Main > Monitor/Video Forum
HDMI to Old CRT
Zebidee:
--- Quote from: abstract3000 on October 29, 2023, 05:31:08 pm ---@Zebidee or @buttersoft
I find myself in between a rock and a hard place, as I sit as the middleman passing information between them with my knowledge painfully lacking. I have a friend who knows electronics and can do what's asked, but may not necessarily understand the context of what's being asked or what's trying to be achieved. Whereas here on the forums We have experts that know the context and what is trying to be achieved.
Today i got some disappointing news in which my friend believes is limitations of the software not outputting a Digital signal but rather analog signal. Here are the points he made:
* Software only appears to produce analog RGB output, Suspected that a Card capable of Digital RGB would make the CGA option show up and switch the signal from analog to digital.
* The Jungle IC Input appears to be locked at Digital (Has no way of knowing if it will take analog) Though if it did it would require a software change that is not in the service menu custom reprogramming of the EEPROM
* Circuit interface works fine with a true CGA (TTL) Signal, though does not believe analog signal was ever considered as there is no decoupling capacitor or shunt resistor in the RGB feed from the CPU to Jungle, Just a Single Voltage dropping Resistor inline.
* Believes the only way to do it would be to send a true CGA signal or interface directly to the neck board (Which he does not recommend)
I'm not sure in particular why he keeps bringing up CGA?
--- End quote ---
Your friend is confused. CGA is an old computer graphics/display output standard that allows 4-bit color (16 colours). For convenience, it is designed to be compatible with domestic CRT TVs @ 15.7khz. This is why old games/software have limited colours and chunky resolutions.
Some people think that CGA = 15.7khz display, but this is simply not true.
Colour CRT TVs themselves are analogue and not limited by CGA, and can display essentially infinite colours, within practical limitations. This is why we could enjoy movies on them, with all kinds of shades and colours, back in the day. We can display higher resolutions than CGA on a CRT TV. CGA defines what some (old) computers can output, not what the CRT TVs can do.
So, the whole CGA and "digital RGB" discussion misses the point and is just distracting. The CRT TV definitely 100% takes an analog RGB signal.
The computer's video card takes the digital video information it creates, and converts that into analogue space using a dedicated onboard DAC (Digital Analog Converter) chip. Computer have done this since the dinosaurs. It is only more modern video cards and GPUs that have dispensed with DACs and output digital-only formats (like HDMI). This is why, for CRT_emulator and similar software, we need to use older video cards with analogue outputs like VGA. That is the whole point.
The sync itself is a TTL-level logic signal, though it might be thought of as both digital and analogue. On an oscilloscope it appears a squared wave, essentially either on or off.
--- Quote --- I have photos of messing with the board and briefly getting the signal to show up so I know the Software does have the capability of moving the Image to the TV but his response to that was as Follows:
When i was touching the board I was passing the 5V (From turn on circuit) to the signal pins effectively pulling up the signal to close to the TTL Levels CGA used.
He stated he could add resistors to do that in the circuit but he sees 2 potential issues:
- I could potentially lose a lot of Contrast
- He also is worried about running injected voltage back through the video card may cause damage.
(He could decouple with capacitors, but stated that might but us back where we started)
--- End quote ---
Move the in series capacitors on the RGB signal inputs. They should be on the other side of those resistors, and as close to the IC as practically possible (distance matters).
The RGB signal inputs Need 75R termination (resistor to ground) for the RGB signals.
The sync input needs a resistor in series, as the video card output is too high (around 3.5-5.0vpp), and the TV industry standard expects 0.286vpp. If you assume VGA sync peaks at 5v (probably a bit lower), and there is 75R termination at the TV end, then a 1K resistor gives you exactly 0.349v (close enough).
Note that sync has *nothing* whatsoever to do with contrast or image brightness. It carries no picture information. It simply tells the TV when to draw new lines and when to start the next screen refresh.
I don't understand why he's worried about "running injected voltage back through the video card". There is no voltage "injected" to the card anywhere, even with the alternative sync circuits suggested. The TV and video card would only share a video ground connection, which is standard/normal.
You would probably want to de-couple the H&V sync inputs, to the custom composite sync circuit, with small ceramic capacitors that are not on the schematics I linked (I'd suggest 0.1uF/104 or 1uF/105 values). This de-coupling is more about making sure the composite sync circuit works properly than about protecting the video card from "injected voltage".
