What ohms value for potentiometer for ES15 brightness limiting?
 

I am considering making up a potentiometer to put into the Composite video line to lower the brightness from my ES-15. I currently use a rheostat box designed for an audio line-level cable but it's really touchy and very sensitive.

I understand that a proper potentiometer would be better for this sort of thing but I don't know what "size"/ohms to get.

The options at my local electronics shop are:

https://www.jaycar.com.au/spend-30-o...ularity-desc&q

I would be grateful if someone could suggest which one I should get.

Thanks!

Here they talk about some values (1x 8KOhm for composite, and 2x 8KOhm for Y/C): https://gleitz-info.translate.goog/f...=no#post461112

Best is probably to ask in the German forum or wait until Bogilein replies here ;)

If you wish to attenuate the composite video signal (both signal and sync equally) the better approach is to use a T-pad configuration that preserves the impedance seen by both the gear. Components used should be able to accurately handle signals beyond 6 MHz. for VHS. The normal impedance for video I/O and cable is 75 ohms.

Below is a link to a calculator for T-pads.
https://www.allaboutcircuits.com/tools/t-pad-attenuator-calculator/

You can find pre-made as well but generally not low cost.

Thanks Lollo and Dpalomaki, much appreciated.

From that tool, a 3dB attenuation (1.4 linear reduction for voltage signal), gives R1=8.5KOhm and R2=142Ohm.
In line with what said in the germal forum, but I suspect 1.4 reduction is too much for the need to "slightly" reduce brightness.

The values you posted below look way too high for a 75 ohm impedance T-pad to be used for a for a standard video signal.

3 dB would be more like 12.8 and 213 ohms
2 dB (1.25 att.)about 8.6 and 323 ohms
1 dB (1.12 att.) about 4.3 and 650 ohms.

As noted in other discussions, any downstream AGC that keys in sync pulse or color burst height would tend to undo the attenuation.

Note that the reason for a T-Pad is to ensure impedance match which should help minimize the possibility of reflections/standing waves at the high frequencies that can hurt image quality. If connections are very short you might get away with a simple resistor in series with the signal; e.g., 10 ohms would drop the signal by about 8%

Agree. There is a K factor not needed :)

Unfortunately, I can't help much here because I am not an electrician.

I played around with the video processors Hama AV-128 & Vivanco 3066 as well as with 2 Kramer Video Distribution Amplifiers to prevent the clipping.

I had to attenuate the signal by about -2.4 db so that the AGC of the Panasonic DMR does not readjust anymore.

If your cassettes are not in bad condition I would try it with a Sony/Pioneer (from 2007-2010) DVD recorder if you can find one cheap in your area.

Thanks Bogilein.

Given the 2.4dB attenuation (I do not know why, but I was expecting a much lower value), the calculator provided by dpalomaki gives 10.30Ohm / 268Ohm

Thanks for the further info everyone.

Restarting this project. I've found these little fellas which will cover the range Lollo has come up with in the post above:

https://www.jaycar.com.au/500-ohm-pi...impot/p/RT4352

My question now is: do I have to put one of these in each "line" of the Composite cable ie the centre wire, and the shield as well? I don't know what travels down the two internal wires of a Composite video cable.

A lot of important advice here, yes it's critical to preserve the characteristic Z=75ohm or lots and lots of issues will make themselves very apparent. Ghosting and reflection will become a huge issue.

Don't worry about the shield, consider that 0V, and terminate that to ground. The 'arms' of the 'T' should be in series with the centre conductor.

If you bring the shield in, essentially everything will pass through two terminators (what goes in, must come out) you will effectively double the attenuation.

In general the shield side of consumer outputs (e.g., RCA connectors) is system/signal ground and should have no added resistance in it. The attenuation should only be in the center (hot) lead.

Note that the reason for impedance matching of input to output is to preserve expected signal levels. Important where the system is expecting a specific signal level, or where one is attempting to maximize power transfer to a load. In the case of radio frequency signals it also reduces reflection in the line that may cause "ghosts" in the signal. When adding attenuation to a videos signal it is a good idea to monitor the signal wave form to ensure proper levels including sync.

I think we posted at the same time!

Reflections can be very bad for equipment too if they're gross and/or continual (I know you know this dpalomaki, I'm posting for a general audience). I know I mention it a lot but I think a lot of issues with distribution amplifiers and similar equipment are probably traced to chained amplifiers continually trying to transfer to ∞Ω where the circuits are desgined for 75Ω.

A side note, but I'm a firm believer in terminating anything that isn't in use to prevent interference and equipment damage.

Interesting that some systems, such as the AG-1980 achieves a 75 ohm output impedance by having a single amplifier with a very low output impedance connected to each of the two two composite output jacks through a 75 ohm resistor. The low impedance of the amp effectively isolates the outputs for many small signal issues.

Happily consumer VCR level video is generally a small signal, very low poser application so impedance mismatches are not likely to damage gear (just the image). However, gear failures and user errors (e.g., connecting mains to video/audio terminals) can be dramatic.:smack:

Ah yes you're entirely right, I'm not talking about domestic video-machines, more additional equipment, professional gear, dist. amps etc.

Obviously the domestic stuff is designed without termination in mind, it's good that you clarified.

Got myself organised on this, but not sure if it is all worth the effort (note I'm using Composite only at this stage). I used a 200ohm trimpot (500 was too sensitive) and soldered it in to the centre core of my Composite cable.

The first video is with the ES15 in the flow (no trimpot), but with the GV-USB2's contrast level adjusted down to bring the right edge of the histogram within the 235 limit. The detail in the whites eg clouds became visible.

