DIGITAL CABLES- Technology

Discussion in 'Digital Sources' started by Negotiableterms, Mar 8, 2006.

  1. Negotiableterms

    Negotiableterms Administrator Staff Member Admin Subscriber

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    This Sticky Thread starts off with a really great summary of the concepts behind transmission of digital signals via cable by member, House de Kris.

    WARNING:

    This thread is intended as a technical reference (for learning) , and we reserve the right to edit the hell out of any post. No non-technical opinions, please? Meaning that this is not a place to compare cables, or to express your long-held belief that analog is better, or silver is better, or whatever. All such comments will be edited out.

    All comments amplifying, explaining, clarifying, questioning, or expanding on Kris' comments are very welcome, as are technical questions of all kinds. They're not only welcome, they're encouraged!
     
    Last edited: Mar 10, 2006
  2. Negotiableterms

    Negotiableterms Administrator Staff Member Admin Subscriber

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    Intro/Summary by House de Kris

    DIGITAL INTERFACES USED IN AUDIO – SPDIF COAX

    There are a number of interfaces used in the audio world for moving digital audio data from one component to another. Like chess, understanding these interfaces is simple on the surface, but complexity escalates exponentially as the layers of the onion are pealed away. To keep things simple, this thread will fixate on the SPDIF on coaxial implementation of digital transfers only. Others methods include SPDIF on Toslink, AT&T glass optical, AES/EBU professional. This is not intended to be an engineering tutorial on the subject, but rather a layman’s aid in understanding the terms and issues involved. As such, engineers may not be too satisfied with this discussion, but remember, the intended audience is the layman.

    THE DATA
    The intent of the digital interface is to get data from one location and ship it to another. In this case, SPDIF is a protocol that defines what the format of the data is, and how it should be interpreted. In addition to the bits that define the instantaneous sampled levels used to reproduce the audio, the SPDIF interface also includes the clock that tells when each of the samples should be used to recreate the audio signal. Therefore, the destination is always a slave of the source. SPDIF can support multiple clock rates and multiple word widths. SPDIF uses a bit modulation technique called bi-phase. Bi-phase is just that, a modulation scheme for the data, it provides no error detection and correction.

    THE PHYSICAL INTERFACE
    The coaxial SPDIF interface specifies a 75ohm controlled impedance environment for the signal. This implies a 75ohm source, a 75ohm interconnect, and a 75ohm destination. This “75ohm” designation is of the characteristic impedance of the transmission line that carries the signal of interest. Being an impedance, it is an AC phenomena, not DC. It cannot be measured with DC instruments. But, it is a very real 75ohm load being presented to the incident waves traveling down a PC board trace or wire. In any controlled impedance digital interface where edge placement accuracy is desired, mismatches between any of these three primary components (source, interface, destination) will cause edge distortions which could cause timing related errors later on downstream. If all three primary components are perfectly matched, the waveshape developed at the source will appear exactly at the destination assuming a lossless interface, no matter how long the interface. Lossless cables don’t exist in the real world, though, and the edge degradation due to bandwidth limitations will degrade the edge fidelity.

    REFLECTIONS
    In a controlled impedance environment, when the edge of an incident wave propagates down the line, it is both an incident voltage wave and an incident current wave. In a 75ohm line with a 1V edge, it will have a 13.3mA edge associated with it. If the characteristic impedance of the line changes (called a discontinuity) as the signal travels the line, a different current is required for the same voltage in order to satisfy Ohm’s Law. The excess voltage or current that results will be reflected from the discontinuity point and send smaller versions of the incident wave back down the line in the opposite direction from where the discontinuity occurred. This reflected energy will keep going back and forth down the line until it is entirely absorbed in the source, destination, or interface. If there are a number of discontinuities in a line, there can be many reflections heading in either direction at any point in the line.

    TRANSMISSION LINES
    A transmission line occurs anytime a signal carrying conductors has a specific relationship to ground. The signal will create magnet flux lines between the conductor and ground. If the space between the conductor and ground is consistent, and the dielectric properties of the insulating material are consistent, then a characteristic impedance is developed. Thus, on a PC board it is relatively easy to create traces with a specific characteristic impedance. By specifying the thickness of the board between signal and ground, the width of the signal trace, and the board material, just about any impedance is possible (in reality, it is difficult to get greater than 80 ohm traces with today’s processes). Keeping everything else constant, making a trace wider will lower its impedance (capacitive discontinuity), while making it thinner raises it (inductive discontinuity). From this it should make sense that a typical via in a PC board appears to be a capacitive discontinuity in the transmission line. This could happen when a trace moves from one layer to another.

