HDMI Cable®

Understanding HDMI®

HDMI® (High-Definition Multimedia Interface) has recently become the definitive standard for audio/video connectivity between various types of data and multimedia devices. HDMI ports are found on everything from HD TVs and blu-ray players to computers, smart phones, tablet PCs, video game consoles, and camcorders.

The HDMI interface provides a medium which makes it possible to transfer high quality, digital, multi-channel audio signals along with video at extremely high resolutions and color bit-rates. Since HDMI is a digital signal with data error correction, it is much less susceptible to electromagnetic and low-intensity radio frequency interference, which cause snow, ghosting, or other signal distortions in an analog signal.

Understanding HDMI® Video - Resolution

All HDMI cables and devices are able to transfer and process a basic set of video resolutions including the basic Standard Definition (SD) video resolutions of 480i (NTSC) and 576i (PAL) and some of the High Definition (HD) video resolutions, specifically 480p, 576p, 720p, and 1080i. Each of these numbers represents the number of video lines displayed on the screen. The more lines, the clearer and sharper the resulting video image will be.

Each of these resolutions has a horizontal number associated with it. The most important to know are those of the more common HDMI resolutions - 720p and 1080i. which are 16:9 ratios of horizontal to vertical. So if you take the vertical resolution, divide by 9, then multiply by 16, your result will be the horizontal resolution in pixels. 720p resolution is 1280 pixels wide, while the 1080i resolution is 1920 pixels wide. This is important if you want to connect a standard HDMI monitor to a computer because you'll need to select the proper resolution on the computer to match the monitor, either 1280x720 or 1920x1080.

The "i" and "p" letter suffixes for each resolution tell you whether the screen is "interlaced" or "progressive" scanned. An interlaced scan indicates that the video image is drawn on the screen in alternating lines. So, odd-numbered lines (1, 3, 5, 7, etc.) are drawn first. Then, when the last available line is displayed, the even-numbered lines are filled in (2, 4, 6, 8, etc.). The interlaced image is good enough for most media, but with faster moving media, it can produce some "flickering" because the individual lines can be perceived.

Alternatively, a progressive scanned image is drawn sequentially, line by line which produces a much cleaner image, without the flickering that can sometimes be caused by interlacing. Progressive scanning requires that the image be drawn at twice the speed of an interlaced image, so equipment must be capable of transferring data twice as fast as interlacing requires. To put it in technical terms, it needs to be able to handle twice the "bandwidth", which is the amount of data that can be transferred each second.

High Speed HDMI also requires the support of more advanced resolutions such as 1080p and 4K. So, referencing the discussion above, you know that 1080p refers to a progressively scanned image with 1080 lines of resolution and 1920 pixels wide.

However, 4K is not as intuitive. The 4K resolution has the same number of lines tall and pixels wide as you would have by placing four monitors together, two wide and two tall. So the 4K resolution is 1920 x 2 = 3840 pixels wide and 1080 x 2 = 2160 lines tall. The 4K resolution is not commonly available yet except in the more advanced cinematic displays and source devices, but with larger-sized displays beginning to appear, the introduction of 4K resolution into the mainstream consumer market is on the horizon.

Support for 3D video is another High Speed HDMI feature that is becoming more commonplace. With the recent trend towards 3D movies and 3D support in video game consoles, the demand for 3D video support is also increasing. Processing 3D signals requires more bandwidth, but generally if a device supports 1080p and 4K resolutions, it should also be able to handle the bandwidth required for 3D video signals.

Understanding HDMI® Video - Color Depth

In addition to the basic consideration of video resolution, another important factor in video displays is Color Depth, which is the number of colors capable of being shown. The real world an infinite number of different color shades, but video equipment is necessarily limited in the number of colors that can be shown. Color Depth is a measurement of the number of bits of data that can be assigned to produce different color shades - the more bits, the more colors that can be displayed.

For HDMI the Color Depth is usually measured as the number of shades of color intensity assigned to each of the primary color channels: red, green, and blue. Minimally, each channel must be capable of handling 8-bits of color intensity for a total of 24-bits of information for each pixel, or each dot on the screen. A 24-bit Color Depth equates to over 16 million different colors, which produces vivid, lifelike color images.

More advanced Color Depths use more bits per channel, including 12-bits per channel (36-bits total) for over 68 billion colors and 16-bits per channel (48-bits total) for over 280 billion colors! Among these are various "Color Spaces", which are predefined sets of colors, which include x.v.Color™. x.v.Color is a promotion name given to the products that have the capability to realize a wide color space based on the xvYCC specifications and is a trademark of Sony Corporation. Currently only AVCHD™ camcorders and PlayStation 3 consoles support the x.v.Color color space.

Understanding HDMI® Audio

Audio signals require significantly less bandwidth than video signals, even for the highest quality, so all HDMI equipment can handle high-quality audio signals, including uncompressed 2-channel PCM with sample sizes up to 24-bits and a 192 KHz sampling rate in addition to support for compressed 5.1 and 7.1 channel audio.

