Re: The Man From Auntie He Say: Making the switch to digital TV
- From: "Max Demian" <max_demian@xxxxxxxxxxx>
- Date: Mon, 29 Oct 2007 18:11:56 -0000
"Java Jive" <java@xxxxxxxx> wrote in message
news:470qh3d3vdpta9tn02hl0m5tbcib56ga9o@xxxxxxxxxx
I'm minded to reply to their article in the following vein ... Any
comments before I let fly?
Making the switch to digital TV By Spencer Kelly, Click presenter
http://news.bbc.co.uk/1/hi/programmes/click_online/7052229.stm
You are flagged on this very page as "The BBC's flagship technology
programme". If this is indeed so, then you have a duty to get things
right. I am by no means an expert in this field, yet even apparent to
me there are a number of points in this report that are at best
misleadingly over-simplified, at worst wrong. In all, it shows a lack
of analytical thinking unworthy of a "flagship technology programme".
TV pictures have been broadcast as analogue radio waves for decades,
and they take up a lot of room in the radio spectrum.
*Radio* is broadcast as radio waves, which lie in a different range of
the *electro-magnetic* spectrum. Analogue and Digital Terrestrial TV
are broadcast in the Ultra-High Frequency (UHF) band of the
electro-magnetic spectrum.
I suppose TV is broadcast as television waves.
It's quite correct to say that televison is broadcast using radio waves.
Even satellite. The radio waves part of the EM spectrum is everything below
infrared, including microwaves.
Newer digital television is a much better broadcast system
It is misleading to say that digital TV is better than analogue
without stating your criteria, and is likely to lead to loss of
confidence in the veracity of your report, because there are many
aspects of digital TV that are clearly *worse* than analogue TV, and
some of these will be painfully obvious to viewers. For example:
* There have been times when over-compression artifacts on digital
TV have been so bad that I've reverted temporarily to analogue. See
the discussion on compression below.
He did mention the trade-off between compression and speed/detail of the
picture (as you reproduced below).
programme listings
These were available on analogue, at least by teletext, but I also
seem to recall custom mechanisms from satellite and cable providers.
You generally can't set the recording from teletext (though you could on my
old Philips VR703 VCR).
and many
more channels efficiently packed into the space occupied by just a few
analogue ones.
This is due to COMPRESSION rather than DIGITISATION. See the
discussion on compression below.
Well you can't use digital compression without digitising it first.
Now there is a good chance that you will already be able to receive
the replacement digital terrestrial signal through your existing
aerial.
Well, I don't have figures to support this, but local experience
suggests that there is a chance, but not really a good chance.
There is supposed to be 74% coverage, so that a probablility of 0.74, which
is a lot better than an even chance.
Analogue transmissions from a given transmitter were grouped into
frequency bands so that local aerials could be optimised to receive
them, and so that neighbouring transmitters could use different groups
to avoid their signals interfering with each other in areas of
overlap. However, the DTT transmissions added later deliberately used
frequencies outside the local groups to avoid interfering with the
existing local analogue transmissions, and their power was
deliberately crippled to avoid interfering with existing analogue
transmissions from neighbouring transmitters.
If you add that to what I pointed out about digital receivers
requiring the signal to be at a threshold level, and to the fact that
the coverage and take-up figures so far suggest that most of those who
can have already made the switch, then a potential problem seems
likely for those who remain on analogue, at least until analogue
switch-off in their area allows an increase in digital transmitter
power.
Which is more or less what chummy said - most people can get it now, and
nearly everyone after DSO.
When I first moved in here, even analogue TV was pretty dire. I now
get DTT because, although it's not really my field, I took an interest
in the technology and made a determined attempt to understand it, as a
result of which I bought a wideband (that is, non-grouped) aerial, a
masthead amp, a UHF-band filter to cut out as much interference as
possible, and shinned up a ladder. My pensioner neighbour would
probably have to pay an aerial installer about £3-400 to get the same
result.
You're unlucky. In the London area Crystal Palace uses the same part of the
band for analogue and digital, so a standard group A aerial should be fine,
and the digital transmitters will increase their power 10x after DSO.
Information Theory states that it will take a certain minimum
bandwidth to transmit a given amount of information by whatever means,
analogue or digital. If the bandwidth falls below this level,
information is lost.
The video compression techniques used in digital TV broadcasting are
also lossy, information lost during compression can never be restored
on decompression. There are lossless compression techniques available
in other digital fields, where the information compressed can be
completely restored on decompression - everyday examples are zip and
rar files on your PC, and the FLAC and APE lossless audio codecs.
Presumably such techniques could be applied to video data as with any
other type of digital data, but, to the dire detriment of broadcast
quality, lossy compression was chosen instead.
If they used lossless compression we would get hardly any channels per
multiplex. There's nothing the matter with lossy compression for video (or
photographs for that matter) provide a good method is used and not too much
compression.
When I digitise analogue TV into my PC with a capture card, a process
which uses no compression, the file size is around 14.85MB/sec,
whereas a channel broadcast Free-To-Air from 28.2E and recorded
without alteration by my satellite receiver creates a file size of
around 0.45MB/sec (both figures megabytes per recorded second). Thus,
we have a guesstimate that the compression techniques used in digital
TV broadcasting are reducing the information conveyed to around 3% of
the analogue equivalent.
It is difficult to see how 3% of anything can ever be an acceptable
substitute for the original 100%!
It's OK if the information lost is not used by the human senses - you might
as well complain that video systems only reproduce EM radiation within the
visible range - and not 1,000,000 frames per second.
--
Max Demian
.
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