[linux-dvb] record multiple channels

Carsten Koch Carsten.Koch at icem.com
Sat Apr 9 12:14:52 CEST 2005

Nico Sabbi wrote:
>> They also told me that the "receiver unit" on the dish needs to be of 
>> a type that can handle multiple channels if you want to watch 2 (or 4) 
>> different channels _at the same time_. Is this true?
> BS. The LNB just receives whatever is transmitted.

Well, mostly true.

Let's answer the original question systematically:

Satellites transmit data from the sky in a certain range of frequencies
(around 11 Ghz) and in two distinct polarizations (horizontal and vertical).
The frequencies are too high to be efficiently processed electronically,
so right at the dish there is a component called LNB (low noise block
converter) that converts a block of frequencies and a certain polarization
down into a much lower frequency (around 1 GHz).
Most current LNBs can be switched between both polarizations and two
ranges of frquencies.
So, roughly speaking, the LNB forwards "one selected quarter" (one
polarization and one range of frequencies) of whatever is transmitted
to the tuner in the DVB-S card.
The tuner now selects exactly one of these frequencies ("tunes" to it) for
further processing. This frequency may or may not carry multiple channels.
I believe that all modern "budget type" DVB-S cards (cards without too
much own intelligence) can forward the whole stream to the driver.
If the driver can handle that (recent Linux drivers can), you will be
able to receive all channels on the stream that is carried on the
frequency the tuner is tuned to, which is part of the block of frequencies
the LNB has selected and has the polarization the LNB selected.

If you want to simultaneously record channels that are transmitted on more
than one frequency, you need one DVB-S card per frequency and one LNB per
block of frequencies and per polarization (makes up to 4 LNBs).



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