Component HD Cables how to measure?

Discussion in 'Beginners, General Questions' started by Peter L, Jul 14, 2004.

  1. Peter L

    Peter L Extra

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    Hi,

    I know that there has been some discussions about the different types of component cables for standard, progressive and HD video and different Mhz bandwidths.

    Is there a device out there that measures the level of mhz of a component cable that an average home theater Enthusiast can buy similiar to a sound level meter to measure the sound distance?

    Thanks,

    Peter
     
  2. Bob McElfresh

    Bob McElfresh Producer

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    I'm afraid you need some equipment that costs a lot more than a $50 SPL meter.

    You need a source that can provide fairly accurate wavelengths from about 1 to 100 Mhz - not too expensive.

    But then you need an oscilloscope or a waveform analysis instrument that can show you how the waveform decreases as the frequency increases. This is not a inexpensive device.

    You basically want something that can give you a graph like this:


    [​IMG]

    And then - you need to know something other than "GOOD/BAD". You need to be able to compare readings from several cables to decide which is best.

    But notice something: that graph is for the coax alone and over 100 ft.

    Your testing results are likely to be affected by the RCA plugs on either end of the coax. And, the readings will likely change if you move the cable from a straight line to a "U" or "S" bend.

    So it's expensive and gets complex really fast. This is also why the AV magazines dont have laboratory tests for cables: there are too many variables (not to mention the potential loss of advertising from whoever comes in last.)
     
  3. Chu Gai

    Chu Gai Lead Actor

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    Well Peter, the sound level meter doesn't measure distance. Instead it measures the sound pressure level from whatever the source is to where it is.

    Aside from that, the bandwidth of a cable is roughly proportional to its gauge or thickness. There are other factors in play here also but if they were somehow held roughly constant, this would apply.

    In the graph that Bob presented above, which plots signal loss vs. frequency for a series of Canare cables (75 ohms), we see that the signal strength of the V-5CFB has been reduced by 3 dB or 1/2 at about 200 MHz. Now the signal coming out of your DVD player can be as high as about 35 MHz if you're outputting 1080 resolution. This means that you could run a considerably longer length of cable, provided your DVD can drive such a length, before your signal lost 1/2 its strength. For the lengths most of us use, a few meters at best, one can choose from a wide variety of cables with different internal diameters and have neglible signal loss. Invariably what signal loss there may be can be compensated for by adjusting the controls on the TV.
     
  4. Allan Jayne

    Allan Jayne Cinematographer

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    THe material used to keep the center conductor centered within the braid conductor has a great deal to do with the bandwidth of the cable. It is almost impossible to tell the bandwidth by looking at the outside, the few instances where you can get a good guess is when the cable has exposed labeling markings such as RG6 stamped here and there along the length on the outside.

    If you had a good horizontal resolution test pattern you can detect degradation due to cable bandwidth limitations when comparing one cable against another. Unfortunately I don't know of any easily obtainable HDTV patterns that push the limits of 1080i or 720p.

    Video hints:
    http://members.aol.com/ajaynejr/whyten.htm
     
  5. Peter L

    Peter L Extra

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    Thank you to all who replied. I guess the reason for my post is wanting to know more about good component cables. I've heard lots of great things about monster cables but they are very expensive. I also read from other forums to go with custom made cables. How do I know if I am getting high quality cables aside from those fancy stickers and labels on the packaging?

    Peter
     
  6. Chu Gai

    Chu Gai Lead Actor

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    Well all that tells you is that you've got fancy stickers and labels. Packaging and presentation are wonderful ways to create or further the impression of quality and allow a vendor the ability to charge more. Simply taking the same video cable, changing its color and using a different looking RCA connector, and now calling it a digital cable allows for a signficant markup.

    To get a quality cable depends on many factors. I'll just choose two: Define your requirements and construction.

    One needs to define where the cable will be used and this will define the specifications or what to look for. In your case, it seems you're looking for component video cables. So naturally, you want a cable that will pass signals from your DVD player over to your TV or receiver with neglible signal loss. That in turn is related to the length needed and whether you're outputting regular DVD or 480 or 1080. For most of us, we've got a run of maybe 1-4 meters and we can easily get by with a 75 ohm video cable that's got a center conductor that is 25 gauge or so. While a larger center conductor (RG59 which is about 20 gauge or RG6 which is about 18 gauge) is technically better it will not necessarily result in a better picture.

