Low Pass Filtering from THS7374 / SCART2DVI / more stuff...

[kitty666cats]

Seems not many people ever picked up these units that Bob @retrorgb designed for Datapath capture cards (they were available on the Insurrection Industries website) but have plenty of function with all sorts of devices that implement analog RGB on DVI-I, DVI-A, VGA & BNC inputs -

https://www.retrorgb.com/scartcleaner.html



I have had one for ages (originally, I used it to either connect to my old Extron 580xi and/or my old Kramer FC-14... sometimes both, in a chain. Hah), and I'll often stick a male DVI-A to male VGA adapter onto the end of it & connect to things like my various projectors, my GBS-C unit and various other things (my Dido LT processor, but that uses DVI-I for I/O).

I have found a lot of these devices benefit from the switch on the SCART2DVI which enables the optional LPF from a THS7374. Ironically, one particular thing I own that doesn't get any noise/interference is my Startech USB3HDCAP. For monitors/gear that *don't* get any noise/interference, I'll sometimes use a SCART2VGA ( https://www.retroupgrades.co.uk/product ... a-adapter/ ) as those don't require external power if the solder blob is adjusted to pull power from connected SCART cables.

ANYWAY, I was wondering if anyone here has ever danced with the idea of making some sort of passthrough adapter (with VGA connectors on both ends, for instance if you want to chain GBS-C with an OSSC's VGA input, or some other secondary processor, etc) with a embedded LPF. Could be a kinda neat design. Perhaps this hypothetical VGA to VGA device could also have an optional sync combiner... I think a real neat idea would be something like @buttersoft 's sync combiner circuit, but with a LPF added into it.

http://forum.arcadecontrols.com/index.p ... msg1733596




A bit unrelated, but is it feasible/of any particular use if someone were to design something like an external RGB amp w/ optional LPF using said THS7374? Or can you only use that IC for amplification *or* low pass filtering, and not both in tandem? If it can do both, then a external VGA to VGA box with dipswitches for amplification, low pass filtering & sync combining would be pretty neat! I know that's not too different from an Extron RGB interface, but those (or at least the 164Xi that I own) still have noise on the output w/ the SCART2VGA, but not with the SCART2DVI.

Once I get these PlayStation 1 & 2 cables a friend is making me. which implement VGA connectors & LM1881s, I'll no longer have use for any of my remaining SCART gear. That's why I brought up the hypothetical VGA -> VGA adapters. But I think the SCART2DVI was a nice little design & definitely hope to see someone produce some more of 'em someday

[NewSchoolBoxer]

This is stuff is relevant to my interests. Been thinking about what sync and conversion products the scene needs the past month. I forgot the SCART2DVI was a thing since I ride the SCART hate train. Best feature is it correctly 75 ohm terminates the R, G and B lines before hitting the THS7374 chip. You know that basically every PCIe RGB capture card takes RGB over DVI-A, so may as well add sync stripper and the same RGB amp every mod uses and break out the audio while we're at it. Can you tell me what it sold for? Would be more appealing if it had a DVI male connector in image.

ANYWAY, I was wondering if anyone here has ever danced with the idea of making some sort of passthrough adapter (with VGA connectors on both ends, for instance if you want to chain GBS-C with an OSSC's VGA input, or some other secondary processor, etc) with a embedded LPF.

YES. VGA in and out with additional BNC (or RCA) in and BNC out to expand beyond niche RGB capture card crowd. I'd use electrical switching to allow USB power supply or +5V supply off VGA pin.

A bit unrelated, but is it feasible/of any particular use if someone were to design something like an external RGB amp w/ optional LPF using said THS7374? Or can you only use that IC for amplification *or* low pass filtering, and not both in tandem?

This is a very good question. The THS7374 does have a disable pin to save on power consumption but it (I assume) also disables the low pass filtering. So you can amp + filter or solely amp but not solely filter. Filters have existed for over 100 years, plenty of options there, but limited SD video amp chip options. The "amping" here doubles the voltage levels in the low MHz range, has been done since vacuum tube days. Splitting up filter and amp into different components has pros and cons.

I'll sometimes use a SCART2VGA ( https://www.retroupgrades.co.uk/product ... a-adapter/ ) as those don't require external power if the solder blob is adjusted to pull power from connected SCART cables.

I can squint my eyes enough to see that it doesn't properly 75 ohm terminate the R,G,B channels and I'm about to smack down the LM1881. Interesting you bring up it not needing external power supply. I get the RetroRGB design using external power that most people don't like because you can't guarantee every console is going to have +5V on the right SCART pin with enough current to power the sync and amp chips. Additionally, console +5V power could be very noisy.

[NewSchoolBoxer]

Let me critique the chips in SCART2DVI and most other things:

LM1881 sync stripper by TI
Most famous chip in our scene? $3.45 apiece if you buy 10. Turns composite video or luma into csync and h+v sync at TTL level. By extension, can separate csync input into h+v.
Takes +5-12V supply and dissipates a high 1.1W. Annoyingly needs a 1% 680 kohm Rset resistor with 0.1 uF capacitor to set the "Vertical Default Sync Delay Time" aka 1 / (horizontal sync rate of 15.7 kHz). If you had 25 KHz or 31 kHz sync to clean or separate then you need a smaller resistor and thus have to solder in a replacement.

