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Thread: The Sound of a Digital Cable: Bandwidth and Jitter

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    Member Sponsor [Technical Expert] DonH50's Avatar
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    The Sound of a Digital Cable: Bandwidth and Jitter

    Can your digital cable impact the sound of your system? Like most things, the answer is “maybe”… Reading some of the cable advertisements I see all kinds of claims. Some make no sense to me, such as “use of non-ferrous materials isolates your cable from the deleterious effects of the Earth’s magnetic field”. Hmmm… Some appear credible but describe effects that I have difficulty believing have any audible impact, such as “the polarizing voltage reduces the effect of random cable charges”. True, a d.c. offset on a cable can help reduce the impact of trapped charge in the insulation, but these charges are typically a problem for signals on the order of microvolts (1 uV = 0.000001 V) or less. Not a problem in an audio system even for the analog signals (1 uV is 120 dB below a 1 V signal), let alone the digital signals. But, there are things that matter and might cause problems in our systems.

    Previously we discussed how improper terminations can cause reflections that can corrupt a digital signal. This can cause jitter, i.e. time varying edges, that are dependent upon the signal, which is really an analog signal from which we extract digital data (bits). As was discussed, this is rarely a concern for the digital system as signal recovery circuits can reject a large amount of jitter and extensive error correction makes bit errors practically unknown. The problem is when the clock extracted from the signal (bit stream) is directly used as the clock for a DAC. The DAC will pass any time-varying clock edges to the output just as if the signal was varying in time. It has no way of knowing the clock has moved, and the result is output jitter. Big enough variation and we can hear it as distortion, a rise in the noise floor, or both.

    One thing all cables have is limited bandwidth. Poorly-designed cables, or even well-designed cables that are very long, may limit the signal bandwidth. This causes signal-dependent jitter. How? Look at the figure below showing an ideal bit stream and after band limiting. I used a 1 us bit period (unit interval) for convenience; this is a little less than half the rate of a CD’s bit stream.

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    There is almost no difference between the ideal and 10 MHz cases. Since 10 MHz is well above 1/1 us = 1 MHz we see hardly any change. Dropping to 1 MHz, some rounding of the signal has occurred, and at 0.25 MHz we see that rapid bit transitions (rapidly alternating 1’s and 0’s) no longer reach full-scale output. Looking closely you can see that the period between center crossings (when the signal crosses the 0.5 V level) changes depending upon how many 1’s or 0’s are in a row. With a number of bits in a row at the same level, the signal has time to reach full-scale (0 or 1). When the bits change more quickly, the signal does not fully reach 1 or 0. As a result, the slope is a little different, and the center-crossing is shifted slightly in time. This is signal-dependent jitter.

    A better way to see this is to “map” all the unit-intervals on top of each other. That is, take the first 1 us unit interval (bit period) and plot it, then shift the next 1 us to the left so that it lies on top of the first, and so forth. This “folds” all the bit periods into the space of a single unit interval to create an eye diagram (because it looks sort of like an eye). See the next figure.

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    The top (ideal) plot shows a wide-open eye with “perfect” edges. As the bandwidth drops to 10 MHz we see the edges now curve somewhat, but there is effectively no jitter seen. That is, the lines cross at the center at one point, no “spreading”. With 1 MHz bandwidth the slower (curving) edges are very obvious, but still the crossing happens essentially at a single point. In fact jitter has increased but it is not really visible.

    At 0.25 MHz there is noticeable jitter, over 41 ns peak-to-peak. That is, the center crossings no longer fall at a single point in time, but vary over about 41 ns in time. Why? Look at the 1 MHz plot and notice when the signal changes, rising or falling, it still (barely) manages to reach the top or bottom before the next crossing begins. That is, the bits reach full-scale before the end of the 1 us unit interval (bit period). However, with only 0.25 MHz bandwidth, if the bits change quickly there is not time for the preceding bit to reach full-scale before it begins to change again. You can see this in the eye where the signal starts to fall (or rise) before it reaches the top or bottom (1 V or 0 V) of the plot. This shifts the time it crosses the center (threshold). If the DAC’s clock recovery circuit does not completely reject this change, the clock period will vary with the signal, and we get jitter that causes distortion at the output of our DAC.

    How bad this sounds depends upon just how much bandwidth your (digital) cable has (a function of its design and length) and how well the clock recovery circuit rejects the jitter. It is impossible to reject all jitter from the clock recovered from the bit stream, but a good design can reduce it significantly. An asynchronous system that isolates the output clock from the input clock can essentially eliminate this jitter source.

