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Tech and Mods

The hitchhikers guide to SCPC reception [06 June 2001]


  If your satellite receiver bears the DVB logo and a common interface CAM slot then it should meet the minimum technical standards required to receive the DVB-S digital broadcasts from the major league players in the anywhere in the broadcasting world. That is, of course if your service provider hasn't deliberately restricted your receiver's abilities or limited the availability of a suitable CAM.

Similarly any 'digital' lnbf working in partnership with a generic offset dish of sufficient size should have no trouble at all in giving trouble free reception within the service provider's intended service area.

But we are enthusiasts aren't we?
We want a little bit more than that.

But as soon as we try and receive a signal that is not intended for our part of the world, we can start to have problems. The received signal strength drops as we move away from the centre of the satellite footprint. The effects of unwanted signals and noise picked up by the dish and added by the electronics in the lnbf and receiver, begin to make themselves felt. The picture starts to pixellate or freeze, and the sound starts to break up too.

The cure is, of course, to get yourself some more of the signal that you do want, or to try and reduce the signals and noise that you don't want.

The best solution is almost invariably to get yourself a bigger dish, because this does both!

A larger dish will collect more of the signal that you do want. But it will also reduce the noise and signals you don't want, because its beam width is narrower. A narrower beam will pick up less interference and noise from nearby satellites, the sky, and the ground.

A lower noise lnbf by comparison, can only reduce its own electrical noise contribution. So a 'magic' low noise lnbf is only likely to show a significant improvement in performance once all the other noise sources have been reduced to the point that the lnbf noise contribution is larger than any of the others.

It's a bit, but only a little bit, like collecting rainwater (the signal) in a leaky bucket, where each of the leaks represent the possible sources of noise. The wider the bucket, the more water you can collect, but if you want to plug the leaks too, it makes sense to plug the biggest leak first!

So now, with a lighter wallet, we should be back to clear pictures and sound again.

But we are enthusiasts aren't we?
We want a little bit more than that.

Moving further away from the centre of the footprint, the same problems recur - time to buy a bigger dish?

There comes a time when a bigger dish is no longer practical, what can we do now?

Perhaps now it's time to look at the lnbf again. Have you noticed how careful I've been in writing lnbf all the time? It's because it stands for low noise block downconverter and feed.
The feed is the part of the system that couples the lnb to the dish, and refers to the feed horn or microwave lens fitted to the front of the lnb.

The lnb has to be able to 'see' all of the surface area of the dish, but not be able to look over the edge of the dish and pick up unwanted noise radiated from your house or the ground, and this is what the feed is supposed to do. Unfortunately, in most systems the lnbf manufacturer and dish manufacturer have never spoken to each other! They rely on the fact that there is a generic offset dish design, using a dish focal length to dish diameter ratio of 0.6 to ensure that things are not too bad. When you look at all the different makes, shapes and sizes of dish there are, it's a wonder that this works as well as it does! But generally speaking, when you are in the centre of the footprint, a few inefficiencies are acceptable and everybody stays happy.

When you are looking for the ultimate in performance this is not quite good enough. It's more sensible to buy your feed from the same manufacturer that made your dish, or from a specialist manufacturer of adjustable feeds. This gives you a fighting chance that your lnb will see the dish, the whole dish, and nothing but the dish. This type of feed normally requires that you use a C120 fitting type lnb.

Whether or not you decide to go for a separate feed or a lnbf, a little bit of tweaking at the dish can pay dividends. You should make sure that the feed actually 'looks' at the centre of the dish and that the feed is positioned at the right distance from the dish and on its centre line. Most supports provide some adjustment for this. Performing these adjustments is difficult and time consuming, and many installers do not bother about it at all! But the gains are there to be had.

Checking that the feed looks at the centre of the dish can be done by sliding a suitable diameter dowel through the feed bracket and noticing where it touches the dish surface. I adjusted mine by shining a torch through the feed itself. There is a professional tool does the same job that uses a laser pointer, which is more accurate still, but I found that a torch worked well enough.

Adjusting the lnb distance is much more problematical, as the position of maximum signal is harder to find, and may in fact prove not be the best position in practice. It's largely a case of "suck it and see".