So popular that a challenger appeared. Renesas really wanted this market since it released the EL1881 drop-in replacement. Has lower supply current to take just 0.4W. Discontinued but can still buy for $4.61 apiece.

Renesas, down but not out, struck back with the ISL59885 that is semi-well known. Both maxtherabbit and Chipnetics use it in their SNES and PlayStation VGA multiouts. Auto-detects and cleans sync on all resolutions without an Rset resistor, takes more practical +3.3-5V supply, dissipates 0.4 W, cleans CVBS or luma as before with same csync and h+v sync TTL outputs. Has additional feature of sending low logic on the HD detector pin if horizontal sync is > 25 kHz. The datasheet shows an optional SD LPF on the sync input that low logic is used to turn off to protect HD high horizontal sync frequencies. Costs more at $5.82 apiece. Price is fine when scene has been content to pay $10 more for a sync stripper chip in their $35 SCART cable. Datasheet lacks the propagation delay, THD and Bode plots as seen with LM1881 though.

tl;dr would change design to use ISL59885 instead

THS7374 by TI
Takes RGBS (and I assume sync over green) or YPbPr input and basically doubles the video voltage levels for a +6 dB gain. Best of the THS chips by making the 6th order LPF have a bypass pin to turn off versus be mandatory. Basic filter knowledge is that chaining more than one filter together significantly pushes the cutoff frequency back and will attenuate into the frequencies you use if low pass. Takes +3-5V supply at 0.35-0.87 W power dissipation. Less than $1 apiece. Some of the key specs to me are -65 dB total harmonic distortion at MHz (more negative is better), channel to channel crosstalk at 1 MHz of -55 dB (more negative is better) and 70 (at 1 MHz) to 78 (at 6 MHz) dB signal to noise ratio (higher is better). Seems sus how datasheet leaves out THD and crosstalk at 6 MHz but at that price what can you do?

Hard to beat that but easy to skim through 4x op amps on a chip datasheets and see they have better signal to noise ratios and crosstalk characteristics. Op amp is itself up to a 2nd order filter that can easily hit same gain of 6 dB. Would want to chain on a video buffer chip and that adds a little noise. Seems worthwhile test for sake of due diligence?

SPECIAL BONUS to Console5's propaganda on their out of stock SNES Mini/Jr and N64 bypass kit for saying their using THS7316 over THS7374 is better since the 7374 is harder to solder. I'm sorry they bought a reel of 7316 before they knew 7374 existed. What's bad about the 7316 for our scene is the filter allows 480/576p through and cannot be turned off, meaning if you send to digital converter you're vulnerable to all the noise at that band on the HDMI without tacking on a second filter that would cut into and soften 240p/480i video / PAL equivalent. Of course they don't tell you that.

In 15 min I found the ADA4417-3 and MAX11500 with filters meant for HD video and one meant for 15 kHz like the THS7374: the ADA4430. Is ONE channel so we need three at 3 x $1.52 = $4.56 and some delay needed on sync for video to catchup. Datasheet shows a much better 90 dB signal to noise ratio, obviously less crosstalk when R, G, and B run through separate chips and half the group delay (much better). None of the chip filters can be turned off.

tl;dr THS7374 is fine but testing may show superior option by specializing in SD or HD video amp chip or using high speed op amp + video buffer chip.

Another thing, you could use 31€ Sync Strike for SCART to VGA. Has same LM1881 chip but no amp and manual versus electrical switch for +5V from screw terminal or SCART pin 8. I realize you don't need but maybe someone in same boat already has one.

[maxtherabbit]

I designed and produced an inline VGA LPF

[maxtherabbit]

tl;dr would change design to use ISL59885 instead

That's what I use in my dongles that require a sync stripper. Costs more than the 1881 tho

[kitty666cats]

WHEW these are some good responses, haha. Fun, informative reading

[NewSchoolBoxer]

Thank you! Kind of a relief to know I'm not the only who cares about these things.

I designed and produced an inline VGA LPF

I didn't know the THS7368 existed. That's pretty cool and useful to have selectable video filter for every analog resolution out there: [480/576i], [480/576p], [720p/1080i/1080p30/1080p24] and [1080p60/1080p50]. I get rows of noise from PS2 480p on my Plasma TV (but not LCD). Last I read on 1080p analog was most every television blocks it on purpose as a smackdown against non-HDCP content. Computer monitor VGA, our salvation.

I have the hardest time finding what I'm looking for on Digi-Key and Mouser but I did find a way to list every sync chip that exists. The priciest ones from TI and Renesas will clean sync over green but PS2 480p is about as niche as it gets.

Here is my untested design to have a primary power supply such as barrel jack or USB with backup on the +5V console pin. I drew in MS Paint for the feels:



The idea is if both power sources are available, the primary works and +5V pin is shutoff. Only if primary is shutoff can the pin supply power.

