How is it possible to have so many multiplexes now?

RichardcoulterRichardcoulter Posts: 30,153
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AIUI, there was only room for four (five at a push) analogue channels in the UK.

If I understand correctly, each former analogue channel changed into a mux transmitting various channels at DSO.

So, my question is how can we have more than four or five muxes?

I'm sure there must be a technical reason why there can be more digital muxes than analogue channels; i'm just curious as to what it is.

Many thanks.
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  • Nigel GoodwinNigel Goodwin Posts: 58,329
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    AIUI, there was only room for four (five at a push) analogue channels in the UK.

    If I understand correctly, each former analogue channel changed into a mux transmitting various channels at DSO.

    So, my question is how can we have more than four or five muxes?

    I'm sure there must be a technical reason why there can be more digital muxes than analogue channels; i'm just curious as to what it is.

    It's not difficult - they 'stole' the space from the vast majority of transmitters, those that only have three multiplexes.
  • RichardcoulterRichardcoulter Posts: 30,153
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    It's not difficult - they 'stole' the space from the vast majority of transmitters, those that only have three multiplexes.

    Thanks for taking the time to reply, but I don't understand what you mean :confused:
  • technologisttechnologist Posts: 13,334
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    Also because of the narrowness of the spectrum of digital Mux you can put them in adjacent rf channels ..- -and hence no taboo channels as well.

    The oringinal is plan had about two hundred 6 mux sites,
  • Nigel GoodwinNigel Goodwin Posts: 58,329
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    Thanks for taking the time to reply, but I don't understand what you mean :confused:

    It's pretty simply, almost all sites had four analogue channels - they now only have three multiplexes.
  • anthony davidanthony david Posts: 14,461
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    Also because of the narrowness of the spectrum of digital Mux you can put them in adjacent rf channels ..- -and hence no taboo channels as well.

    The original is plan had about two hundred 6 mux sites,

    To expand on that, from the early days of am radio, broadcasters realised that receivers had to be affordable and their tuners would therefore have short comings. Frequencies were therefore chosen to avoid the problem, problem frequencies are known as "Taboo Channels". As far as analogue TV was concerned the taboo channels were known as n+/-1, n+5 and n+9 where n is a channel in use. So if one of your local channels was 30 you couldn't use.

    29 (n-1, adjacent channel)
    31 (n+1, adjacent channel)
    35 (local oscillator frequency for TVs tuned to channel 30)
    39 (image/alternate channel for a TV tuned to channel 30)

    With careful planning the taboo channels for one service can be made to fall on the taboo channels for another thus increasing the number of available channels. Digital TV is relatively unaffected by these problems and therefore taboo channels can be used greatly increasing the number of channels available.
  • [Deleted User][Deleted User] Posts: 435
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    To expand on that, from the early days of am radio, broadcasters realised that receivers had to be affordable and their tuners would therefore have short comings. Frequencies were therefore chosen to avoid the problem, problem frequencies are known as "Taboo Channels". As far as analogue TV was concerned the taboo channels were known as n+/-1, n+5 and n+9 where n is a channel in use. So if one of your local channels was 30 you couldn't use.

    29 (n-1, adjacent channel)
    31 (n+1, adjacent channel)
    35 (local oscillator frequency for TVs tuned to channel 30)
    39 (image/alternate channel for a TV tuned to channel 30)

    With careful planning the taboo channels for one service can be made to fall on the taboo channels for another thus increasing the number of available channels. Digital TV is relatively unaffected by these problems and therefore taboo channels can be used greatly increasing the number of channels available.

    Deary me Dave

    Not kept up with the evolving technoligy on telly tuners eh.

    You can use the image ,L/O, and adjacent channel frequencys on analogue broadcasts ,cos I have done it for years .

    Take a butchers at the performance of modern TV tuners.
    If you need lille ole me to explain I will do so .

    Plus digital modulation is more immune from co channel interferance so this allows more spectrum to be used.

    Check out the immunity of digital modulation!

