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HDMI Corner: Playing the Numbers Game with New Rev 2.1

Under Rev 2.0b the 18Gbps pipe offered 4K 60Hz Deep Color and HDR. Now, with increased bandwidth we can achieve resolutions out to 5K, 8K and 10K with refresh rates much faster than ever.

Jeff Boccaccio · February 13, 2017

We realized a while back that HDMI was likely to expand to support an 8K video environment, but the announcement of Rev 2.1 at CES was still somewhat of a surprise to many.

Historically, any major revision change was bandwidth related and typically doubled in size.

Take for instance HDMI’s original Rev 1.0’s bandwidth’s aggregate was a mere 5Gbps. A few years later, under Rev 1.3, it doubled to 10.2Gbps. Then in 2013, Rev 2.0 bumped it up again to 18Gbps. This was in fact a doubling in bandwidth from 1.3 since the actual consumed bandwidth under Rev 1.3 was 9Gbps and not 10.2.

However, instead of following the interface’s historical bandwidth increase, HDMI hit this one out of the park with a whopping 48Gbps.

Why?

8K is coming on pretty fast, but you have to consider Display Port as it provides a means of transmission that in some ways is better than Rev 2.0. So the need for HDMI to up the ante had to be addressed.

Bigger Pipeline to Flow Features Through

Let’s detail what we know and predict how things will shake out. In this case it is all about the bandwidth and additional features it will support.

First, understand the interface is just a transport mechanism that carries signal data from point A to point B. Whatever content is carried through will be up to the industry. This will allow more features and benefits to be used — and more for manufacturers to implement— with far fewer obstructions by way of a much larger pipeline.

Take resolution for example. Under Rev 2.0b the 18Gbps pipe offered 4K 60Hz Deep Color and HDR signal transmission. Now, with this increased bandwidth we can achieve resolutions out to 5K, 8K and 10K with refresh rates much faster than ever.

Under Rev 2.0b the 18Gbps pipe offered 4K 60Hz Deep Color and HDR. Now, with this increased bandwidth we can achieve resolutions out to 5K, 8K and 10K with refresh rates much faster than ever.

For instance, 4K will now have the ability to increase its refresh rate to 120 fps — unheard of under Rev 2.0.

HDR continues to improve, and high quality Audio Return qualities are now called eARC.

This is very cool, but can the transmission line support all this? HDMI provides a backwards-compatible interface, so it can support all the lower quality video back to Rev 1.0. The plan is to keep the current HDMI cables and connectors’ overall configuration the same, but bump up the requirements to a new level called 48G cable. This is massive and, in DPL’s opinion, would have to be manipulated by both source and sink.

It is similar to what happened to Rev 2.0. Making the jump to 18Gbps is achieved via some additional instructions sent from the source through the DDC channel to the sink to determine if the sink can support 600MHz. If that comes back positive, the clock’s operating ratio is switched from one-tenth the data rate to one-quarter the data rate.

In addition, a different scrambling technique is implemented to keep the risk of emissions down to safe levels.

Now don’t get bent out of shape; we’ve been there before with standard speed (Cat 1), High Speed (Cat 2) and now 48G. It just means that there will probably be some tighter requirements for wires within the cable.

This could include timing, impedance, crosstalk and others. These are all very doable.

Active Cables Continue to Play a Role

In our opinion, the extra set of instructions used under Rev 2.0 is minor compared to what will be required under Rev 2.1. Since the cable assembly will be pretty much the same overall configuration we have been using from day one, we predict a need for improved insertion loss figures over the cable and some form of manipulation within the cable’s internal signal channels that must be addressed.

In addition, it would have to be assumed that a different equalization spec would have to be employed into each sink, similar to what was done with Rev 1.3 and 2.0.

The need for active transmission lines will increase — if cable lengths get beyond 2 or 3 meters, some kind of 48Gbps active cables will be required. We are beginning to see these new devices surface. And fiber will have to up its bandwidth to follow suit.

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