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• Sharing some explanations on video cable specs ... - SteveLim 22:56:56 08/01/06 (1)
  In Reply to: Need help in understanding Specs of Belden video cables posted by SteveLim on August 1, 2006 at 04:27:15:

  Hi ,

  Managed to find some good explanation/discussions at the following website
  http://archive.avsforum.com/avs-vb/showthread.php?s=&threadid=239963
  where Steve Bruzonsky shared from his readings of Widescreen Review's "Imaging Science Theater 2000" issue , and also from Belden website.

  I found it beneficial and so extracting to share some key explanations. : -

  *** IMPEDANCE ***
  The primary goal is to transfer the maximum amount of signal power from one device (component) to another. This is achieved when the source impedance matched the load impedance, which uses Z as a symbol, is a complex combination of reactive components, otherwise known as capacitance and inductance, and the resistive characteristics of the wire. It is determined by the ratio of the size of the center conductor and the distance between the center conductor and the outer shield.

  The impedance of all video devices should be fixed at 75 ohms. Therefore, good coax video cables should also have a 75 ohm impedance, ideally with no reactive components. This will provide the maximum transfer of power from video components all the way to the monitor or video display.

  A mismatch in impedance between the source, cable and load might cause reflections of the signal, within the cable. These reflections would cause ringing, or extra edges, to occur in the picture.

  Connectors have a characteristic impedance of their own. Ideally, it would also be 75 ohms. None of the video RCA connectors surveyed were 75 ohm - but several manufacturers in the years since the survey corrected this with true 75 ohm RCAs, Even different size RCA connectors have a different characteristic impedance. This had a disastrous effect on the measurement of the test signals. However, it was not significant in picture material, most likely due to the short lengths that WSR tested (1.5M RCA cables and 25 foot BNC cables).

  
  *** RESISTANCE ***
  A good cable should have as little resistance as possible. A cable and connector can have 75 ohm impedance, but attenuate all of the signal as it passes down the wire. This is caused by series resistance in the wire. Any cable system requires energy to move electrons through a material, and all materials have an inherent force that tries to prevent the electrons from passing through the material. This is called series resistance. The amount of series resistance in a wire is affected by the diameter and length of the center conductor. Resistance increases as length increases or as size decreases. Likewise, resistance decreases as size increases or length decreases. Inotherwords, a short, fat cable has less resistance than a long, skinny one. Excessive resistance in a cable will attenuate the signal. If the video signal goes through a lot of devices before reaching the display, it can be destroyed by a lot of little cables. In the WSR tests, they measured resistance as the amount of attenuation of the signal at 5.5 MHz and at 40 MHz. (Keep in mind that now we would test higher with DVD and especially HD - this WSR test was back when LD was the best source material.)

  
  *** FREQUENCY RESPONSE ***
  If a cable is purely resistive in its characteristics, frequency response will be flat. This means that it will be equal in amplitude for all frequencies. The response might be down from the original signal, because of the series resistance, but still remain flat throughout the frequency band.

  As the WSR cable survey then focused on LD, and the then nearly brand new DVD, as the best video source, frequency response was considered as a measure of the video signal from 30 Hz to 5.5 MHz for a LD or DVD signal.

  Capacitance and Inductance are the two reactive components that will affect the frequency response capability.

  
  *** CAPACITANCE ***
  A coaxial cable, by its nature, is a form of capacitor, and therefore, has capacitance. A capacitor is an element made up of two conducting materials separated by an insulating material. It can store direct energy, block direct current and yet pass alternating current. A coaxial cable has capacitance because it is made up of a center conductor with a shield around it. This forms two conductive materials. They are separated by a dielectric, which is an insulating material. The combination makes it a capacitor, with capacitance. Once again, capacitance blocks the direct current, or low frequency information, from traveling to the ground, and is essentially, not seen. However, as we go higher in frequency, the capacitance of the cable does become a path to ground for the signal. This can present the high frequencies with a lower impedance than the low frequencies, which will cause a roll off, or attenuation, in the high frequency information. The result is a loss of both fine picture detail, and possibly, color information. The loss in color will not be seen because all consumer sets re-amplify the color signal before decoding it. However, amplifying the color signal can also amplify noise along with it, depending on the quality of the video amplifier. The loss in high frequency information will noticeably reduce picture resolution. Any cable surveyed that produced a flat frequency response, will have little capacitance.

