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  #1  
06-06-2022, 09:18 AM
Hushpower Hushpower is offline
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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!
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  #2  
06-06-2022, 09:28 AM
lollo2 lollo2 is offline
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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

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  #3  
06-06-2022, 06:45 PM
dpalomaki dpalomaki is offline
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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.
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  #4  
06-07-2022, 01:37 AM
Hushpower Hushpower is offline
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Thanks Lollo and Dpalomaki, much appreciated.
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  #5  
06-07-2022, 03:51 AM
lollo2 lollo2 is offline
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Quote:
Below is a link to a calculator for T-pads.
https://www.allaboutcircuits.com/too...or-calculator/
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.

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  #6  
06-07-2022, 05:46 AM
dpalomaki dpalomaki is offline
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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%

Last edited by dpalomaki; 06-07-2022 at 06:04 AM.
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  #7  
06-07-2022, 08:06 AM
lollo2 lollo2 is offline
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Agree. There is a K factor not needed

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  #8  
06-07-2022, 12:09 PM
Bogilein Bogilein is offline
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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.
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  #9  
06-07-2022, 12:52 PM
lollo2 lollo2 is offline
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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

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  #10  
06-08-2022, 12:24 AM
Hushpower Hushpower is offline
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Thanks for the further info everyone.
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  #11  
07-04-2022, 09:26 AM
Hushpower Hushpower is offline
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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.
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  #12  
07-04-2022, 09:48 AM
RobustReviews RobustReviews is online now
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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.
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  #13  
07-04-2022, 09:52 AM
dpalomaki dpalomaki is offline
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Quote:
Originally Posted by Hushpower View Post
... 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.
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.
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07-04-2022, 10:04 AM
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Quote:
Originally Posted by dpalomaki View Post
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.
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  #15  
07-04-2022, 10:43 AM
dpalomaki dpalomaki is offline
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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.
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  #16  
07-04-2022, 10:59 AM
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Quote:
Originally Posted by dpalomaki View Post
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.
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.
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  #17  
07-05-2022, 08:37 AM
Hushpower Hushpower is offline
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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.


Attached Files
File Type: avi LG990 Comp ES15 GV Levelled 7sec.avi (92.45 MB, 4 downloads)
File Type: avi LG990 Comp Trimpot down ES15 GV levels default 8sec.avi (94.85 MB, 5 downloads)
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  #18  
07-05-2022, 11:16 AM
lollo2 lollo2 is offline
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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
1.jpg

trimpot
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  #19  
07-05-2022, 09:58 PM
Hushpower Hushpower is offline
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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):
Trimpot_900.jpg

GV Procamp at 84 (was 88):
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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.


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  #20  
07-06-2022, 02:10 AM
Hushpower Hushpower is offline
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I've settled on this 100ohm pot:

100ohm pot.jpg

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?


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