The USB cable mega thread. Walking off the plank

Discussion in 'Digital: DACs, USB converters, decrapifiers' started by Dot, Oct 16, 2015.

  1. mickeyvortex

    mickeyvortex Friend

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    You're creative. Never let any one tell you otherwise.
    My suggestion- save yourself a pretty penny by using the good old spit and air-dry method.
    Try A/B ing this....
     
  2. rayfalkner

    rayfalkner Not to be confused with Roy Fokker - Friend

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    Or you could pour a big pile of audiophile-grade dust into the ears and gradually scoop them out to shape the canal until it reaches the optimal acoustic resonance pathway that you like. No more messing away with hardware, we go the organic way, green technology. :D
     
  3. feilb

    feilb Coco the monkey - Friend

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    More whiskey consumed by 3am than 7pm.

    To bring this thread back to a place that addresses its subject, and to try to do so in as constructive a way as can be done on a discussion about cables of any sort, lets talk about what things might actually influence the USB signal.

    I'll probably write a few of these on different actual cable properties that might influence the signal, but lets start with one:

    1. Characteristic Impedance
    Characteristic impedance is a property of transmission lines that describes the impedance that a change in voltage sees as it propagates down the line. It is not something that you measure with a multimeter (if you were to do so, you would get the resistance of the conductors), but instead a time-domain reflectometer.

    The USB spec calls out a 90ohm differential impedance formed between D+ and D-.This characteristic impedance is determined by geometry of the transmission line: conductor size, spacing, and proximity to a reference plane.

    Data transmission standards specify a characteristic impedance to ensure that all parties designing transmission lines target the same value. This is important because when a change in characteristic impedance is observed by the propagating signal, some of its energy is reflected back to where it came from. The magnitude and polarity of the reflection depends on whether the change in impedance is low-to-high or high-to-low and by how much. This reflected energy can cause signal integrity issues such as undershoot and overshoot which (especially for longer cables) can be symbol dependent. Symbol dependent errors can show up as jitter,

    The magnitude of the reflection is also dependent on the voltage gradient on the cable. Propagation speed of twisted pair cable is (order of magnitude) about 2 * 10^8 m/s. USB 2.0 has a 480Mbit/s data rate, which thus, has a nyquist frequency of 240MHz (the lowest frequency component of the data). In order to fit a full rising or falling edge on the cable all at the same time (i.e. the voltage at the transmit side of the cable is logic high and the voltage at the opposite end is logic low *simultaneously*), the cable must be at least 20cm long. If the cable is shorter, less than the full scale voltage will be shown across it (i.e. logic high on the transmit side and logic high * 0.5 on the opposite end). Thus, the shorter cables are, the less opportunity for error.

    As cables increase significantly past 40cm, the opportunity for this reflected energy to degrade future bit values increases (as multiple bit values will be present on the cable simultaneously). This is the symbol dependent interference i talked about above (usually referred to as ISI or intra-symbol-interference).

    How much does this matter? The answer might be not much. Unless the interference is so bad that a wrong value is interpreted these reflections will likely have very little impact. The SERDES simply needs to meet its minimum eye (timing and amplitude requirements negatively impacted by these reflections) in order to consistently interpret correct data.

    In USB async, the actual pacing of the samples (i.e. what the DAC cares about) is not impacted by bit-level jitter because frames are buffered and transmitted to the DAC based on an on-board or on-chip oscillator separated completely from USB.

    "Good Cables"
    Good cables wrt characteristic impedance will maintain consistent conductor spacing and twist rate, exhibiting a consistent characteristic impedance for the entire length of the cable.



     
    Last edited: Oct 22, 2015
  4. The Alchemist

    The Alchemist MOT: Schiit - Here to help!

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    I use a cheap $10 Monoprice USB cable - works good for me!
     
  5. ultrabike

    ultrabike Measurbator - Admin

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    I'm a monoprice whore. And I like it. :D
     
  6. Thad E Ginathom

    Thad E Ginathom Friend

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    Haha, I saw the pic and thought, ohno, it's going to be one of those audiophile cables here. The laugh's on me: enjoy!

