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uk.tech.digital-tv (Digital TV - General) (uk.tech.digital-tv) Discussion of all matters technical in origin related to the reception of digital television transmissions, be they via satellite, terrestrial or cable. Advertising is forbidden, with no exceptions.

Just a reminder about the sensitivity of modern DTT boxes



 
 
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  #1  
Old November 20th 13, 08:25 PM posted to uk.tech.digital-tv
Bill Wright[_2_]
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Posts: 9,381
Default Just a reminder about the sensitivity of modern DTT boxes

I've been setting up some Humax boxes in the workshop. I wanted to use
the raw Emley Moor aerial feed because there's a fault on the RF system
that I haven't got round to fixing. The raw feed has no filters.
Bilsdale comes in with very uneven muxes at around -6dBmV. I attenuated
the feed by 30dB and the boxes were still tuning Bilsdale, and reception
was perfect. That's ridiculous.

Bill
  #2  
Old November 20th 13, 08:42 PM posted to uk.tech.digital-tv
Woody[_5_]
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Posts: 1,820
Default Just a reminder about the sensitivity of modern DTT boxes

"Bill Wright" wrote in message
...
I've been setting up some Humax boxes in the workshop. I wanted
to use the raw Emley Moor aerial feed because there's a fault
on the RF system that I haven't got round to fixing. The raw
feed has no filters. Bilsdale comes in with very uneven muxes
at around -6dBmV. I attenuated the feed by 30dB and the boxes
were still tuning Bilsdale, and reception was perfect. That's
ridiculous.

Bill




Remember Bill time was that the 'preferred signal strength for
both TV and FM radio was 1mV, and 250uV was classed as fringe.
The manufacturers thus didn't need to build sensitive tuners.

Nowadays with modern low noise components and SMD manufacturing
techniques it is often easier to build a tuner with high
sensitivity than it is to build a deaf one (unless you are
building a Sky box of course!)

It is my (limited) experience so far that modern TV's are at the
very least 20dB and in most cases 30dB+ more sensitive than their
predecessors - that when we have effectively increased the Tx
power by about 4dB in DSO.

Someone somewhere soon will be making a lot of money out of
building automatic attenuators methinks!



--
Woody

harrogate three at ntlworld dot com


  #3  
Old November 20th 13, 08:45 PM posted to uk.tech.digital-tv
Mark Carver
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Posts: 7,608
Default Just a reminder about the sensitivity of modern DTT boxes

Woody wrote:
that when we have effectively increased the Tx
power by about 4dB in DSO.


We did ? it might have been +4dB, but how is that figure of 4dB arrived at ?


--
Mark
Please replace invalid and invalid with gmx and net to reply.
  #4  
Old November 20th 13, 08:55 PM posted to uk.tech.digital-tv
Woody[_5_]
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Posts: 1,820
Default Just a reminder about the sensitivity of modern DTT boxes

"Mark Carver" wrote in message
...
Woody wrote:
that when we have effectively increased the Tx power by about
4dB in DSO.


We did ? it might have been +4dB, but how is that figure of 4dB
arrived at ?



Well its actually as a result of the nature of the signal and how
it is measured.

In analogue the Tx output power was only ever achieved on peak
black, but as it was AM the average level was way below peak,
typically 10dB or more. The received signal strength would thus
tend to be read from this average level. Thus Emley for instance
whilst a rated erp or 870KW per channel the average level that
gave the receiver signal strength was more like 87KW or less.

With a COFDM the amplitude registers as essentially constant at
or near peak level, so the signal strength reads that level. The
rated erp of Emley on DTTV is 174KW, which is as far as the tuner
is concerned at least 3dB higher than it was using before.

At least that is as I understand it but of course I may be well
wrong and am prepared to be corrected. :-))


--
Woody

harrogate three at ntlworld dot com


  #5  
Old November 20th 13, 09:10 PM posted to uk.tech.digital-tv
Steve Thackery[_3_]
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Posts: 1
Default Just a reminder about the sensitivity of modern DTT boxes


Nowadays with modern low noise components and SMD manufacturing
techniques it is often easier to build a tuner with high
sensitivity than it is to build a deaf one (unless you are
building a Sky box of course!)


