THS7374 vs THS7314 + 1 chip brightness fixing

[radaxian91]

^^^ image broken

Fixed the image link.

[Syntax]

Hey guys,


I've installed the borti RGB Bypass board into a 1CHIP PAL SNES but I can't get any output. The instructions say to take CSYNC from pin 151 but that didn't work. I've also tried taking CSYNC directly off the S-RGB chip pin 18 with no luck.

I've followed the instructions here but they don't seem to work... or I've missed something.

Removed so far -

R15, R16 and R17 from the SNES mainboard
R28 and D1

Excuse my long test wires...

[/img]

Even with no sync you should at least get a rolling Image on a crt. If using ossc or gscart then you wint as they need sync to process signal.

If you are using ossc or similar try composite video for sync. If you get a picture then its def your sync that's the issue

I notice between your install and the one below it the RGB lines are on opposite sides of the resistors. Maybe double check that.

[borti4938]

This is a modified version of the GitHub repo, to make the overall usability more comfortable for the end-user while having a single design.

He has to close J3 for getting CSYNC passed to the MultiAV pin 3: https://www.videogameperfection.com/pro ... ypass-amp/
And by the way: S-CPUN pin 151 has an easy access at the bottom side

[Ikaruga11]

Do I need a low-pass filter for an RGB-modded SNES Mini going to a CRT?

[radaxian91]

This is a modified version of the GitHub repo, to make the overall usability more comfortable for the end-user while having a single design.

He has to close J3 for getting CSYNC passed to the MultiAV pin 3: https://www.videogameperfection.com/pro ... ypass-amp/
And by the way: S-CPUN pin 151 has an easy access at the bottom side

Thanks for that.

J2 and J3 were quite unclear from the instructions, but on that new link you sent I see what you mean.

So with J3 closed I now get sync, but still no image.

The RGB points are taken from the vias as in the photo.

FYI - Using this cable wired for TTL pass through sync - https://www.retrogamingcables.co.uk/nin ... gold-scart

[unmaker]

This is a modified version of the GitHub repo, to make the overall usability more comfortable for the end-user while having a single design.

He has to close J3 for getting CSYNC passed to the MultiAV pin 3: https://www.videogameperfection.com/pro ... ypass-amp/
And by the way: S-CPUN pin 151 has an easy access at the bottom side

Thanks for that.

J2 and J3 were quite unclear from the instructions, but on that new link you sent I see what you mean.

So with J3 closed I now get sync, but still no image.

The RGB points are taken from the vias as in the photo.

FYI - Using this cable wired for TTL pass through sync - https://www.retrogamingcables.co.uk/nin ... gold-scart

What is your setup like? Are you going straight to a PVM or some kind of upscaler?

[radaxian91]

What is your setup like? Are you going straight to a PVM or some kind of upscaler?

I've tested it via an RGB CRT and an OSSC.

[borti4938]

If you get sync, there might be something different wrong. Does composite video still work?

[borti4938]

Do I need a low-pass filter for an RGB-modded SNES Mini going to a CRT?

Probably not

[FBX]

FYI - Using this cable wired for TTL pass through sync - https://www.retrogamingcables.co.uk/nin ... gold-scart

That's the link to the NTSC version. I assume you know the sync pin on NTSC cables would end up going to a power pin in your PAL console, correct? Also is your console a 1CHIP revision? I'm sure you already covered these bases, but I just wanted to be sure on both counts. Edit: I see that you say it is a 1CHIP console, so that's covered then.

[borti4938]

He has removed R28 and D1 from his PAL 1Chip mainboard. This frees pin3 from 12V and makes it useable for CSYNC.

But in general you are absolutely right! People in PAL regions have to be aware of that and have to mark there cables carefully if they have CSYNC cables lying around. Falsely plugged in a unmodified PAL console and powered on ... ouch ...
This is also the reason why all my cables are 'Pseudo' sync-on-luma cables. Pseudo, because my consoles are modified to output csync on the luma pin.

[FBX]

He has removed R28 and D1 from his PAL 1Chip mainboard. This frees pin3 from 12V and makes it useable for CSYNC.

But in general you are absolutely right! People in PAL regions have to be aware of that and have to mark there cables carefully if they have CSYNC cables lying around. Falsely plugged in a unmodified PAL console and powered on ... ouch ...
This is also the reason why all my cables are 'Pseudo' sync-on-luma cables. Pseudo, because my consoles are modified to output csync on the luma pin.