Even Standard HDMI Cables have sufficient bandwidth to carry the more advanced audio types that require a bit more bandwidth such as Dolby TrueHD™ and DTS-HD Master Audio™, as well as the basic HDMI video resolutions (720p/1080i).

One more audio consideration is the Audio Return Channel feature in the HDMI specification. The Audio Return Channel is a way to carry audio signals out to be played over external devices. For example, when you have an HDTV using its internal tuner and you want the audio to play over your surround sound speaker system. Audio Return Channel provides a way to carry the audio signal from your TV to the receiver/amplifier.

Keep in mind that only equipment that is specifically designed to process and/or transfer the Audio Return Channel can take advantage of this feature. A piece of equipment must specifically list support for the Audio Return Channel or it is not supported by that equipment.

Understanding The HDMI® Ethernet Channel

Another major HDMI feature that bears mention is the HDMI Ethernet Channel. The HDMI Ethernet Channel is a method by which internet connectivity can be shared between properly equipped devices, without the use of additional wires. For example, if your blu-ray player is connected to the internet, your television can also be connected through an HDMI Cable With Ethernet.

As with the Audio Return Channel, all the equipment and devices must specifically be designed to process the HDMI Ethernet Channel. Unlike the Audio Return Channel, normal Standard and High Speed HDMI Cables are not capable of carrying the Ethernet signal - you must use Standard or High Speed HDMI Cables With Ethernet.

With many devices now equipped with wireless network adapters, the usefulness of the HDMI Ethernet Channel is somewhat diminished. However, a wired Ethernet connection can produce greater speeds and throughput, so the HDMI Ethernet Channel is worth using if your equipment supports it.

Understanding HDMI® - Hz, MHz, and Gbps

Definitions:

Hz is an abbeviation for Hertz, which means "cycles per second".

MHz is the abbeviation for Megahertz. 1 MHz is 1 million cycles per second.

Gbps is short for Gigabits per second. 1 Gbps is 1 billion bits per second. So, how do they relate? Without getting too deep into the technical aspects, when you see an HDMI number with Hz next to it, it is usually something like 24, 60, 120, or 240 Hz. This is referring to the refresh rate, which is the number of times the screen is drawn per second, also known as frames per second. The common television refresh rate before HDMI came along was 29.97 frames per second for NTSC and 25 frames per second for PAL.

With HDMI, the Hz rating can vary from 24 to 60 Hz for a normal 1080p display. Televisions may refresh their screen more often, so they will end up redrawing the same frame of video signal several times. For example, a television with a 240 Hz refresh rate will redraw each HDMI 1080p@60Hz frame four times.

The MHz and Gbps numbers are closely related. They refer to the clock rate (MHz) and the associated bandwidth (Gbps) of the HDMI signal. A 165 MHz clock rate equates to a bandwidth of 1.65 Gbps. The two terms are generally interchangeable when discussing the video amplifier and how much data it can handle in any given second. The minimum clock rate/bandwidth required to transmit a 1080p@60Hz signal with 8-bit per channel color depth is 165 MHz/1.65 Gbps per channel, or 495 MHz/4.95 Gbps total.

The most important thing to understand is that a higher Hz rating means a clearer signal with less flicker, while a higher MHz/Gbps rating means that more information can be transmitted and processed each second. More data means more features - higher resolutions, Deep Color with higher bitrates, 3D video signals, and higher quality multichannel audio.

HDMI® Devices - What To Look For?

When shopping for HDMI devices, you should look for specific support for the features that are important to you. Whether they be source devices (blu-ray players), synch/display devices (monitors, televisions), or repeater devices (switches, splitters), if you do not need 4K or HDMI Ethernet Channel support, there is no need to spend money on equipment or cables that include those features. For "repeater" class devices like switches, splitters, or extenders, another primary consideration is the video amplifier bandwidth. There are a couple of important numbers to keep in mind when looking at these devices:

2.25 Gbps - The minimum bandwidth required to transmit a 720p/1080i signal at 60Hz with 8-bit per channel color depth is 0.75 Gbps per channel, or 2.25 Gbps total.

4.95 Gbps - The minimum bandwidth required to transmit a 1080p signal at 60Hz with 8-bit per channel color depth is 1.65 Gbps per channel, or 4.95 Gbps total.

6.75 Gbps - The minimum bandwidth required to process a Full High Definition 3D signal (1920x1080 resolution for each eye at 24 Hz refresh rate) is 2.25 Gbps per channel, or 6.75 Gbps total.

10.2 Gbps - The current minimum bandwidth requirement of the High Speed HDMI standard is 3.4 Gbps per channel, or 10.2 Gbps total.

HDMI® Cables - Basic Choices

With HDMI Cables things are much simpler. For home theater applications, there are two choices with a single option for each. The two choices are:

High Speed HDMI Cable - Cable is designed to handle 1080p and 4K resolutions. It is required to have a bandwidth of at least 10.2 Gbps, which is enough to process all but one of the features in HDMI at the same time, including 3D, 48-bit Deep Color, Audio Return Channel, and any type HDMI supported audio signal. The one feature it cannot handle is the HDMI Ethernet Channel.

