UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post - Bandwidth of Standards Converted Pictures (625-405)

flyback · 23rd Feb 2005, 1:51 pm

Just to clarify what you will see at the output of a DOMINO Standards Converter…

625-Line Test Card C via Domino. (Viewed after conversion, on 405-line TV set or monitor)

My Test Card C generator has the definition gratings at (MHz) 4.5, 3.75, 3.0, 2.25, 1.5
The original BBC 625-Line Test Card C had definition gratings at (MHz) 5.25, 4.5, 3.75, 2.5, 1.5

When viewing 625-line Test Card C (from my generator) the 4.5MHz gratings will not be visible, after conversion to 405-line they will be at 2.9MHz, the filter in the input stages of Domino has to remove the chrominance sub carrier at 4.433619 MHz (this has to be done to avoid pattering on the converted picture) so the 4.5MHz grating gets filtered out as well.

So the top grating will be grey, due to deficiencies in this filter there is attenuation at 3.75MHz as well, this leads to a slight loss of contrast of those gratings, this effect is hardly noticeable on real pictures.

The converted signal from Domino is better that most sets from the 1950’s could display, the TV22 can just manage to display the 2.5 MHz bars from a real 405-Line Test Card. (maybe a little more if tweaked!)

The BBC standards converters had very good filters in them, Domino’s filter circuits are a compromise as is the sampling frequency used (10MHz) but this does not detract from the end result.

For a fuller version of this information, other examples of the bandwidth limitations and a note from a BBC RD report can be seen on my web page here.. http://www.domino405.co.uk/bandwidth.html

And now a mini-whinge…

Before the days of DVD players which can supply full PAL bandwidth the common source of material was the VHS machine, that could only manage a video bandwidth of 3.0MHz if you were lucky and that was downhill with a tail wind!.
Have a look at a VHS tape of a natural scene such as trees, they have green leaves you know, but VHS does a fabulous job of turning them into green blobs!, and for a real giggle look at a recording of Test Card F, remember this is 625-Line.
If using VHS tape as a source for a standards converter you won’t see much above 2.0MHz on the 405-Line set, but then there are the tape crinkles and dropouts to watch as well !.

Maybe I should have used Gold Plated BNC connectors and Oxygen Free wire in Domino, maybe used a higher sampling frequency, or…well you get my point.

The end result was a 625 to 405-Line standards converter that has got at least 60 peoples old tellies going, turned old bits of junk into something that brought back memories of hiding behind the settee from DR WHO and the Ice Warriors and gave some interesting challenges to a lot of people.

The old TV sets themselves have lots of built in faults, quirks, mean level AGC to “help” with the black level and some of them even have flyback blanking!..

Malcolm
(Sad git now 44 and getting greyer my the second!)

23rd Feb 2005, 1:51 pm	#1
flyback Retired Dormant Member Join Date: Apr 2003 Location: Swindon Wiltshire Posts: 26	Bandwidth of Standards Converted Pictures (625-405) Just to clarify what you will see at the output of a DOMINO Standards Converter… 625-Line Test Card C via Domino. (Viewed after conversion, on 405-line TV set or monitor) My Test Card C generator has the definition gratings at (MHz) 4.5, 3.75, 3.0, 2.25, 1.5 The original BBC 625-Line Test Card C had definition gratings at (MHz) 5.25, 4.5, 3.75, 2.5, 1.5 When viewing 625-line Test Card C (from my generator) the 4.5MHz gratings will not be visible, after conversion to 405-line they will be at 2.9MHz, the filter in the input stages of Domino has to remove the chrominance sub carrier at 4.433619 MHz (this has to be done to avoid pattering on the converted picture) so the 4.5MHz grating gets filtered out as well. So the top grating will be grey, due to deficiencies in this filter there is attenuation at 3.75MHz as well, this leads to a slight loss of contrast of those gratings, this effect is hardly noticeable on real pictures. The converted signal from Domino is better that most sets from the 1950’s could display, the TV22 can just manage to display the 2.5 MHz bars from a real 405-Line Test Card. (maybe a little more if tweaked!) The BBC standards converters had very good filters in them, Domino’s filter circuits are a compromise as is the sampling frequency used (10MHz) but this does not detract from the end result. For a fuller version of this information, other examples of the bandwidth limitations and a note from a BBC RD report can be seen on my web page here.. http://www.domino405.co.uk/bandwidth.html And now a mini-whinge… Before the days of DVD players which can supply full PAL bandwidth the common source of material was the VHS machine, that could only manage a video bandwidth of 3.0MHz if you were lucky and that was downhill with a tail wind!. Have a look at a VHS tape of a natural scene such as trees, they have green leaves you know, but VHS does a fabulous job of turning them into green blobs!, and for a real giggle look at a recording of Test Card F, remember this is 625-Line. If using VHS tape as a source for a standards converter you won’t see much above 2.0MHz on the 405-Line set, but then there are the tape crinkles and dropouts to watch as well !. Maybe I should have used Gold Plated BNC connectors and Oxygen Free wire in Domino, maybe used a higher sampling frequency, or…well you get my point. The end result was a 625 to 405-Line standards converter that has got at least 60 peoples old tellies going, turned old bits of junk into something that brought back memories of hiding behind the settee from DR WHO and the Ice Warriors and gave some interesting challenges to a lot of people. The old TV sets themselves have lots of built in faults, quirks, mean level AGC to “help” with the black level and some of them even have flyback blanking!.. Malcolm (Sad git now 44 and getting greyer my the second!)