SNES output circuit question - Page 1
Forum section: electronics / beginners

author: BlackEagle
author_rank: Newbie
timestamp: January 29, 2022, 08:52:20 am
content: Hi thereI have a question about the Super Nintendo (SNES) circuit (attached). There is the AV multi out which some of you will know very well. So, I can explain the Signals a bit:Red, Green, Blue => RGB Signals 5Mhz max 1V pp according to SCART standardCSYNC => Composite Sync Signal TTL 5v ppY-SV, C-SV => S-Video Signals 5Mhz maxCV => Composite Video Signal 5Mhz 1V pp according to SCART standardVCC => 5V DCAudio-L, Audio-R, Audio Signals 20Khz max 1V pp according to SCART standardSo, now my question. Why are there shorted capacitors to ground in every signal? How is the size of the capacitor determined? What happens if you leave them out? I noticed that in the N64 there is a similar circuit, I just don't understand it. Thank you for your help.

author: Capernicus
author_rank: Frequent Contributor
timestamp: January 29, 2022, 01:56:01 pm
content: The caps would do something to do with some power short protection, or some power anomoly for protection?Because its a dc signal, so they only charge one way.I bet u can leave them out, but I dont know for sure.

author: Gyro
author_rank: Super Contributor
timestamp: January 29, 2022, 02:05:57 pm
content: Quote from: Capernicus on January 29, 2022, 01:56:01 pmNo, nothing at all to do with short protection (of any kind).The caps are there to reduce EMC emissions - to allow the product to pass the relevant standards. The capacitor values are sized to minimise the emissions of high frequencies, eg. The CPU clock, while being small enough that they don't impact the wanted signals, hence the capacitors for audio (and VCC) are higher than those for high frequency video signals.The actual values of the capacitors depend on several factors - the source impedance of the signal (lower for 75R video than audio), it's required rise/fall time (faster for video signals), and probably to some extent, experimentation during emissions testing.P.S. Leaving the caps out would mean that the product would no longer meet its CE / FCC etc certification.

author: tooki
author_rank: Super Contributor
timestamp: January 29, 2022, 06:14:40 pm
content: Quote from: Capernicus on January 29, 2022, 01:56:01 pmStop replying to things you knownothingabout.

author: Benta
author_rank: Super Contributor
timestamp: January 29, 2022, 08:12:47 pm
content: Quote from: Gyro on January 29, 2022, 02:05:57 pmI agree partially, but that's not the main function.The caps are there to bandwidth-limit the signals so your monitor (or TV) doesn't go crazy. The direct outputs from the SN are digital and far too fast for normal video/audio.A nice side effect is of course the EMI part.And I can only agree with tooki's last post.

author: Gyro
author_rank: Super Contributor
timestamp: January 29, 2022, 08:24:15 pm
content: Hmm, that would be leaving things rather late in the chain. I would have hoped the the on-board post audio/video DAC filter networks would have taken care of that. Maybe games consoles are different from the consumer A/V gear that I am familiar with, but I would have thought that quality would still be important.

author: Benta
author_rank: Super Contributor
timestamp: January 29, 2022, 08:38:54 pm
content: Quote from: Gyro on January 29, 2022, 08:24:15 pmBy modern standards you're absolutely right, but this is a 1990s device with 64 KB of video RAM and RGB443 output. Don't expect sophisticated video D/As. More likely a resistor chain driven directly from the digital logic. And for this kind of thing, every fraction of a cent counts on the bottom line.

author: Gyro
author_rank: Super Contributor
timestamp: January 29, 2022, 08:46:44 pm
content: A sad truth, it's a good few years (sh*t, a couple of decades!) since I was designing consumer DTV stuff. I well remember pitting BB and AKM against each other on pricing to take account of their different sub cent component count post-filter requirements (that was on 100k+ volumes though). The video filters were proper controlled impedance CLC Pi filters too.EDIT: Not being into video games, I didn't realize the SNES was so old!

author: tooki
author_rank: Super Contributor
timestamp: January 29, 2022, 09:51:48 pm
content: Quote from: Gyro on January 29, 2022, 08:46:44 pmIt was released over the course of late 1990-late 1994 (!) depending on where in the world. But yeah, essentially late 80s technology.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 30, 2022, 02:55:06 pm
content: Almost certainly EMI. You may find ferrite beads, shielding, etc. nearby as well.So, those signals will have some source impedance (which may be due to the circuits they come from e.g. TTL or CMOS logic, or added impedance e.g. series resistor or ferrite bead), and the capacitor shunts against that at high frequencies, forming a lowpass filter. If the impedance is say ballpark 100 ohms, then 47pF rolls off at ~34MHz. The cable I'm guessing isn't shielded/screened, nor does the connector have a metal shell?, so this helps to reduce radiated emissions (emissions above 30MHz are measured with an antenna, with the equipment in some standard configuration, inside a semi- or anechoic chamber) without having much of any effect on the signals themselves (std def analog video uses under 10MHz bandwidth).Note that a lower cutoff may have undesirable effects on the video, for example skewing the hue and saturation of composite or S-video. A lower cutoff would give noticeable phase shift and attenuation of the video signal; 34MHz seems high enough to be safe from this. So, we can guess they used about as much as they could get away with here.As for what they are trying to filter, it's switching noise and harmonics from the CPU and other onboard logic. Though the CPU runs at, whatever, 3.58MHz or a fraction of that (I forget what exactly; also the SPC700 runs faster, doesn't it?), but the switching transitions are what matters, and they are typically fast enough (a few ns) to include significant energy to 100-200MHz. Most of which should be confined to the main board, but due to various limitations (including cheap construction for a mass-market product!), some inevitably gets out along connector boards, cables, etc. So additional shielding and filtering tends to be necessary.The same is true of the controller ports, which are also unshielded cable (AFAIK). I don't have an SNES, but I recall my NES controllers actually use a CD4000 series shift register on board, with no bypass capacitor on that long cable! CD4000 at 5V, is very slow, evidently slow enough to get away with this -- and a similar interface on the mainboard can keep EMI low, allowing emissions to pass.A similar mechanism is used to this day, on USB HMI -- keyboard, mouse, etc. typically have unshielded cables, using a particularly slow communication mode that is tolerant of noise and less prone to emission. Or, for USB Full Speed (12Mbps), limitations are very similar to the video signals above (but without analog levels!), so it can be filtered in a similar way (though this is less useful, as USB is still quite sensitive to common mode interference, and is almost always carried on shielded cables and connectors).Tim