Alternative Sources of 405 line and 240 lines
A number of
people have developed the use of PC graphics cards for
presenting 405 line and 240 line signals to old
televisions. Below is a description of the method I have
I would like to
acknowledge and thank some of those whose work has made
this possible: Jeroni Paul for his PC application for
graphics card control and for his very helpful assistance
with the specifics, Kat Manton for conceiving the method,
Jim Beacon for demonstrating the feasibility and Graham
Rabstaff for demonstrating it from a PC.
The scheme I have
been using is simply a development of that devised by
Graham Rabstaff. My methods differ regarding the sync
pulse generation and also in the application of sources.
The present scheme works well but doesn't generate the
correct sync pulse stream in accordance with either the
EMI 405 line standard nor the 240 line standard of the
Baird system. The Baird
system did not specify any line sync pulses during
the frame pulse and receivers needed a method of
maintaining correct line frequency in the absence of sync
pulses. In the case of the EMI sets some additional
circuitry and a valve (V8) were required to be
switched in for 240 line operation. The graphics card
source does continue to generate line sync pulses during
the frame sync pulse so there is no need for the
additional V8 circuitry.
The graphics card
syncs do differ in two respects from the EMI
405 line standard and this can cause rather critical
line frequency adjustment or hooking at the top of the
picture in older sets that don't have flywheel sync.
Sorting this is work in progress. The existing composite
sync is simply generated by exclusive OR of the line and
frame pulses but the resulting inverted line pulses
within the frame sync are slightly delayed relative to
their correct positions. There is also no half line pulse
stream. The latter is not essential and a simple
monostable circuit can correct the pulse positions. See Here.
The use of monostables is not very exacting but the odd
and even frame syncs shown below are what I am using and
are a fair representation of the spec.
To use this
circuit the Modelines need to be changed such that the
Composite Sync box should be unticked and the +hsync
references should become -hsync.
A graphics card
with two VGA outputs.
In practice this
generally involves a card with a DMS-59 connector and
splitter cable giving the DMS to 2 x VGA connectors.
One VGA output
will support the PC whilst the other will connect to your
television via a small VGA to composite video adaptor and
a modulator to present the signal at the aerial socket.
The adaptor can be as simple as the circuit shown in
Jeroni Paul's notes:
circuits are available I am using the design from Darius'
The sound signal
is simply taken directly from the PC speaker output to
the input of the sound modulator.
A PC with a PCIe
x16 motherboard slot.
Jeroni Paul's Modeline
VLC Player for Windows
the Modeline Editor you will need to sign in to your PC
as administrator and have ability to edit the registry.
Having loaded the Modelines
using Jeroni Paul's editor as in the first picture below,
you don't need to use the Modeline Editor from now on.
If you want to switch line rates
then you use your Windows Control Panel, Display, Change
Display Settings then select your Number 2 display and
select the resolution you require.
want to display video material the freeware application
VLC Player works very well. You do need to make a little
change to the pixel aspect ratio and so I have two
shortcut icons on my Windows Desktop both pointing to VLC
loaded the Target in the shortcut might read:
For my 405 line
shortcut to VLC Player I changed the Target to read:
And for my 240
line VLC Player the Target is:
Now if I want to
display a DVD on a 405 line receiver I connect my Number
2 VGA port on the PC to my Channel 1 modulator and click
on my 405 VLC Player. I select Media, Open Disc.. then in
the new window "Play" and the DVD menu comes up in the
window. I then drag that window over to the right and off
my PC screen and into my TV screen. Then hit f to get
full screen then position the cursor over the video to
play and hit return.
controls can be conveniently detached from the full
screen image such that they remain in the PC desktop
screen by unticking "Embedded video" in the Video
Static images can
also be displayed and for that I use Irfanview.
An aspect ratio
change is also required here and the resulting images can
be seen display on the 1937 HMV901 below:
405 line Test Card C:
240 line Test Card C:
This gives better
resolution of the finest bars because the video bandwidth is less
demanding but the fewer lines and lack of interlaced scanning is
Looking at a photograph of a 240 line Baird image it appears to
compare quite favourably with the 405 line EMI images but what
you are not seeing in the still photograph is the very noticeable
flicker from the Baird 25 Hz frame rate. See how engineers at
Philips perceived flicker in November 1936
Unfortunately flicker was not the only defect to affect Baird's
transmissions. The image below may be flattering the Baird
standard. A. C. Cossor Ltd were particularly concerned with
departures from the published waveform and complained of the
sync pulses with exponential falling edges that initially rose to
black level followed by another rise and fall resulting in a
black strip down the left hand side of the picture.
holes in the scanning discs were not spaced equally resulting in
ratio of picture to sync pulse should have been 60:40 but in
practice ranged from 50:50 to 40:60 resulting in insufficient
frame pulses lasted for 23 lines instead of the 12 specified and
there was no black barrier following the frame pulses.
I don't have any means of capturing the flicker of the
240 line display but the video below shows part of the
1946 BBC demo film converted to 405 lines in a PC
graphics card and modulator to channel 1 and presented on
a 1937 HMV901 television. Unfortunately the video
capturing masks the good resolution that the graphics
card is actually displaying on the television screen as
can be seen in the images above.