The second video is with the GV-USB's contrast set at default. I used the trimpot (between the VCR and the ES15) to bring the contrast down to fit into the histogram. It was quite easy to finely adjust the contrast with the trimpot-around 30° of rotation). I did notice that during initial backing-off of the contrast, the level would move back up against the 235 point. Then, I'd back the trimpot off even more and the level would reduce and stay down. I guess that would have been the ES15 AGC driving the contrast up, until I overpowered it?

Anyway, it seems to me that I can achieve the same picture, visually, with the contrast proc-amp in the GV-USB2. There doesn't seem to be much point in the trimpot.

Thanks Hushpower, very interesting experiment.

There is a difference in the height and the distributions of the levels between the two methods.

When comparing on a frame second is confirmed to look "better", but maybe you can equalize a bit more: https://imgsli.com/MTE1MzY0

procamp
Attachment 15382

trimpot
Attachment 15383

Thanks Lollo, yes, I see the difference, especially in the clouds. I've done another capture without the trimpot. I had to wind the GV Procamp contrast way below what I would normally use as a limit. It was well inside the 235 point on the VDub histogram.

Here's the result. Very similar but the 3 shirts on the left still favour the trimpot capture, I think.

Trimpot (as before):
Attachment 15393

GV Procamp at 84 (was 88):
Attachment 15394

A question: what are your thoughts on the overall higher level/shape of the trimpot histogram verses the GV procamp histogram? Better or worse? For example, the GV histo has that huge spike right at the end but is much flatter in the middle.

I've settled on this 100ohm pot:

Attachment 15395

https://www.altronics.com.au/p/r2156...ire-wound-pot/

It has a nice wide range for fine adjustment of the contrast.

I'll be able to put this in a little box with RCA in/out connectors on each end. I'll now look for S-video splitters.

Will I only need to run the luma (I think it's called) through this gizmo?

I look forward to seeing how this works. :)

Those ES10/15 type units are not TBCs, with multiple undesirable affects on video, so the ability to nuke one of the problems will be a win.

I'm now going to put a S-video socket on each end of the box. Could somebody please tell me which wires hold the luma signal?

Attachment 15396

https://www.jaycar.com.au/4-pin-mini...ocket/p/PS0376

Thanks!

Edit: Found it. https://www.leadsdirect.co.uk/techni...-video-wiring/

I do not think that higher levels means better. The "quality" is more related to the distribution and absence of holes.

That can be a problem, if the GV-USB is clipping the whites before the signal reaches the internal procamp, and then are lost and you cannot do anything. But the capture range of the GV-USB is close to 252, so there should be no loss in principle. BTW, you should allow the card to capture up to its higher limit, and not limit the range to 235.

I share your impression that the trimpot methodology is somehow "better". Waiting also comments fro Bogilein and hodgey, they are familiar with the ES-10/ES-15 levels ;)

Made up my combo Composite/S-Video trim pot box. I'm glad I don't solder S-video sockets for a living! :eek: Took me hours. The 100ohm pot works well, with lots of rotational movement for small amounts of signal reduction. Certainly easier to set the contrast than using the proc amps of the capture sticks.

I'll do some interference tests next. Watching the AGC in my GV-USB2 working is fascinating. It really fights when the signal is strong but eventually gives up as I reduce the signal.

Attachment 15418

The Panasonic AGC boosting but then giving up below some level is what I've noted as well, and that's been notet on those german forums too. You don't want to lower it too much either though of course so need to locate the sweet spot where it's lowered enough to avoid clipping but not so much that the panasonic gets issues with signal locking.

You sure this isn't the ES15 clipping? It is how it's looked like when I've gotten clipping on the ES10 and EH57 and used the analog outputs atleast (With HDMI out from the EH57 it goes up to 255 but image content can go above and clip without level limiting on the input of some sort). Or does lowering the GV-USB procamp even more help?

Looks like a smart little project. It should really use coax (goes back to tedious impedance control discussions) but if this works, it works brother.

I haven't the schematic to hand for the Panasonic AGC but I can well imagine it could be something as simple as Zener-clamped, meaning if the level drops below the VZ it could well just bypass the AGC for stability reasons.

I don't have the circuit in front of me though, but you're right if you run the level too low you could well cause various nagging issues with stability.

I'm looking at a tiny picture, but I guess you've got the Y signal of the S-Video bypassing everything in the box?

Well done on building it though, the soldering from what I can make out looks absolutely fine. If you wanted a bit of a extra credit you could have gone for some heat-shrink etc (or a dab of dielectric grease even) on the open solder joints but that's only finesse stuff, this still looks like a rather neat little project.

I'm very much in the 'biggest tip, smallest solder' camp for things like this, if you're fighting the soldering switching to a nice chisel tip and 0.6mm might make things a bit easier. It's a personal thing though, there are no one-size-fits-all answers to how people choose to solder hobby projects. Also, I don't know if you used it, but some good old fashioned rosin flux really makes connections of things like wires to tabs much easier.

No, just my guess; I trust more your experience ;)

Maybe HushPower can experiment that

Yes, straight from In to Out across the top.

I did consider buying S-Video splitter cables so only the Luma would be in the box going through the trimpot (like that PCB on the Gleitz forum) but that was going to cost me $60 Aussie! Better to spend 6 hours of fiddling...:smack:

The main hassle, apart from the stiffish wiring I used (probably a bit too high-gauged, with hindsight) was actually seeing everything. I need a pair of magnifying glasses (those ones on FB are a WOFTAM, BTW).

Re the levels, when doing some more tests with a different tape today, I couldn't get the contrast to bounce as much, either direct to the GV or via the ES-15. Maybe it was that awful tape or used initially.