Easier? How's that, then? If you'd said "much the same" I'd understand, but "easier"?


--
SteveT

  #6  
Old November 20th 13, 09:24 PM posted to uk.tech.digital-tv
tony sayer
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Posts: 4,988
Default Just a reminder about the sensitivity of modern DTT boxes

In article , Woody
scribeth thus
"Mark Carver" wrote in message
...
Woody wrote:
that when we have effectively increased the Tx power by about
4dB in DSO.


We did ? it might have been +4dB, but how is that figure of 4dB
arrived at ?



Well its actually as a result of the nature of the signal and how
it is measured.

In analogue the Tx output power was only ever achieved on peak
black, but as it was AM the average level was way below peak,
typically 10dB or more. The received signal strength would thus
tend to be read from this average level. Thus Emley for instance
whilst a rated erp or 870KW per channel the average level that
gave the receiver signal strength was more like 87KW or less.

With a COFDM the amplitude registers as essentially constant at
or near peak level, so the signal strength reads that level. The
rated erp of Emley on DTTV is 174KW, which is as far as the tuner
is concerned at least 3dB higher than it was using before.

At least that is as I understand it but of course I may be well
wrong and am prepared to be corrected. :-))



Agree with that in general.,.

It does seem that reception on DTV these days is better, least I can
receive DTV signals usually ones out in the office on a simple indoor
aerial whereas I could not do that before on analogue...

--
Tony Sayer

  #7  
Old November 20th 13, 09:34 PM posted to uk.tech.digital-tv
Mark Carver
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Posts: 7,608
Default Just a reminder about the sensitivity of modern DTT boxes

Woody wrote:
"Mark Carver" wrote in message
...
Woody wrote:
that when we have effectively increased the Tx power by about
4dB in DSO.

We did ? it might have been +4dB, but how is that figure of 4dB
arrived at ?


Well its actually as a result of the nature of the signal and how
it is measured.

In analogue the Tx output power was only ever achieved on peak
black,


Peak Sync, not 'peak black'.

Negative modulation was used, so the sync bottoms related
to maximum output. However (ignoring the equalising pulses
during the VBI) that was only sustained for 4.7us in every 64us

but as it was AM the average level was way below peak,
typically 10dB or more. The received signal strength would thus
tend to be read from this average level.


APL ? Average Picture Level. Well that depends on picture content
at any given moment, but let's say that most of the time, it's
probably about 0.2 volts above black (where sync bottom is 0 volts,
black is 0.3volts above sync bottom, peak white 1.0 volts.)
So that's 0.5 volts above sync bottom, or when turned upside down (or not !)
50% of peak output. To be 10dB down on peak sync, would imply that the APL
is 0.9 volts above sync bottom routinely, which unless the picture is
predominately peak white, it can't be ?


Thus Emley for instance
whilst a rated erp or 870KW per channel the average level that
gave the receiver signal strength was more like 87KW or less.


You mean Tx ERP ? My estimation is it would have averaged out
at 40 to 50% of max ERP, so for Emley that's 350 - 430 ish kW ?

With a COFDM the amplitude registers as essentially constant at
or near peak level, so the signal strength reads that level. The
rated erp of Emley on DTTV is 174KW, which is as far as the tuner
is concerned at least 3dB higher than it was using before.


Except it's not PSB mux ERPs are predominately a fifth or -7dB
of their analogue predecessors. I fail to see how that figure was
arrived at using either your, or my calculations, there must be
another set of assumptions and parameters that have been used
to arrive at -7dB providing equivalent coverage to analogue ?
And I still don't see where your original figure of 4dB comes from ?