    The dust is essential. Even Sherlock Holmes inew of the value of dust in data storage.

    Simple: the more the better.

    By the way, I also had some magic liquid from Russ Andrews for wiping CDs, and it was supposed to be good for wiping on all sorts of cable. That laugh is on me too! I can't remember if it had any effect on the green marker pen on some of my CDs

    Ray has the answer. This is the way to go. CIEW --- Custom In Ear Wax, of which that dust would be a vital ingredient.

    And a big thanks to Feilb for the real technical stuff. :bow:
     
  7. feilb

    feilb Coco the monkey - Friend

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    Continuing on with discussion about actual cable properties.

    2. Shielding
    Most people understand the concept behind and the purpose of shielding. Fundamentally, shielding is a means of preventing electric field interference both from getting out (i.e. your DAC to the rest of the world) and getting in (i.e. the rest of the world to your DAC).

    Electric fields are formed between two objects in space that have differing electric potentials and has the unit Volts per meter (V/m). The higher the difference in potential the stronger the field. The smaller the distance between two potentials, the stronger the field.

    Electrons flowing through an electric field experience a force compelling them toward the more positive voltage. An electron experiencing a changing electric field (whether it varies with time or distance traveled) will similarly experience a changing force. Thus if electrons flowing on a conductor experience external perturbations due to a varying electric field, the signal carried by the flow of those electrons similarly experiences perturbations.

    Shielding takes advantage of the fact that the electric field inside a conductor is 0. Rather, the charges on the surface of a sufficiently conducting material rearrange themselves in order to make the electric field inside the conductor 0. By wrapping a signal wire entirely in a conductive material, an electric field forms between the conductor and shield and between the outside world and the shield. Charges mobilize on the inside surface of the shield in response to the potential of the encased conductor, and on the outside surface of the shield in response to the outside world, so as to make the e-field between their surfaces 0 and thus preventing the fields from interacting.

    In order for the fields not to interact, however, the shield must be held to a stable potential. Typically, this is accomplished by having the shield "grounded"; in other words at the same electrical potential as the return path for the signal.

    Cables are typically shielded with either a foil wrap or a mesh braid, though some coaxial cable is continuously extruded, producing a continuous metal shield.

    "Good Cables"
    Good cables wrt shielding will have as continuous as possible shield, either foil or mesh with a very high coverage percentage (very small openings in the braid). They must also have the shield properly terminated to the connector on both ends. If the shield is not terminated to 1 of the two ends, its effectiveness is greatly diminished. If it is terminated to neither end, it has no protective effect.​
     
  8. JewBear

    JewBear Almost "Made"

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    I am strongly of the mind that any USB cable from a reputable company (like monoprice) will work the same as an ultra expensive cable. However, USB audio does not have error correction so that leads me to one question that I would need answered before making a definitive conclusion. What would data loss sound like? I am pretty sure it would be random static rather than any subtle changes to the sound stage or frequency response.

    Anyone have any thoughts?
     
  9. George Mhmmm

    George Mhmmm Acquaintance

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    Bit flips will cause large impulses or VERY AUDIBLE pops in your music. It depends on how "significant" a bit is. If one of the first significant bits in a number get flipped, that will cause a very large change to the data, where as less significant bits will cause less noticeable issues.

    Imagine you have 0001 (1) and the first bit gets flipped. Now you will have 1001 (9) instead of (1). This effect will be much more noticeable with 24 bit numbers where a flip on the 20th bit for example could be the difference of 2^20 in your digital signal.

    You probably have not experienced data loss in audio, otherwise you would DEFINITELY know when digital stuff goes wrong.
     
  10. ultrabike

    ultrabike Measurbator - Admin

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    The digital filter might smooth out too instantaneous a change (to some extent). But yes, a flip in a a bit might probably roughy be equivalent to the needle of a record player hitting some dust.

    Easy enough to simulate. Load a wav file into Matlab or something and flip screw up a bit in a few samples, wav it back and there you go. Could do that even in Octave for cheaps.
     
