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G8MNY  > DIGTV    13.06.26 11:02l 108 Lines 5013 Bytes #58 (0) @ EU
BID : 62302_GB7CIP
Read: OE5RCO
Subj: Digital TV so far
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Sent: 260613/0843Z @:GB7CIP.#32.GBR.EURO #:62302 [Caterham Surrey GBR]
From: G8MNY@GB7CIP.#32.GBR.EURO
To  : DIGTV@EU

By G8MNY                                      (Updated Jan 08)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)

In Croydon, South London, I am line of sight to main Tx for London, but DVB-T
(Freeview) multiplex channels are still about 25dB weaker here than the 1MW ERP
analogue signals.

       Analogue     DIGITAL MUX
   <-----8MHz-----><----8MHz----->
    Carrier                         v
    ³         Sound
    ³          ³                   25dB
    ³   Colour ³Nicam
    ³       ³  ³ Û ÛÛÛÛÛÛÛÛÛÛÛÛÛÛ   ^
  ÄÄÁÄÄÄÄÄÄÄÁÄÄÁÄßÄßßßßßßßßßßßßßßÄÄ

To cut down on the number of boxes connected to the TV, I use a Freeview VCR
(now obsolite as the freeview Data format has changed) in front of the main
16:9 Plasma 42" TV, fed from a large 25 element loft Group A aerial which I
have broadbanded a bit by trimming elements for a more even signals display on
the STB/VCR, giving 25-27dB S/N on all muxes. Note that 17dB S/N is the no go
error threshold.

  Mux                        Mux
 Signal   Original          Signal   Modified Aerial
  30dB´                       30dB´
  25dB´ Û Û Ü Ü               25dB´ Ü Û Û Û Û Ü
  20dB´ Û Û Û Û Ü             20dB´ Û Û Û Û Û Û
  15dB´_Û_Û_Û_Û_Û_Ü_\Freq     15dB´_Û_Û_Û_Û_Û_Û_\Freq
       Ã GROUP A ´  /              Ã GROUP A ´  /

I used another identical STB/VCR a 1st floor bedroom from a small 4 element set
top aerial into an standard 4:3. This gives similar good signal results.

PICTURE QUALITY
On the big TV, picture definition is generally higher than I expected, with the
16:9 format apparently giving about 7MHz (720 pixels/line) resolution over the
RGB SCART feed.

Of course there is no visible noise (snow) with a digital picture, just other
artifacts! In fact real falling snow can't be encoded in Mpeg as there are no
similar frames to allow for compression!

However the pictures are not @ the 50Hz new frame rate of true interlaced
analogue. e.g. the net frame rate is often very low! So parts of the most
visible picture are updated 1st then a few frames later the darker bits!
A actor's face half in the dark turning, will appear s t r e t c h e d   o u t
until the darker bits get updated. A very strange effect when it happens &
sometimes repeatedly!

     Û²±° ³         Û²±°   ³         Û²±° ³
     Û0 0 ³         Û0 0   ³         Û0 0 ³
     Þ U ³          Þ U   ³          Þ U ³
      Ý= ³           Ý=   ³           Ý= ³
      ßß             ßß                ßß
      HEAD       HEAD < MOVED     1/2 Sec later

But this is a rare affect, the more usual effect will be highlighted forehead
detail not following the head movement at all well, or a shakey camera shot
with parts of the picture cut out and moving independantly.

Also there is what seems to be data bandwidth competition (bidding) on any of
the broadcaster MUXs, so that occasionally a NEEDED new "I" (full) frame for a
shot change has no bandwidth available for say 1 second, so the picture just
hangs for that time with no errors or sound faults.

As far as I know these 2 affects are all the result of the EXTREME MPEG OVER
COMPRESSION that the broadcaster has chosen to use for that programme, and are
nothing to do with weak signals and error rate that cause the [] blocks to
appear and "glitch" noises in the sound. I no longer get those after a new coax
feed to the loft aerial.

DATA RATES
From a multi Mux Tx site stats printout I saw (published on the Internet) the
typical data rates per channel vary from an extremely low 50kB/S to a peak of
6MB/S in any 5 mins giving means of 1-2MB/ch. This is still extremely low
compared to the uncompressed >200MB/S from a studio camera source. (a HDTV
source is 1200MB/S)

BBC LIPSYNC
Since using the larger TV, poor lipsync problems are far more noticeable. Other
programme makers don't seem to suffer from this, it definitely is a BBC thing.
I know the plasma TV might add a 1/50 or 1/25S picture delay in its picture re-
formatting and hence add to the problem, as might the larger screen make the
lip movements more apparent, but why should this be just a BBC thing? I have
even seen reasonable lip sync degrade during a BBC interview, why? I have even
seen the same problem with digital cable system on BBC channels! Are the other
broadcasters' running 1/25S delayed sound to compensate for large screen TVs,
or is the BBC output just inconsistent? I noticed they have improved their act
recently.

DVB-T TV channels.
Since the addition of loads more junk TV channels the STB's programme guide is
corrupted once/twice a day if it is put into standby. This seems to be centred
on some of the channels that keep swapping their allocations (on & off service
in the menu guide). I assume they actually go missing from the Tx guide menu
for that mux I am using for a few seconds and that my STB instantly sees the
change and flags up it need for a rescanned (early STB software fault?) each
time.


Y don't U send an interesting bul?

73 de John G8MNY @ GB7CIP


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