    Apologies for teasing ,can't help myself .
  • [Deleted User][Deleted User] Posts: 435
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    Winston_1 wrote: »
    It works (on modern TV tuners) provided the level of all the channels is pretty much the same. The cable companies did for years as they can easily control the levels. When broadcast over the air however the levels can vary due to propergation issues.

    Say a tuner has an adjacent channel rejection of 40dB. But if that adjacent channel is 15dB stronger the adjacent channel rejection drops to 25dB.

    Then not everyone had the latest TVs either.

    As Mike says DTT is less affected by these problems.

    It is nothing to do with relays only having 3 MUXES either. Before DSO main transmitters had 4/5 analogue and 6 DTT MUXES and the relays 4 analogue channels.

    Adjacent ch rejection is higher than you mention these days me thinks.
    But I will test to confirm .
    Also telly tuners have had good agile front end tracking filters for 20 years or so
    If I am wrong I will make a grovelling apology on this thread .

    Grovelling is something I have learnt to do with my wife,she who must be obeyed .
  • anthony davidanthony david Posts: 14,461
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    Deary me Dave

    Not kept up with the evolving technoligy on telly tuners eh.

    You can use the image ,L/O, and adjacent channel frequencys on analogue broadcasts ,cos I have done it for years .

    Take a butchers at the performance of modern TV tuners.
    If you need lille ole me to explain I will do so .

    Plus digital modulation is more immune from co channel interferance so this allows more spectrum to be used.

    Check out the immunity of digital modulation!



    Apologies for teasing ,can't help myself .

    Firstly I doubt that you have done it for years as you are far too young and we have had digital since 2009 in this area, earlier in others.

    Secondly as winston points out, and common sense should tell you (although again I accept you have the arrogance of youth), broadcasters have to take in to account the fact that people have/had very old equipment that complied with the standards at the time they bought them. I see you still put your commas in the wrong place by the way making it difficult to take you seriously. You also claim to be living in the USA (remember that?) perhaps it is different there.

    By the way it's interference not interferance, technology not technoligy and no adult types cos.
  • [Deleted User][Deleted User] Posts: 435
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    Firstly I doubt that you have done it for years as you are far too young and we have had digital since 2009 in this area, earlier in others.

    Secondly as winston points out, and common sense should tell you (although again I accept you have the arrogance of youth), broadcasters have to take in to account the fact that people have/had very old equipment that complied with the standards at the time they bought them. I see you still put your commas in the wrong place by the way making it difficult to take you seriously. You also claim to be living in the USA (remember that?) perhaps it is different there.

    By the way it's interference not interferance, technology not technoligy and no adult types cos.
    Me spellin is naff ,I accept that .

    Firstly telly tuners in the UK used to radiate the LO,at 38.9Mhz above the incoming frequency in the days of valve TV tuners , so you couldn't use channells spaced five channels above as the LO would radiate from the viewers antenna and interfere with nearby users .
    .Plus the image frequency (9 channels away) would also convert to the IF frequency due to the poor input filtering in the tuner .
    The VSB at 1.25 MHz would mean that the lower chanell sound carrier at . 75 MHz away would be too close for adjacent ch operation.
    1.25MHz was originally needed due to the relative poor performance of LC IF filters in the TV demodulator circuit as these rolled off at 1.25 MHz on the inside of the IF frequency .

    TV sets many years ago when solid state tuners emerged started using active input filters which filters out the image frequency and LO from feeding back up the antenna .
    I have designed these type of active input filters .
    The IF SAW filters introduced well over thirty years ago provided much sharper filtering ,plus the need for the sound subcarrier to be about 5 or 6 dB,s down from the peak sync vision carrier wasn't needed because of the ceramic sound traps used in more up to date demodulators , so the problem of caption buzz was eliminated.
    I have designed TV demodulator circuits as well as TV transmitters.
    In a nutshell tuners and IF circuits got much better after the five and nine adjacent ch rule was established.
    At our local university when I was in the UK on this subject the lecturer was making the point about these rules to some of my friends who were in attendance at the lecture.
    My friends pointed out that we were operating transmitters in other countries on multiple adjacent channells by reducing the lower VSB to .75Mhz and level of the sound carrier of the transmitter to 15dB relative to peak sync.
    This enabled analogue transmitters to overcome the established thinking on five,nine and adjacent ch operation .
    Of course the radiated power of the transmitters had to be the same on all frequencies.
    But the relevent fact is I have done it several times.
    The other relevant factor is propagation differences at diffrent frequencies ,but this is minor on adjacent ch frequencies as the diffrence in refraction and path loss is minimal.