    Likewise, in coax cables the impedance is set via physical parameters. In this case, it is the distance from center conductor to shield (which is set by the thickness of the insulator), and the properties of the insulator (dielectric constant). Any variation in the distance from center conductor to shield will show up as a change of impedance, or discontinuity.

    COPPER OR SILVER?
    The type of metal used for the center conductor has no effect on the speed of propagation down the line. Silver does have greater conductivity than copper, and this is why it is sometimes specified. Silver is more expensive than copper, though, so the typical use of silver is to plate a copper center conductor to minimize losses due to skin effect at high frequencies. Losses due to skin effect look like bandwidth limitations in the frequency domain.

    TEFLON OR POLYPROPYLENE?
    The dielectric choice directly impacts the speed of propagation of the incident wave. It will also have an impact on the distance from center to shield. All other considerations being the same, a cable with a polypropylene dielectric will be thinner than one with Teflon. The polypropylene dielectric cable will also have longer propagation times. Air-impregnated Teflon, or Teflon foam, is superior to solid Teflon for speed purposes.

    REALITY SETS IN
    With the spec for SPDIF on coax so well defined, and no new science being developed for this interface, it would seem logical that all manufacturers of audio equipment and cables would be able to abide by the simple rules of maintaining a 75ohm path throughout the equipment. Unfortunately, this is not always the case. When source, interface, and destination don’t adhere to the standards that would ensure the best possible situation, we are left with always dealing with trying to optimize the sub-par. Thus, changing things that should, on the surface, have no logical reason to affect the sound, does.

    MEASURING CABLES
    There are ways to measure the characteristic impedance of cables. The most common is TDR (Time Domain Reflectology). A TDR instrument will send a fast edged step function down a line, and then interpolate the reflected energy into equivalent characteristic impedances at any physical point in the line. That way, the displayed trace then directly represents the cable being measured. A TDR can also make measurements as to the minimum and maximum reflected energy. Another similar test is TDT (Time Domain Transmission). This instrument will observe the signal on both the souce end (like TDR) but also at the destination end. This allows easy measurements of propagation delay and loss.

    It should be noted that lumped capacitance values for coax used as a transmission line has no value. The capacitance is balanced by the inductance to yield the characteristic impedance.
     
    Last edited: Mar 8, 2006
  3. DKak

    DKak Active Member

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    In a related series of posts, House De Kris is kindly measuring many digital cables via TDR. Look in the "Thinking Out Loud" forum and read threads beginning with "TDRing..."
     
  4. wsjoe

    wsjoe Well-Known Member

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    Impedance mismatch and reflection has more to do with much higher frequency range such as video in the mega and giga hertz. Yes there is plenty of reflection going back and forth from source to amp to speaker but it's happening at such a high frequency that our audio range is not essentially affected.

    Transmission of long ranges (hundreds and thousands) of feet is where the problem starts for video people. In our audio world, our cables are in the 10's of feet or even less. There is not much to worry. For the purists, yes, there might somethings you could do but not real worth the money. And even if you were affected by these waves, you will never hear them. The average male even in the 30's is below 20khz.

    wsjoe
     
  5. House de Kris

    House de Kris Loud-n-Deep

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    True, the notion of reflections is very appropriate for high frequency systems, not audio. But, a digital audio interface has bandwidth requirements way beyond audio, into the video range. In addition, it is necessary to have sufficient bandwidth to ensure accurate edge placement at the receiver. Having enough bandwidth to make the data look square-ish is not enough. When sub-nanosecond timing accuracy is desired, extra care must be used when shipping digital data from one point to another.

    I completely agree with you about any reflections between amp and speakers as having no effect at all. But, that's not what this thread is about.
     
  6. Stanga

    Stanga Active Member

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    I don't support the notion that having a digital signal where each frame is as square as possible in the corners would contribute to a better signal decoding at the D to A end. The digital signal is fed to a level shifter, or Schmitt Trigger, that decides if it is a valid 0 or valid 1. The actual incoming signal is NEVER used, but serves as a guide towards recreation of a new data stream that reflects the incoming 0s and 1s.
    Where the problem arises is when the clock at the source end is unstable, so the waveform jitters about in time. The Schmitt Trigger cannot correctly identify the start or end of the jittery square wave. Sometimes this problem is tackled through the use of a Phase Locked Loop clock circuit, but selection of the maximum and minimum range over which a PLL can accurately lock in and onto a jittery waveform is not for the faint hearted designer involved in precision clock designs. Harmonics can easily defeat a PLL. Cable reflections and standing waves are the kind of waveforms I have in mind.
     
