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  #21  
07-26-2019, 02:12 PM
dpalomaki dpalomaki is offline
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Some leisure reading for a start. (There is significant math involved.)
https://en.wikipedia.org/wiki/Skin_effect
https://en.wikipedia.org/wiki/Telegrapher%27s_equations
https://en.wikipedia.org/wiki/Reflec...nducting_lines
https://en.wikipedia.org/wiki/Oxygen-free_copper
Beyond the above overviews you can delve into the chemistry, physics and related engineering (material science) if you wish more depth.

Note that frequency and wavelength are related by the speed of the electrical signal through the media. For air this is essentially the speed of light in a vacuum (~3x10^8 m/s). For wire/cable it is less; e.g., typically 79% of the speed of light for RG-59/59 coax, different speeds for different cables).
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  #22  
07-26-2019, 04:53 PM
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Quote:
Originally Posted by Koreth View Post
I feel like I'm derailing this thread.
Not a derail. Carry on.

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  #23  
07-26-2019, 06:37 PM
Koreth Koreth is offline
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Okay, you've got me reading and learning things, so thank you. You're right, there is some non-trivial math involved in this. Even though some of the math is over my head, I think I'm starting to have fun.

I did some more reading about transmission lines. You're right. Skin effect, characteristic impedance, reflections, it's all important. It's what I was intuiting and alluding to in my prior statement:
Quote:
Originally Posted by Koreth View Post
Every single cable in existence does this, because physics.
However, reading up more on transmission lines, there's a caveat here, the characteristic impedance of a transmisison line, and its consequent effects (reflections, etc) only matter when the transmission line is longer than the 1/4 of the wavelength of the highest frequency of concern.[1]

Recall that our highest frequency of concern is 6.75 MHz[2]. The formula for wavelength is λ = v/f, where λ is the wavelength, v is the velocity and f is the frequency. Your reference to the speed of light in coax was another clue I needed, apparently electricity in a cable does not propagate at the same speed as light in a vacuum, it can be a good deal less, depending on the properties of the cable. The Wikipedia article on Velocity Factor[3] contains some examples.

Assuming the speed of light in a vacuum, the quarter wavelength of 6.75MHz is 11.1 meters, or 36.4 feet. However, as dpalomaki correctly mentions, our signals are not propagating at the speed of light, but at some fraction thereof. Per [2], data cables can have a velocity factor of as little as 42%. To get an idea of a worse case, let's assume we have cheap, low quality cable with a velocity factor of 40% of the speed of light. 40% of 299792458 m/s is about 119916983. Plugging that into our wavelength formula above, we get a 1/4 wavelength of 4.44 meters, or about 14.5 feet.

Shorter than this distance, it will be the interaction between the the input of the capture card and the output on the S-VHS deck that dominates the quality of the signal your capture card, well, captures. This assumes that there are no appreciable transmission losses, or additional noise coupled in caused by the cable itself.

So as long as our hypothetical cheap cable is shorter than 14.5 feet, has losses less than 1 dB at 6.75Mhz, and doesn't let more than 1 dB of noise to get into the signal, and doesn't have any other problems like breaks in the insulation or shorts between conductors, than this cable is good enough, electrically. It could have other problems, like flimsy connectors at both ends that are going to break the next time you look at them wrong, but for the goal of getting the analog signal from your S-VHS deck into your capture card, you are not going to be able to tell the difference between it and a fancier, more expensive one with a better bandwidth or velocity factor.

I would be willing to pay money to someone who could prove otherwise in a properly done, double-blind ABX test.

Now, that doesn't mean all cheap cables are good enough, nor all expensive ones wasted money. More expensive cables should (hopefully) have quality connectors that are going to survive being connected. And note that 14.5 feet isn't so crazy a distance for a cable. Most consumer video electronics I've worked with have come with cables that range from 3 to 6 feet long, and honestly most of them aren't constructed like ideal transmission lines. Well, that's a reasonable distance for the most common scenario of the Xbox or DVD player being placed on a shelf just below the TV. At that short of a distance, you don't have to necessarily worry about characteristic impedance as long as you're not allowing noise to leak in or causing appreciable signal losses. However, if you need to put your capture computer on the other side of the room as the S-VHS deck, because you live in a small apartment and have to play Tetris with your furniture layout thus, you could find yourself needing a cable longer than 14.5 feet pretty quick.

Well, 14.5' was an estimated worst case, with a cheap, crappy cable. Playing with the numbers from my "definitely good enough" candidates of Belden 7700A, 1807A, or 1808A, we get a lengths of 28.4' (They all list a velocity factor of 78%). That's certainly a lot better, but a still a distance you could conceivably run up against in a practical situation (maybe the S-VHS deck has to sit in the next room). Fortunately, the characteristic impedance of all three of those is 75 Ohms, which happens to be the common standard source and input impedance for input/output ports for video signals.

