Background: The main advantages of HDMI, high definition multimedia interface, over standard analog video transmission is that it is 100 % digital from source to display. However, transmitting HDMI over long distances is not so easy. In this article we will describe some of the issues encountered during HDMI transmission and present a solution that can significantly improve HDMI transmission: HDMI Extender.Basic HDMI transmission characteristics: HDMI is based on TMDS, transmission minimized differential signaling, developed by Silicon Image. In order to support non-compressed HD video quality, HDMI needs to support data rates up to ~1.65 Gbps for each of 3 transmission pairs. In addition, TMDS is based on a characteristic impedance of 50 Ohms ( 100 ohms differential). The math of HDMI signaling: Although TMDS is a base-band “digital” signal, mathematically it can be thought of as a summation of harmonically related sine waves. The great French mathematician, Jean Baptiste Fourier (1768-1830), showed that any signal waveform including a square wave (think digital here) could be resolved into a summation of harmonically related sine waves. As the data rate of a digital signal increase, so does the frequency of the individual sine waves that make up the “composite” digital signal. For example, for a 50% duty cycle perfect square wave, the fundamental frequency (lowest frequency component) is ½ of the data rate. In summary, a digital signal is simply composed of the fundamental frequency sine wave and harmonics of this fundamental sine wave. The amplitude of the individual sine waves is determined by Fourier transforms. The challenges of HDMI signaling over long cables: Transmitting high-speed HDMI signals is not easy because the frequency of the individual sine waves is such high frequency. As frequencies of the signal increase, the design of a good HDMI transmission product falls in the realm of good RF/microwave engineering. The challenges of high-speed digital transmission can be broken down to : Attenuation: as frequencies increase, signal attenuation of the cables increases due to the “skin effect.” Essentially, the “skin effect” describes the phenomena that as the frequency of a sine wave increase, the wave tends to travel on the outer portions of the cable conductor. Whereas ordinary 60Hz AC signal might travel through the entire cross sectional area of the conductor, a high frequency may only travel along the outer edges of the conduct therefore the “impedance” to signal travel is greater. Think of this as a water pipe where the center of the pipe is blocked and fluid can only travel on the outer edges of the pipe. In addition, the attenuation of a cable increases with the cable length. Intra-signal pair skew: recall that higher frequency sine wave also have shorter wavelengths. HDMI is based on differential signaling, therefore the “skew” ( any delay between a transmission pair ) of each the signal must be minimized. Any differences in trace/cable lengths will tend to skew the arrival time of the signal to the receiver. As frequencies and cable lengths increase, the errors will be more pronounced. Impedance matching: : HDMI is based on 50-Ohm ( 100 ohm differential) impedance. All cables, traces must be matched to this characteristic impedance. Any “mis-match” causes reflections, which degrades transmission performance. Using a HDMI extender to mitigate transmission problems over long cables: In many front projection installation where cable lengths are significant, the problems described above will surface in the form of “pixelization” or even a complete picture drop out. An external HDMI Extender can be used in order to correct and compensate for the long cables that causes the impairments. The HDMI Extender automatically adjust and compensate for cable attenuation up to 40 dB. In addition, an adaptive equalizer compensates for the time-based skews. A limiting amp “squares “ up and re-shapes the input signal before sending to the output through output buffers.
Jeff Su is product development manager at <a href="http://www.octavainc.com">www.octavainc.com</a>. 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 <a href="mailto:info@octavainc.com">info@octavainc.com</a>
