A friend and I have been working on adding RGB to a CRT tv with some degree of success however we have hit a dead end and were hoping that you guys could possibly help us out. We have the data sheets of the tv chasis, osd chip and the video chip and are aware of the dangerously high voltages in a crt and do discharge it prior to each attempt. Up till now we've been going off of this guys guide for doing the mod http://133fsb.wordpress.com/2010/05/22/adding-an-rgb-input-to-a-cheap-generic-ntsc-tv/#more-123
Whoever wrote that guide has the right general idea, but his actual implementation shows he is actually quite ignorant of the electrical propertied of video signals and how they are used in practice. You can be dead certain he hasn't tested his gorious new RGB input on anything other than his own Sega Genesis. Essentailly this TV can now accept RGB video arcade monitor style -- ie. every time a different device is connected the input he'll need to re-adjust those resistor values on the input.
Right, narke, your first step to acheiving your goal is to wipe everything you read from the article after the heading "Modifying the 3Y03 Chassis for Direct RGB Input" from your brain. You see, there are actually standards for these sorts of things.
1) A video input must be terminated with 75 ohm resistor.
2) After terminating, the video must have its DC level (black level) restored. This is most often done with a coupling capacitor and a video clamp circuit.
3) The standard signal level for a video signal is 0.7 Vpp (vots peak to peak) into a 75 ohm load. If the video signal contains sync information (which is 0.3 Vpp below the video) making a total of 1Vpp into 75 ohms.
There is nothing particularly mysterious or secret about this information. This is the kind of thing you'd best up in a techical book or paper, not the internet.
So you want to connect your Sega Genesis console to this Samsung TV. First thing to do is make sure your Genesis is actually generating a standard video signal. Check your RGB cable, in include some capacitors and resistors as shown in this diagram:
http://members.optusnet.com.au/eviltim/gamescart/gamescart.htm#megamstr If you, you'd better put them in place. Without a knowing what kind of signal you are dealing with, you will never acheive your goal.
Right, now you have a standard video signal, three of them actually, but I'll refer to one as they're all treated the same. We know that the signal is 0.7 Vpp when we terminate it at 75 ohms. Take a look at the datasheet of the Jungle IC (TDA8843) and see under the heading "quick reference data" the parameter V
iRGB happens to be 0.7 Vpp. That's not to say that an RGB input to any given Jungle IC will absolutely be designed to accomidate 75 ohm standard video. Some are digital (TTL) inputs which can only be switched on or off, as this is all that is required by an OSD anyway. Sometimes a Jungle IC has both digital and analog RGB inputs - the digital ones being used for the OSD, the analog pins just sit there, bypassed to ground. Sometimes you find analog inputs which are spec'd at 1 Vpp or something else silly but it's fairly rare.
Back on track - BLACK LEVEL RESTORATION. That sounds complicated and a lot more involved that installing a termination resistor but that is very often not the case. If the analog RGB input is connected to the OSD you can check to see if there are any coupling capacitors in the signal path. If there are, then the jungle IC has an integrated video clamp, you need only supply the capacitors. The value is usually 10uF or 0.1uF, electrolytic and ceramic respectively. The value of the coupling capacitor is important and depends entirely on the characteristics of the jungle IC - older parts generally use larger coupling capacitors. If the analog RGB input is unused the capacitors are usually still present in the circuit, they just bypass the input to ground to prevent oscillation. Common sense should dictate the polarity of the coupling caps if they are of a polarised sort. If one end it tied to ground with a 75 ohm resistor and the other clamped at three volts or so it shouldn't be too perplexing to work it out.
Standard video input, check
video terminated, check
video coupled and clamped, check
Synchronisation is next. For this step connect the composite video signal output of the Genesis to the composite video input of the TV. Very easy. This is what the Europeans have been doing for decades and it works for them. Don't use the Genesis' composite sync output for anything, ever.
All that is left now is to enable the RGB input. There's usually a pin next to the other three video inputs called 'Ys' or 'FB' or 'RGB_INPUT_ENABLE_ACTIVE_HIGH' or similar. Often this is the equivalent of the RGB select pin on a SCART socket. It's the base of a transistor, applying a volt or so will bias it on and activate the RGB input. Sometimes it's just a digital TTL input. Also take note of other components in the circuit before introducing any foreign voltage.
INTEGRATION with the rest of the TV, assuming you don't want to make it a dedicated RGB monitor. Can be acheived with the use of mechanical switches. Toggle and rotary, and latching pushbutton switches are all good choices, keyswitches and knife switches are not.
To summarise:RGB input on the jungle IC of low end TVs are common.
Sometimes they are perfect match for standard video and designed to accomidate a SCART input (all LG and Samsung do - I'm fairly sure) and sometimes it's a useless digital video input. Sometimes it's a non-standard analog input. If it's this type it may not be worthwhile proceeding unless you're really keen, it may require actually learning about video signals to accomplish this.
Inputs are terminated with 75 ohm reisistor. Always.
Right after the resistor comes the coupling capacitor. Always.
Composite video is your synchronisation source.
Have a thorough look at the circuit and use rational thought to determine how the RGB input is activated.
It's easier to just find a cheap RGB monitor than worry about any of this non-sense.
