Electronics > Projects, Designs, and Technical Stuff
Composite video issue
maginnovision:
Yea, I don't doubt something isn't right. I'm attaching the schematic for the modulator for anyone interested.
Ultron81:
OK, so can you clarify some stuff for me?
1 - You can't make any modifications to the main board (I'm guessing this is a Bally Astrocade, right?)
2 - You don't have access to the RF modulator
In reality, you have the 3 signals that make up component video (Y, R-Y, B-Y) going to the RF modulator. If you have a TV with component inputs, you can just take these signals off the RF modulator connector and pass them through the THS7314 amp and into your TV, should work.
I'm not sure how easy it will be to get composite with passive components.
EDIT: Looking at it again, yeah grab the signal from pin 13. That should be your composite video. Send that into the buffer and should be good to go. The chroma is on pin 13, the luma is added to pin 13 with a 3.3k resistor inside the Astrocade.
maginnovision:
Yea, it's not the cleanest signal but it is a valid composite signal. The THS7314's should show up tomorrow or the next day. After swapping to a bigger cap and reversing polarity and adding the proper series term resistor the output looks pretty good, but the THS7314 being a proper amp with the integrated low pass filter may help clean up a little of the noise and fix the remaining tilt; At the very least it's cheaper than the transistors. This is just a small part of the project(value add because it's cheap) but as it's close to finished I'm trying to polish some of this stuff as much as I can.
vk6zgo:
--- Quote from: maginnovision on January 15, 2019, 04:47:44 am ---Yea, it's not the cleanest signal but it is a valid composite signal.
--- End quote ---
Sorry, but it does not meet any of the specifications for a "valid composite signal", & nobody in Television Broadcasting would accept it as such.
Those of us "greybeards" who learnt the characteristics of a "standard composite video signal" by heart, & in our day-to-day work for decades, tested equipment & systems relying upon those standards,tend to be very critical.
In your case, it works OK for the purpose, & incidentally, shows how forgiving the decoding circuitry is in the later generations of analog TV receivers.
Something like a iPhilips K9 would probably produce rubbish from a signal that looked like that.
--- Quote ---
I'm trying to polish some of this stuff as much as I can.
--- End quote ---
I'm sure you have heard the saying about polishing some things! ;D
maginnovision:
--- Quote from: vk6zgo on January 17, 2019, 02:17:32 am ---
--- Quote from: maginnovision on January 15, 2019, 04:47:44 am ---Yea, it's not the cleanest signal but it is a valid composite signal.
--- End quote ---
Sorry, but it does not meet any of the specifications for a "valid composite signal", & nobody in Television Broadcasting would accept it as such.
Those of us "greybeards" who learnt the characteristics of a "standard composite video signal" by heart, & in our day-to-day work for decades, tested equipment & systems relying upon those standards,tend to be very critical.
In your case, it works OK for the purpose, & incidentally, shows how forgiving the decoding circuitry is in the later generations of analog TV receivers.
Something like a iPhilips K9 would probably produce rubbish from a signal that looked like that.
--- Quote ---
I'm trying to polish some of this stuff as much as I can.
--- End quote ---
I'm sure you have heard the saying about polishing some things! ;D
--- End quote ---
I really couldn't say for sure, not being a greybeard, but the maxim pdf on cvbs seems to say it's going to be similar to what I'm seeing although they have no actual waveforms. :-//
The good news is the waveform is much improved with the ths7314. Yellow is from the unit, orange is to the TV. Also the video output looks very good.
EDIT:
At your behest(kidding), I compared the output to that of a Sega Dreamcast. I'll admit it is a cleaner signal, but I don't think the output is any less valid and easily explains the noise on screen and the more limited color space.
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