    What sometimes is the issue is those who make the decision on the broadcast rules sometimes have little or no design experience and the knowledge they have is what they have read in books and consequently it takes a long time before the rule makers become aware of emerging technology or don't understand all aspects .
    The consequence has been the poor use of available spectrum.
    This was evident in the initial roll out of of DVB-T ,a total disaster in the UK in my view.
  • nvingonvingo Posts: 8,619
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    Also some mux are broadcast a little above (+) or below (-) the centre frequency for the channel.
    And i expect quite a lot has been learned about packing mux into bandwidth from digital satellite and digital cable technologies.
  • [Deleted User][Deleted User] Posts: 435
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    nvingo wrote: »
    Also some mux are broadcast a little above (+) or below (-) the centre frequency for the channel.
    And i expect quite a lot has been learned about packing mux into bandwidth from digital satellite and digital cable technologies.
    The offset is due to the constant impedance combiners used at transmitter sites .
    A little nudge in the frequency to create a wider gap for the filters helps.

    Unless I have missed something I cannot see any reason for offset to improve cci on digital modulation?

    But hey I don't know everything 😇

    Can some other anorak enlighten me !
  • [Deleted User][Deleted User] Posts: 435
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    The offset is due to the constant impedance combiners used at transmitter sites .
    A little nudge in the frequency to create a wider gap for the filters helps.

    Unless I have missed something I cannot see any reason for offset to improve cci on digital modulation?

    But hey I don't know everything 😇

    Can some other anorak enlighten me !

    Dave,Nigel and Technolagist .

    I was expecting a bit of input from ye ?
  • Nigel GoodwinNigel Goodwin Posts: 58,329
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    Dave,Nigel and Technolagist .

    I was expecting a bit of input from ye ?

    I would imagine the offsets are to help reduce interference between multiplexes from different transmitters?, but we haven't been provided any info for years now.
  • [Deleted User][Deleted User] Posts: 435
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    I would imagine the offsets are to help reduce interference between multiplexes from different transmitters?, but we haven't been provided any info for years now.


    Go on Nigel

    You must know !

    What about analogue then ?

    Offsets were very common then with no adjacent ch combining filters to worry about .
  • [Deleted User][Deleted User] Posts: 435
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    Winston_1 wrote: »
    Offsets on analogue, either 5/3 or 10/3 of line frequency , if I remember correctly, were to reduce the effects of co channel interference. With no offset there is a slow flickering, with correct offset there is a defined horizontal banding which is less distracting.


    Well done Winston

    I mark you 6 out of 10
    I will give you a full score of 10 if you can describe how this was done 40 years ago and how it is easier to do know.
    Also why line frequency offset reduced cci and by how much in dB,s

    In fact a bonus point is possible 📡📺🔥
  • Mark CMark C Posts: 20,725
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    Well done Winston

    I mark you 6 out of 10
    I will give you a full score of 10 if you can describe how this was done 40 years ago and how it is easier to do know.
    Also why line frequency offset reduced cci and by how much in dB,s

    In fact a bonus point is possible 📡📺🔥

    It didn't reduce it, just reduced the visible effects :D. Most of energy in an analogue TV transmission is at multiples of the line frequency (15.625 kHz for 625/50 transmissions)
    view it on a spectrum analyser and you'll see the spikes at multiples of 15.625 right through the pass band. Therefore off set a co channel transmission by a fraction of the line freq, and the peaks won't coincide.
  • [Deleted User][Deleted User] Posts: 435
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    Mark C wrote: »
    It didn't reduce it, just reduced the visible effects :D. Most of energy in an analogue TV transmission is at multiples of the line frequency (15.625 kHz for 625/50 transmissions)
    view it on a spectrum analyser and you'll see the spikes at multiples of 15.625 right through the pass band. Therefore off set a co channel transmission by a fraction of the line freq, and the peaks won't coincide.


