The High-Definition Multimedia Interface (HDMI) was originally launched in 2002 to replace the aging and bulky SCART connection or RCA cabling. Since then, more than 2 billion HDMI-enabled device have shipped and HDMI technology has extended into an increasingly wide array of devices, applications, and industries, including cell phones, automobiles and commercial applications such digital signage and airport installations.
To further expand HDMI’s reach and build on its success, the original seven founding companies decided to increase industry participation in the standard by forming the HDMI Forum, a non-profit corporation to further development of HDMI. Since its formation in October 2011, the HDMI Forum now has 88 member companies, clearly establishing HDMI as the most widely accepted interface for uncompressed audio and visual applications.
After nearly two years of behind-the-scenes development, the HDMI Forum officially unveiled HDMI 2.0 in September of 2013, the first specification to be developed by the new group. The heir apparent to HDMI 1.4b, HDMI 2.0 is an improved standard for AV connectivity with the performance needed for high-end video playback, such as 4K Ultra HD.
While HDMI 1.4b has a throughput of 10.2 Gbps, it's only enough to support a 4K resolution of 3840 x 2160 pixels at 24, 25 and 30Hz or to display full 4K (4096 x 2160 pixels) at 24 Hz. The 2160p24 and 2160p30 formats might be good enough for movie playback, but they fall short of the 50/60Hz required for 4K TV broadcasts and future Ultra HD gaming. HDMI 1.4b also only supports 8-bit color at 4K. HDMI 2.0 improves upon this, offering 10 to 12-bit color depth at Ultra HD resolutions.
HDMI 2.0 doesn't fully double the throughput of HDMI 1.4b, but it boosts it up to 18 Gbps. This enables a number of important enhancements and features including:
• 4K Ultra HD at 50/60Hz
• Up to 32 uncompressed digital audio channels (compared to HDMI 1.4's eight)
• Up to 1536 kHz audio sampling
• Simultaneous delivery of dual video streams to multiple users (on the same screen)
• Simultaneous delivery of multi-stream audio to multiple users (up to four)
• Support for 21:9 aspect ratios
• Dynamic synchronization of video and audio streams
• Additional CEC extensions for remote controls
Despite the many advantages provided by the new specification, it’s important to stress that HDMI 1.4b is far from obsolete. HDMI 2.0 is fully backwards compatible with HDMI 1.4b and uses the same 19-pin connectors. Higher speeds such as 4K at 50/60Hz require a Cat 2 HDMI cable. And to ensure backward compatibility, the HDMI Forum is requiring that HDMI 2.0 products must pass HDMI 1.4b compliance testing.
The basic HDMI structure after 1.4 is shown in Figure 1. The architecture includes three data lanes and a unique separate clock lane. These form the high-speed lanes of the HDMI interface. HDMI also has a low-speed hand shaking bus known as EDID plus encryption, a capability that has become popular with service providers and contributed significantly to HDMI’s success over the past decade. It also has a consumer electronic control to provide remote control capability using a single digital interconnect. HDMI 1.4 added the bi-directional Ethernet and audio return channel. This architecture carries over to HDMI 2.0.
HDMI 2.0 source testing
The source electrical tests for HDMI 2.0 compliance testing include the following:
· Test ID HF1-1: Source TMDS Electrical – 340-600Mcsc – VL
· Test ID HF1-2: Source TMDS Electrical – 340-600Mcsc – TRISE, TFALL
· Test ID HF1-3: Source TMDS Electrical – 340-600Mcsc – Inter-Pair Skew
· Test ID HF1-4: Source TMDS Electrical – 340-600Mcsc – Intra-Pair Skew
· Test ID HF1-5: Source TMDS Electrical – 340-600Mcsc – Differential Voltage
· Test ID HF1-6: Source TMDS Electrical – 340-600Mcsc – Clock Duty Cycle
· Test ID HF1-7: Source TMDS Electrical – 340-600Mcsc – Clock Jitter
· Test ID HF1-8: Source TMDS Electrical – 340-600Mcsc – Data Eye Diagram
· Test ID HF1-9: Source TMDS Electrical – 340-600Mcsc – Differential Impedance
Compared to HDMI 1.4b there are a number of notable differences. As we will discuss in more detail, the clock jitter and data eye diagram tests are now performed at test point 2 (TP2) rather than test point 1 (TP1). This change led to the need for a differential voltage test, which is a new requirement. Also new with HDMI 2.0 is the source differential impedance test which is performed using a sampling oscilloscope with the device under test (DUT) turned on.
From a test instrumentation perspective, higher data rates translate into a need for higher bandwidth oscilloscopes. For HDMI 1.4b, an 8 GHz real-time oscilloscope was the typical recommendation. For HDMI 2.0, which has a 42.5 ps rise/fall time on the signal, the recommendation moves to a 16 GHz bandwidth oscilloscope in order to achieve a 1 percent error rate. That said, it is possible to perform pass/fail compliance testing with a 12.5 GHz bandwidth oscilloscope. Other requirements include high-speed differential probes, HDMI 2.0 test fixtures and an HDMI 2.0 test software package.