  11. Madaboutaudio

    Madaboutaudio Friend

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  12. Judeus

    Judeus Facebook Friend

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    Haha those cnc connectors are the same as the ifi ones
     
  13. feilb

    feilb Coco the monkey - Friend

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  14. JewBear

    JewBear Almost "Made"

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    I'm actually a bit surprised no manufacturer has come out with a DAC that uses error correcting. Obviously you'd need to write custom drivers and what not, and it wouldn't work with android or ios, but it would seem that if data errors was really something that happened, then a manufacturer would have done it by now.
     
  15. mtoc

    mtoc SBAF's Resident Shit-Stirrer

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  16. drez

    drez Acquaintance

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    My current understanding of why USB cables, computers etc matter is firstly due to noise riding the signal and not completely removed by galvanic isolation, and secondly due to indirect mechanism of the USB signal integrity causing the USB receiver "PHY" to do more work. Or something. I would do better to leave the technical explanation to John Swenson: http://uptoneaudio.com/pages/j-swenson-tech-corner

    It is, however, important to acknowledge that USB gear that sounds different is not always better. There are a lot of hand made cables out there, out of spec, which although still maintaining lock, achieve a different sound by impairing the performance of the USB connection. Equally there are some cables which improve upon the materials, tolerances, and construction of typical USB cables and provide superior technical performance for example far exceeding the bandwidth requirements of the USB standard and also providing better shielding.

    Over the years I have tested many cables, and the one to beat IMO is the Wireworld Platinum Starlight USB. This is not a sales plug, just if you want the most accurate and nuanced USB cable - IMO that is it.
     
  17. ald0s

    ald0s Facebook Friend

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    Ive used a Schiit USB cable and a 5 year old printer cable and they sound exactly the same to me out of a Gungnir Multibit/V2/HD800 rig. For what its worth I'm currently using the printer cable because its longer and more flexible.
     
  18. mtoc

    mtoc SBAF's Resident Shit-Stirrer

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    I've said many times, Wireworld Platinum Starlight 7 is the standard cable in Asia, some really crazy guy bought Russian Sleath usb cables (3 versions, from $999) or Kubala Sosa (from $1.4k), a guy bought ten Kubala...
     
  19. Thad E Ginathom

    Thad E Ginathom Friend

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    That disgusts me.

    But, the thing is, my disgust is a gut feeling, but I don't have the technical knowhow (and no, of course I haven't heard it). When it comes to ethernet cables, I don't think one needs a very great deal of techie knowledge to say that that in-spec cables are utterly sufficient, and that the expensive audiophile stuff is indeed audiophoolery. I'm ready to eat my TCP/IP text book* if it isn't. But I have the ketchup handy just in case.

    [​IMG]

    With USB, I still have gut-feeling concerns about the power side of it. But not gut feelings that need more than $10 to quieten.

    A potential problem with this is that if your box of cables dates back as far as mine does, it just might contain some USB1.0 cables. According to my dim memory, USB1.0 was really not good for audio. There was a big jump to the USB2.0 specs. I'm guessing the cable specs changed too. It probably doesn't matter except to chronic horders like me, but can feilb (or anyone) confirm that?


    *Yes, it is very old. Way out of date, in fact, dating back to when I first started doing that stuff. I really, really, doubt, however, that newer editions have an extra chapter for audio
     
  20. BioniclePhile

    BioniclePhile The Terminal Man - Friend

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    I've got PYST usb for my Macbook to Modi. I'm not sure how exactly a digital cable could make any difference cause I don't know much about the physical properties of electricity, but I know that I'm not missing out on anything at the moment. Maybe with my future Gungnir Multibit I'll test some cables out if I can find somewhere cheap to loan em.

    Also, how the feck is any kind of cable going to make a schiit of a difference if the conductive metal in the circuit board is cheap-ass copper? I mean, if someone managed to make a computer, DAC, amp, cables, connections, and all manner of other conductive material out of gold, pure copper, silver-plated whatever so there was continuity of the same metal all across the whole platform, would it really make an audible difference compared to a regular setup? I don't know the answer to that, but if anyone has several million bucks hanging around, they could always get on it. For science, you know?
     

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