  7. wifihifi

    wifihifi Member

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    Thanks, its a good article to read and understand. Much appreciated.
     
  8. xQuebecx514x

    xQuebecx514x Active Member

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    Hi, why 75 ohms? And how can I test my wire? Also if the wire is 0 0hms, can I simply place a 75 ohms resistor?
     
  9. House de Kris

    House de Kris Loud-n-Deep

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    Why 75 ohms? Good question, and hard to definitively answer. Here's some history, about 100 years ago, 50 ohms was chosen to minimize the attenuation in coax cables for a fixed outer diameter. The neighborhood around 50 ohms really is the best place for characteristic cable impedance. For whatever reason, the video industry settled on 75 ohms for its interconnect characteristic impedance. This lowered power dissipation and made driver design easier, at the expense of fatter cables for the same amount of loss. I suppose when the SPDIF protocal was being defined, the existence of plentiful 75 ohm video cables terminated in consumer RCA connectors was a compelling influence. For whatever reason, that's where we are.

    You can test you cable with a TDR (as mentioned earlier), or with the VSWR method (if you have a lengthy cable to measure). A simple DMM (or even a complex DMM) won't be able to do this for you.

    I can assure you that the characteristic impedance will not be 0 for any cable. And no, simply adding a resistor will not do anything for the characteristic impedance. The characteristic impedance comes from the physical construction of the cable. Thickness of center conductor, dielectric thickness, and dielectric constant all play into setting the characteristic impedance.
     
  10. xQuebecx514x

    xQuebecx514x Active Member

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    Ok, great! What does TDR stand for? And ill do some research on the wire im using, its the Canare GS-6 if im not mistaken :)
     
  11. House de Kris

    House de Kris Loud-n-Deep

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    As mentioned in the second post of this thread, TDR stands for "Time Domain Reflectology." I had a series of threads where I measured a number of cables and reported on them at AK using a TDR/TDT. In one thread, here, I described the functions of a TDR somewhat - complete with pictures and whatnot.

    I looked up Canare GS-6 cable. This appears to be a guitar cable. The manufacturer doesn't specify the nominal impedance, even when a place is provided in the spec sheet. Nor do they specify the inductance, so we can't even make a good guess as to its impedance. From this, I would deduce that this cable is not designed for use in a controlled impedance application, like a digital interconnect. I'd bet you money that it would 'work,' but it may not be the best choice in cables you could make in this situation.
     
  12. 2string1

    2string1 Super Member

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    Do you have any thoughts on this cable vs the Blues Jeans cable? I know you don't want this thread to be a whats good whats no,but you seem to have a lot of wisdom on digital cables...
    http://www.ebay.com/itm/301698356520?_trksid=p2060353.m1438.l2649&ssPageName=STRK:MEBIDX:IT
    http://www.bluejeanscable.com/store/shopbycable/1505A.htm
     
    Last edited: Nov 5, 2015
  13. botrytis

    botrytis Trying not to be a Small Speaker Hoarder Subscriber

  14. saratoga

    saratoga Member

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    interesting,i just discover tonight,before i come to audiokarma,that my Cardas clear line made in USA. USB CABLE,cost me 190 aud 1 y ago,from computer to the DAC,was outperformed,from the shielded USB cable ,made in China, and cost me 3.00 aud from ebay!!!!The sound is much clear,bass is more define,and coherence of the music is much more define, WTF:yikes: 20160607_223950.jpg :bowdown::rant::idea:
     
  15. Lesterbest

    Lesterbest Super Member

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  16. Lesterbest

    Lesterbest Super Member

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    Talk about short & sweet.
     
  17. wa2ise

    wa2ise Super Member

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    Any coax cable designed for cable TV work (like RG6) will be fine for SPDIF. Only thing with such coax is that the shield braid is often made of aluminum (which cannot be soldered). You'd need to use crimp on RCA connectors. Other coax, like RG59, can vary in quality from good to bad, but most RG6 I've seen and used was always good.

    Reflections due to inaccurate termination at the load should be absorbed by the source impedance of the SPDIF output of the CD transport. Bad cable reflections probably won't be absorbed like this, and you want to avoid bad cable...
     
  18. Lesterbest

    Lesterbest Super Member

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    Digital cables either work or they don't. Do you need double speak by cos pimping a product? In network TV they don't bring white papers to the toilet to read unless they're out of toilet paper. All cables are copper shielded. To save $, Monoprice & Markertek have copper shielded cables galore.
     

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