Transmission Line Effects in Video Cables
The Do's and Don'ts of Component Video Cables Supplemental
We established a pretty good foundation of the important metrics governing component video cables in our Component Video Cable Definitive Guide article. However, we recently stumbled on some very poorly designed component video cables which according to the manufacturer are best suited for audio and video applications because of minimization of "eddy currents" and a host of other unproven pseudo sciences contained herein. Out of respect to the manufacturer they will remain nameless in this article. However, we will examine their cable to determine its validity for High Definition (HD) video applications.
This cable was examined with scrutiny only after I noticed resultant poor video quality (ghosting and loss of resolution) in application on my HDTV system. It wasn't until I dissected this cable that it all began to make sense.
Exotic Component Video / Esoteric Audio Cable
Cross section (left); Side view of cut cable, outer conductor separated (right)
Inner conductor spread apart, notice the plastic material in the center; Outer jacket removed, this is their idea of an effective shield
Exotic Manufacturer Description
This cable minimized eddy currents and is suitable for high end audio and HDTV video as well. The wire is considerably more advanced than other cables used for this purpose using OHNO Continuous Cast Single Crystal Copper and is one of the most expensive raw wires. There is no center conductor in this cable. The inner core is an XLPE monofilament. The signal conductors are spiral wound around the inner core. A thin layer of polypropylene dielectric separates the inner conductors from the return conductors which are spiral wound around the signal conductors but in the opposite direction.
The insulation is polypropylene. Cheap cables use PVC. The outer conductors on coax cables are considered to be the shield. Adding an extra shield on an unbalanced cable would serve no purpose in this case and would be harmful to proper signal transfer especially in the time domain.
Our lesser designed cables are 75 ohmcharacteristic impedance. These higher quality video cables are 50O. Since lengths shorter than approximately 24' are not transmission lines at video frequencies exact impedance matching is irrelevant. In this case, a 50 ohm characteristic impedance may be better due to less signal loss.
Inexpensive OEM Video Cable (Patch Cord )
The cable depicted here is the typical free cable supplied by the Satellite/Cable provider. Notice the inclusion of a foil shield in addition to the braided shield. The shield coverage of this cable is significantly better than the $1200 exotic video cable in the previous example.
Also notice the solid core center conductor and thicker dielectric material. The conductor spacing between inner and outer conductors, in conjunction with the dielectric is what determines characteristic impedance of the cable.
Pursuing the Truth…
Unfortunately the manufacturers claim about impedance matching being irrelevant at video frequencies is incorrect.
The sample rate for 1080i HDTV is 74.25 Mhz. This is arrived by taking 1920 horizontal pixels plus 280 samples for sync per line times 1125 lines including sync (1080 +45) times 30fps. With a factor of 2 (based on Nyquist theorem) the highest analog frequency you can theoretically achieve is 37.125 Mhz.
This approximates to a real world number of about 30 Mhz for 1080i with about 20% overhead.
Coming up with the highest frequency component possible in a digital HD system is very straightforward. Here is how it is calculated:
- Step 1: Figure the pixels per line (including sync). For a 1920x1080 system, SMPTE-274M specifies the "total sample periods per line" to be 2200 (1920 Active and 280 non-Active.). For a 1280x720 system, SMPTE-296M specifies the "total sample periods per line" to be 1650 (1280 Active and 370 non-Active.).
- Step 2: Find the total number of lines (including sync). For a 1920x1080 system, SMPTE-274M specifies the "total lines per frame" to be 1125 (1080 Active and 45 non-Active.). For a 1280x720 system, SMPTE-296M specifies the "total lines per frame" to be 750 (720 Active and 30 non-Active.).
- Step 3: Figure what frame rate you are figuring out. For this example I will use 30 fps for 1080 line (corresponds to 60 field interlace (1080i/60) or 30 frame Progressive or Progressive Segmented Frame (PsF)(1080P/30 or 1080PsF/30)) and 60 fps for 720 line (corresponds to 60 frame progressive (720P/60)).
- Step 4: Figure the sampling frequency. This is found by this formula: pixels per line x lines per frame x frames per second = sampling frequency For 1080: 2200 x 1125 x 30 = 75,250,000 samples per second or 75.25MHz For 720P: 1650 x 750 x 60 = 75,250,000 samples per second or 75.25MHz
- Step 5: Lastly, remember Nyquist. For digital sampling, you need to sample at a rate over twice the frequency of the highest frequency component in the waveform that you are trying to sample. Taking 75.25MHz, the absolute theoretical maximum frequency that can be carried by this sampling rate would be just under 37.125MHz. Now in the real world, we don't often deal with perfect theoretical maximums so the common accepted bandwidth for these two common HD formats is between 30 and 35MHz. (The SMPTE specs noted above define a nominal bandwidth of 30MHz for analog interfaces of 1080i/60, 1080P(sF)/30 and 720P/60. It also specifies 60MHz for the 1080P/60 standard which is sampled at 148.5MHz).
An update proposal to SMPTE 274M can be found at: http://www.smpte.org). The other thing to remember is that this is in a perfect world where digital processing is not messing with the signal. In addition to the large amount of compression done to an HD signal for distribution (which is what most people see), one common HD production format, HDCAM, "pre-filters" (subsamples) the 1920 active pixels in each line to 1440 reducing the bandwidth to about 24MHz at the production stage.
For additional references, check out the glossary that Scott Thomas (Director of Engineering - The Victory Studios) helped write for HighDef.com and the Highdef-411 directory found here: http://www.highdef.com/library/glossary.htm
For more information about Video Bandwidth, I recommend the excellent article found here:
What about Uncompressed HDTV?
According to Belden, uncompressed High Definition Video Signals is greater than 1500MHz and is considered more of an RF signal than video. The bandwidth of a single uncompressed high definition video picture now exceeds that of standard multichannel CATV/broadband signal. Further, the bandwidth limit of HD is often higher than the transmitted channel frequency!
Thus when dealing with uncompressed HDTV the cable characteristic impedance and termination impedance is critical for even the shortest cable lengths. Other factors such as Return Loss must be considered.
Transmission Line or NOT?
"Since lengths shorter than approximately 24' are not transmission lines at video frequencies exact impedance matching is irrelevant." Exotic Cable Vendor Statement
Pursuing the Truth….
The argument of transmission line relevancy, or lack thereof at video frequencies must be considered.
If we use classic and proven transmission line theory to determine the importance of cable impedance matching we derive the following:
wavelength (in meters) = v / (f*sqrt(er))
where f is the frequency of the signal in Hz, v is the velocity of the signal =
3x10^8 meters/second (vacuum) and er = velocity of propagation factor
d = outer diameter of inner (center) conductor (approximate value for stranded)
D = outer diameter of dielectric
e = dielectric constant (e=1 for air)
This equation supports the fact that the characteristic impedance of a coax cable is directly related to the diameter of the conductor and the dielectric. For component video cables, this characteristic impedance should be 75-ohms. With characteristic impedance (Zo) held at a constant 75-ohms, the variables are the diameters and dielectric constant.
Once an understanding of the engineering principles behind 75-ohm cables is established, practical issues that apply to designing, or considering the purchase of a 75-ohm video cables include conductor material and diameter, dielectric material and diameter, grounding shield, noise protection, termination (solder joints) and RCA connector design.
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