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: the HDMI Extender.
Basic HDMI transmission characteristics:
HDMI is based on TMDS, transition minimized differential signaling, developed by Silicon Image. In order to support non-compressed HD video quality, HDMI needs to support data rates up to ~2.25 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.
In the Digital domain, the figure of merit that represents the quality of the Digital Transmission Link is called the "EYE Pattern." An EYE Pattern measured by sending PRBS ( pseudo random bit sequence) at the desired data rate ( 2.25Gbps for HDMI example) and overlapping a "histogram" of the data series. The EYE Pattern represents a series of bits, so a perfect EYE would show perfect square sequences.
Any physical medium such as HDMI cable will have some finite resistance, capacitance and impedance mismatch. The finite resistance of the cable will minimize the height of the Eye, capacitance in general will "slow" the rising and falling edges of the bit sequence and impedance mismatches will manifest itself in a "ripple like" pattern on the EYE.
A "open" EYE pattern ensures that the signal at the HDTV receiver can correctly decipher the digital signal for error free signaling. For example, if a "1" is sent from the transmitter ( HD source) then the HDTV will receive a "1" . A "shut" EYE pattern that has been degraded will cause the HDTV receiver to misinterpret the digital sequence and will generate "bit errors" ( a "1" may be interpreted as a "0") which manifests itself in incorrect pixel display on the screen. A completely "closed" eye pattern will be unrecoverable at the HDTV and render the picture un-viewable.
Open EYE Pattern . Error Free
Shut Eye Pattern .
Completely Closed Eye Pattern .
Using HDMI Extender to mitigate transmission problems over long cables:
In many front projection installations 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. In short, the HDMI Extender will correct the HDMI signal to ensure a OPEN Eye Pattern for Error Free transmission.
In addition to video data, it is important that the DDC data channel ,less than 400KHz( i2c), is not corrupted. HDE extender should incorporate DDC channel signal compensation to ensure error free transmission.
The HDMI Extender is low power and can be attached to the display end ( after the long cable) to completely rectify any errors caused by transmission over long HDMI cables. The extender is housed in a small ease to use package. Power can be supplied directly from the HDMI source, however an external AC adapter is provided and recommended. The HDMI Extender can be used with DVI-D products simply by using a HDMI to DVI-D cable or adapter.
HDTV technology is changing rapidly. HD connections such as HDMI will become the de facto standard in HDTV connections. Transmitting HDMI signals over long cables is a challenge and often causes pixelization and signal drop out. An external HDMI Extender can be placed at the display end of the cable to completely compensate for signal degradation caused by a long HDMI cable.
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