  Another explanation of Capacitance : -

  A capacitor is a device that holds an electrical charge. It consists of two metal plates with insulation in between. Well, isn’t that exactly what a cable is? Two metal plates (or wires) with an insulator (dielectric) in between. Figure 9 shows the specific parts of a twisted pair and coax cable that are involved with capacitance.

  While cables do have capacitance, it is very small, due mostly to the fact that the wires are also small. Capacitance in cables is almost always measured in picofarads-per-foot. (pF/ft.) A picofarad is a trillionth of a farad, the unit of capacitance. So why would we have any interest in an effect that small? Because you don’t use just one foot of cable. Most often you are using tens, hundreds, even thousands of feet. And this capacitive effect adds up. That is, a 1,000-ft. cable will have 1,000 times the capacitance as a one-foot piece. Then you can get up to some serious capacitance!

  The real problem with capacitance is that it is affected by the frequency of the signal on the cable. The higher the frequency, the more the capacitance "stores" that signal as a charge. This "reaction" to frequency creates "capacitive reactance" also measured in Ohms, like resistance. But the effect changes with frequency, which resistance does not. Being "Frequency-dependant", capacitance is responsible for the "Frequency response curve" of any cable.

  *** INDUCTANCE ***
  Inductance in a cable will also affect the frequency response, though not as much as capacitance. Inductance is the property that opposes any change in the existing current in a wire. For example, a fixed magnetic field is created around a wire when a wire is energized with a DC voltage. If the value of that voltage were to change, as in an alternating current, the magnetic field would expand and contract. Likewise, an AC or DC current can be generated in a wire by placing the wire within either an alternating, or fixed, magnetic field around the wire which in turn generates its own voltage into the wire. This added voltage would attempt to stabilize the change taking place by the alternating current. The voltage created on the wire, or the field around it, can be amplified by wrapping the wire in a coil, as an inductor. An example of this would be in DC power supplies where an inductor is placed in series with the DC line. As current demands try to reduce the DC voltage, the magnetic field around the inductor will collapse, inducing additional voltage back into the line, reducing the chance for any changes in the current. This means that a straight wire will have less inductance than a coiled wire. Inductance in a coaxial cable attenuates the low frequency information. It will create little reactance between the center conductor and shield, and will give the low frequencies a path to ground. As the frequency increases, the adverse effects of the inductance decreases. If inductance is high, it will reduce the low frequencies, and the overall amplitude of the signal. This will lower the contrast, or white level, of the picture. If the contrast on the monitor is set too high, as most are, a high inductance cable could fool one into believing the picture had improved. On a properly adjusted monitor, however, the high inductance would darken the picture. As with capacitance, a cable with a flat frequency response will have little or no inductance.

  Another explanation of Inductance : -

  The electrical signal down a wire also creates a magnetic field down that wire. This effect is called "inductance". However, on most cables, the inductive effect is so tiny, that it is never listed in a catalog. The effect, with a frequency running on the cable, is called "inductive reactance".

  Because the inductance is tiny on most cables, the inductive reactance is also tiny. Inductance and capacitance are reverse effects. Therefore, they cancel each other out. But, in almost every cable, the capacitance and capacitive reactance and so much greater that they cancel out the inductance and inductive reactance/ But there is still capacitance, and capacitive reactance, left. This is why capacitance is a critical number in almost every cable type from analog audio to high-speed UTP, and inductance is essentially ignored.

  Inductance is based mainly on the size of the wire (AWG) and can be most easily changed by changing the size of the wire.

  
  *** TIME DELAY DIFFERENTIAL BETWEEN FREQUENCIES ***
  Capacitance and inductance not only affect frequency response, but they also can create time delay differentials between frequencies. This delay will cause different frequencies to arrive at the source at different times. This will result in a smearing of edges, a loss of picture detail, and a shift in the position of color, relative to the borders of objects in the picture

  
  *** RETURN LOSS ***
  The Truth About Coaxial Cable : Precision of impedance is important but an imperfect way to tell cable performance. One of the most important parameters for any cable transmission system is Return Loss (RL). For High Definition Television (HDTV) cables a band-with of 750 MHz (3rd harmonic frequency is 2.25 GHz) is required with recommended RL limit of > 15 dB from SMPTE.