Having the feed further away might allow the feed to see the whole area of the dish more effectively, and the effect of the lnb seeing past the edge of the dish might not be too serious. On the other hand, in a noisy environment, you could well be better having it closer to the dish and maybe not seeing the edges of the dish so effectively, but reducing the noise pickup from behind the dish.

So now we can receive everything, or can we?

Well, perhaps not. There are narrower bandwidth DVB-S signals out there that might give your system some trouble, let us give your system a real workout!

Troubles with single channel per carrier signals.

Imagine that you run a small TV channel and you want distribution of your service across a wide geographical area, perhaps for onward cable system distribution, but you can't afford the millions of euros that a full satellite transponder will cost you. You have two choices.

1. You can pay for carriage on a major player's transponder, but this means that you have to get your signal to the company satellite uplink site first, so this might not be too cheap after all, and unless you encrypt your signal, anybody can watch it for free.

2. You can approach a satellite operator and lease a smaller amount of bandwidth and run your own uplink site to the satellite.

Let's look at what the Turkish channel Pop TV have done on Eutelsat W1 at 10 degrees east.

They have been allocated a channel on 11.019 gHz, horizontally polarised on transponder B1.
They are using a symbol rate of 2170 megasymbols per second with an FEC of 7/8, the video pid is 308 and the audio pid is 256. They also carry Radyo Eksen with an audio pid of 257.

But wait!

On the same transponder at 11.015 gHz, also horizontally polarised is CNBC-E!
And if that's not bad enough, there's Olay TV, also on the same transponder, this time at 11.024 gHz. and horizontally polarised too!

Oh mercy! These channels are only 5MHz apart! Compare the Single Channel Per Carrier signals with the Multiple Channel Per Carrier and Analogue signals in the picture below.

Perhaps your system is going to have a hard time here.

From a signal strength point of view, things don't look too bad, the footprint shows a reasonably strong signal at your location.

Click to enlarge
Picture © Eutelsat

You enter the channel details into your receiver and hope.

In the best of all possible worlds, after a few seconds the receiver locks and displays in glowing colour a pop video (or what passes for a pop video in Turkey).

More likely to happen however is

1. You get no lock at all, and the signal quality display lurches all over the screen displaying wildly varying values.
2. You get nothing at all.
3. You get lurching juddering freezing mosaic blocks, that might just be a pop video, but might just as well be a newscast, you can't really tell.
4. Strangest of all perhaps, you get a perfect picture, but its not Pop TV!

What's happening? Well, your receiver is having a very hard time.

It has been designed to accept the wide bandwidth of standard broadcast signals, something in the region of 30 MHz wide, so there are at least three digital signals plus various noise signals being presented to the demodulator at once. It's a pretty clever demodulator though and can even capture slightly mistuned signals (3MHz or so off tune), so it will seek and try to lock onto the signal at the centre of the passband. It might just make it if everything else is in its favour.

Unfortunately, if you look at the specification of your lnb, you will see that the frequency stability is quoted to be something like +/- 3MHz, plus a bit more for temperature variations. So the figure you programmed into the receiver may not be exactly the frequency it is trying to demodulate, pretty important when the frequencies we are interested in are only 5MHz apart! When you factor in the frequency stability of your receiver too, which can have a similar variability, it's all to easy to see how this could cause the demodulator to lock intermittently, not to lock at all, or to lock on to a signal a few MHz away.

This problem is of no real consequence on wide bandwidth signals, but the narrower the signal bandwidth, and the closer each signal is to the next one, the more serious it becomes. Very narrow bandwidth, low data rate signals require the use of special narrow bandwidth receivers coupled with phase locked loop or crystal controlled lnbs, for fear of missing the signals altogether!

So, if you really must watch that channel, try altering the local oscillator frequency in the receiver set-up by a couple of MHz and try again!

The other problem your system has to contend with is that the FEC of the transmission is set to 7/8 rather than the 2/3 or 3/4 that the major networks use. Once the demodulator has locked, the data has only 1 error-correcting bit for every 7 bits transmitted, giving very little protection against the effects of a noisy signal. The only sensible cure for this is a bigger dish, but hopefully not quite as big as the one they use at the cable system head end!


The Professor





 

 




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