While researching capacitors, hoping to find which variety are typically used for video use, I came upon a datasheet for a Fairchild S-Video Filter which makes mention of the characteristics which are desirable in a video grade capacitor. It points to low equivalent series resistance, low inductance, low leakage and a constant capacitance over a frequency range of 20Hz - 6MHz. With their specific IC in mind they also state that it is desirable for said capacitors to have an ESR of less than one megaohm at 6MHz and that the capacitance not change more than 2% over the 20Hz - 6MHz frequency range.
Upon further googling, I come up with article TA0310 from STM which gives more interesting info about this frequency range requirement:
SD(standard defenition) or 720 x 480 (480i) uses up to 6MHz bandwidth.
PV(progressive video) or 720 x 480 (480p) uses up to 12MHz bandwidth.
HD(High Definition) uses up to 30MHz bandwidth and fits 1280 x 1920 (1280p) or 1920 x 1080 (1080i)
I guess that means that S-Video is always 480i, right? :) So does this mean that the ideal capacitor could cover the range of the format it's using? After reading some older posts from users trying to run high definition over the same box they use for their older consoles, hearing about the problems they had, I wonder if inadequate components were the problem. Also, if perhaps that's why some people have odd problems with the DC VGA Box even though they all followed the schematic correctly.
I could not find any leaded caps that meet these requirements. >_<
Wow, see what happens when you rely on google for information :). Nothing in your post makes any sense at all. When you put a capacitor in series with the video signal coming out from a console it's because you want to remove the DC (0Hz) part of the signal and makes it possible to make a decent, low power video output stage without having to use a negative voltage supply.
I know you're typical of the shelf electrolytic capacitor isn't perfect (with tolerances like +50% -20% what do you expect) but these are only 0.7Vpp signals. Most TVs have a cheap'n'nasty disc ceramic cap in series with the video signal right after it's terminated anyway.
The capacitors you describe do not exist because they would cost a fortune to make (though I think ESR of less than one megaohm is acheiveable :))
btw bandwith required by a 15.7Khz/50Hz (TV) RGB signal is about 10MHz
QuoteI know you're typical of the shelf electrolytic capacitor
haha...that should have been:
...I know your typical 'off the shelf' elec...
excerpt: Fairchild Semiconductor - Application Brief 42018
"Finding video quality -- i.e., low ESR, Equivalent series resistance, low inductance, low leakage, and constant capacitance over a frequency range from 20Hz to 6MHz -- 0.1uF capacitors presents no problem. Surfance mount capacitors meeting these requirements are both inexpensive and plentiful."
I could find any of them. However, the manufacturers I checked don't often mention the impedance vs. frequency of their caps or they are made for a much lower frequency range. I'm not thrilled about using surface mount chips either. >_<
It would be nice if someone would mention some good caps to use for video use. A type and brand name would be nice. This info is so easy to find in the audio world. When researching video caps, I get mixed statements. Some say that tantalum caps are a quality choice for video, but then others will say tantalums are a cheap (i.e., lame) choice for video use and make reference to a film type cap, while still others claim that aluminum electrolytics are a good choice due to typical high-frequency capabilities.
It doesn't matter, it really doesn't. Only old consoles with extra voltage in the RGB lines (ie Super NES) need them, and the specs are irrelevent. ANY 220uf cap will do as long as the Voltage is more than about 2. I used 220uf 10v 'cause they're smaller than 16V, but it doesn't matter a whit.
I've read the ap note, it describes a very typical video ic arangement.
I think you're spending too much time reading about this stuff and not enough time doing practical testing. What kind of bad things do you expect to happen if you use an inmperfect cap somewhere in the video path?
In practice, I really doubt that the signal attenuation through a coupling capacitor could be comparable to, say, the losses through the transmission line (you are using 75 ohm coax with a -3db attenuation point much higher that 10Mhz arn't you?), which probably isn't all THAT lossy anyway. Were're only talking 10Mhz here anyway, some computer video stuff goes all the way up to 100Mhz. I just had a look at couple of computer monitor chassis I've got rusting away. At the video input one uses aluminium 1u capacitors (not shit hot EL caps, just the cheap shit) and the other uses 0.1u ceramics (cheaper still!) and i'm sure they were perfectly adequate when they were alive.
For the input I suggest 0.1u ceramic disc caps or perhaps greencaps (polyester). For the output just use standard aluminium electrolytic capacitors. I dunno if tantalums go as big as 220u but even if you did did find some, they'd be big, expensive and unnecessary. If you want to use low leakage alluminium electros for the output coupling then they shouldn't be hard to get, they're used a lot in the repair of switching power supplies. I really don't see how a few MEGAohms of leakage is going to effect a circuit with an impedance of 75 ohms or how a few MILIohms of ESR is going to make any difference when you've already got a 75 ohm resistor in series!
QuoteI think you're spending too much time reading about this stuff and not enough time doing practical testing.
Yep yep, that's pretty much the jist of it. I took a crack at building my own DC VGA Box one time, but got frustrated and bought a real Sega box. It's neat but it's a little dark on my monitor. I would really like to jump on the bandwagon and get a 1084 and goof with my consoles, but I'm not sure where to look. I don't want to buy one off of eBay... although there was a new-in-box 1084S on there the other day.
Anyway, I had pretty much figured I was being overly critical on component choice for things like the DCVGA and the LM1881 circuit, especially since like you said, whatever I'm running into it and running it out to probably have crudola for components anyway. I guess it's not worth trying to make chicken salad out of chicken crap. Although, I have always expected to see someone do a rebuild on a 1084, but not yet. Probably the same thing, "Why?"
p.s. Lawrence, please get motivated to post all those SPDIF mods, soon! ^_^