HDMI Corner: Navigate Noise & Voltage When Vaulting to 4K/60

DPL Labs tests the integrity of an HDMI 5-volt line, which is the lifeblood of HDMI and ‘should never be messed with.’

Jeff Boccaccio

One major benefit we have at DPL Labs is that we are not governed by bureaucracy. HDMI continues to grow with more and more features that will introduce idiosyncrasies that will sometimes poison its ecosystem.

It is difficult to control the many makers that use and build HDMI products and that is where most of the trouble comes from. However, DPL’s discovery process can find out what is going on and when possible, add procedures to its testing Matrix.

The Matrix is everything, especially now that we are knee high in Ultra HD 4K video. By adding arguments to the Matrix we can stay ahead of new anomalies that pop up especially now when “making the jump” to 18Gbps.

Most know and understand the importance of the HDMI 5-volt line that is carried over the bus. But many consider it a “supply voltage” when it really isn’t.

By design it is used to support the return HotPlug, EDID and HDCP. This voltage operates with very low current levels and should never be used to power up devices. Some companies tend to break this rule to cut costs, not paying any attention to the reliability.

DPL’s test expanded to find anomalies that can harm the integrity of this 5-volt line, which is the lifeblood of HDMI and should never be messed with.

Think about it this way; the 5-volt line can be looked at as a common conduit of power throughout the system environment. Every HDMI output product (source) must support this, which means each product must properly regulate, clean and filter this voltage when sharing the interface. It is imperative that this voltage is on target and be free of noise, otherwise it can spread throughout the system like a cancer.

So What Can Cause Noise & Voltage Issues?

The list is long, but here are a couple of problems we have discovered that can cause the interface to fail.

One is the voltage value itself. It must be within certain a value and must be able to maintain that value under defined load conditions. In this case, it is about 50ma, not much.

This means that whatever load is put on the bus it must be under 50ma and the source supply cannot deviate from 5 volts. If the 50ma is exhausted, the user is on his own. This problem has existed for some time, the only difference now is the 18Gbs.

In most semiconductor devices frequency throughput can be directly proportional to the current; the higher the current the bigger the load. We are finding that some products that rely on this precious 5-volt line did not or failed to consider the demands inflicted upon it when you go up to 18Gbps. The current goes up, the voltage goes down and so can the display. But it gets deeper because the 5-line supports many parts of the interface that can be affected.

An example is HDCP — if this voltage goes south, so will HDCP. Same for HotPlug and EDID and that doesn’t even consider what the voltage requirement is to run the external peripheral that is robbing the bus for fuel.

Then there is the noise issue. Fig 1 shows an oscilloscope pattern of a solid 5-volt rail one would expect in an HDMI environment.

 

Fig 2 shows the same voltage only amplified so you can see noise (not much, only about 20mv).

 

Fig 3 shows the unit at 18Gbps jumped to 35mv.

 

Fig 4 shows the noise that you would experience from a USB volt supply brick — greater than 100mv.

 

All of these products must build in adequate noise reduction — it’s cheap, but you’d be surprised how many companies try to save 5 cents and not provide adequate filtering.

Remember, this changes from system to system; some do it right and some don’t do it at all. Accessories like fiber products, extenders, fixers, and even switches can introduce this contaminant. Adding such tests to HDMI products sets up safeguards in supporting a seamless and straightforward installation.