  For this reason, Belden coaxes:
  1855A (0.6/2.6 O.D. 4.03 mm)
  1505A (0.8/3.7 O.D. 5.97 mm)
  1694A (1.0/4.6 O.D. 6.99 mm)
  7731A (1.6/7.2 O.D, 10.3 mm)
  intended for HDTV applications are currently 100% sweep tested up to 3 GHz.

  And a Belden quaranteed RL specification limit :
  > 23dB (for 5 to 850 MHz)
  > 21dB (for 850 MHz to 3 GHz)

  Using Belden coaxial cable will result in a minimum 6 dB of Headroom.
  For example the actual RL data of Belden 1505A is typically 30 dB with another 9 dB of Headroom to allow for losses introduced by connectors, transition devices, patch panels and improper cable installation or handling, in the transmitted signal.

  Broadcast engineers are familiar with return loss. They know it as Voltage-Standing-Wave-Ratio (VSWR), sometimes called SWR. In high frequency systems, when a signal goes down a cable which the wrong impedance, the signal will reflect and "return" to the source. This is sometimes mistaken for natural attenuation, or even resistive loss in the cable. However, it looks like a very long cable is attached, when the cable is not long at all. In digital systems, it can increase bit errors, even to the point of signal failure, if the impedance problem is severe enough.

  
  *** VELOCITY OF PROPAGATION ***
  Velocity of Propagation (Vp) compares the speed of a signal down a wire to the speed of light. The speed of light in a vacuum is the standard by which all other signals are measured. The reason there is such an effect is because the signal consists of an electromagnetic field around the wire. That field travels in the plastic or other insulation on the wire.
  Velocity of Propagation is a cousin to dielectric constant.

  Velocity of Propagation is what you most often see in a catalog, and comparing VP between cables gives you some idea of which cable performs better at high frequencies. A faster velocity means less high-frequency loss and flatter frequency response overall.

  Here is a list of dielectrics with their velocities.

  Vacuum = 100% (by definition)
  Air = 99.18% (100%)
  Teflon = 70%
  Polyethylene = 66%
  Polypropylene = 66%
  PVC = 45 to 60%

  
  ********************************************************

  SUMMARY :

  Based on my readings , my understanding (and i stand to be corrected as I am just learning these things) summary of important video cables specs are : -

  Impedance - 75 ohm impedance end to end (for HDTV Impedance critical 75 +/- 3)

  Resistance - as low as possible.
  Large gage coax such as 20 AWG or 18 AWG suggested.

  Frequency Response - as flat as possible at all frequencies. The lower the resistance, capacitance and inductance, the less attenuation and flatter the frequency response.

  Capacitance - as low as possible. The higher the capacitance, the more roll off as frequencies get higher. (for HDTV, low capacitance <20pF/ft)

  Inductance - as low as possible. The higher the capacitance, the more roll off as frequencies get lower.

  Return Loss
  > 15 dB SMPTE standard to 2.25 GHz
  > 20 dB suggested to 2.25 GHz
  Precision video cables swept to at least 2.25 GHz minimum

  [Note Belden quaranteed RL specification limit :
  > 23dB (for 5 to 850 MHz)
  > 21dB (for 850 MHz to 3 GHz)
  And Belden coaxes intended for HDTV applications (1855A,1505A,1694A,7731A) are currently 100% sweep tested up to 3 GHz.]

  Time Delay differential between Frequencies – as low as possible.

  Velocity of Propagation - A faster velocity means less high-frequency loss and flatter frequency response overall.

