HDMI Switch: Proof of Performance
 The typical setup is shown to the above. The HDMI source is connected thru the HDMI switch with two 6ft HDMI cables( input, and output of switch). Note that with the HDMI switch in between, the total length of the HDMI cabling has doubled from 6 feet to 12 ft. A well-engineered HDMI switch will take into account the degradations introduced by the HDMI cables. For projector systems, the output cables may run from 30-100 ft. For cable runs above 30 ft, a HDMI extender is recommended. Before we get into the details, lets first look at what physically happens to a HDMI signal as it propagates over a HDMI cable. HDMI Digital Signal at the Originating Source The picture to the below shows the EYE Diagram of a 1.65 Gbps HDMI signal coming directly from the HD source.  An EYE pattern shows the baseband transmission quality of a digital signal. An EYE pattern is essentially a histogram of digital data bits. A well- behaved system will have a wide EYE area such as the one shown to the right. This is what you want for error free digital transmission. However we will see that as the HDMI signal propagates over cable, the attenuation and "group delay" introduced by the cable will degrade the EYE pattern. As the EYE pattern degrades, bit errors will be incurred which lead to "snow" in the HD picture. HDMI Signal after 15 ft and 30 feet of HDMI cable  Here we see the degradation of the HD signal after 15 ft and 30 feet of HDMI cable. After 15 ft, the HDMI EYE pattern degrades and closes. After ~30 feet, the EYE pattern closes completely. A diminished EYE opening introduces bit errors in the HDMI receiver which leads to "snow" in the HD picture. In the case of the 30 ft cable, the video signal will be un-recoverable at the HD display. Two main causes of HDMI transmission degradation over cable are attenuation and propagation speed variations of the cable. Cable will attenuate the HD signal levels which we can observe by the reduced height of the EYE. A second effect is timing jitter. Timing jitter is due to propagation speed differences of each frequency components of the HDMI signal. The speed of an electrical signal thru a medium is a function of the dielectric constant of the medium with the dielectric constant being a function of frequency of the signal. For example, the speed of light is 3x108m/s in a vacuum ( dielectric constant =1), but will travel slower in a fiber optic cable( dielectric constant >1)with each frequency component of the signal traveling at slightly different speeds since the dielectric constant itself is a function of frequency. The byproduct of timing jitter and attenuation leads to Inter-Symbol Interference or ISI. ISI describes the "spreading " of adjacent bits to the next data bit. Both attenuation and timing jitter will cause bit errors in a HD digital signal. The impairments described above can be solved by 1) compensating the "delay" of the HDMI signal so each frequency component arrives at the HD source at the same time and 2) the Signal level can be re-leveled with a data-slicer circuit and HDMI limiting amplifier. The EYE pattern diagram to the below shows the same HDMI signal after it has been compensated by the Octava HDMI switch utilizing Clear EyeTM. The recovered signal clearly improves the signal shown above and returns the HDMI signal back to the original quality.  Bottom Line A HDMI switch is an essential component for expanding your HD system. Some products on the market have not been designed to take in to account all the factors shared here. Octava's HDMI switches with Clear EyeTM. have been engineered to flawlessly switch and IMPROVE HDMI signal quality. Clear EyeTM. is based on the latest generation chipsets and ultra wideband design techniques meeting HDMI V1.2. We design with the HD system in mind. Jeff Su is product-marketing manager at Octava Inc. http://www.octavainc.com. Jeff is a graduate of the Georgia Institute of Technology (BSEE, MSEE) and has over 10 years of experience in CATV systems, RF, and microwave designs. He may be reached at info@octavainc.com |
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