Try putting a pot on the 5v blanking signal so you can "tune it in", then replace the pot with a normal resistor. 5v may work for some TVs and not others. True blanking voltage required may be more like 2-3v, every model TV can be different. Schematic might help you to narrow it down.
--- Quote ---The last bit of advise was not to confuse the signal itself with the resolution. CGA is a TTL level signal meaning the 3 colors only have two modes (on and off) The CGA color pallet can only produce 16 colors which is fine for a TV's OSD.
--- End quote ---
This is irrelevant, The TV is not limited to CGA pallet.
--- Quote ---The interface injects the signal from the computer into the jungle at the same spot the CPU injects the OSD.[/i]
--- End quote ---
This is fundamentally correct. It is an OSD-injection RGB mod.
--- Quote ---So at this point I'm not sure what to do, would it be helpful to scan the TV's Schematics I have?
--- End quote ---
Probably not necessary, though I guess it won't hurt anything. Referring to the schematic could help us determine the correct values for in-series resistors on the RGB signal inputs and correct blanking voltage.
--- Quote ---Does CRT Emudriver have the ability to push out a digital Signal opposed to an Analog Signal?
--- End quote ---
This is irrelevant, forget about it.
--- Quote ---Is he right in his assumption that injecting the 5V would cause issues with the contrast and potentially ruin the Video Card?
--- End quote ---
I honesty do not fully understand what he wants to do here. There is no need to "pull up" the RGB or sync. Don't do it.
--- Quote ---I really appreciate any time or assistance :)
--- End quote ---
No worries.
I have more to say, but am going to finish this post as it is long enough already. Next, I'd like to move on and introduce you to the OSD RGB mux method developed by Mark Ozlad and Syntax (with help from others) from the Shmups forum.
abstract3000:
Thank you so much for you Response. I will try to make more sense of this in the meantime He sent me link to how he re did the board and has been looking here https://crtdatabase.com/modding/rgb-via-mux, and it appears we are looking for the CGA Preset is the TV....
I really appreciate the quick response :)
Zebidee:
How about you upload that schematic you have? I can see if it is the same as what I've got.
EDIT: or you could just tell me if it is the same as this one
https://www.manualslib.com/download/780770/Sharp-27r-S50.html
abstract3000:
Sorry there is a Fold in Page A/B, If the fold lands on info you need I can get the info for you :)
Zebidee:
--- Quote from: abstract3000 on October 29, 2023, 11:32:26 pm ---Thank you so much for you Response. I will try to make more sense of this in the meantime He sent me link to how he re did the board and has been looking here https://crtdatabase.com/modding/rgb-via-mux, and it appears we are looking for the CGA Preset is the TV....
I really appreciate the quick response :)
--- End quote ---
OK, yes that is basically the same approach I suggest. That site you linked doesn't seem to acknowledge that different TVs are... different.
I've attached that popular RGB OSD MUX diagram. However, you will need to change some resistor values.
You can work this out by looking up the schematic and plugging some values into this OSD mux RGB calculator spreadsheet. The spreadsheet linked is read only, you need to copy it then plug in values on your own copy.
The schematic I have says that your OSD RGB inline resistors are 6.8k, so over-write 5600 with 6800.
EDIT: the schematic you just uploaded seems to have the same values, 6.8K
There are no diodes on the OSD RGB inputs, so we'll over-write 0.7 with 0.0
We WILL add 75R termination resistors, as marked in the diagram, so we can leave the 75 there.
We want the output voltage to be 0.7 or over. If leave the default at 1k (1000) for the Ext RGB inline resistor, we get 0.68v. Close. If we bump it up to the next standard value of 1.2k (1200) then it will be 0.78, a little high. So, choose 1K or 1.2K for the ext RGB inline resistors.
The OSD blanking signal has a 6.8K resistor on it. The spreadsheet suggests we use 3.75K to the 5v blanking, to achieve 1.8v. It also suggests just using the same value resistor, 6.8K. Confusing much? Try a 10K pot (with only two terminals attached), find what value works, then replace with a fixed resistor. Don't just assume that 5v blanking will work.
You'll probably want to put a switch on that blanking voltage, so you can retain input/OSD options. I'm not sure where you'll be getting your 5v, there are a few options. Try starting from the Vcc/power pins, on those main jungle and OSD ICs, and working back towards the voltage regulator(s).
EDIT thanks for the schematic. Different to what I linked, but basic info seems the same.
I see that someone has pencilled in some diodes on the OSD RGB lines. If they are there, then the Ext RGB inline resistors will need to be 1.3K, just like your friend has already done. But I don't see the 75R termination resistors.