Two different HDMI signal isolation schemes: optical fiber vs dedicated chip

In the field of industrial control, it is often seen that HDMI cables are used to connect displays to monitor various key indicators throughout the production process. Sometimes the display is 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 enable the signal to be transmitted over a longer distance.

This article will introduce two different HDMI signal isolation schemes – optical fiber signal isolation scheme and dedicated chip signal isolation scheme – hoping to inspire 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). In general, the cable length of HDMI can support 15 meters, so that it can be conveniently routed to the touch screen display and the control room.

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

But in harsh industrial environments, extending HDMI cable lengths further can present challenges as EMC hazards can affect the 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, thereby extending the transmission distance.

Traditionally, adding signal isolation to high-speed interfaces has been a non-trivial task. Typically data rates in excess of 250Mbps are beyond the capabilities of optocouplers and most digital isolators.

For example, for a video link that transmits 24-bit color at 60Hz and has a resolution of 1920×1080 (1080p), 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 transmission of video.

Optical Fiber Signal Isolation Solution

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

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

The fiber optic signal isolation technical resources that Digi-Key can provide are as follows:

● Digi-Key HDMI Optical Cable

● More Digi-Key HDMI cables

Dedicated signal isolation chip

Of course, this function can also be implemented using standard digital isolators. We run at 150Mbps per standard digital isolator. This scheme also needs a serializer and a deserializer. If the total bandwidth of 4.4Gbps is to be realized, more than 30 standard digital isolators are required. Obviously, this solution is unreasonable, so we will not continue the discussion.

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, use ADI’s ADN4654 series of low-voltage differential signaling (LVDS) digital isolators.

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

By utilizing the Gigabit data rate of ADN4654, the complexity of the system can be reduced, and only two ADN4654 can be used to achieve 4. 4Gbps bandwidth. Each device has two channels for a total of four channels, and the operating rate on each channel can be as high as 1.1Gbps.

● ADN4654 Demo board EVAL－CN0422－EBZ

EVAL-CN0422-EBZ is a plug-and-play evaluation board for HDMI 1.3a port, which can realize signal isolation. With iCoupler? Isolation technologies combine to transmit the necessary power and high-speed video and control signals through the insulating 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 (Transfer Minimized Difference) channels: three data channels (corresponding to red, green and blue) and one clock channel. Each lane must be segregated individually.

Figure 6: HDMI video data is transmitted through the TMDS channel

As shown in Figure 7, although TMDS is slightly different from LVDS (Low Voltage Differential), CML (Current Mode Logic), that is, some simple passive components, can be used to achieve compatibility with LVDS compatible devices.

Figure 7: TMDS and LVDS compatibility using simple passive components (Image source: ADI)

These passive components are used in conjunction with two dual-channel Gigabit ADN4654 isolated LVDS transceivers to isolate the four TMDS channels. A pixel clock frequency of up to 110MHz can be achieved, supporting 720p resolution at a frame rate of 60Hz.