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Thread: What does it technically mean to get a lock on teh satellite?

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    Member Optima Collins's Avatar
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    Default What does it technically mean to get a lock on teh satellite?

    I know this may sound like a newbie question so please don't go telling me it's when the little bars go green...

    What I'd like to know from someone with more technical knowledge is what does its actually mean to get a lock on a satellite? Is it signal strength and quality thesholds, specific network/transponder information etc. In other words, how does the STB (Strong 4856x) actually decide it has a lock.

    The reason I'm asking is that sometimes I do a blind scan after moving the lnb (T90) or dish to a position close enough to where I expect the satellite to be (without using a satfinder and just for the fun of it). Usually, I get reception of most if not all TP's and often at reasonable levels, but just as often these scans disappear once the box is switched off. Why, considering it knew where they were despite the "no lock"?

    Just curious and wanting to understand the technology better.

    Cheers
    OC



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    Quote Originally Posted by Optima Collins View Post
    I know this may sound like a newbie question so please don't go telling me it's when the little bars go green...

    What I'd like to know from someone with more technical knowledge is what does its actually mean to get a lock on a satellite? Is it signal strength and quality thesholds, specific network/transponder information etc. In other words, how does the STB (Strong 4856x) actually decide it has a lock.

    The reason I'm asking is that sometimes I do a blind scan after moving the lnb (T90) or dish to a position close enough to where I expect the satellite to be (without using a satfinder and just for the fun of it). Usually, I get reception of most if not all TP's and often at reasonable levels, but just as often these scans disappear once the box is switched off. Why, considering it knew where they were despite the "no lock"?

    Just curious and wanting to understand the technology better.

    Cheers
    OC

    G'Day Cobber,
    To keep it as simple as possible, the STB has a Free running Local Oscillator configured as a PLL. (Phase Lock Loop). When you select a TP Freqency, as well as Symbol Rate, the Local Oscillator shifts to accommodate the Sat Signal. Simply put, it locks onto it. Within limits it will follow the Sat signals minor drifts.
    As to why they disappear, perhaps you have not saved them or switched off without properly exiting the menu.
    Perhaps they have been saved under another Satellite data.

    Kindest Regards, " The Druid ".


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    Quote Originally Posted by beer4life View Post

    ...As to why they disappear, perhaps you have not saved them or switched off without properly exiting the menu.
    Perhaps they have been saved under another Satellite data.

    Kindest Regards, " The Druid ".

    Thanks for your explanation. That's exactly what I wanted to know!
    As for the services dissapearing, I should have been clearer. Its not the data entries that disappear, its the sat signal that disappears. At first I thought that maybe the LNB(s) on the T90 may have shifted position(s) because I had them clamped with springclips to stop rotation (continuous unscrewing would have damaged the threads), but as far as I can see that is not the case. As per your explanation, couldn't it be that, being out of phase (no lock) when I scan, that the box LO loses the ability to accommodate the sat signal when I switch it off and then back on?

    OC

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    Quote Originally Posted by Optima Collins View Post
    Thanks for your explanation. That's exactly what I wanted to know!
    As for the services dissapearing, I should have been clearer. Its not the data entries that disappear, its the sat signal that disappears. At first I thought that maybe the LNB(s) on the T90 may have shifted position(s) because I had them clamped with springclips to stop rotation (continuous unscrewing would have damaged the threads), but as far as I can see that is not the case. As per your explanation, couldn't it be that, being out of phase (no lock) when I scan, that the box LO loses the ability to accommodate the sat signal when I switch it off and then back on?

    OC

    Can't understand why you would want to do that, unless you're a Masochist.
    I've no interest in duplicating your experiment as it does not prove a thing.
    However, there is one possibility. You may have done a network scan and picked up the TP datum from the NIT. (Network Information Table). However not good enough lock to be able to scan the channels in.



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    Quote Originally Posted by beer4life View Post

    Can't understand why you would want to do that, unless you're a Masochist.
    I've no interest in duplicating your experiment as it does not prove a thing.



    Yes, you're probably right about me being a masochist (or a NIT wit ). I have no problem admitting that's how I learn about things. I'm not trying to prove anything with the "unlocked" scans other than winging it without a satfinder. Had I not tried I wouldn't have asked about it and wouldn't have known about the LO PLL. Just part of my convoluted learning curve.

    Thanks again
    OC

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    The PLL locks onto the frequency and SR, but the actual 'decoding' lock, or what enables reception, is the bit error ratio (BER) and modulation error ratio MER). You can have heaps of signal strength, or the correct term for digital is digital channel power (DCP) but if there are too many bit errors in the data stream, the tuner cannot correct them fast enough (or not at all) and the 'lock' is lost, resulting in no reception. This happens even though the PLL still knows the frequency and symbol rate.

    There are two levels of bit error correction.... viterbi and reed-solomon.
    tuners will have a pre viterbi BER at their input, which it then attempts to correct any corrupt data packets, which then results in a post-viterbi measurement. Reed-solomon is a secondary error correction system.

    This process happens automatically and it's why there is a slight delay when changing channels as the receiver goes through the signal analysis process and error correction, before it 'locks in".

    My spectrum analyser will display digital channel power, carrier/noise, pre & post viterbi BER, reed-solomon, noise margin, MER and constellation.

    All of these measurements are used to accurately align dishes/LNB's (and terrestrial antennas.... links, etc) with a TDR (Time Domain Reflectometer) to measure multipath, single frequency networks (SFN) and a few other tricks, for tuning filters and locating cable/connection faults, etc.

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    Hi MTV

    Thanks for the right answer.

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