This “Product How-To” article focuses how to use a certain product in an embedded system and is written by a company representative.
For the equipment suppliers, the key in HDMI 1.4 testing is to ensure the product's interoperability to achieve excellent customer experience.
The physical layer consists of three pairs of differential data and one pair of differential clock. Test of the physical layer of HDMI consists of test equipment test, cable test and receiving equipment test.
Besides the physical layer, protocol layer and HDMI's additional Ethernet channel and the audio return channel should be also tested.
These tests could better your knowledge of the tested equipment. If this knowledge is obtained in the early phase of design, the corresponding improvement measures can be executed in time.
As an adopter, you must submit your HDMI products to the ATC of HDMI for validation tests, while Agilent could provide you with supports for the commissioning and characteristic tests before validation.
Profiled in this article paper are the high-speed source equipment, cable and the equipment's electrical tests, the protocol, audio and video tests, and subsequently the details of Ethernet and audio return channel tests (for these are all the additional functions).
Testing the physical layer
As a traditional test item of HDMI, it has varied a little from the previous plan, with the main variation on the updates of the software and clamps.
Tests of the source equipment of HDMI 1.4 . Consistency test items include low electrical level and differential rise/fall time etc. in addition to the eye diagram and vibration tests with the clock recovery method. The oscilloscope is the center of Agilent source equipment test (Figure 1, below ).
|Figure 1: Consistency test items include low electrical level and differential rise/fall time etc. in addition to the eye diagram and vibration tests with the clock recovery method|
The high bandwidth, low noise and the probe technology of these real-time oscilloscopes promises clean eye diagrams and the real surplus capacity tests. For example, the eye diagram test is used to validate the minimum receiving electrical level of the receiving equipment.
If the oscilloscope has a relatively poor noise index, the test result might cheat you by giving smaller electrical level surplus capacity, which shall result in a wrong estimate in the design.
Since the peak signal velocity of HDMI 1.4 has not changed from 1.3, the necessary bandwidths of oscilloscope and probe have neither changed from the original, i.e. 8GHz.
However, as a result of the additional HEAC channel, new clamps are needed to derivate the HEAC signal, and some new test software are also needed to support tests of HEAC signal. The instruments listed in Figure 2 below are recommended for tests of HDMI 1.4 source equipment.
|Figure 2: It is vital to ensure the quality of cable and connector, for they shall affect the quality of displaying.|
Tests of the cables, connectors of HDMI 1.4. It is vital to ensure the quality of cable and connector, because they shall affect the quality of displaying. Time delay, crosstalk, attenuation and impedance etc. should be tested.
Tests to the time parameters (e.g. time delay) and eye diagram may adopt realtime or sampling oscilloscope, while tests to the attenuation and impedance prefer to vector network analyzer. Compared with HDMI 1.3, the main addition in this section is the connector of TypeD and TypeE, and thus new clamps are needed for tests of these interfaces and cables, which could be purchased from Bitifeye, cooperator of Agilent. The instruments listed in Figure 3 below are recommended for tests of HDMI 1.4 cables and connectors.
|Figure 3: Tests of the receiving equipment need the TMDS's signal generator to produce test code types in various frequencies.|
Tests of the receiving equipment of HDMI 1.4 . Tests of the receiving equipment need the TMDS's signal generator to produce test code types in various frequencies. The operator shall judge by vision whether the image has been displayed properly.
Test code types should be scrambled to test the consistency and characteristics of the equipment. Popular problems in the receiving equipment tests include noise points, horizontal or vertical vibrations and incorrect colors etc.
The Agilent E4887A TMDS signal generator can provide all the parallel clock and data signals in little inherent vibration and rapid edge jumping. Measuring precision of receiving equipment lies in the signal source's noise bottom (inherent vibration).
The more accurate the signal source is, the profoundly you shall know about the receiving equipment. In the vibration tolerance test, controllable and accurately calibrated vibration should be introduced.
Some other requirements exist, including the clock and data channels need individual signal sources, which shall enable introducing various vibrations in the clock and data channels separately to realize precise testing.
Compared with HDMI 1.3, main addition in this section is the support of deep color, 3D and high definition etc. in the software, so the upgrading of N5990 automatic testing software plays the main role. Besides E4887A, many other test accessories are also needed for tests to the HDMI 1.4 receiving equipment, which are omitted hereby for the limitation of context.