Okay cool, yeah I was just concerned and wanted to make sure he didn't actually fry something in his console. It is strange that he's not getting a signal from R, G, or B. With all 3 lines not working, there must be something fundamentally wrong with his mod and not just a loose connection.

Anyway, that's a good idea to use the safer luma pin for re-routing csync. Semi-related: I found out the other day that the Neo Geo RGB cables I got from retro access were using composite for sync access. While that works fine most cases, My OCD got the better of me, and I opened the cable up and swapped the csync wire to the csync pin. Works great, and it seems to be a bit easier to dial in perfect sampling phase on the OSSC.

[kel]

Now that you have jumper J3 closed try connecting Csync to the via that borti suggested or pin 7 of the S-RGB chip or even S-CPUN pin 151 as you did previously. Pin 18 on the S-RGB chip may not be sufficient in some cases with this mod. I have measured pin 18 on two "S-RGB A" chips and and it was 4.6Vpp which might just about be enough at about 281mVpp once it has been through the amp circuit if you're lucky but the one "S-RGB" chip that I measured was about 4.1Vpp on pin 18 which is a bit low at about 250mVpp once it has been through the amp circuit.

[radaxian91]

He has removed R28 and D1 from his PAL 1Chip mainboard. This frees pin3 from 12V and makes it useable for CSYNC.

But in general you are absolutely right! People in PAL regions have to be aware of that and have to mark there cables carefully if they have CSYNC cables lying around. Falsely plugged in a unmodified PAL console and powered on ... ouch ...
This is also the reason why all my cables are 'Pseudo' sync-on-luma cables. Pseudo, because my consoles are modified to output csync on the luma pin.

Yes R28 and D1 were removed at the start.

The console now only produces a grey screen with composite after installing this board. It was working beforehand.

[kruuth]

Kind of OT but does anyone have these in stock? Everyone I've checked is out.

[P.H.U.]

I have an interest in connecting a few of my consoles to an arcade monitor, most likely a Wells Gardner 25" K7000 or a Nanao MS8. With the K7000, it seems it can accept the AC coupled video output from the consoles but the MS8 cannot. As of now, I am using Arcade Forge's Sync Strike to harvest RGB, VGnd, and CSync from my retro-access sourced SCART cables then connecting that to a JAMMA fingerboard, then connecting to my JAMMA cab. I want this to work on my Nanao MS8-based cab as well. DC coupled video is required and I am holding out for viletim's next revision of SCART2Jamma whenever that becomes available. Would like to put something together to hold me over until then.

What I would like to know is if the THS73xx, particularly the THS374, series chips output a DC coupled signal that will work properly for an arcade monitor. If so, how do I enable DC coupled output considering that output from the consoles is AC coupled? Thus far, all I know of is Bob's guide here and here. Does simply following Bob's guide achieve this?

[P.H.U.]

Spent some time reading the data sheet over the weekend. Seems Bob's guide captures what I am asking but in more curt terms. So if I am not mistaken, the only thing required for DC coupled output is 75 ohm termination on the output. Page 24 on the data sheet says:

Code: Select all

Note that the THS7374 can drive the line with dc coupling regardless of the input mode of operation. The only requirement is to make sure the video line has proper termination in series with the output-typically 75 ohms.

So basically, if I am not mistaken, follow Bob's guide and I should be fine. Also, if this is true then, doesn't seem that this Sony BVM/PVM monitors are all that much different than arcade monitor when it comes to RGB 15khz input? Learning this stuff is pretty cool.

[obitus1990]

Mods, is it possible that images could be uploaded to the forum itself, as attachments, so that photos in this less year old thread would still be present? I have encountered a few of these so far as a new member, and, I really am not trying to be a PITA about it, but, it does result in some lost knowledge in the threads. Several other forums I frequent no longer allow image linking at all, and insist on direct uploads of images (properly size limited to no more than 1024x768, or sometimes 800x600 to limit bandwith usage) to protect against broken or removed image links from third party image hosting sites.
Many thanks for considering it.
-o

[Blonu]

I have installed SNES RGB Bypass Board Vers. 4.1a with S-Video in my SNES mini and am noticing the brightness to be really high. Every component is soldered on the pcb and I have it wired for csync.

Is this expected?

[Link83]

I have installed SNES RGB Bypass Board Vers. 4.1a with S-Video in my SNES mini and am noticing the brightness to be really high. Every component is soldered on the pcb and I have it wired for csync.

Is this expected?

