the years because I recall all the channels were 'H' polarisation a few years ago.

I bought my first sateliite kit in 1985 long before Sky existed. It was an analogue system, 1.8m dish, NEC2022 receiver/positioner, C120 mount LNB and separate mechanical polariser and dual band LNB for Ku and the French Telecom bands. Band switching was by changing the LNB power supply from 12V to 18V for the higher Telecom band. There was a Sky channel, encrypted using the Oak Orion system which could be hacked using a few ICs and a tuned circuit. The fun was in decrypting the porn channel Red-Hot Dutch which simply swamped the signal with a fixed frequency carrier. All you had to do was apply the same frequency and amplitude, but 180 degrees out of phase to null the interference. My first phase locked loop using TTL I recall. Other channels were Music Box, Supechannel, Premiere and a few others, uplinked from the UK.

BSB came along using the DMAC encryption system and circular polarisation. You needed a 'squarial' to receive the programmes which were British rather than the Sky American rubbish that fills every channel nowadays.

When BSB went bust, squarials became freely available along with DMC decoders which were considered obsolete. However, the more clever folk changed the crystal to double the frequency and fitted a new EPROM so the box became a D2MAC decoder for hacking the Scandinavian channels that used that format for transmisison. The films were broadcast in the original language and subtitled into Swedish. The 'key' could be found by trial and error and lasted for a few days until the company changed it. Problem for them was that only a certain number of 'keys' could be stored in the decoder so they kept rotating 'keys' usually on a monthly basis. This was not considerd to be piracy as you could not pay for these programs in the UK so were not depriving the channel provider of any income. Some of the channels changed format around 10pm and became very 'interesting' if you know what I mean.....
Happy days indeed. (before Sky came along).

[boiss]

I worked for the IBA and was part of the CMAC dev team. I tested personally all the CMAC Codecs for the European operators. This was between 1982 to 1984. in 1983/4 there was only 1 channel on Ku band (No C or S band) called Europe up to 1985, this was a beige screen with the word Europe on it, for most of the day that was it with some news in the evening. System was analogue with sawtooth dispersal signal at 50Hz. There were some frequencies used by the operators for tests but there was no timetable or programming.
I built my own Rx and LNB in 1983 because an LNB cost 3000$ from Sony and had a
3dB noise figure. In 1986 the prices had started to fall whole systems in Europe (Italy) were around 2000$, We put together systems to sell in Italy to a major distributor for 1000$. Taking LNBs from Korea Rxs from Japan and 1.2m turned dishes from the States.
Mike

[dennishoy]

I worked for the IBA and was part of the CMAC dev team. I tested personally all the CMAC Codecs for the European operators. This was between 1982 to 1984. in 1983/4 there was only 1 channel on Ku band (No C or S band) called Europe up to 1985, this was a beige screen with the word Europe on it, for most of the day that was it with some news in the evening. System was analogue with sawtooth dispersal signal at 50Hz. There were some frequencies used by the operators for tests but there was no timetable or programming.
I built my own Rx and LNB in 1983 because an LNB cost 3000$ from Sony and had a
3dB noise figure. In 1986 the prices had started to fall whole systems in Europe (Italy) were around 2000$, We put together systems to sell in Italy to a major distributor for 1000$. Taking LNBs from Korea Rxs from Japan and 1.2m turned dishes from the States.
Mike

My 1.8m dish was 'petalised' and bolted together as six identical sections. The actuator was a 'Superjack' which worked on 36V DC from the positioner. Each and every channel had an adjustable 'skew' setting saved in the receiver. There was a company called HiTech Xtravision in Camberley, Surrey that sold this kit at a reasonable price and also decoder kits. It was an expensive hobby in those early days.

[boiss]

I've still got a couple of polarotors one with a scalar feed, can't think of anything to use it for these days.
As for CMAC it was the choice for Europe beating PAL E. It would have been adopted but the Italians hadn't made up their minds as they were heavily in involved with the Japanese 1250 line system. After the italian dillying and dallying for over a year most of the European countries went their own ways such as Dmac and various Analogue encryption systems. Eventually The Italians saw the light, their system on which they had reputedly spent 200M$ was untransmittable over the current and planned range of satellites requiring some 70MHz of bandwith. Things went very quiet on their side and that was basically the end of the common transmission standard in those days. Many variations of the MAC system were used in the UK, Scandinavia, Australia and the AFN until the advent of digital.