N64 RGB Mod NUS-CPU-04 Help. Have sound, but no Video xrgb-mini

Hi Calpis. I formerly used 75 ohm resisters with that amp, however I found it to be too bright on all my displays. I also received many emails from people who said it was also too bright on theirs, so I tested 75, 100, 121, 130 and 150 ohm. The 100 ohm seemed to make the RGB amp output the same brightness level as stock consoles outputting RGB. I switched all my consoles with the THS7314 amp over to 100 ohm and they've been perfect ever since.

Could you please elaborate as to why that's not advised? I've tested the combination of THS7314 & 100 Ohm on quite a few displays, consoles, cables and switches and the picture always looks perfect.

 

The signal level (amplitude -> brightness) is standardized, so it doesn't make sense for it to be too bright unless your displays are confused by the amplifier's DC offset. This is more likely with old monitors--modern TVs generally have robust restoration/clamp circuits.

If the level really were too high, which I'm pretty sure isn't the case, then the proper course would be to attenuate with a resistive voltage divider at the input of the amplifier, and keep the 75 ohm resistors at the output to properly terminate the line.

 

Hi Calpis,

Sorry for the delayed response, but I contacted Texas Instruments, as well as an electrical engineer friend of mine for more info. First and foremost, both TI and my EE friend assured me that using 100 ohm resistors won't damage any equipment. I just wanted to make sure I was clear about that, so anyone reading your posts won't misunderstand and think they'll fry their monitor by using resistors higher than 75 ohm.

That being said, both people I contacted agreed with you and suggested 75 ohm. Basically, what they said was (and I'm paraphrasing in layman's terms) that using resistors other than 75 ohm could result in picture quality issues, but if everything seems to be working the way I need it to with 100 ohm, it wouldn't do any harm leaving things the way they are.

That's not good enough for me though. The entire reason I started the work that turned into RetroRGB.com was to get the absolute best possible quality out of my favorite game systems. As a result, I've changed the guide links to 75 ohm resistors and will test this further.

For the record, I personally never thought the brightness was "too high" when 75 ohm resistors, just brighter than stock console output. What sparked me to try different resistors was a few complaints I had emailed to me (and also saw on forums) saying it was too bright and even recommending using 150 ohm (which I found to be WAY too dark). That's why I tried different resistors until I found ones that made the brightness as close to stock NTSC console output as possible. Maybe the brightness issue people complained about was with modded PAL consoles that didn't have the proper cables?

What I find interesting is that every combination of THS7314 + 100 ohm resistors has looked flawless to me. I have a few close friends with RGB setups, so I'm able to test my mods on all of their consoles and equipment: At the moment, that amp/resistor combo is running on over ten consoles (mix of SNES 1CHIP, SNES Mini and N64), which are displayed on five different RGB monitors (different brands & models) and an XRGB mini. Each setup is extremely different, with a mix of different cables, switches and video processors (some are routed through an Extron 201Rxi). In each of these setups, the output quality is flawless and exactly the same as when using 75 ohm, just slightly darker. That being said, I didn't test any PAL consoles and I can't test every combination of displays out there, so there is a potential for quality issues.

The N64 page has already been updated with the new links, but I'll also be updating all the pics soon with nicer ones. Thanks again for all your help.

 

I have mentioned before to Calpis that its too bright with only the 75ohm on the output.

 

This is why only EEs should publish circuits, most people (many incompetent and lazy EEs included) don't know what they're doing wrong, and/or simply don't care as long as it works for them, on their device, at that moment. What modders need to learn and take to heart is that video signals aren't analogous to audio signals which many technical people have a faint concept of. Video signals are complex, and their high frequency content means they cannot be transmitted as carelessly or distorted without consequence as audio can. Video standards are in place because it's the ONLY way to ensure consistency (here in the form of brightness) or even inter-operation across all devices, and in fact ignoring the standard guarantees some degree of incompatibility with devices that are simply built to specification. Video input circuits in most televisions are very forgiving at the expense of some signal degradation. This is the only reason most mods published online even work acceptably for people. What many modders don't know is that the TV is conditioning the horrific signals to extract useable video, and this conditioning results in increased noise (picture grain), decreased bandwidth (sharpness) and very often distortion (color non-linearity such as over/under saturation and clipping). To me it's hypocritical for modders to exalt the virtues of RGB when their signals more or less have the integrity of circuit bending and are only usable by the grace of complex restoration circuits or manual picture adjustment.

