HDMI signal isolation: optical fiber vs. dedicated chip, which of the two solutions should you choose?

Latest update time：2022-01-05

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In the field of industrial control, we often see HDMI cables used to connect monitors to monitor key indicators throughout the production process. Sometimes the monitor is very far away from the entire production line, and the HDMI cable is not long enough. We often need to introduce a signal isolation solution to allow the signal to be transmitted over a longer distance.

This article will introduce two different HDMI signal isolation solutions - optical fiber signal isolation solution and dedicated chip signal isolation solution - I hope it will be inspiring to everyone.

Under what circumstances does HDMI require signal isolation?

As shown in the figure below, this is an application that uses an HDMI cable to connect a display to a human-machine interface (HMI). Generally, the HDMI cable length can support 15 meters, which can be easily wired to the touch screen display and control room.

Figure 1: Industrial control, connecting the display via HDMI (Image source: ADI)

However, in harsh industrial environments, extending the length of HDMI cables may pose challenges because EMC hazards can affect wiring and interfere with signal transmission. At this time, it is often necessary to consider signal isolation for HDMI to reduce the impact of EMC on signal transmission and thus extend the transmission distance.

Traditionally, adding signal isolation to high-speed interfaces has been a nontrivial task. Data rates exceeding 250Mbps are typically beyond the capabilities of optocouplers and most digital isolators.

For example, for a video link that transmits 24-bit color at a resolution of 1920×1080 (1080p) at 60Hz, if we want to isolate the signal of this video link, we need an isolator with a total bandwidth of 4.4Gbps to ensure smooth video transmission.

Fiber Optic Signal Isolation Solutions

In the fiber optic signal isolation solution, the conversion from copper media to fiber requires a serializer, a deserializer, and an electro-optical converter. The fiber also requires sufficient bandwidth. This solution is fast and has a long transmission distance. It can transmit 4K video over long distances, but the relative cost is also very high.

Figure 2: HDMI signal to optical fiber (Image source: ADI)

Dedicated signal isolation chip

Of course, this function can also be achieved using standard digital isolators. We run at 150Mbps per standard digital isolator. This solution also requires a serializer and a deserializer, which is equivalent to more than 30 standard digital isolators if the total bandwidth is to be 4.4Gbps. Obviously, this solution is not very reasonable, so we will not continue to discuss it.

Figure 3: HDMI signal to digital isolator (Image source: ADI)

Another technical option is to use a dedicated chip to solve this problem. For example, ADI's ADN4654 series of low voltage differential signaling (LVDS) digital isolators.

Figure 4: HDMI signal to Gigabit LVDS isolator (Image source: ADI)

By leveraging the Gigabit data rate of the ADN4654, system complexity can be reduced and a 4.4 Gbps bandwidth can be achieved using only two ADN4654s. Each device has two channels, for a total of four channels, and can operate at up to 1.1 Gbps on each channel.

ADN4654 Demo Board EVAL-CN0422-EBZ

The EVAL-CN0422-EBZ is a plug-and-play evaluation board that implements signal isolation for HDMI 1.3a ports. Combined with iCoupler ^® isolation technology, it transmits the necessary power and high-speed video and control signals across the insulation barrier.

Figure 5: ADN4654 evaluation board: EVAL-CN0422-EBZ (Image source: ADI)

The video data in the HDMI 1.3a protocol is transmitted through four TMDS ( Transition Minimized Differential ) channels: three data channels (corresponding to red, green and blue) and one clock channel. Each lane must be isolated separately.

Figure 6: HDMI video data is transmitted via TMDS channels

As shown in Figure 7, although TMDS is slightly different from LVDS (Low Voltage Differential), it is possible to use CML (Current Mode Logic), which is some simple passive components, to achieve compatibility with LVDS-compatible devices.

Figure 7: Using simple passive components to achieve TMDS-LVDS compatibility (Image source: ADI)

These passive components are used in conjunction with two dual-channel Gigabit ADN4654 isolated LVDS transceivers to isolate four TMDS channels. Pixel clock frequencies up to 110 MHz can be achieved, supporting 720p resolution at 60 Hz frame rates.

Conclusion

The use of signal isolation methods can extend the transmission distance of HDMI. In particular, the use of dedicated signal isolation chips can effectively achieve a safe and noise-free connection between the HDMI source and the HDMI sink, providing developers with a new option to isolate high-bandwidth interfaces.

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