Standard HDMI Cable - Cable is designed to handle 720p/1080i resolutions and capable of a total minimum bandwidth of 2.25 Gbps. Standard HDMI Cables can handle some of the HDMI features, but not all at the same time like a High Speed HDMI Cable can. The one feature it cannot support is the HDMI Ethernet Channel.

The following cable types feature the addition of support for the HDMI Ethernet Channel:

High Speed HDMI Cable With Ethernet - This cable can do everything that a High Speed HDMI Cable can do, with the addition of support for the HDMI Ethernet Channel.

Standard HDMI Cable With Ethernet - This cable can do everything that a Standard HDMI Cable can do, with the addition of support for the HDMI Ethernet Channel.

There is a fifth cable type, which is the Standard Automotive HDMI Cable, but that doesn't apply to home theater applications. If you've got a car HDMI installation, you will use this cable and won't have any other choices to make.

Generally, if you're hooking up a 1080p 3D-ready blu-ray player to your 1080p 3D-ready TV, you're going to want a High Speed HDMI Cable. If your TV that is only capable of 720p and you won't be using 3D, you can use a Standard HDMI Cable.

However, it can get a bit more complicated...

HDMI® Cables - AWG Explained

There are more choices than just Standard and High Speed HDMI Cable. There are the different AWG (American Wire Gauge) ratings of the different cables. There are 28 AWG High Speed HDMI Cables and 22 AWG Standard HDMI Cables.

First, AWG (American Wire Gauge) is a measure of the thickness, or gauge, of a wire. The AWG system is based on the number of times a wire could be wound around a spool of a given width, so a 30 AWG wire could be wound 30 times, while a 20 AWG wire could handle only 20 windings. Therefore a 20 AWG wire is thicker, with a larger diameter, than a 30 AWG wire.

Wire gauge directly relates to the amount of electrical current that can be carried on the wire. The larger diameter of wire, the more current it can carry. In terms of HDMI this means that a larger gauge wire (smaller AWG number) is capable of higher bandwidth than a smaller gauge (larger AWG number). Therefore, a 22 AWG wire is capable of higher bandwidth than a 28 AWG wire.

Keep in mind that as the length of a wire increases, so does the overall resistance of the wire. Increased resistance means decreased current capacity and therefore decreased bandwidth in HDMI terms, so the longer the wire, the lower the performance capacity even thought the same kind of wire is used.

The cutoff points for High Speed HDMI Cables of each AWG rating are:

28 AWG = maximum 10 feet

26 AWG = maximum 12 feet

24 AWG = maximum 15 feet

22 AWG = maximum 25 feet

So a 12-foot 28 AWG HDMI Cable is not rated for the full 10.2 Gbps required for the High Speed designation. However, it doesn't just jump down to the minimum 2.25 Gbps required for Standard HDMI Cables. It may be capable of 9.5 Gbps, which is almost enough for the High Speed rating, but because it isn't 10.2 Gbps, it must therefore be classified as a Standard HDMI Cable. The longer the wire, the lower the bandwidth it will be able to handle.

HDMI® Cables - More Considerations

There are a couple more considerations when choosing an HDMI Cable.

Wire Length

First is the application. If you are connecting a cable directly from the source (blu-ray) to the sync/display (TV), you can be assured that if the cable is rated for High Speed, you can get the full 10.2 Gbps, especially if your length is less than the cutoff length.

However, if you are installing a repeater device, such as an AV receiver, an HDMI switch or splitter or anything else in between, you should consider that each connection adds some amount of resistance to the total connection. So, even if your total length is less than 10 feet, you may have too much total resistance for a set of 28 AWG cables to handle High Speed signals.

For this reason, we always recommends the use of minimum 24 AWG cables when connecting any intermediate device between the source and the sink/display. It may even be advisable to go with 22 AWG cables, just to be safe.

Silver Plating

Another consideration is Silver Plating found on some HDMI Cables. HDMI signals are at such high frequencies that the signal itself rarely penetrates to the core of a wire. Instead it rides along the outside of the cable in what is known as the "skin effect".

Silver is a better conductor than copper (it has lower resistance, so can handle higher bandwidths), but is too expensive to make solid wires from. However, plating a wire in silver is much cheaper and takes advantage of the skin effect. Since the signal is only riding along the outside of the wire, the silver plating gives the signal a lower-resistance path, which increases the bandwidth capability of a cable. If a normal cable is on the edge of being able to reliably handle the full 10.2 Gbps bandwidth required for High Speed, the silver plating can make the difference between a reliable signal and one which occasionally cuts out or that produces digital artifacts.

Conclusion

While the technical aspects of HDMI can be a bit confusing, we hope that this guide has helped make some sense of what is most important when shopping for HDMI equipment and accessories. If there is something that you are still unsure about, please feel free to contact our Technical Support representatives at support@primuscable.com. We will be happy to answer any questions you might have and help you choose the right equipment for your needs. Most importantly, they can help you get the most value and usefulness from your limited entertainment dollars!