It'd be commendable if you can get this across on your site because it's been mostly an uphill battle for me on message boards. I don't care what people do for personal use, but it really irritates me that people will deface 10s or 100s of consoles with crappy mods, make notoriety or income from them and pat themselves on the back. IMO since game consoles are a finite resource they should be respected and modded with care, if at all.

That is true, using a 100 ohm resistor is easier on the amplifier because the load as seen by the amplifier (100 ohms + 75 ohms inside the TV) has increased from 150 to 175 ohms, meaning less current will need to be sourced/sunk for a given voltage (Ohm's law). Also it's a perfectly valid way to attenuate (decrease by fractional division) the signal, but it's inelegant because not only are the levels/voltages defined by specifications but the signal's output impedance is too so that it may be matched with the cable and TV for best transmission quality (transmission line theory).

Doing it right will increase the total cost by 6 resistors, two on each input. Alternately you could attenuate the signal at the output to theoretically save 3 resistors and still match the 75 ohm impedance but this is more difficult and requires very precise resistor values (achieved with multiple values in parallel, negating the savings).

The next step is to check the signal level with an oscilloscope. If the level really is too bright (meaning the white voltage swings more than 714 mV peak-to-peak), a voltage divider (two resistors) can attenuate the signal until it matches exactly 714 mV p-p at the input or 1.43 V p-p at the output of the amplifier (before the 75 ohm resistor), which will be correct.

If the signal at 714 mV p-p is too bright, then the display's brightness (black level offset) and/or contrast (signal gain/amplification) needs adjusting so that black is black and white is white.

 

Thanks for the information.

I'll try my best. At least in the short term, I can try to help with the THS73124: It might be a help if in the mods, instead of describing how to build the amp, I showed a picture of the amp and said "click here for details on how to build the amp, as well as information on brightness and the proper components" (or something like that). Then, the THS7314 would have it's own page, where I could try to explain why 75 ohm resistors are used (as we've discussed here). If you don't mind, I'd like to quote some of what you said in these posts...but I can't make the page too technical, as the goal of my site is to try and present things in a way where almost anyone could understand it. Also, a large portion of my readers don't speak English as their main language, so I try to keep my descriptions as simple as possible. At the very least, it might be a good page for me to have, simply to show more detail and tips for how to build the amp and test with a multimeter that it was soldered correctly.

Unfortunately, I no longer have access to a scope. Do you have a modded N64 with a THS7314 you could test?

 

I have a scope and THS7314 amps. I can try to get the required measurements when time allows.

 

Feel free to quote!

The amp is irrelevant to the measurements because its high input impedance effectively makes it "out of circuit". If Bad doesn't get to it I can measure the DAC's output but I strongly suspect that the level will be very close to the ideal 714 mVpp anyway--the mid 90s isn't exactly the Stone Age when it comes to analog chips. AFAIK the N64 also adheres to video standards more closely than any console before it.

 

Awesome. Thank you both.

 

If you plug in an N64 with composite and compare to the same N64 with RGB (with THS7314 amp, with 75 ohm on ouput) the RGB is noticeably too bright. The intro with the age rating on golden eye is completely white, rather than with a red checkered pattern in the circle. etc. (check at 7 seconds on this video: https://www.youtube.com/watch?v=Bj1z7F5BkyM).

Its certainly not correct as-is.

 

The DAC will have a DC offset and the THS adds additional offset. It seems just as likely to me for typical SCART inputs to have a much more naive DC restoration circuit instead of a black level clamp like composite, or maybe different brightness/contrast settings per input (my latest TV). I guess it's suspect if a handful of TVs perform identically.

Also if you're using a LM1881/logic level sync instead of composite the TV won't have a coherent reference for the RGB signal levels (in the case it puts RGB through AGC)

Have you tried the THS amp with AC coupling capacitors on the input and output?
(You could test it with 100 nF on the inputs--no pull-ups necessary, and 220 uF as standard for the outputs)

I guess I'll unpack a N64 soon.

 

Not tried with the caps, that was what we discussed last time - just not got around to it.

The colours are out, even on old TVs with no per source brightness/contrast/colour adjustments. Not using a LM1881 or logic level sync either.

I will dig out the RGB modded console over the weekend hopefully.

 

The measurement won't be from me, I don't have a N64 early enough.