So you could take an S-video cable made from Belden 7700A, 1807A or 1808A, terminated well to high-quality min-din 4 connectors on either end, and that cable could be almost 100' long before you had to start worrying. If you need your S-VHS deck to be more than 100' away from your capture card, I think the easier and cheaper thing to do would be to find a way to move them closer together, rather than worrying about cable lengths.

Now all we need to do is find some quality mini-din 4 connectors. Terminating the cable to the connector will still be a crapshoot, as not everyone can or cares to learn how to solder well, but there's nothing I can do about that.

1. https://www.allaboutcircuits.com/tex...mission-lines/
2. http://www.digitalfaq.com/forum/vide...html#post62771
3. https://en.wikipedia.org/wiki/Velocity_factor
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  #24  
07-26-2019, 06:55 PM
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Just an an interjectory reminder, the cables that came with DataVideo TBCs and JVC S-VHS VCRs are what I use almost exclusively.

- The JVC have worn out in time, especially seeing as most of the came from the late 90s and early 00s.
- But the DataVideo have not. However, some DataVideo were just bad right away, tossed aside.
- Other branded cables have been all over the place, with the main issue being visual signal noise, mostly on runs longer than 3 ft.

I've never really found a digestable/understandable reason for this, so I'm reading your conversation here.

Carry on...

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  #25  
07-26-2019, 08:28 PM
dpalomaki dpalomaki is offline
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Some potential sources of noise with cables:

- Poor shielding allowing stray signals to enter.
- Cold solder joints/connections that might rectify RF that is picked up by the cable.
- Weak connections that generate static as there is microscopic relative movement of contact points, perhaps even microphonics induced by ambient sound, building vibration, or mechanism vibration.
- Ground loops
- failure to have a low impedance common ground for all connected gear.
- fields from nearby power lines, light dimmers, switching power supplies, etc.

https://incompliancemag.com/article/...out-shielding/
provides some interesting reading on cable shielding.

Lots of factors come into play, and a lot depends on the specifics of the installation at hand. Bottom line is go with what works for you. Pricey Monster cables and their ilk do come with bragging rights .
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  #26  
07-28-2019, 01:36 AM
Koreth Koreth is offline
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I suspect the reason you're seeing noise in some cables that are longer than 3', is the top reason that dpalomaki lists there: poor shielding.

Per the pinout of S-video the pins are Y, Y Ground, C, C ground. Both the Y line and the C line are supposed to be individually shielded.

I suspect no small amount of S-video cables are made wrongly. I've also seen the pinout listed as Y+, Y-, C+, C-, which would imply that the Y and C lines are differential instead of single-ended. Someone could see this and think that he can use 2-pair unsheilded twisted pair to make an s-video cable. Unsheilded twisted pair does have noise rejection properties, when the transmitting and receiving ends are wired to use it as a differential transmission line. In such a case, the noise is common mode, and gets cancelled out, as the receiver receives the difference between the voltage on both wires. If you've ever worked in audio and dealt with XLR cables on microphones or other studio gear, those work on the same principle. However, since the Y line and C line are each supposed to have their own grounds, both are supposed to be shielded, twisted-pair isn't going to do much for rejecting noise when the input it's connected to was designed assuming the line would be shielded.

There's another way a cable could end up poorly shielded, and that's by cheaping out on the on materials. Instead of using a pair of shielded wires, one might end up using simple 4-wire cable. Both shielded and unshielded variants of such exist. In an unshielded setup, noise is gonna couple into that line pretty quick. But, even in a shielded 4-conductor cable, since the Y and C lines aren't shielded from each other, you could easily get cross-talk between the two. That'd be the Y signal coupling into the C signal as noise, and the C signal coupling into the Y signal as noise.

I kinda want to take apart a bunch of S-video cables now to see what they do right and what they do wrong.

Another crazy thought: maybe I should pick up a spool (or long length) of 7700A, and make a few cables of varying lengths for people to test.
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  #27  
07-28-2019, 06:37 AM
LightWorker01 LightWorker01 is offline
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I will agree with lordsmurfs and other posts regarding this. For S-Video, expensive cables are a waste of money. I find the same with things like HDMI cables. I wouldn't buy the cheapest 99p cables either, mostly for durability issues as opposed to functional issues with composite as an exception.

I paid £2 for my S-Video cable and £2.50 for my component ones, and they feel thick and sturdy and has given me no problems. With S-Video, you are not as likely to have problems as composite, as both Y/C is not mixed together on the same cable.

