Author Topic: Freeview v Freesat Feeds  (Read 299 times)

CAMRAMan

  • Formerly A Warwickshire Lad
Freeview v Freesat Feeds
« on: September 16, 2019, 03:07:06 pm »
Can someone explain in layman's terms why a Freesat box needs separate inputs from the satellite dish for each tuner, but a Freeview box needs only one antenna feed for multiple tuners?
Haggerty F, Haggerty R, Tomkins, Noble, Carrick, Robson, Crapper, Dewhurst, Macintyre, Treadmore, Davitt.

Re: Freeview v Freesat Feeds
« Reply #1 on: September 16, 2019, 03:58:52 pm »
its because the LNB is only capable of doing one thing at once, and what it doesn't do is to simply spit the raw signal from the dish down the coax; it simply can't for one thing, not least because the signal into the dish can be polarized and the signal down the cable can't be.

  The LNB is instructed (by the receiver) to receive one frequency band (from two) and one signal polarisation (from two), amongst other things.  This means (I think) that the signal that comes from the dish only contains one channel's worth of information, because that is all a single LNB is capable of spitting out.

You can get LNBs that allow reception from two or more  satellites, with a particular angular separation in the sky.  These are (I think) capable of feeding as many (different) receivers as there are output cables from the LNB(s).

By contrast a freeview antenna is capable of feeding the raw antenna signal to a network of receivers, each one tuned into a different frequency. But then a freeview signal only has to travel about twenty or thirty miles before it is picked up, but a satellite signal comes from about x1000 times further away.  If broadcast at the same signal strength (which I don't think it is), the satellite signal is liable to be about a million times weaker; I'm surprised it works at all, in fact.

cheers

Kim

  • Timelord
Re: Freeview v Freesat Feeds
« Reply #2 on: September 16, 2019, 05:01:18 pm »
its because the LNB is only capable of doing one thing at once, and what it doesn't do is to simply spit the raw signal from the dish down the coax; it simply can't for one thing, not least because the signal into the dish can be polarized and the signal down the cable can't be.

The LNB is instructed (by the receiver) to receive one frequency band (from two) and one signal polarisation (from two), amongst other things.  This means (I think) that the signal that comes from the dish only contains one channel's worth of information, because that is all a single LNB is capable of spitting out.

Almost.

The LNB will operate in one of two frequency bands (downconverting the entire band to the same intermediate frequency on the cable), and in horizontal or vertical polarisation.  DC power and a low-frequency switching signal go up the cable to select which mode it's in, according to what mux[2] the receiver wants to receive.

You could happily split the signal (throwing away one of the switching signal) and receive multiple muxes in the same band/polarisation, but that would complicate things for the end user in terms of what is available at the same time.  Receivers that can tune to more than one mux at the same time (eg. to record other channels) generally have n LNB inputs.  Distribution systems feeding large blocks of flats etc will use an LNB with fixed outputs of the four possible combinations, and do the selection elsewhere.


The terrestrial transmission, on the other hand, occupies a much smaller band (hence there are fewer channels at generally lower bitrates), and uses group and polarisation to differentiate the signals from different transmitter sites, to prevent interference in the overlap zone.  You'll normally only have an aerial aligned and grouped/filtered for the nearest one, so the receiver can see everything in one go without re-programming the aerial.


Quote
But then a freeview signal only has to travel about twenty or thirty miles before it is picked up, but a satellite signal comes from about x1000 times further away.  If broadcast at the same signal strength (which I don't think it is), the satellite signal is liable to be about a million times weaker; I'm surprised it works at all, in fact.

The satellite signal has the overwhelming advantage that it's only really competing with cosmic background radiation, whereas the terrestrial signal has all sorts of natural and man-made noise to drown out.



[1] In DVB-T and DVB-S, each individual digital signal contains half a dozen TV channels and an assortment of radio.  The receiver decodes all of them, and throws away whatever data it isn't interested in.  Given a sufficiently competent receiver you can record/watch the entire mux with a single tuner, but you still need a tuner for each simultaneous mux.
To ride the Windcheetah, first, you must embrace the cantilever...

Re: Freeview v Freesat Feeds
« Reply #3 on: September 16, 2019, 07:03:34 pm »
I had freesat once. It was OK once I'd tuned out the multiple channels I got for each region. However it didn't work if it rained so it went after a while

Kim

  • Timelord
Re: Freeview v Freesat Feeds
« Reply #4 on: September 16, 2019, 07:08:26 pm »
I had freesat once. It was OK once I'd tuned out the multiple channels I got for each region.

When we had a MythTV box, those duplicates would occasionally come in handy for resolving scheduling conflicts, as they sometimes show the same programme at different times in different regions.
To ride the Windcheetah, first, you must embrace the cantilever...

CAMRAMan

  • Formerly A Warwickshire Lad
Re: Freeview v Freesat Feeds
« Reply #5 on: September 16, 2019, 07:22:30 pm »
Thank you for the responses, some of which I understand  :-D
I've had Freesat for 10 years, but am moving to a flat, so a dish won't be an option, sadly. My Raspberry Pi fettling skills will then be tested to the max...
Haggerty F, Haggerty R, Tomkins, Noble, Carrick, Robson, Crapper, Dewhurst, Macintyre, Treadmore, Davitt.

Re: Freeview v Freesat Feeds
« Reply #6 on: September 16, 2019, 07:25:26 pm »

Almost.

trust me to get in a muxing fuddle... ;)

cheers