Friday, May 27, 2005
What is a Quality Cable?
There are several important factors to keep in mind when choosing cabling products:
- There is no one all around best cable type for every application, even when the cables will be used to carry similar signals
- The electrical performance is not the only significant factor in choosing a cable, and can often be less significant than mechanical and jacket type specifications
- Even a super high quality cable can have poor results if the interconnection of the entire system is not correctly designed
When choosing a cable for an application the first thing to consider is signal type, this will lead you to a general class of cables. For example, if you are connecting two devices with component video connections you will want to use either a component video cable which consists of 3 75 ohm coax lines, or 3 individual composite video cables of the same length.
Next you should consider the length of the cable needed. For most signal types short cables (for simplicity we will call this anything less than 25', though it varies depending on the signal type) have different electrical requirements than longer runs. Typically cables designed for longer signal runs have larger conductors to minimize signal loss in addition to lower capacitance designs to control high frequency attenuation. These cables are somewhat inflexible and heavy. Short interconnects, especially those used in the back of equipment racks need to be more flexible than longer runs.
For the shorter cables signal loss and capacitance issues are often trivial since any signal loss in the cable over the short length is virtually undetectable. Signal loss is a factor of cable design and length, with attenuation in coax cables usually measuered in dB per 100ft. With typical values in the range of 1.5dB per 100ft in the component video frequency range for a precision coax cable. At 6 ft. this would be about .09dB of attenuation. To put this in perspective anything less than 3dB would probably not result in a visible change in image quality.
An important issue for short interconnects is bend radius. If a video cable is bent smaller than its bend radius the dielectric can be damaged which can cause an abrubt change in impedance at that point. This can result in a ghosting or blurring effect on the video image. Bend radius is directly related to cable diameter, so in the case of very short interconnects with tight bends a smaller cable is definitely the best choice.
Consider where the cable will be run. Will it be run through a wall, over a ceiling, buried, in conduit, or just strung behind a rack. Different cable jacket compounds are made specifically for these different applications. Consult your local building code for the required UL rating if the cable will go in a wall or above the ceiling. Special compounds are made for decreased flammability, UV resistance, direct burial, and extra flexibility.
Finally look at the entire interconnect system as a whole. Consider an overall grounding scheme (too big a topic for this post), route low level signals apart from AC power connections, use balanced audio connections when practical and needed. If you have a problem such as an audio buzz or video hum bar check grounding issues first before running a new cable, the cable is rarely the problem in this situation.
Many vendors will try to push an exotic metal, super expensive, questionable science cable product on you without even discussing your application. Find a cable supplier that will go through the details of your installation and then recommend the right products. The result will be better sound and video at a much better price.
Monday, April 18, 2005
1/4" Stereo, Balanced or Insert?
An example of a 3 contact 1/4" plug, Switchcraft part number 297, is shown above. This part is also called a 1/4" TRS connector, where TRS stands for tip, ring, and sleeve, referring to each of the 3 contact surfaces on the connector. While this type of connector is often called a "stereo" connector it is important to understand that the stereo configuration is not the only way the 1/4" TRS connector is used.
Following are the 3 most common signal configurations for this connector:
- The first is as a stereo connection, most often used with headphones. In this application the tip connection is used for the left *unbalanced positive signal. The ring is used for the right signal. The sleeve is the ground connection for both the right and left. This type of connection is commonly found on high end consumer audio equipment and on professional mixing consoles and recording equipment.
- The second common use for the 1/4" TRS connector is as a *balanced audio connector. In this case the tip connection is used for the "hot" connection, the ring is the "cold" connection, and the sleeve is connected to the cable shield. The balanced 1/4" connection is often used as a line level output on professional audio equipment.
- The third use is as an insert connection, often found on mixing consoles. In this application the tip is used as the send, or output signal from the mixer channel. The ring is used as the return, or input signal to the mixer. On some mixer models the tip and ring are reversed. The sleeve is used as the ground connection for both signals. In this application the signal path in the mixer is broken when the 1/4" plug is connected to the insert jack causing the signal path to flow through the equipment attached to the insert cable. An insert cable would normally have the 1/4" TRS connector on one end and split out to two seperate connectors to attach to the input and output of the outboard equipment.
Don't assume that just because the cable fits the interconnection will work. It is in many cases possible to connect one of the types listed above to one of the other types with an adapter cable. Look closely at equipment manuals to find out exactly how the inputs and outputs are wired. If the two pieces of equipment don't have the same connection type you may need an adapter cable or additional equipment to make your connection.
*Balanced vs. Unbalanced signals - The complete explanation of balanced audio wiring is too big a topic for this post, so I will be doing a complete post on this subject in the near future. Suffice it to say that balanced wiring is a way of reducing external interference in an audio line that requires 2 signal wires twisted together, and in the case of most audio installations surrounded by a braided or foil shield. For a balanced cable to be effective both the signal source equipment and receiving equipment must have balanced connections.
An unbalanced line relies solely on the shield for protection from interference and only requires 1 signal line plus the shield conductor which also acts as the signal ground.
Monday, April 11, 2005
Subwoofer Cables, Fact or Fiction?
The input of the powered sub is a high impedance type which means only a tiny amount of power is transferred through the cable, the power amplification occurs inside the subwoofer at its internal power amp. Because there is so little power transferred through the cable, its conductor size is insignificant. Typically 22, 24, or even 26 gage conductors are appropriate, and will not have noticeable signal loss even at lengths well over 100 ft.