The HDMI 1.4 protocol layer
In addition to the forging discussions of physical layer tests to high-speed TMDS signals, we shall focus on some other test items as below, including protocol tests, and HDMI's Ethernet channel and audio return channel.
We shall hereinafter refer to the HDMI's Ethernet channel and audio return channel as “HEAC.” Besides the protocol tests (e.g. deep color test) which have been already included in HDMI 1.3, HDMI 1.4 has been added with HEAC test, which is independent from the high-speed signal and protocol tests and could be realized via low-cost test equipment.
Equipment of 882CA from QuantumData is still applicable in tests of EDID/DDC/CEC, while SL-309 from Simplay still applicable to realize tests of CEC/CDC. For protocol tests of high-speed audio/video formats etc., Agilent's N5998A protocol analyzer, which has been applied in HDMI 1.3, would be still used for those in HDMI 1.4.
The N5998A protocol generator and analyzer test setup includes a protocol analyzer, video sequence analyzer, video image analyzer, audio sequence analyzer and audio/video protocol generator. This equipment shall support deep-color including protocol and audio code types, with its software providing predefined audio/video files.
Compared with HDMI 1.3, the main addition in this section is the support for 3D and high definition. For example, 3D videos in more complex format than that of 2D have included two times more image info. New function tests require upgrading the test equipment's software to support HDMI 1.4.
HDMI's Ethernet & audio return channels (HEAC)
Similar to the physical layer test plan of a high-speed signal, if the new HEAC function is supported by your equipment, the physical layer tests on Ethernet and audio return channel shall be required.
As shown in Figure 4 below , tests of the signal quality of HEAC's transmitting terminal could be accomplished by the upgraded HDMI test software of oscilloscope plus the additional test clamps of HEAC.
|Figure 4: Settings of HEAC receiving tests are shown, by which you may verify whether the equipment under test could work normally in the stipulated worst conditions.|
The test procedure requires the ability to enable transmitting of HEAC via CEC/CDC control channels, which means CEC/CDC controllers are also required to assist the test; the CEC/CDC controllers could be realized by 882CA from QuantumData or SL-309 from Simplay still applicable to realize tests of CEC/CDC.
And the signal sources of HEAC are just needed for tests to HEAC receiving terminals. Signals of HEAC could be produced by Agilent's 81150A dual-channel arbitrary- waveform generator, and two sets of 81150A shall be necessary for simultaneous generations of Ethernet signal and video signal.
Settings of HEAC receiving tests are shown in Figure 4, by which you may verify whether the equipment under test could work normally in the stipulated worst conditions.
In addition to the necessary tests of the physical layer functions of HDMI's Ethernet channel (HEC) and audio return channel (ARC), ARC should also have its protocol layer functions tested. Ethernet as a differential & bidirectional link in 125Mbit/s could receive and transmit data simultaneously.
This test needs to generate three levels of electrical signals. The test code type is a request packet of ARP, which means to transmit ARP packet to the tested equipment and check the return by oscilloscope.
The minor differences from general 100MB Ethernet are that: (1) the general 100MB Ethernet uses two pairs of differential cables for receiving and transmitting signals individually, while HDMI's signal receiving and transmitting are realized via one pair of differential cables; (2) as the link is bidirectional, the transmitted signal should be subtracted from the retuned signal.
Furthermore, as the audio signals are transmitted simultaneously in the common mode, the first 81150 shall generate differential Ethernet signal at the same time as the 2nd 81150 generates the scrambling signal in the common mode.
For audio tests, it shall be the second 81150 to generate audio test signal in the common mode, and the first 81150 to generate differential scrambling signal and tested equipment should not let out noises during the test. One certain specialty of 81150 is that when the signal parameter is being changed, no burr shall ever arise in the signal to result in abnormal situations.
Tests of exchanging functions of Ethernet. Besides HEAC, if HDMI's display is also provided with the Ethernet hub and exchanging functions, tests of the exchanging functions of Ethernet shall be necessary, which are now of fully developed type for traditional data exchanging equipment (e.g. routers and exchanges) and for which the existing Ethernet test equipment could be directly used.
Agilent has offered correspondingly the exchange for N5610 between Ethernet and HDMI, which could convert HDMI's Ethernet port into the standard Ethernet port to enable subsequent test with particular Ethernet test instrument.
Thus, the standard Ethernet test items could be applied for tests to the Ethernet function of the object equipment. N5610A could be configured as the source or receiving equipment of HDMI to facilitate the connection with the test object.
Li Kai is an Application Engineer at Agilent Technologies Inc.