--
Mark
Please replace invalid and invalid with gmx and net to reply.
  #8  
Old November 20th 13, 09:57 PM posted to uk.tech.digital-tv
PeterC
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Posts: 790
Default Just a reminder about the sensitivity of modern DTT boxes

On Wed, 20 Nov 2013 20:25:40 +0000, Bill Wright wrote:

I've been setting up some Humax boxes in the workshop. I wanted to use
the raw Emley Moor aerial feed because there's a fault on the RF system
that I haven't got round to fixing. The raw feed has no filters.
Bilsdale comes in with very uneven muxes at around -6dBmV. I attenuated
the feed by 30dB and the boxes were still tuning Bilsdale, and reception
was perfect. That's ridiculous.

Bill


And modern tuners in TVs. For DTTV I use Oxford; I'm on the fringe at
about43km N, but Sandy is t'other side of a railway embankment. On the 3-yo
Sammy there's no trace of Hannington (about twice as far as Oxford) but
there is Sandy to some extent.
Bought a Panny a few weeks ago and it offered me Oxford, Sandy and
Hannington. I don't know if there was a lift, but 90-odd km on an old
contract aerial...?
--
Peter.
The gods will stay away
whilst religions hold sway
  #9  
Old November 20th 13, 10:42 PM posted to uk.tech.digital-tv
Ian Jackson[_2_]
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Posts: 2,968
Default Just a reminder about the sensitivity of modern DTT boxes

In message , Woody
writes
"Mark Carver" wrote in message
...
Woody wrote:
that when we have effectively increased the Tx power by about
4dB in DSO.


We did ? it might have been +4dB, but how is that figure of 4dB
arrived at ?



Well its actually as a result of the nature of the signal and how
it is measured.

In analogue the Tx output power was only ever achieved on peak
black, but as it was AM the average level was way below peak,
typically 10dB or more. The received signal strength would thus
tend to be read from this average level. Thus Emley for instance
whilst a rated erp or 870KW per channel the average level that
gave the receiver signal strength was more like 87KW or less.

The maximum analogue TX power is during sync tips - and of course,
regardless of picture content, these occur every 64us (and then there
are also the broader field syncs). A decent signal level or power meter
should be sensibly peak detecting, and not change its reading (much)
with picture content. IIRC, in Pal-I, black is 2.4dB less than sync tip,
and white is 14dB less (20% residual RF envelope). Fully saturated
yellow 'peaks' even deeper, leaving around 7.5% residual RF envelope.

With a COFDM the amplitude registers as essentially constant at
or near peak level, so the signal strength reads that level. The
rated erp of Emley on DTTV is 174KW, which is as far as the tuner
is concerned at least 3dB higher than it was using before.

At least that is as I understand it but of course I may be well
wrong and am prepared to be corrected. :-))

I'm no expert, but information from the UK SCTE says that the
differences between the average noise-like power (what most measuring
equipment responds to) and the typical peak power is as follows:
16QAM +2.55dB
64QAM +3.68dB
256QAM +4.23dB
However, there may be occasional transient spikes up to 10dB higher the
average power.
--
Ian
  #10  
Old November 21st 13, 04:40 AM posted to uk.tech.digital-tv
Bill Wright[_2_]
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Posts: 9,381
Default Just a reminder about the sensitivity of modern DTT boxes

Woody wrote:


Well its actually as a result of the nature of the signal and how
it is measured.


The point I was making was this we have traditionally thought of -20dBmV
(40dBuV) as being the absolute minimum for DTT (lets forget the QAMs and
what-have-you just for a moment; I'm only talking ballpark). Even at
that sort of level reception would not be expected to be reliable.
This afternoon I had a signal that was already distorted (due to being
received on an aerial pointing the wrong way) giving solid reception, at
a level of about -36dBmV. OK, what was helping? I suppose the s/n ratio
would be pretty good. The aerial, which is is the garden, is screened
from all directions except the arc NW to NNE. The signal was weak not
because the field strength was poor but because it had been attenuated
30dB. Even so I find it surprising that the box was able to work on such
a signal.
After my original post I had another box on the bench. Well, I had a
fair few, but there was this particular one. It took ages to autotune
and the programme guide wouldn't populate. The intention was to use the
box for radio but on test the audio did the stop/start thing. It turned
out that with a signal input of less than 0dBmV the box reported 'no
signal'! So that one is going back to meet its maker.

Bill
 




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