  ********************************************************

  Comparing the popular Belden 1505A, 1506A,1694A, 1695A, 7731A, 7732A precision video cables on their specs , then 7731A (and it is not Plenum) clearly excels : -
  Nominal Capacitance (Conductor to Shield) - lowest at 52.496 pF/m
  Nominal Inductance – 2nd lowest at 0.318 uH/m
  Nominal Velocity of Propagation – highest (best) at 85%
  Nominal Delay - lowest at 3.97 ns/m
  Nominal Conductor DC Resistance @ 20C - lowest at 8.202 ohm/km
  Nominal Outer Shield DC Resistance @ 20C - lowest at 4.922 ohm/km

  The above is purely looking from specs point of view.
  Of course there is still the practical and cost/value considerations of how visible is the difference of 7731A compared to the others cables. But for the purist , who simply must have the very best at any cost , this is food for thought.

  [Again, just want to end here by saying that the above is from my web-search and understanding. I am no expert in this area and stand to be corrected].

  Cheers ,
  SteveLim
  
  
  

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  • Those pesky specs - Jon Risch 21:24:56 08/03/06 (0)
    In Reply to: Sharing some explanations on video cable specs ... posted by SteveLim on August 1, 2006 at 22:56:56:

    Specifications are all very well and fine, but they do not tell the whole story.

    All of the specs mentioned above were obviously looked at with strictly video/RF in mind, and the importance of those specifications for video/RF use, and generally, for use with fairly long lengths (hundreds and thousands of feet), rather than the several feet to 20-30-50 feet that is used around the house in consumer applications.

    First of all, most of those specs, in and of themselves, mean absolutely NOTHING with respect to the cables use for analog line level signals and the sonic performance they will have.

    Second, even if we restrict ourselves to the intended use of those specs, and examine their application to the analysis of cables that WILL be used to transfer video or RF, not all of the specs have the same priority, or put another way, they should not all be weighted equally. Some of the cable parameters are only incidentally of concern, and others are of concern, but for reasons not immediately obvious, nor directly connected to the basic or primary parameter metric.

    There are also some questionable statements made within the text you copied from the AV site, and again, their comments ONLY apply to video/RF use, and not for audio use AT ALL, for instance:
    " None of the video RCA connectors surveyed were 75 ohm - but several manufacturers in the years since the survey corrected this with true 75 ohm RCAs, .... "

    To the best of my knowledge, the ONLY RCA that actually reaches a true 75 ohm impedance, is the WBT Nextgen plug used in conjunction with the matching WBT Nextgen jack. You MUST use BOTH the WBT Nextgen plug AND the jack in order to maintain the benefit of a true 75 ohm characteristic impedance. Use a regular RCA jack, and the plug's benefit's are lost.

    So their statement is in error, and only confuses the issue of RCA plug Z.

    AND
    "Excessive resistance in a cable will attenuate the signal."

    This is true, but is typicaly not a signficant factor for home use.
    The most strenuous use of a coaxial cable in the home is the run from a satellite dish, and this is an extremely high RF frequency, AND a fairly long run is involved, say 50 to 100 feet. As such, the overall cable parameters have more to do with the loss of signal at these frequencies than JUST the resistance. Yes, if you use a VERY small center wire, then there will be some losses for runs that are longer than a few dozen feet, but usually these losses will occur all across the band, and will not favor one frequency or another that much.

    Typically, if the run is NOT a 50 foot run, and/or the signal is NOT from a satellite dish, the the resistance of the center wire will not be a significant factor. There are always special circumstances, or specific situations, such as the satellite dish, that might actually benefit from less resistance, but on the whole, even for vidoe and RF, unless you are doing something atypical, the resistance of the coax center wire will not be a problem.

    "Velocity of Propagation is what you most often see in a catalog, and comparing VP between cables gives you some idea of which cable performs better at high frequencies. A faster velocity means less high-frequency loss and flatter frequency response overall."

    This is just plain wrong. A higher velocity of propogation DOES NOT mean less high frequency loss and a flatter frequency response. WRONG.

    There are other questionable statements about the various parameters, but these are the worst.

    I am short for time right now, but I will post a continuation of my reply as time permits, probably this weekend sometime, and try to cover some of the coaxial cable parameters, and what they have to do with video, and how they might relate to audio use (if at all).

    I will also address your summary points, as it would probably be helpful to you as well as others reading this.

    
    Jon Risch
    
    

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