The signal most commonly used by a powered subwoofer is a limited bandwidth line level audio signal. In this case limited bandwidth means that high frequency information is removed from the signal either at the a/v receiver - preamp/processor, the subwoofers input circuitry, or in some cases both. Capacitance is the primary factor in high frequency loss in audio cables. Since high frequencies are not used by the subwoofer the capacitance of the subwoofer cable is not as important a specification as it would be for interconnecting full range components.
So, don't be fooled by high priced "subwoofer" cables, use a normal audio interconnect and your sub will sound just as good. For powered subwoofer applications at AVCable we recommend our PPC - precision audio patch cable, for in-wall applications the Gepco 61801EZ is a great bulk cable choice.
Friday, April 01, 2005
Can you make a component video to RGB cable?
Before I get into the details of what we can and can't do I think it would be helpful to define some of the relevant video standards and terms:
- Component Video - Also known as Y/Pb/Pr, Y/Cb/Cr, YUV, and EIA/CEA-770.
The component video standard calls for three parallel channels on 75 ohm impedance coaxial cables. Component video cables are typically terminated with RCA connectors for consumer applications or BNC connectors for professional applications. The three channels are known as Y (luminance) essentially a black and white representation of the image, Pb (blue color difference) , and Pr (red color difference). All three of these signals are derived from different combinations of the red, green, and blue primary colors present in the image.
The component standard describes several different signal formats which differ in resolution and scanning method (interlaced or progressive). The standard explicity defines the formats as 480i and 480p in both 4:3 and 16:9 aspect ratios - called standard definition, and 720p and 1080i - called high definition. The number indicates the visible horizontal lines of resolution in the image, the letter indicates progressive or interlaced scan. Additional formats are also present on some equipment.
- RGB Video
RGB uses 3, 4, or 5 parallel channels to seperately carry the red, green, and blue primary color signals and timing information. The 4th and 5th channels are used for different implementations of the timing (sync) signal. The 3 channel version has sync combined with the green signal and is also know as sync on green. The 4 channel version has the horizontal and vertical sync pulses combined on a seperate line, this version is known as RGBS or RGB composite sync. The 5 channel version has the horizontal and vertical sync pulses on two seperate lines, this version is called RGBHV or RGB with seperate sync. RGB cables are made with 75 ohm coaxial cables and usually have BNC connectors though RCA connectors are occasionally used.
VGA and its relatives SVGA, XGA, and UXGA are very similar to RGBHV. This standard is used in computer displays and some projection systems. In addition to the seperate Red, Green, Blue, Hsync, and Vsync signals present in RGBHV the VGA family also have some additional lines for digital data communications between the display and computer. VGA cables have 5 coax lines and a varying number of twisted pairs and single conductors for digital data. The standard VGA connector is a 15 pin high density d sub (hd15).
So, how do you convert from RGB to Component or Component to RGB?
Simple enough, all you have to do is combine the RGB signals according to a specific formula to derive each of the component video signals. For example Y = .299R + .587G + .114B. The Pr and Pb signals are derived with similar equations. The reverse conversion is also accomplished with a similar set of equations starting with the Y,Pb, and Pr values. Also, the horizontal and vertical timing signals (sync) have to be processed and combined with the appropriate channel.
While it looks simple on paper it is too complex to be achieved with a simple cable, fortunately there is a type of device called a transcoder designed to do this sort of conversion. Transcoders vary in price from approximately $250 to $1500 depending on signal quality and features. Many scalers combine transcoding with more advanced switching and format/resolution conversion.
How about RGBHV to VGA?
Good news, this can usually be done with a simple adapter cable with 5 BNC or RCA connectors on one end and a HD15 connector on the other. For this to work the devices on both ends must be set up for the same resolution and format. This cable is just adapting, not converting.
How about Component to VGA?
Many video projectors and plasma displays accept a component signal on an HD15 connector, in this case you can use a simple 3 RCA to HD15 cable to make the connection. It's critical to make sure the display is specifically equipped to accept component on this input since the cable isn't converting component to VGA, it is just adapting the connection for the HD15 input.
The wide variety of signal formats and standards can make integrating a system challenging. Fortunately it is possible to make the necessary conversions with minimal or no loss of signal quality using the appropriate adapters, cables, and transcoders.
Please share your insights and experience with RGB and component interconnection issues. If you need help with a specific project send me an email at firstname.lastname@example.org.
Monday, March 28, 2005
Where audio/video myths are unmasked and cable quality is defined.
My name is Jim Woodier, and I will be your host.
As the owner of AVCable.com, I have been astonished again and again by the sorts of questions that come in over our support lines, and the sometimes confusing, incomplete and inaccurate beliefs that even the most seasoned professionals have about a/v cable. I blame the lack of clarity on four dynamics in the a/v market:
- the speed of today's technological evolution;
- the lack of clear-cut standards and a single governing body to set them;
- the subjective nature of image and sound quality, which makes it difficult to define what is "true";
- and a smattering of borderline deceptive marketing efforts unleashed on the industry, promoting "unique selling propositions" that may or may not have anything to do with actual performance.
The mission of the blog is to shine some light on what is definitely true, what is seemingly true, and what is relevant about a/v cable options. Together, we will explore:
- what are the standards
- what are the myths
- what are your cable options
- and how should you weigh them.
Each week, we'll post a new article derived from the questions we hear most often from professionals just like you who call AVCable to sort through the confusion and arrive at the perfect cable for the job at hand.
We also welcome your contributions!!! Please submit your questions, comments and suggestions -- write early, and write often! Blogs are more fun when they are an open forum for debate and learning.
I hope you enjoy this forum as much as I know we will enjoy putting it together -- we look forward to hearing from you!