DIY digital coaxial cable

Discussion in 'DIY Discussion' started by harrygrey382, May 15, 2007.

  1. harrygrey382

    harrygrey382

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    I know there is considerable DIY cable experience on here (no names!)... so I thought I'd ask about making a DIY optical cable.
    What is the difference in the physical requirements between an RCA optical cable and a single channel phono cable? What would happen if I used a single TNT shoestring (which I like as a phono cable)?
     
    Last edited by a moderator: May 15, 2007
    harrygrey382, May 15, 2007
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  2. harrygrey382

    sq225917 Exposer of Foo

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    er, optical cables are made of glass or very high grade optical plastic,they pass light down their length, not electricity.

    so unless you have F/O glass cutters and available fibre optic cable, and the ability to polish the ends to an near mirror polish finish, there's not really much you can do.

    if you work in F/O telecoms as an engineer it's probably no big shakes, but otherwise you'll be missing a few 1000's worth of equipment needed to finsh a cable properly.
     
    sq225917, May 15, 2007
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  3. harrygrey382

    harrygrey382

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    sorry my mistake i meant the coaxial RCA ones. I know a single side of phono cable works, just wondering what's involved making one
     
    Last edited by a moderator: May 15, 2007
    harrygrey382, May 15, 2007
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  4. harrygrey382

    MMike

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    MMike, May 15, 2007
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  5. harrygrey382

    harrygrey382

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    yeah that looks good. However, it's got me thinking, he really has a thing against RCAs. It's not the first time I've heard it either, apparently BNCs are much better.
    But what is the 75ohms a measure of, the cable? The cable plus the female sockets. And surely it would depend on cable length.
    I guess what I'm asking is what needs to be 75ohms and is there a way I can make sure my cable is?
     
    harrygrey382, May 15, 2007
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  6. harrygrey382

    zanash

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    yes .....you fit a buffer resistor to "fool" the reciever chip between dig sig and dig ground.

    The simples way is a twisted pair with said resistor at one end ....

    silver sounds good but copper will do ..a lot depends on how much you want to spend ..

    personally I've not found bnc's to be any better than a good rca ....all things are relative though.

    remember it doesn't need to be coax ....people use this because its cheap to make !

    pm me if you need more details
     
    zanash, May 15, 2007
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  7. harrygrey382

    Tenson Moderator

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    Isn't the 75 ohms measured at a certain AC frequency? Thats why its an impedance, not a resistance. So you need the right measurement gear to be sure your resistor is just right and that it takes in to account the impedance of the cable that already exists – at the relevant frequency. The best thing to do I'd have thought would be to use 75 ohm coax cable as you will find in any DIY shop for connecting your TV aerial.
     
    Tenson, May 15, 2007
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  8. harrygrey382

    felix part-time Horta

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    The idea is that it's 75ohms flat across a wide bandwidth. Hence coax with a characteristic impedance, and a termination that matches that impedance. 75ohms resistors are probably the most accurate bit of that spec, provided they aren't wire-wound types! SMT parts are very good here (as 'RG6' can be for the cable - cheap and effective)

    Phonos, on the other hand, are comparatively hopeless - nothing like 75ohm characteristic impedance, even those that claim they are (they can't be, it's a simple matter of ratio of conductor diameters for coaxial connectors - the RCA spec precludes 75r being possible!). Which doesn't mean they can't work subjectively, of course... but if you have a choice...
     
    felix, May 15, 2007
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  9. harrygrey382

    STELLABAGPUSS Happy Chappy

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    I have DIY both using a high quality Low loss Coax and twisted 0.5mm Silver, IMO the Silver twisted version was better alot better, infact I'm still using it.
    I unsoldered the resistor on mine, as I couldn't detect any difference with it on, this will of course all depend on your CD-P/DAC set-up.
    One things for sure, DIY is certainly cheaper and more fun and rewarding
     
    STELLABAGPUSS, May 15, 2007
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  10. harrygrey382

    felix part-time Horta

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    Oops, that's a good point. If you are making a cable to ue between two existing bits of kit you DONT want to add a resistor in there - the 'termination' is handled internally at the receiver
     
    felix, May 15, 2007
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  11. harrygrey382

    harrygrey382

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    so hang on, adding a resistor with the socket is not good? This'l be going between DAC and computer
     
    harrygrey382, May 15, 2007
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  12. harrygrey382

    felix part-time Horta

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    ...For which a phono-to-phono digital cable requires no resistor, that's correct.
     
    felix, May 15, 2007
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  13. harrygrey382

    rmorris

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    characteristic impedance

    you have to get into wave / rf theory a bit to appreciate the '75 ohm' characteristic impedance thing.
    What it comes down to electrically is that if you terminate a signal with the correct impedance ( set by the source impedance ) then you get no reflections back down the wire ( these will combine with the transmitted signal hence problems if too big ). Cabling and connections should maintain this characteristic impedance as far as practicable.
    There is no 75ohm to be measured on a multimeter etc for a cable or connector.
    Other impedances are commonly used also - eg 50 ohm BNC ; 110 ohm on AES-3 digital audio signals ( although there's nothing 110 ohm related on the pre-existing XLR connector ).
    Voltage level is halved due to equal source and receiver impedances forming potential divider.
    effect depends on frequency vs cable length. In general relevant at digital audio frequencies - more so with increased bit rates - and not applicable to audio frequencies.
     
    rmorris, Dec 21, 2007
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  14. harrygrey382

    Paul Ranson

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    One way to help understand what the 75 Ohms means is to imagine an infinite length of the cable, then connect a resistance meter to it. The meter will read 75 Ohms. If you have a non-infinite length, like most of us, then the meter will read 75 Ohms until the voltage it puts onto the cable has reached the far end, turned around and come back, at which point the meter will read 'open circuit'. In most circumstances this is a really short time. If you 'terminate' the cable with a 75 Ohm resistor then the meter will read 75 Ohms and the cable will appear to be infinitely long. This is what you want to achieve. The termination is inside your DAC, it's not something you generally have to provide*.

    So simply buy 75 Ohm cable. This will be in the specs for any suitable wire.

    *professional video applications frequently require external termination, this allows you to distribute the signal to multiple devices and terminating it just once at the end of the chain. Similarly for old style Ethernet and SCSI.

    Paul
     
    Paul Ranson, Dec 21, 2007
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