TV + Satellite + Video: How The Channels Work 
			- by Julian Stacey

The system is a sequence of boxes.  Understand what happens to the signal
at each stage in the sequence, & you'll understand the whole:

The standard domestic configuration is this:
   - The satellite receiver first merges 2 inputs to 1 output:
   	Input 1: All channels simultaneously received from normal roof aerial.
   	Input 2: Any _One_ solitary satellite channel you tune in on,
   		 (all other satellite channels Not simultaneously available.)
   	Output:	Goes to VHS video recorder.
   		The terrestial(roof aerial) channels are left on
   		unchanged frequencies, the single satellite frequency
   		you chose to tune in on is modulated (put on) to
   		a fixed carrier frequency (set by the TV technician
   		or similar) which the Video & TV know as the
   		satellite channel.
   - The Video Recorder (in Play mode), merges 2 inputs to 1 output:
   	Input 1: Everything sent it by satellite box (IE all roof aerial
   		 channels + the single modulated satellite channel).
   	Input 2: Output from VHS tape.
   	Output:	Goes to TV, as a combination of all original frequencies,
   		plus a single extra channwl, set to whatever the
   		vidoe is currently tuned to. This extra new channel
   		is modulated on a new fixed carrier frequency (which
   		is set by TV technician or similar) which the TV
   		know as the video channel.
   - The Video Recorder (in Record mode), passes on everything it receives
   	Input:	(IE all roof aerial channels + one satellite
   		channel), & also selects any one channel to record.
   		The video has no knowledge of which single satellite
   		channel it receives.
   	Output:	As above.
   - TV	A normal TV only tunes in on 1 channel at a time,
   		(except new expensive TVs with small extra preview
   		frames showing other channels at bottom of screen,
   		which will have 1 channel tuner per picture).
   	The TV can be tuned to any of these:
   		- A terrestial channel
   		- A video 
   			(sometimes TV button 0 or 8 is used as the
   			TV has extra PLL (phase lock loop) hardware
   			to show picture with less wobble deriving
   			from video tape motor speed variation.)
   		- A terrestial channel tuned by the video, while TV is
   		  tuned to video channel.  Reduced picture quality.
   		  Generally times out & screen goes blank after
   		  aprox. 15 mins when video goes into standby mode.
   		- The satellite channel the satellite receiver is tuned to.
   		- The satellite channel as tuned into by the video.
   		  Picture quality less good.  Times out etc.

Extra Expensive Luxury Configurations & Complications:
- You can connect 2 satellite receivers & connect them in sequence,
  or 2 videos, or both etc.  Each additional tuner (box) needs
  another spare channel for its carrier frequency, & losses more
  signal quality. (Just as you lose more money if converting US
  dollars to Australian to British to Russian Roubles, rather going
  direct from USA to Russian).
- Non domestic, more expensive satellite systems, EG for block of
  flats & larger families, have satellite receivers that can
  simultaneously receive multiple satellite signals, not just one.
  Such satellite systems are useful for someone who wants to EG
  record satellite sport & simultaneously record a satellite film,
  or for families to let kids watch MTV, while adults watch something else.
- Some satellite receivers can allow simultaneous reception of EG
  channels 3 & 5, or 6 & 10, but you need a dual LNB (low noise
  block) on the end of the satellite dish, to allow simultaneous
  reception of EG channels 9 & 12. This is because typically odd
  numbered channels are horizontally polarised signals (on Astra)
  & even are vertical.