    The method of construction, which I take to be the way the plug is attached to the cable varies depending upon whether you're buying custom made or mass produced. In the former case, the connections are usually made by hand and are often crimped but sometimes you'll also see compression fittings or soldering. In the latter case assembly is done by machine which may involve crimping with ultrasonic welding. Places like Radio Shack, Monster, and I'd imagine most other machine made cables that have a logo attached to them (RCA, Magnavox, Phillips, etc.) perform quality control tests at the factory such as pull tests where a certain weight is repeatedly placed on the cable to determine the strength of the connection. This simulates a consumer yanking on a cable instead of gripping the RCA connector and twisting it off. I seriously doubt such tests are performed by custom places but its my guess that the ones who have been in business for some time such as the forum sponsors or say Markeertek or AVCable make competent connections.

    A large part of buying cables is quite frankly snob appeal or technical oneupmanship. Many feel that it's just tacky to have a Radio Shack cable or something you bought from Walmart or Target. They can point to their solid silver conductors with foamed Teflon dielectrics and their Rhodium plated connectors as some sort of mark of class of distinction. The mark of the audiophile if you will. Others will do that one better and say yes, but my cables have arrows that indicate which way the wire was drawn. Damn, Audiophile group #1 got beat. Then someone says, well that's fine, but mine were cryogenically treated. Damn Audiphile group #2 got beat. Then someone says well I've got all that but mine have micro welded RCA's. Oops, Audiophile group #3 is now out of the running. Then someone says, well mine are just made by RCA but Pam Anderson came by to install them, she wasn't wearing any underwear and she did to me what she did to Tommy Lee in that video. Sounds good to me.

    Look, define what you need. After that it really becomes a matter of personal preference and then spend what you need to in order to meet those wants.
     
  7. Vince Maskeeper

    Vince Maskeeper Producer

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    I can only speak for myself, but I do extensive testing on each item I make to insure crimp strength, signal flow and proper impedance. Although i would still (and do still) recommend proper handling of cables (including plugging/unplugging via the gripping the plug)-- I am confident that the crimps will take some pretty serious abuse.
     
  8. Chu Gai

    Chu Gai Lead Actor

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    I understand where you're coming from Vince. What I was getting at goes along the lines of statistical quality control where random production samples are taken and analyzed in a standardized fashion to see if they conform to certain standards. Since you're doing something along those lines, you might want to keep records and then figure out a way to use them in advertising your product. If you have any questions, drop me a PM.
     
  9. Leo Kerr

    Leo Kerr Screenwriter

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    One of the easiest ways to know what sort of quality cable you're getting yourself into is fairly simple.

    Basically, the safest fashion is to buy cables where the 'manufacturer' doesn't strip off the real manufacturers' cable branding.. like the "Belden 1672A Digital Brilliance RG-6 55028' .... Belden 1672A Digital Brilliance RG-6 55029' " or Canare, or West Penn, or...

    I've known people who've had good luck in communicating with the engineers at Belden...

    Leo Kerr
     
  10. Bob McElfresh

    Bob McElfresh Producer

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    Actually, the stickers are a GOOD way to tell. But you have to under stand some things:

    "..made for todays high-performance AV systems"

    This does not mean the cable is High Definition capable. It is marketing-speak to fool you into buying Component cables, but thinking you are getting high-bandwidth HD cables.

    If the package only says "Component Video" - the contents only has to be compatible with the 1940's interlace video standard.

    Look for labels that say things like "90 Mhz or 100 Mhz bandwidth" or "High Definition Capable". If it says other things - ignore it.

    Another way: look at the frequency response chart. Learn what it means. Buy cables built for your application.

    Here is what an engineer would do: he would pick the max frequency he was going to send through a cable. Multiply this by 3 or 4, then look for a coax that at 100 ft, has no more than 50% drop in signal. (50% drop is -3 db - you will often hear people talk about the -3db point for this reason).

    For 1080 HD video - the max frequency is about 35 Mhz.

    3X - 105 Mhz
    4X - 140 Mhz

    So look at the frequency chart above. See the "3" line? This means the signal strength has dropped by 50%. See the 3 different coax's? Each one has a bit different response. But the all qualify as "HD Compatible" using the 3X rule and 2 of them qualify using the 4X rule.

    Hope this helps.
     

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