HDMI Rev. 1.4: 4K x 2K Resolution – Mission [Im]Possible

The amount of confusion and paranoia that HDMI™ Rev 1.4 has stirred up is just amazing. There is a heck of a lot of great features that HDMI Licensing’s new specification is bringing to the table. Top new features include Ethernet capabilities for networking, bi-directional audio transmission, content enhancement for 3D TV, increased resolution supporting 4096 x 2160 (called 4K x 2K), and support for increased color space – specifically for DSLR image playback.

Yet the morning that HDMI Licensing made the announcement of the new revision 1.4 (Rev. 1.4) – the stage was set for a 200% increase in our tech support phone call volume. It was like World War III had started. We had a huge surge with more questions and concerns related to Rev. 1.4 than our usual high daily volume of tech support calls. 

So what is the big deal? 

The big deal is that people were mistakenly interpreting the features and benefits of Rev. 1.4 negatively, as if the new revision was a rushed attempt to improve previous mistakes related to the HDMI interface. Even worse, some callers were panicking because “they heard” that Rev. 1.4 was going to obsolete all the HDMI™ products that have already been deployed. Of course neither of these concerns is true. So let’s set part of the record straight.

Is 4K x 2K even possible?

It is beyond the scope of this article to answer ALL questions associated with HDMI™ Rev. 1.4. Rather we will focus on one key new feature that is causing a lot of buzz – Rev. 1.4s support for a new dramatic new video resolution of 4K x 2K. 

Let’s review what we had with the previous HDMI™ revisions and what we will get with the new Rev. 1.4.

In the beginning…

Originally, commercial television started off supporting a video screen resolution of 480i. The number “480” refers to the total number of lines of vertical resolution. The “i” stands for an interlaced scanning system. 

In interlaced scanning, the system makes two sequential scans to complete one frame. It starts by scanning the first 240 (odd numbered) lines, and then continuing with the second set of 240 (even numbered) lines. Each set of odd and the even lines are designated as a “Field”. When the scan of both the odd fields and the even fields is completed they create what is known as a frame. 

Up until now, these frames were generated at a rate of 30 cycles per second (Hz). The fields were generated at 60Hz. This system…the foundation upon which modern television was originally created…was designed with slow frame rates in an effort to reduce bandwidth . However, at the same time this system would limit some vertical resolution as well. 

Goodbye flicker…

One byproduct of an interlaced scanning system is that it can cause a small amount of flicker. On television signals, which are mostly full-motion video, this flicker is not very noticeable. However, when computers entered our lives and people spent hours peering at mostly static images, it became desirable to reduce the inherent flicker of the interlaced system. 

Enter progressive scan…

Progressive scanning was introduced by scanning all 480 lines one after the other (progressively), thus doubling the frame rate to 60Hz. This new process also doubled the bandwidth and the resolution. That increased resolution is why line doublers…so called because they doubled the frame rate…were so popular some years back. This concept – progressive scanning doubling the resolution by doubling the frame rate (and bandwidth) - is important to remember when trying to understand how HDMI™ was able to achieve a 4K x 2K resolution in a Rev. 1.4 system that maintains backwards compatibility.

Remember that doubling the vertical frame rate doubles the bandwidth.

Bandwidth: going up!

With that foundation, let’s now take a more detailed look at those higher resolutions. Today we mostly consider 1080i and 1080P as the standard resolutions. These resolutions require bandwidths of 74 MHz and 165 MHz respectively. Did you notice what happened? Simply by moving to progressive scan 1080P from 1080i we had to double the bandwidth. HDMI™ specification revision 1.2 fully supported these bandwidths and resolutions.

However, the introduction of Rev 1.3 provided a major development. In HDMI™ Rev. 1.3, the bandwidth was increased from 165 MHz (1080P) to 340 MHz (1080P Deep Color). Notice again that bandwidth was doubled to allow for Deep Color – a wider and more accurate color gamut. 

So did HDMI™ double the bandwidth again to achieve 4K x 2K resolution? Nope! 340MHz is the limit. There was no further doubling of the bandwidth. And it is that incongruity that has so many end users, dealers and installers lost. 

Seems like it should be different, but stays the same…

Under Rev 1.4…bandwidth stayed the same! Yep, it remains set at 340 MHz. How can that be since HDMI Rev. 1.4 boasts a 4K x 2K resolution? Doesn’t that quadruple the bandwidth? The truth is a big fat NO. Dig down deep into 4K x 2K and you will find that the details in the specification allow the system to perform within the 340 MHz currently supported under Rev 1.3.

In order to achieve the higher 4K x 2K resolution, Rev. 1.4 didn’t increase the bandwidth…instead; it decreased the frame rate to 24Hz – less than half the currently more common 60Hz. Cutting the frame rate to less than half and using only 8 bits of color depth allows the 4K x 2K resolution to function while maintaining the existing 340MHz bandwidth. 

So, as you can clearly see, our panicky tech support callers had assumed that something terribly bad was happening with HDMI™ Rev. 1.4. But in reality, it was simply their misunderstanding of the technology. These kinds of misunderstandings take place every day, not only in HDMI™, but in all aspects of life. 

Change is scary. Many people have much invested in the status quo. And there are many so-called experts in our industry who make a living out of scaring people about technological change. They are false prophets, who attempt to increase their own reputation by offering seemingly authoritative, but often incorrect misinformation. Don’t listen to them.

Seek out independent and authoritative resources, such as DPL Labs, for the straight scoop on today’s digital signaling solutions.