I looked at pinout sheets of consoles that go beyond Composite video. Jaguar is funky with +9V on a 100 mA limit but the rest do +5V. Maybe no one knows the current limits on console +5V pins but I haven't heard of a hungry LM1881 blowing anything out yet.

Kind of an interesting test for a capacitor to keep the chips powered on against a 50-500 ns switching speed and if you need another 1-2 capacitors to filter out the AC response of what amounts to be two step functions. Turning on and off a switch forms a trapezoid because switching isn't infinitely fast. Both the switching speed and slew rate show up in the Fourier series.

[kitty666cats]

I designed and produced an inline VGA LPF

Would it be relatively easy to implement an optional sync combiner into that design? I don’t know much about circuit design but I would assume “yes”… especially since you’ve already got dipswitches implemented into the design

Definitely think it’s a cool idea, but unfortunately very very niche unless you’ve been stuck waist-deep into the muck and mire of this stupid/wonderful hobby for quite a while, heh

[maxtherabbit]

yes, if you want "ghetto" sync combination using basic combinational logic

for "proper" c-sync you need sequential logic, and there's no room for that on this design

[buttersoft]

yes, if you want "ghetto" sync combination using basic combinational logic

for "proper" c-sync you need sequential logic, and there's no room for that on this design

Can you explain the difference you're talking about, there? I'd love to learn more.

[maxtherabbit]

yes, if you want "ghetto" sync combination using basic combinational logic

for "proper" c-sync you need sequential logic, and there's no room for that on this design

Can you explain the difference you're talking about, there? I'd love to learn more.

I'll let Ste Kulov explain it:

https://www.hdretrovision.com/blog/2018 ... -the-stage

[SuperSpongo]

Been thinking about what sync and conversion products the scene needs the past month.

Off-Topic but one thing that immediately comes to mind is a horizontal and vertical image shifter.
There's Extron Interfaces, but vertical shifting is a bit more rare and in my part of the world Extron products are not that common to begin with.

For horizontal shifting, there's this:
https://playoffline.wordpress.com/cable/rgbshifter/
and this from viletim:
http://members.optusnet.com.au/eviltim/ ... tysync.png

I've never found a DIY circuit for vertical shifting, but some Scart switcher products on Aliexpress have the feature. They have unfortunately since almost doubled in price.

[NewSchoolBoxer]

Editing because I think I wasn't helpful and just adding fluff.

Logic gates useful for sync combining are probably the most simple and therefore cheapest (and shortest propagation delay) chips that exist. An introduction to digital logic will have you make CMOS nand and nor gate using 2 transistors and a resistor. Then you get to use 7400 series chips for easy mode. But yeah, not fitting that onto the device.

Gating to combine sync won't work at 720p/1080i/1080p because then things switch to superior tri-level sync. I'm not certain if 480/576p is always bi-level or not. What I'd do for HD is more much expensive easy mode of Renesas EL4511 chip that accepts every sync on earth and outputs csync, hsync and vsync.

Been thinking about what sync and conversion products the scene needs the past month...

Maybe you saw the shifting thread that has viletim's PCB linked to order and a different finished product on AliExpress.

There is an Extron datasheet that describes horizontal and vertical shifting. Only part I can remember is saying horizontal shift to the left and vertical shift up are impossible because you can't predict the future.
Notice the circuits are only for RGB. Seems easiest format to work with when you have separate sync. Constantly send sync to television to keep "no video" notice from appearing and I think the two designs send artificial black level RGB as the delay. Staying in analog is rough but the horizonal shifter design is extremely cheap to make.

Solution I would think of is using a 1 frame max buffer to shift horizontally. Maybe just 1/4 a frame if only vertical shifting on the presumption no one needs more than that. This forces analog to digital conversion and back again but digital signal processing makes things easier if much more expensive. Maybe you want to match Super Game Boy border instead of just black.

[ryankduong]

I designed and produced an inline VGA LPF

Am I able to purchase one from you? This would help out alot for Datapath capture card users like me!

[Physikant]

Hey, i found an issue in the Voodoo 1 -Signal of my Ms-Dos PC, you can find a video of the issue and some discussion about it here:
https://www.voodooalert.de/board/forum/ ... /&pageNo=1
I know its mostly german, but there is one english post describing the issue in between.

We came to the conclusion that it is just a noise issue that modern devices pick up from the card. One user suggested trying a LPF and linked to this thread.
@maxtherabbit I see that the schematic of your pcb is quite simple and could be redone in a few hours, but would it be okay for you to share your design/layout so that i could make one for my problem and try to solve the issue with your filter? Or do you sell this thing somewhere and try to protect your design? If you are scared of someone monetizing your design, you cold read my posts over at voodooalert or vogons to see that im just a hobbyist myself.

[kitty666cats]

Looks like Bob is re-designing the SCART2DVI, using a VGA connector this time around.

[Physikant]

It's okay. I designed one myself.

[RuffNEC]

Id like to buy a Scart2DVI with a Low passfilter
also interested into Vga2Dvi with a Low passfilter or Vga2Vga like posted in this thread.

can't find anything online any suggestions?

it seems those adapters on eBay don't have a LPF inside....