Hi all,
I got it working... somewhat! I hooked the Raspberry Pi's NTSC composite
up to the LM1881 sync separator, and after fiddling with the horizontal
hold it snapped into place. A few problems:
* As mentioned before, the screen coverage is a bit poor (lots of
space to the left and right).
* The brightness is rather low (what is full brightness at TTL?
Probably 5V? Composite maxes out way below that, so maybe I should
hook up an amplifier?)
So here is the schematic of a very similar module:
http://oz1db.dk/hp/omnivision_man_lp06xx_rev0_0611.pdf . Can't I just
change the timing capacitors for the horizontal and vertical sync? (I.e.
mess with R17 at U1 and R35 at U2)? Based on what Adrian wrote below it
seems it might be possible, within reason? Since it is already working
(kind of, limited screen coverage) I probably don't ahve to tweak much?
So if anybody can help on the brightness issue as well as on the
frequency modification I would be very grateful!
Best wishes
Jens
On 2021-04-11 10:41 p.m., Adrian Godwin wrote:
The line oscillator has to run close to the line frequency and is then
phase-locked to it by the horizontal hold mechanism. If the oscillator
is tuned for 19kHz it's a big stretch to get it to sync to a 15kHz
input signal.
Yes, you might be able to modify some components to slow it down, or
even just change the horizontal preset frequency to get it closer but
that line oscillator is the most complex thing in as monochrome
monitor. It generates a linear ramp for the horizontal coils and is
also wound up with line output transformer which generates the high
voltage for the tube. I guess this makes sense as during retrace the
line coils are getting a big pulse to shove the spot to the other side
of the screen. But drastically changing the line frequency is likely
to have an effect on the line linearity and the EHT voltage.
I'm not saying don't try it - it would be very educational, though
best if you can get hold of a schematic. But a lot easier to get hold
of an NTSC monitor, I think. The people who maintain arcade machines
have been scraping up all the large colour monitors recently but I
doubt they'd have much use for a small monochrome.
If the stored energy in the line coils can be dumped into the line
output transformer at the right phase of the cycle, it's saving a lot
of power that would otherwise require a separate circuit. In the old
days. the more use you could get out of any one vacuum tube the
better, and the style persisted until multisync monitors needed to be
more flexible (and even the simple earlier ones of those had more
complex LOPTs to deal with the variation).
On Mon, Apr 12, 2021 at 3:22 AM jb-electronics
<webmas...@jb-electronics.de <mailto:webmas...@jb-electronics.de>> wrote:
Still new to the whole composite video thing---could you clarify
what is the stretch? Do you think the driving electronics might
not work at the lower NTSC frequency? Or is there a mechanical
impediment, like the windings of the CRT's yoke? Thank you!
Jens
On 2021-04-11 7:19 p.m., Adrian Godwin wrote:
It sounds a bit of a stretch - 15kHz to 19kHz. You could try
running the monitor without an input signal and adjusting
horizontal frequency (if there is one) to see what range it can
manage.
Another possibility is to fix it in software by creating a custom
monitor signal. Some information is at
https://github.com/raspberrypi/documentation/blob/master/configuration/config-txt/video.md
<https://github.com/raspberrypi/documentation/blob/master/configuration/config-txt/video.md>
On Sun, Apr 11, 2021 at 10:51 PM jb-electronics
<webmas...@jb-electronics.de
<mailto:webmas...@jb-electronics.de>> wrote:
Thank you! I think it should be possible to modify the
horizontal and vertical deflection circuitry on the board to
make it a proper NTSC frequency, yes?
Jens
On 2021-04-10 10:52 p.m., 5-ht wrote:
Jens,
The horizontal sync requirement of this monitor (52 uS /
19.2Khz) is somewhat higher than a standard NTSC signal
which is about 63 uS / 15.7 Khz.
Mark
On Saturday, April 10, 2021 at 8:04:15 AM UTC-5 Jens Boos wrote:
Thank you! I did some more digging and found the service
manual for the display (Panasonic TR-60S1A, see here:
https://www.opweb.de/english/company/Panasonic/TR-60S1A
<https://www.opweb.de/english/company/Panasonic/TR-60S1A>)
There is is a timing chart (see below). It looks an
awful lot alike NTSC to me, can somebody confirm?
Best wishes
Jens
On 2021-04-10 12:28 a.m., Adrian Godwin wrote:
It's easy enough to extract the sync signals, as you've
seen. It may not be particularly difficult to modify
the TTL input to analog, or to create a TTL level
signal if you don't need a grey-scale. On many
general-purpose monitors like that they could often be
built for either standard - the microvitec Cub popular
with the BBC Microcomputer had that option.
But the critical thing is that it runs at the right
speed. CRT circuits are built around the line
oscillator which generates the horizontal scan AND the
EHT voltage. It was only when multisync monitors came
along that the optimisation was split to reduce the
dependency. And if the frame frequency is wrong, you
will often have problems getting a full frame scan when
you force it into sync.
So the first thing is that you need to make sure the
8920 monitor ran at either PAL or NTSC rates so you can
choose a raspberry pi format to match. There's a good
chance it's NTSC but later monitors like the ones in
the 54 series of digital scopes were more like the IBM
standards MDA and EGA (CGA was NTSC).
On Sat, Apr 10, 2021 at 4:28 AM jb-electronics
<webm...@jb-electronics.de> wrote:
Hi all,
This is a bit of an off-topic question, but I hope
there will be
somebody here that can help. I have bought a
new-old stock monochrome
CRT for a HP Agilent 8920A, basically this unit here:
https://www.ebay.com/itm/273930914548
<https://www.ebay.com/itm/273930914548> .
It looks to me that it is just a rebranded
OmniVision 6" display:
http://www.omnivisionusa.com/Industrial-LCD-CRT-Monitors/replacement-crt/crt-monitors/6-inch-kit-.html
<http://www.omnivisionusa.com/Industrial-LCD-CRT-Monitors/replacement-crt/crt-monitors/6-inch-kit-.html>
Now unfortunately it takes TTL video as input. But
I have a composite
source (a Raspberry Pi). How can I convert
composite into TTL? I
basically need to extract Hsync and Vsync and feed
it separately to the
unit, okay. There is an old circuit here that does
just that:
https://www.elektormagazine.com/magazine/elektor-198812/47485
<https://www.elektormagazine.com/magazine/elektor-198812/47485>
But I know there are also dedicated chips, like the
GS1881:
https://www.mouser.com/datasheet/2/761/GS1881_GS4881_GS4981_Datasheet-769183.pdf
<https://www.mouser.com/datasheet/2/761/GS1881_GS4881_GS4981_Datasheet-769183.pdf>
So basically here is my question: before I dive
into this any further,
is there any chance of success? What is the optimal
choice? Or is it
quite unlikely that I will be able to convert the
signals? I mean, even
if I manage to extract Hsync and Vsync, is it
likely to work?
I am sorry for the naive question, but I am no
expert on video signals,
and it would be nice if anybody more experienced
could chime in with a
few words of caution/experience.
Best wishes
Jens
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