digital threshold

llss

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#1
hi , fellow fanatics installers whatever just curious what threshold for dtt do you guys use when balanceing signal to noise ratio , can t find national guide but i think buy experience 23 db threshold , tricky can be , i just wish the change over was here and digital upped in power , my readings north east , pontop pike , run digital at good readings 45 db each multiplex in good areas
 
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#2
For decoding MPEG2 bitstreams, the post Viterbi bit error rate must be less than 1 in 1000, which measures as 17dB sig to noise ratio (Rayleigh modal value to variance ratio).

Unfortunately, COFDM performs very poorly on impulse interfrenece, check there's no nearby electrical machinery or radio interference sources.

See my Freeview guide below, FAQs section.

(also, try a UHF bandpass filter on the receiver aerial input).
 
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My Satellite Setup
Pace 2200 Sky digibox with ftv card, Comag SL65 FTA sat receiver, 40cm Sky minidish, Setpal terrestrial receiver (for free uk tv only!).
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#3
Sorry, that probably wasn't too helpful!

For digital broadcasting to work, you need a reception bitrate error of approx 1 in 1000 or better. Below that and the error correction system can cope, above and it can't.

For orthogonal carrier systems, as the number of carriers "tends towards" infinity, in a Gaussian ("white noise") channel, this errror rate requires a carrier(s) to noise power of at least 17dB, a "magic number"! (below that, the bit error rate very rapidly increases as carrier power falls).

For COFDM, either 2k or 8k carriers is "close enough" to infinity for this rule to apply.

For COFDM, channel noise is approximately equal to measured variance, and average carrier power approximately equal to measured modal value.

(with COFDM Rayleigh scattering - radio wave reflection from nearby buildings - some carriers cancel out while others reinforce and strengthen. The strongest carrier gives the modal value. Although SNR improves on reinforced carriers, it worsens on cancelled out ones, so the overall SNR remains roughly the same).

This is only true under "best" reception conditions, and only the Setpal receiver (Novapal tuner) seems to almost manage it. If there's any i.f. channel noise - ie insufficient image and spurious rejection - then the received SNR via aerial will have to be better.

Also, the receiver's local oscillator frequency must be very exact, otherwise the carriers are no longer exactly orthogonal, and some further SNR advantage is lost.
 

llss

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My Satellite Setup
technomate 1500 ci super 80 c m dish diseque 1.2 superjack motorised system dragon celebro cas 3
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northumberland
#4
yes , just trying to balance readings accuratley , bettween digital and anolouge , pretty critical in some areas and espesically when useing mast heads or distribution amps , i normally get about 18db reading of noise , which i try to keep to a minuium. , s/n ratio has to be more than 23 db for digital to work i was told , anolouge and digital quite tricky balancing at moment , with anologue being so high and the digital running at low power ,amplifers produce strange results at moment , watching noise levells at this time mate many thanks
 
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My Satellite Setup
Pace 2200 Sky digibox with ftv card, Comag SL65 FTA sat receiver, 40cm Sky minidish, Setpal terrestrial receiver (for free uk tv only!).
My Location
Midlands
#5
Well, 17dB is the theoretical minimum needed. If you're measuring at the aerial - so have to add on cable losses - and then consider that most receivers do "worse than the best", then yes 23 dB CNR (carrier to noise) would be "reasonable" for digital reception to start working.

Yes, agreed, the transmitted power levels for analogue and digital are rather different, and there can be problems, and some people recommend "as standard" separating the digital muxes and amplifying separately before recombining. Which sometimes you might have to. I say "if it ain't broke, don't fix it".

eg see www.teldis.com/TP-headendcomp.htm .
 

llss

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My Satellite Setup
technomate 1500 ci super 80 c m dish diseque 1.2 superjack motorised system dragon celebro cas 3
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#6
cheers mate , many thanks
 
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