All posts by chrisw
Below are all of chrisw's postings, with the most recent are at the bottom of the page.C
More Freeview capacity - COM7, COM8 and COM9 - in the 600MHz baSaturday 24 September 2011 12:34PM
Otley
A good quality group B aerial should still work fairly well at these frequencies. It will be dropping off a little, but in a strong to medium signal area I would not expect any problems.
Chris
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Briantst: As you say:
"Except that these signals are already down perhaps as much as -12dB as it is..."
I don't argue with that, but according to the graphs here: ATV Stock Aerial Tests (Frequently referenced!) an XB10B will have as much or more gain at channel 30 than any wideband aerial.
This is the solution I am looking at to receive Emley Moor where I live, as it has peak gain around channels 48-52 which is where the COM multiplexes are. A wideband would give me no better chance of receiving the new multiplexes when they appear.
It is of course quite likely that I won't get COM7, 8 & 9 whatever I do, due to the likely reduced power.
Chris (LS213PN)
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C
More Freeview capacity - COM7, COM8 and COM9 - in the 600MHz baSaturday 24 September 2011 6:12PM
Otley
Dave Lindsay: Grouped aerials do exist for an engineering reason.
For any given aerial type, there is a tradeoff between gain and bandwidth. This is particularly so with the conventional Yagi. It is possible to widen the bandwidth by adding extra elements - this is what the X beam does to some extent. Copare the Yagi 18 XB10 and XB16 at the ATV Aerial Tests, All have similar peak gain, but the two XBs are wider bandwidth.
The only "True" wideband TV antenna is the log periodic, but this sacrifices gain for bandwidth, only a few elements of a log periodic actively contribute to reception at a particular frequency. All other wideband TV aerials are a compromise, generally with peak gain at about Ch60 and gradually falling from there down.
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C
More Freeview capacity - COM7, COM8 and COM9 - in the 600MHz baSunday 25 September 2011 12:21PM
Otley
Briantist: I don't understand how the frequency of TV signals can be changed siginficantly in frequency by reflection or refraction. The only frequency shift I am aware of is the few kHz doppler shift that can occur if the zone of reflection is moving rapidly. At UHF this is only likely to be seen with Sporadic E propogation.
The reason for having a two channel separation on analogue transmitters would be what you refer to as "the sideband-effect", i.e. adjacent channel interference due to the imperfections of the transmitters and receivers, which I expect would cause patterning on an analogue picture.
While digitalTV is less prone to cochannel interference, I expect we will still experience the occasional problem when the interferring signal becomes larger than the wanted signal. This certainly used to happen when I lived in Essex; sometimes a French channel would comletely wipeout the local signal for several minutes.
Chris
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ChrisW: I am sorry Briantist, I must disagree, co-channel interference does affect DTV. Interference degrades the signal to noise ratio until at some point the error correction cannot cope, and the picture will begin to degrade.
From Ofcom Guidelines for Improving Digital Television and Radio Reception:
"Co-channel interference occurs when signals are received not only from the wanted transmitter but also from one or more distant transmitters operating on the same channel. It can degrade the bit error ratio of a DTT multiplex until the failure threshold is exceeded, resulting in prolonged loss of picture."
Guidelines for Improving Digital Television and Radio Reception
(Sorry, I don't know how to insert links properly on this board)
Chris (LS213PN)
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Briantist: That document was one I found in a hurry. I have done some more research and found some more definitive documents:
Ofcom: The feasibility of DVB-T on-channel repeaters for coverage repair on Channel 60; Section 2.1; Page 2;
http://stakeholders.ofcom….pdf
"In practice, most of the DTT network will be interference-limited, rather than noise-limited. Co-channel interference from DVB-T transmissions must be 19.8 dB below the wanted signal, expressed as a 'protection ratio' of +19.8dB."
Aegis for Ofcom: Preparatory study for UHF spectrum award, Annex A: UHF Technical Compatibility Issues
Sections 1.2.1, 2.2.1, 2.2.6
http://stakeholders.ofcom….pdf
"For co-channel interference between DVB-T services, Reference [D-Book] gives
C/I values of 17dB for 64-QAM (2/3 code rate) and 13dB for 16-QAN(3/4 code
rate), for a Ricean channel."
http://stakeholders.ofcom….pdf
ERA Technology for Ofcom; Conducted Measurements to Quantify Different Types of interference in the DDR Frequency Spectrum; Summary; Page 4; Table 1
Shows measured results for adjacent and co-channel interference. Co-channel protection ratio 15dB
http://stakeholders.ofcom….pdf
Digital Terrestrial Television Steering Committee: REPORT ON THE TECHNICAL TRIAL OF DIGITAL TERRESTRIALTELEVISION (DTT); Pages 26 & 27;Co-channel Interference (CCI) for DVB-T;
http://www.ofta.gov.hk/en….pdf
Laboratory measured results, DVB-T to DVB-T protection ratio 19-21dB. Interestingly this table shows that DVB-T is more susceptible to to DVB-T cinterference than PAL interference, by about 10dB.
Also interesting is this document which does not give figures for co-channel interference but tests the performance of 15 DVB-T receivers for adjacent channel interference.
ERA Technology for Ofcom: Conducted Measurements to Quantify DVB-T Interference into DTT Receivers.
http://stakeholders.ofcom….pdf
As an aside, while I do not have any specialist knowledge of DTT, my "expectations" were based on nearly 30 years as a practicing RF Design Engineer, the last 15 of which have been spent designing (very specialist) RF data links.
While the characteritics of DTT are very different from my professional work, it is essentially just a digital data link.
Chris. (LS213PN)
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Parick: Depending on exactly where you are it MIGHT be possible for you to receive more channels from a different transmitter. I have done some experiments at my house and I can receive Emley Moor, despite being down by the river at the bottom of The Chevin. I will need a high gain Group B outside aerial to achieve this with a reasonable margin. It is likely that reception will be variable.
The only way to be sure is to try at your location.
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Mark, I agree: I have spent quite a while reading ATV Aerials website-very informative and a good read. I have come to the same conclusion, probably the Yagi 18B due to it's smaller size and much lower wind loading.
All I have to do now is find someone to put it up, because I'm certainly not going to do it myself. (my experiments were in the loft).
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C
Channel 5 HD on Freeview cancelled ... again | High definitionFriday 16 December 2011 10:56PM
Otley
I hope the BBC keep hold of the channel, at least until after the Olympics. Other than that possibility, I expect it to go to a third party, the BBC could do with the income.
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Thursday 22 September 2011 11:16AM
Otley
Briantst: I know any licenced channel can operate, and Arqiva have to accept the highest bidder. I just despair that this sort of channel is so successful.
On the other hand I suppose there are only a limited number of entertainment/news/sport etc. channels that wish to and/or can be supported by audience demand.
Chris.