    HTH - Don
    Last edited by DonH50; 02-25-2012 at 08:14 PM. Reason: Redo figures.
    Don Herman
    "After silence, that which best expresses the inexpressible, is music" - Aldous Huxley
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    Addicted to Best! RogerD's Avatar
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    Yes, I use a SPDIF cable and there is a difference in ambient information retrieval.

    I auditioned many digital cables and this review validated my thoughts, after two years my mind hasn't changed. I won't mention my positive ferrite experiment on the Nirvana cable either.

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    Member Addicted to Best! NorthStar's Avatar
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    About digital Coaxial versus digital Optical (Toslink), Don?

    And what is the prefered digital Coaxial cable's impedance?

    Connectors, do they have an influence on the overall sound quality?
    ...In relation to Jitter, or other type of spurious noise? ...Contact noise?

    Dither noise? Thermal noise? About elevation?
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    It is great to have those simulations Don! And wonderful way to explain how it can be data dependent. Thanks so much for writing and posting it.

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    Transparent Ref and Nordost Valhalla digicables were very different on my system. I am a big Transparent fan, use nearly all transparent...and i wanted so much to buy the Tranp Ref...i returned it after 3 fruitless hours of listening. On a lark, i went for the Nordost Valhalla since it was second-hand and in the store that day. i bought it that day, and i still listen to it every day after 2 years with no regrets.

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    Member Sponsor [Technical Expert] DonH50's Avatar
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    Thanks guys. I tweaked the figures so they might be easier to see. I have yet to find an easy way to optimize for legibility...

    Bob (Northstar):
    1. This is about coax. Optical uses a fiber link. The same sort of bandwidth issues can happen in the transmit/receive electronics, of course.
    2. Coax is typically 75 ohms though 50 ohms is used sometimes. This analysis was independent of coax impedance.
    3. Connectors don't usually impact the system unless they are poorly attached or the wrong type (impedance).
    4. Dither is random noise intentionally added to "scramble" the quantization spurs. It is mentioned in other threads but does not apply here. Dither does not impact bandwidth limitations.
    5. Thermal noise is not included and for a cable in audio systems is out of the picture.
    6. Elevation? I don't know what you mean. My house sits at about 7500', does that matter?


    Amir: Figured you'd like the sims! Took waaay longer than it should to get them right. Tried a couple of ways before getting a source I liked (went with random), then had to generate a little impulse sampler (basic T/H did not work) and comparator circuit, then futz with various parameters. I swear it was easier to design at the transistor level than to gen up an ideal circuit for this...
    Don Herman
    "After silence, that which best expresses the inexpressible, is music" - Aldous Huxley
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    Member Addicted to Best! NorthStar's Avatar
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    Thank you Don for your clear reply.

    1. I got you; I was just wondering if it could make a difference.
    2. I asked because some people use 100 Ohms digital Coax cables.
    3. By the connectors I meant their built, material use; metal, gold, copper, brass, silver, etc.
    And XLR Balanced versus RCA Unbalanced.
    4. Got you, thanks.
    5. Good to know, thanks again.
    6. Elevation because at sea level versus 5,000 to 15,000 feet high (or more) can perhaps make a difference (just asking). But my guess is; no, it wouldn't affect the audio transmission through digital Coaxial cables.
    - But low bass frequencies are certainly affected, but this has to do with sound pressure being different at various altitudes.

    Thanks again, and your first post is a source of great info. Very much appreciated.
    All the Very Best, - Bob --------- "And it stoned me to my soul" - Van Morrison --------- AudiophileAudition

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    Member Sponsor Addicted to Best! Ronm1's Avatar
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    Nice work...

    As likely a place as any to add....

    When I first started getting into digital, issues I had were power related. My coax spdif hookups would be affected by diff devices turning on causing noise thru xports/players to external dac. Aes/Ebu, I2s or optical did not, only coax had issues. Eventually a Stealth shielded cable with proper gnd eliminated the coax symptom. It was an older house without a gnd to outlets except where new outlets, circuits had been added. I eventually added a dedicated 20a line and a power conditioner. Heating system turning on, fluorescent lights, dimmers were all culprits at the time.
    A Bug!! Naa...that's a feature!!
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    Quote Originally Posted by amirm View Post
    It is great to have those simulations Don! And wonderful way to explain how it can be data dependent. Thanks so much for writing and posting it.
    +1
    Thanks Don

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    Excellent, Don!

    One other element which is often overlooked is proper resistive termination of the coax. I.E., 75 ohms. I would think that lack of proper resistive termination would have varying effects depending on the length of the run. For S/PDIF or Word Clock runs I use only 75 ohm quad shielded RG-6 with crimp-on BNC or RCA connectors (though I prefer BNC as a true 75 ohm connector) and have had zero problems with digital or WC connections.

    AES/BSU is probably a safer alternative if your equipment has it.

    --Bill

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