The High Definition Multimedia Interface (HDMI) 1.4 specification supports cinema-quality video resolution of 2160p (3840 by 2160 pixels), also known as Quad HD, ¡°4Kx2K,¡± or just ¡°4K.¡± It also can support true 1080p 3D video at a 60-Hz refresh rate. The 2160p format allows for four times as many screen pixels as the ubiquitous 1080p TV format and runs at a data rate of 2.97 Gbits/s (also known as 3Gbps). Until recently, integrated silicon solutions for systems engineers to design 2160p and 1080p 3D 60-Hz video systems have not been available.
Applications for 3Gbps HDMI solutions include audio/video receiver (AVR) boxes, video wall displays, switchers, extenders, 4K projectors, and 1080p60 3D video generation. As the insatiable demand for video resolution continues to increase, more 3Gbps applications will come to market. 3Gbps HDMI interface solutions provide new features to the video space that were not previously available in past versions of the HDMI specification.
The hdmi to usb cable Even with the best silicon solutions, smart layouts and attention to printed-circuit board (PCB) challenges are needed to prevent bit errors in a 3Gbps video system. The fast data rate requirements of the latest video resolutions are pushing the limits of physics on the standard FR4 PCB material. Although higher-performance board trace materials are available, they may not be practical in a consumer electronics application due to their higher cost compared to FR4. But with careful attention to layout detail, a 3Gpbs HDMI system solution using FR4 can still be attained.
Three important board layout issues could arise with a 3Gbps HDMI signal resulting in system failure. The first is poor transition-minimized differential signaling (TMDS) intra-pair matching and poor termination (see ¡°What Is TDMS?¡± below). Next is the capability to recover HDMI data in spite of attenuation loss on cables and FR4 board material. Finally, crosstalk with other noisy signals can cause inter-symbol interference (ISI). Any one or all of these issues could degrade the video signal enough to be inoperable in the system.
With a data period of only 336.7 ps, the timing skew relation between all of the HDMI signal paths must be closely watched during system board design. Each TMDS signal pair should be routed as close together as possible to keep the trace lengths within the pair matched and to maintain 100-¦¸ differential impedance along the entire transmission line. In addition, each of the three pairs of traces should be kept the same length to minimize the inter-channel mismatch at the HDMI receiver chip.
Care should be taken to avoid routing the TMDS data signals adjacent to or over other digital communications and clocks, such as the Display Data Channel (DDC) interface or the Consumer Electronics Control (CEC) control lines. Such routing could induce unwanted crosstalk to couple onto the TMDS signals, causing bit errors.luoqiyin666
Due to the permittivity of commonly used FR4 material of 0.45 to 0.48, ¨C1-dB signal attenuation can be seen for approximately every 8 inches of total trace length within the HDMI transmission line. All other passive electrostatic discharge (ESD) protection devices and connectors will further increase the attenuation of the signals and further distort the impedance matching. Although higher-performance PCB materials are available to the designer, they may be cost prohibitive for high-volume consumer electronics applications. To pass HDMI physical-layer (PHY) compliance testing at 3Gbps, the attenuation on the TMDS signals must be minimized to meet both Tx and Rx system requirements.
The HDMI cable typically acts as the largest component of signal attenuation within a video system. The magnitude of attenuation over an HDMI cable largely will depend upon its length and the quality of the wire gauge used. For example, a 30-meter 24 American Wire Gauge (24AWG) cable will attenuate a 2.97-Gbit/s (1485-MHz) signal significantly (Fig. 1). The signal at the far end of the 30-meter cable will be seen as ¨C28 dB relative to the power at the beginning of the cable.
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