What value resistors are installed for R11, R21, R31 on the bypass board? According to bort4938's readme:-
-for the 'regular' 1Chip model: use 750 Ohm resistors
-for the SNES Mini/Jr.: use 1.2k Ohm resistors

[Blonu]

Here is a picture of the assembled pcb.

https://imgur.com/gx0heiN

I think i assembled the parts as per the BOM. Do the registers look incorrect? I assembled as per the BOM and i think they are https://www.digikey.com/product-detail/ ... -ND/729799

[Link83]

Here is a picture of the assembled pcb.

https://imgur.com/gx0heiN

I think i assembled the parts as per the BOM. Do the registers look incorrect? I assembled as per the BOM and i think they are https://www.digikey.com/product-detail/ ... -ND/729799

Looks like R11, R21, R31 are correct for SNES Mini, but I cant work out what value resistors have been used for R12, R22, R32?

[VajSkids Consoles]

From my own tinkering with making my own bypass board, what I've found is that a bypass doesn't offer much more than simply a brightness fix on 1-chips.

The SNES JR has 150r pulldowns on the RGB lines from factory - these are left in place and 'worked around' with current bypass boards
The phat has 160r pulldowns - these are left in place and 'worked around' with current bypass boards

I 'worked around' the onboard resistors, which (at least on the phat SFC i have) are sloppy from factory (tolerance wise)
I had a 5ohm difference maximum between RGB lines.

I just went back to bread-boarding with an LT6550 - removed the onboard pull-downs and revised my board design to be direct coupled - no more AC coupling caps, direct RGB to the inputs of the amp with pulldowns - thats all you need for attenuation.

I only have 120r and 180r resistors on hand - 180r basically takes you back to the OP factory encoders output, 120r isn't bad, but on 240P test suite colour bars, you are missing vertical line 1 and the colours start to show at vertical 2.

The easiest way to do a 'brightness fix' this entire time was simply swapping out R6, R7 and R8 to your liking....
So I'd hate to say it, but bypasses are bit of a gimmick.

I am running the LT6550 @ 3.3v because the datasheet shows it performs better at this voltage.
Yeah, on paper, any bypasses driver/amp will beat the factory one, but with actual game-play - I'd like to see someone prove it is actually better.

I will grab some more resistors tomorrow and find the minimum required to show the first bar ('1') in colour bars via 240p test suite. 130ohm are available at jaycar. But ultimately, if this is the perfect brightness as per the colour bars in 240p test suite - you may as well have swapped out the factory 160's to 130's with SMD resistors and called it a day.

With 120r, you can JUSTTTT see the first bars of each colour if you glue your eyes to the screen, so I reckon 130r will do it. At least with the board, which I will still adjust my proto and make - you have some extra sync options.

Mine has 470r resistor on board which is able to be bypassed via a jumper depending on your cable, this is also leads to the composite pin with a jumper if you want to send TTL or 75r C sync down the composite line with a sync over composite cable (I just remove the 75r composite series resistor off the mainboard)

[VajSkids Consoles]

[VajSkids Consoles]

[VajSkids Consoles]

Final note: this board doesn't cut the top few scan lines on Street Fighter, but I am using PAL version ROM on JP SFC... I will grab NTSC version and update this post.

yeah, they don't seem to be missing

[NewSchoolBoxer]

Sliding into this legendary thread. Lack of knowledge on opamps and filters so let me contribute.

OPAMPS

First rule is you want gain bandwidth product of opamp to be at least [10 x gain x max frequency in signal]. If we say [Standard Definition aka 15k aka 480i/576i + 240p/288p] video caps at 5 MHz to be extra generous then we need 10 x 2 x 5 = 100 MHz to the -3 dB point you see on a Bode plot. Datasheets usually list the bandwidth but have to infer if it's at unity gain or the fixed gain in video amps. 80 MHz at unity gain is effectively 40 MHz with gain of 2 - too slow when we want 50+ MHz. Basic reason for min bandwidth is the operating rate of the IC is itself a lowpass filter (LPF) because it can't handle signals faster than it can run. We lose 50% power at 3 dB so the 10x margin is 1 decade of safety. Incidentally, +6 dB means doubling the signal's power for a voltage gain of 2.

Slew rate is how fast the opamp draws the output. Math is easy enough, if we need on the output 1.4V then takes 1.4V / [slew rate V/us] = time in microseconds to draw the rise or fall time. In practice, takes longer for a few reasons but can use that to sort opamps for the minimum slew rate you need and then analyze datasheet graphs of rise and fall times. If sinusoidal signal instead of RGB square wave then min slew rate you need is [2 x pi x Vp * max frequency in signal]. Have a margin of safety.

