TV + Satellite + Video: How The Channels Work - by Julian H. 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. Text to merge or strip: History of how technology evolved helps understand what may be possible now: Originaly, old satellite LNBs (Low Noise Blocks, the block on the end of the stick, connected to dish), had just 1 cable. They could simultaneously receive either all odd number channels or all even number channels. Not both at the same time as On Astra, Odd number channels were horizontaly polarized, & even verticaly polarized. Polarization of the LNB was selected by the satellite receiver powering the LNB at 12 Volts or 18 or was that 18 & 12 ? Volts. Each receiver could tune 1 channel. ( A 2nd & 3rd receiver (if connected to signal, but prevented from sending power) could tune a 2nd & 3rd channel, but only from the same odd or even half selected by the 1st receiver's choice of odd or even. ) Twin LNBs were developed. Essentially 2 single LNBs in one box on the end of the stick. Several uses: - To connect 2 independent single channel receivers. - For a single better receiver that could simultaneously record one channel & send another live to TV. - For big blocks of flats, feeding to a multi way box. Quad LNBs were developed, essentially 4 LNBs on one stick. Other LNB variants exist. Conrad.de told me 2012-04 theres no longer horiz & vert polarizing on internal channels. (maybe cos all going digital?)