Friday, May 27, 2005

 

What is a Quality Cable?

Simply put, the highest quality cable is the right one for your application.
There are several important factors to keep in mind when choosing cabling products:

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?

There's a lot of confusion surrounding 1/4" plugs, especially the kind with 3 contacts.

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:

So, what is the important lesson here?
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?

A common myth/misconception propagated by the hi-fi audio cable industry is that a special, usually larger and heavier, audio cable is required to connect a powered subwoofer to an audio system. In truth, any good quality audio cable that can be used for interconnecting audio components will be excellent for use as a powered sub cable.

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?

On an almost daily basis we receive requests for a cable to convert component video to RGB or the opposite. I'd love to be able to build one of these, and would probably sell quite a few of them. However, there are a few technical issues in the way.

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:

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.

Conclusion-

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 avcablog@avcable.com.


Monday, March 28, 2005

 

Welcome to...

The AVCABL-OG
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!

Sincerely,

Jim Woodier
President, AVCable.com


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