Decoding HDMI Bandwidth: What’s the Deal? Is It 10Gbps, 18Gbps or 30Gbps?
What happens when an HDMI cable boasts 18Gbps but can't fight its way out of 10.2Gbps? Let's sort through the question of HDMI speed once and for all.
There was a fair amount of fiber at CEDIA 2016. It tends to focus on longer products, typically 10 meters and up, since copper is pretty much limited to 6 to 8 meters. It was fun visiting these booths, not only to see the products but to listen to presentations.
It was also fascinating to hear companies talk about their bandwidth capabilities. Some of their sample products we’d actually evaluated while designing our fiber testing platform and fixture designs.
Although these products had claims of 18Gbps, during testing some could not fight their way out of 10.2Gbps. But then others were boasting bandwidth much greater.
So what’s the deal; is it 6, 9, 10, 10.2, 18 ... or even 30Gbps?
Most integrators just want the facts and not the confusion, so let’s sort through this.
Solving the HDMI Speed Question
One big obstacle that must be defined is how the claim is made. Are the manufacturers adding up the individual video channels providing a net sum of data rates or are they providing channel-by-channel calculations?
The image below shows a simple schematic of a cable’s video transmission attributes under Rev 2.0. These are all differential (balanced) lanes, which doubles the wire count.
Under HDMI Rev 2.0 each lane must be able to support up to 6Gbps. So it can be interpreted as having four lanes of video at 6Gbps, which equals a whopping 24Gbps! Ah, but this where you have to dig further.
There are not really four video channels, but three: D0-Blue, D1-Green and D2-Red. Because this is a serial interface, it requires a clock, so the fourth channel is the clock channel. There is no real video data here and it operates at ridiculously low speeds, almost not even worth talking about. You can see how a claim can be misinterpreted.
The bottom line is that in most cases the bandwidth is given as the aggregate of the bus, not channel by channel and never includes the clock.
To follow this I have color-coded each number. Anything in red is aggregate and anything in green is channel by channel.
Let’s move on to some of the other talked-about numbers. Where does 6Gbps come from?
Rev 2.0 maximum data rate is 5.94Gbps, rounded off to 6Gbps per channel. In this example, the data rate is a channel-by-channel explanation. With that known it’s easy to figure out where 18Gbps came from: the aggregate of all three channels (3x6=18).
How about 9Gbps claims? This is the aggregate of all three channels under Rev 1.4’s 10.2Gbps total bandwidth spec. We never really used the entire bandwidth in practical applications under Rev 1.4; the actual number is 2.97Gbps per channel, so rounded up and multiplied by three gives us 9Gpbs.
So all things being equal, where the heck does 10Gbps come from? This can be explained a few ways.
Those who play in the technical HDMI world will typically refer to 10.2Gbpsas 10Gbps aggregate sometimes referred to as “engineering slang.” However, under Rev 2.0 this habit can be misunderstood; many at CEDIA mentioned 10Gbps. So which is it?
Well, trying to anticipate what the future may bring, some firms are claiming a max bandwidth of 10Gbps per channel. Do the math, that’s 30Gbps aggregate.
But is it useful or is it just hype?
Whether this particular CEDIA presenter knew it or not, it can be a very useful specification due to the dynamic headroom it offers over 18Gbps. Better yet, and clearly more powerful, it can be construed that these new transmission lines are gearing up for 8K.
In this case, 30Gbps product has some definite benefits such as:
- Providing more bandwidth under current 4K, and therefore improve performance for 18Gbps systems.
- Allowing for future-proofing as newer formats make their way into the consumer space.
But don’t give up on copper just yet. We recently tested a new active cable device that can in fact support 10Gbps over three channels — yes, a 30Gbps long-distance copper cable.
Don’t you just love technology?
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Jeff Boccaccio is president of DPL Labs. Jeff can be reached at email@example.com. Have a suggestion or a topic you want to read more about? Email Jeff at firstname.lastname@example.org
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