    No quite Mark so 3 out of 10

    What about the oskalator .you made no mention of that ,deary me !!!!!!!
  • Mark CMark C Posts: 20,725
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    No quite Mark so 3 out of 10

    What about the oskalator .you made no mention of that ,deary me !!!!!!!

    You are a patronising troll, aren't you ?

    I'm not wasting any more time
  • anthony davidanthony david Posts: 14,461
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    Mark C wrote: »
    You are a patronising troll, aren't you ?

    I'm not wasting any more time

    Why do you think I didn't reply to him. He is a fascinating troll, like spiney2, and like him he is a "cut and paste" merchant with no understanding of what he reads. He claims to be a genius living in the US, so clever he could design and build you a complete transmission system, has already done so in his mind probably, amazing for a 25 year old. Yet despite his alleged abilities he is unable to find the spell check on his computer or grammar check for that matter. Now I know that spellcheckers have their drawbacks but they do eliminate the worst errors so when I see ungrammatical miss-spelt rubbish, not the occasional mistake, I know what I'm dealing with. Still it has on occasions passed the time before my wife serves dinner.

    I assume he is very young, possibly still at school, possibly with "issues". Someone said that the numbers and letters after his name are a UK amateur radio call sign, If anyone here subscribes to a call sign data base they could perhaps check that out.

    He will probably now go away and return under a different name.
  • [Deleted User][Deleted User] Posts: 435
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    Mark C wrote: »
    You are a patronising troll, aren't you ?

    I'm not wasting any more time


    Well call me Dave and Mark

    Don't yer wish to know then.

    Give Yer two clues

    1/ Precision oscillators ,in the old days oven controlled, currently GPS used as a reference .

    2/ Back porch on each line .

    This improves the isolation needed from about 46dB plus to about 34dB

    There yer go I have given it away .

    Stop being so touchy ,we're is your sense of fun ?

    If I am wrong you can take the piss out of me .

    Young folk won't have a clue ,it's the old farts who will know the answer .

    I do live in the US , but I worked in the UK broadcast sector .
    .
  • [Deleted User][Deleted User] Posts: 435
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    Well call me Dave and Mark

    Don't yer wish to know then.

    Give Yer two clues

    1/ Precision oscillators ,in the old days oven controlled, currently GPS used as a reference .

    2/ Back porch on each line .

    This improves the isolation needed from about 46dB plus to about 34dB

    There yer go I have given it away .

    Stop being so touchy ,we're is your sense of fun ?

    If I am wrong you can take the piss out of me .

    Young folk won't have a clue ,it's the old farts who will know the answer .

    I do live in the US , but I worked in the UK broadcast sector .
    .

    Lesson two is how it works on DVB-T

    But yer need to get up to speed on phase modulation otherwise it will zip straight over yer head .

    PS
    Never could spell ,but I am just not bovered
  • anthony davidanthony david Posts: 14,461
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    It's Anthony not David, to an adult that would be obvious.

    You're a teenage troll.

    You were fun for a while but we are bored with you now.

    Goodbye.
  • [Deleted User][Deleted User] Posts: 435
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    It's Anthony not David, to an adult that would be obvious.

    You're a teenage troll.

    You were fun for a while but we are bored with you now.

    Goodbye.

    Sorry I have offended you it was not my intention .

    My sense of humou, gets me in the crap sometimes .

    So I hope you won't take your bat and ball away , I will promise to be nice to you and Nigel

    PS call me Anthony doesn't sound right 😎
    PPS were did you get my age from ?
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