DOMINO 625 to 405 Line Standards Converter

It depends which test card source you use,
My Test Card C generator (note 1) 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
These cover a similar range as the later Test Card F,
Test Card F has six gratings at (MHz) 5.25, 4.5, 4.0, 3.5, 2.5, 1.5

When viewing 625-line Test Card C (from my generator) the 4.5MHz gratings will not be visible,
there is a filter in the input stages of Domino 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.

Due to slight 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 following note taken from BBC Engineering Sheet, 4306(5) July 1974. Television Test Card F.

At the right of the centre picture there are six gratings each consisting of vertical stripes
corresponding to the following fundamental frequencies in MHz.

At 625-Line 1.5 2.5 3.5 4.0 4.5 5.25

At 405-Line 1.0 1.6 2.25 2.6 2.9* 3.4* (approx)

*These bars are likely to be indistinct due to the removal from the transmitted signal of colour information to avoid the degradation of the converted 405-Line picture.

So even the BBC could not convert the full bandwidth 625-Line signal to 405-Line standard,
you can’t do that and remain within the 405-line RF bandwidth, the sound carrier is 3.5MHz
below the vision carrier, so you don’t want any vision frequency components creeping down
there, that’s why 3MHz is the upper vision frequency limit.
It is important to keep the converted video signal bandwidth within the 405-Line specifications
to avoid problems with sound interference (buzzing) on the receiver.

The converted signal from Domino is better that most sets from the 1950’s could display,
the BUSH TV22 can just manage to display the 2.5 MHz gratings 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
converted end result.
On a side by side (My Test Card C as source) comparison with a Dinosaur Standards converter
(note 2) only on careful inspection can the differences be seen and on real pictures
it is hard to see any difference.

Illustration of the effects of filtering 625-Line signals.
Click on the pictures for larger versions.
Captures made on a PC using a Hauppage WinTV card.

625-LineTCC
Note the lack of definition in the top grating,
due to a notch in the capture cards Decoder.

The same effect can be seen on TCF, show in BW here.

These captures are shown just as they were taken, no tweaks or adjustments have been made.
This shows how the processing performed on the 625-Line video signal effects the resolution.
The grabs are 768x576 Square Pixel, The capture card uses a CONEXANT Fusion 878A Device.

With the DOMINO Standards Converter, pictures converted to 405-Line will have been
filtered to remove frequencies that are above 4MHz, the roll-off starts at about 3.5MHZ
The resulant 405-Line picture will not be to dissimilar to the picture of 625-Line TCC above.

I will add some 405-Line pictures taken from the screen of a good quality monitor soon.

A graphic example taken from BBC RD Report
"Digital line-store standards conversion: a feasibility study: No. 1971/44" (Hope they dont mind!)

BBC RD Report from which this illustration was taken BBC RD 1971/44
List of BBC RD Reports, some on Standards Conversion BBC RD Reports

(note 1)
This version of 625-Line Test Card C has been made so that when converted to 405-Line,
the definition gratings frequencies will be close to the original 405-Line version,
which were (MHz) 3.0, 2.5, 2.0, 1.5, 1.0
The frequencies after conversion are (MHz) 2.91, 2.43, 1.94, 1.45, 0.97, close enough!
(note 2)
Dinosaur Standards Converter, made by Dave Grant, works along the same lines as the
BBC digital converters did, with 12.65MHz sampling.