I would not worry, just use what you can, and only maybe buy one a couple of quid more if you need to, but no need to buy super expensive cables.

I know Sony sold these PS2 RGB scart cables, that people buy on eBay for £100+. I found some at a boot sale and flogged them, I use a £3 RGB cable that does the same job with no difference. Not sure if they buy it for collector's value? I was more than happy to sell it at that price if they wanted it at that price, but I did laugh my head off.

Save your pennies and put it towards better capture equipment, though for obvious reasons avoid the crazy cheap 50 pence cables, mostly cause they will break on you quite quickly.
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  #28  
07-30-2019, 03:22 PM
dpalomaki dpalomaki is offline
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Quote:
...differential transmission line...
AKA: Balanced or double-ended. Much of the data networking cable is differential/balanced (e.g., Ethernet) as well as HDMI, USB, Firewire, RS422, (but not RS232), SATA, DVI-D, the old TV 300 ohm twin lead and 450 ohm ladder line to name a few. And of course professional audio connections.

There is a wide variety of shielding on coax cables: foil, foil with a drain wire, braid (with varying degrees of coverage), spiral strands, and combinations (e.g., foil with braid). The low cost s-video cables I've cut into used spiral strand shield around the Y and the C hot lead, the shields being the ground for Y and C. The slightly premium NGX Sapphire s-video cable that has a failed connector used a combination of foil and spiral wrapped strands around 4 wires (red & black hots and white grounds for Y and C). Decent shielding from outside field, but probably not between Y and C. At least it is a pretty color

The 300 ohn twin lead has much lower losses than coax at TV broadcast frequencies, but suffered from other issues.
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  #29  
12-29-2019, 11:43 AM
Vakicious Vakicious is offline
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Has anyone found a good cable that uses Belden 7700A etc. cable? I just bought a good capture card (AIW 7700), TBC, and VCR, and now Iím trying to get good cables.
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  #30  
12-31-2019, 09:23 AM
sanlyn sanlyn is offline
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These old-fashioned plain vanilla 75-ohm svideo's at https://www.amazon.com/BLUECELL-Gold...7805330&sr=8-4 are probably as good as you'll get these days. I've used these and similar for several years. Meanwhile the Belden line isn't what it used to be: I find their latest svideos to be blurry and low-contrast. The worst are overpriced Monster Cable, nothing more than cheap 50ohm stranded core hookup wire with a lot of chroma noise. You might still be able to find a decent set of Acoustic Research Pro II s-video wires around.

Most people claim that the 99-cent cables they use look like all the others. Since they haven't used anything else, I wouldn't take their word for it.
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  #31  
12-31-2019, 12:28 PM
latreche34 latreche34 is offline
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The cables that once were selling for over $75 are for less than $10 now thanks to the internet:
https://www.ebay.com/sch/i.html?_fro...call_filtering
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  #32  
12-31-2019, 03:07 PM
Vakicious Vakicious is offline
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I see two different models for Accoustic Research II on Amazon. Which is the right one, PR-129 or PR-121?
https://smile.amazon.com/Acoustic-Re...s%2C160&sr=8-1
https://smile.amazon.com/Acoustic-Re...s%2C160&sr=8-4
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  #33  
12-31-2019, 04:27 PM
bigkazzyry bigkazzyry is offline
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I too was searching for some solid cables since pearstone has discontinued all cables under 10 feet long. I came across these and ordered a bunch of them so I can wire one of my TBC-4000's up to all of my capture VCR's so I no longer have to unplug cables to use different devices.

https://www.cablesforless.com/cables...o-svhs-cables/

These cables are on closeout so I bought a bunch. I got 10 cables (6 x 3' and 4 x 6') for under $20 and that includes shipping. The cables look decent but time will tell. Mine are supposed to arrive on Friday so I'll report back if they end up being junk. The cables I purchased are the closeout performance cables that are blue in color.
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  #34  
12-31-2019, 06:54 PM
sanlyn sanlyn is offline
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Quote:
Originally Posted by Vakicious View Post
I see two different models for Accoustic Research II on Amazon. Which is the right one, PR-129 or PR-121?
https://smile.amazon.com/Acoustic-Re...s%2C160&sr=8-1
https://smile.amazon.com/Acoustic-Re...s%2C160&sr=8-4

P-129 is a camcorder cable. PR-121 is has standard s-video connectors.
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  #35  
12-31-2019, 07:32 PM
Vakicious Vakicious is offline
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Quote:
Originally Posted by sanlyn View Post
P-129 is a camcorder cable. PR-121 is has standard s-video connectors.
Are the 121 cables the ones people are talking about when they talk about getting Accoustic Research II if you can find them?
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