The reality is THS7316 is nowhere near fast enough to amp analog video at unity gain bandwidth of 36 MHz without distortion. THS7314 hides behind its mandatory LPF to not even list its bandwidth in datasheet but can zoom in on page 9 to see it's slow as balls as it starts attenuating before 5 MHz:

Spoiler

Neither THS7316 nor THS7314 should have ever been used in our scene.

THS7376 and THS7374, those are fast enough for analog video. Can debate on which 'fast enough' amp is better than another due to noise specs, differential gain/phase error, higher speed or faster settling time. I think LT6550 for instance is better than those if you don't need the LPF and you don't mind paying $4-5 vs $1. At luxury $6-7 tier, you can use LT6557 or OPA3692 with insane 2000V/us slew rates. Extra speed does reduce the gain error with diminishing returns and read datasheet for notes on stability. Speed can be too much of a good thing.

If you want to amp luma (Y) with its negative sync voltage without cutting it off, or allow DC coupling of RGB, I'd suggest looking at ISL59837 that has a built-in charge pump for negative voltage. Renesas also makes a superior sync stripper compared to LM1881.

One more thing. Quad channel opamps like THS7376 and THS7374 where you don't use the 4th, you need to terminate it correctly or you add noise and generate more heat. Is a design flaw this scene has had for years that I'd like to see fixed.

--------------------------------------------------------------------------------------------------------------------------------

FILTERS

First, you don't need a filter playing on (analog) CRT. Is a waste and just adds distortion. Noise at 6+ MHz that a 8.5-9MHz LPF attacks is invisible to 480i/576i SD CRT. Most people know but I prefer to have all basic info one one place.
You definitely want a filter before analog to digital conversion to prevent aliasing for HDMI or otherwise scaling from interlaced to progressive. Framemeister, OSSC, RetroTINK 5X Pro have them but not sure about other RetroT scalers or capture cards.

Filters, chaining two independent LPF or HPF together significantly reduces the cutoff frequency and should never be done. Chaining two 2nd orders to make a 4th order filter with worse characteristics than designing a 4th upfront is classic lab exercise. Each opamp can have 1 or 2 orders on it so 4th order requires 2 opamps. Odd orders, sometimes you'll see 1st order simple RC passive filter chained on at the start or end to save an opamp. Passive filters beyond 2nd order aren't useful in practice because their max gain is 1 and have limited passband compared to active that applies power to preserve a constant gain that can be >= 1. Video amps have gain of 2 to compensate for 75 ohm voltage divider on the video processor.

What is an order? Each capacitor and inductor in the filter that doesn't get canceled out, adds 1 order. Can equivalently say that the order is the highest order in the differential equation of the circuit's time response or the highest degree in the polynomial of the denominator in the transfer function. Whew.

Each order increases the rate of attenuation by 20 dB/decade (good) but also increases the phase distortion and overshoot/ringing (bad). Classic tradeoff between better frequency response at cost of worse phase response. We all seem to use Butterworth filters that have balance of good frequency and phase responses. Chebyshev and Elliptic have superior (faster) frequency rolloff but worse phase responses. Can get away with a lesser order for same attenuation. Bessel, common in audio amps, has best phase response but slowest (worst) frequency response that audio world accepts to preserve sound quality.

Rate of change of phase delays is the group delay. What you want to minimize is the non-linear group delay that is shown on opamp datasheets. Effect is stretching and compressing the frequencies by having them arrive at the output at different times. Too much of that and video will be distorted. Linear group delay, is okay, is effectively lengthening the cable distance but too much and sync that isn't filtered with this added delay is, well, de-syncing. Can separately delay the sync with an all-pass filter or other method to match video's arrival time.

So why is SD lowpass filter cutoff of 8.5-9 MHz when signal doesn't go past 5 or 6 MHz? A brickwall filter that passes 5 MHz and completely cuts off 5.1 MHz is physically impossible. Got to start your attenuation past the end of your signal and accept not filtering most of the noise in that 5.1-8.0 MHz band. An unfortunate amount of electrical engineering is a balancing act between the ideal and reality. Need a 355pF capacitor? Doesn't exist, take your choice of 350 or 360.

Inductors aren't used in active filters because of how large and non-ideal they are compared to capacitors, not to mention they cost more. They are used in the form of (passive) ferrite beads that you see on PC power supplies as cheap LPFs and perhaps they could become a thing in our scene.