I am thinking of upgrading my 45' coax digital cable... (1 Viewer)

[Mark Rich]

Interesting stuff!

 

[Bob McElfresh]

brucek: I always thought the rule was to use a cable that could handle TWICE your maximum frequency (using the 15 db drop for 100 feet, even if you are not running 100 feet).

So Video (6 Mhz max) needs a 12 Mhz cable

So HDTV (35 Mhz max) needs a 70 Mhz cable

But my question is this: there has been some debait over the various models of Belden vs Canare coax, some tested to 2 Ghz, another to 3 Ghz. Since we are already following the x2 rule, what does all that extra capacity give us? Warm Fuzzy at the overhead?

 

[brucek]

Bob,
I guess our talking about video is a little off Johns original question (which I think we answered) and I guess we're not allowed to "debate" cable issues, but anyway I certainly agree with you.
I don't think I'd even consider any loss in the video signal at these short lengths and low frequencies. I suggest the Belden cable would be as good as the Canare cable. These frequencies are a cake walk for both these cables (i.e 1694a vs L-5CFB). Are they both 75 ohm characteristic impedance? - yes. Are they both 100% shield coverage? - yes. Do they both have a solid copper core? - yes. Do they both have low capacitance per foot? - yes. Do they both have excellent DCR per 100 ft? - yes. I guess that's about it eh? In fact most RG-59 or RG-6 will pass muster when it comes to passing baseband video and especially when passing digital signals at these short distances of < 15 meters.....
I don't know, perhaps the concern isn't loss, but reflections. Given a typical velocity factor of this coax and a worst case HDTV 35Mhz signal, its wavelength of about 7 meters would be considered a function of the 50 feet in question. This says we should use a proper 75 ohm cable with proper 75 ohm terminations. I don't think the cable is an issue.
brucek

 

[Chu Gai]

now it'd be rather interesting to see identical cables made (keeping length constant) and examine various terminations to see what effect, if any they make...even the so called 75 ohm connectors...is anyone aware of any studies in this area?

 

[Bob McElfresh]

Chu - "Secrets of Home Theater and Hi Fi" did their famous comparison of Progressive Scan DVD players. They used some expensive video-analysis equipment and found that several store-bought cables produced different readings from a "reference" source. Some even changed values if they touched the RCA plug.
In the end, they wired up their own cables with the Canare plugs and when they discovered it made the readings fairly consistant, they used that one cable for all the tests to retain fairness. (I dont recall that they mentioned what brand the coax was, just the connectors.)
I like the idea of your test. The problem is AFAIK Canare is the only manufacturer that promises 75 ohms for it's RCA plugs. I dont see any other manufacturer (Neutrik, Switchcraft, Amphenol, Kings, etc.) making that claim.
The high-precision of the Canare plugs come at a cost of $2.80/plug. The others sell for $0.69, $1.17 - so I'm not sure it would be a fair comparison.
My guess is that you could take a handfull of any one of the other plugs and using the same coax, get different readings from each cable. They are just not built with the same unit-to-unit precision and materials that Canare promises.
But it would be a fun afternoon if we could come up with a way to "test" each cable.

 

[Chu Gai]

actually Bob, its not a true 75 ohm rca as geometrical considerations preclude that from happening...this is actually Canare taking a bit of marketing liberty. As what they actually do is run a sweep test to 200 MHz for the RCAP series and specify it'll have a VSWR of 1.1 or less. And yes a test would be quite interesting.