The connection set-up for the source eye diagram test is shown in Figure 2. The clock occupies one channel on the oscilloscope connected in differential mode with the data lanes occupying the other two lanes connected in single ended mode. This means that there will be some manual switching involved to test the remaining channels. There is the potential to use an automated switch to eliminate the need for manual intervention.
As noted, the eye diagram test is now performed at TP2 with an equalized eye after the signal has passed through a cable with worst-case attenuation and a skew of 112 ps. As shown in Figure 3, this is emulated on single-ended signals using software running on the oscilloscope. It’s important to note that the unused data lanes need to be terminated.
Moving to the source eye itself, as seen in the eye diagram for an HDMI 2.0 6 Gbps signal in Figure 4, the equalized eye does not have a top and bottom mask defined. There is no point in having the top and bottom mask because the signal gets amplified as it passes through the equalizer and invariably would violate the top and bottom mask. However, to overcome this particular aspect, the differential voltage test has been added at TP1 to ensure that the signal level does not exceed 780 mV.
HDMI 2.0 sink testing
HDMI 2.0 signals are scrambled as per the defined scrambling mechanism defined in HDMI 2.0 specification. The signal generators should ensure that they follow the scrambling requirement for HDMI 2.0 signals. The sink electrical tests for HDMI compliance testing include the following:
· Test ID HF2-1: Sink TMDS Electrical – 340-600Mcsc – Min/Max Differential Swing Tolerance
· Test ID HF2-2: Sink TMDS Electrical – 340-600Mcsc – Intra-Pair Skew
· Test ID HF2-3: Sink TMDS Electrical – 340-600Mcsc – Jitter Tolerance
· Test ID HF2-4: Sink TMDS Electrical – 340-600Mcsc – Differential Impedance
The main difference in these tests compared to HDMI 1.4b is that the intra-pair skew test and the jitter tolerance test require a 112 ps delay line. Unfortunately, this increases the test times because there is human intervention required in order to insert the 112 ps delay line first on the positive lanes, complete the test, remove them, and put the delay line on the negative lanes. In may be possible to eliminate this manual step using direct synthesis to simulate the 112 ps delay, but this capability is not currently available.
As with source testing, HDMI 2.0 requires higher performance test equipment including a recommended 16 GHz real-time oscilloscope and appropriate fixtures and probes. In addition, automated sink or receiver testing requires two high-performance arbitrary waveform generators, ideally the latest generation with up to 50 GS/s sampling rate and 14 GHz bandwidth for compliance and margin testing.
As shown in the example test configuration in Figure 5, an arbitrary/function generator is also used to synchronize the two AWGs. The two AWGs are needed because the clock and data lanes all need to be tested simultaneously. The signal goes through a 112 ps delay line and is then fed into the DUT through an HDMI 2.0 plug fixture.
As per the HDMI 2.0 test specification, the signal must pass through a cable emulator to simulate worst-case conditions. This can either be accomplished using a hardware cable emulator or software-based cable emulation in a signal generator. In either case, the cable emulator should be verified using a real-time oscilloscope. With available HDMI sink testing software, the required tests complete automatically and generate a pass/fail report with detailed margin analysis.
HDMI oscilloscope-based protocol analysis
A major challenge for HDMI designers is identifying the root cause of problems across the physical and link layers. There is essentially a blind spot when it comes to isolating where a particular problem may be occurring. One solution to this problem is oscilloscope-based protocol analysis software that provides the ability to correlate protocol errors with the physical layer.
As shown in Figure 6, this software effectively eliminates the blind spot by providing correlated views into what is happening across the different layers. It provides a useful multi-viewer capability that includes frame summary viewer, frame viewer, bus viewer, protocol viewer, data island viewer and event and test results viewer. Automatic cross-linking between all these viewers enables a designer to see and correlate the data in different parts of the protocol stack.
HDMI 2.0 has arrived and is well positioned to build on the success of previous generations of the HDMI standard. It provides a number of important new capabilities including support for high-end video playback including 4K Ultra HD and 18 Gbps compared to 10 Gbps for HDMI 1.4b. However, HDMI 1.4b remains as an important and viable standard since HDMI 2.0 uses the same cabling and retain backward compatibility.
From a compliance test perspective, HDMI 2.0 requires the use of higher bandwidth instrumentation and involves a number of changes in test procedures, most notably eye diagram testing is now performed on an equalized eye after the signal has passed through a worst-case cable. For HDMI debug and troubleshooting, new oscilloscope-based protocol analysis provides insights across the protocol and physical layers.
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About the author
U N Vasudev is a strategic product planner at Tektronix. He also serves as chairman of the HDMI Forum Test Sub Group.