When dealing with an end-to-end video system, we are increasingly confronted with the fact that end of the chain uses the HDMI (High Definition Multimedia Interface), (e.g., a Camera, Set Top Box or DVD player). This standard incorporates into its structure an encryption system which can severely limit the possibilities for analysis if the measuring device is not HDCP (High Bandwidth Digital Content Protection) compliant. Developed and adopted by an industrial consortium led by Intel, this content protection algorithm prohibits displaying images on non-complaint HDCP displays.
It is based on an accurate identification of the source and the display, leading to a single encryption key that will only benefit the two devices. There is an exchange of data between the source and destination, and is structured into the EDID (Extended Display Identification Data) packets. This EDID data can be analyzed with the Auxiliary Data Status of the WFM5250 Waveform Monitor.
The following describes how it works and how to interpret the data.
Principle of the HDCP Pairing Procedure
- Upon connection, the source sends an identification code to the receiver.
- To demonstrate eligibility, the receiver must respond within 100ms with its code
- With these two identifiers, the source and the receiver calculate internally a unique key called a session key (unique and secret 56-bit key)
- The media is encrypted with this key that only the connected receiver can decode. A synchronization sequence is then exchanged every 128 frames.
Data identifying a source or display is stored in a block of 256 bytes encoded in hexadecimal (V2.0). The data always begins with an easy to recognize header (in red below). Followed by various data such as resolution and standard of the media, brand, type, serial number, date of manufacture, supported resolution, the color space, the timing etc. Careful analysis of EDID data often helps to identify the nature of the pairing defects. That is why the waveform monitor displays three separate pages: A page summarizing the main parameters decoded in the clear, raw EDID received from the receiver, and those sent by the source (see Figure 1 for example – page 3 of 3).
- Select the STATUS button on the waveform monitor for one of the tiles and push and hold this button to display the menu select the Aux Data Display under the Display Type menu.
- There are three pages within this display and pressing the Left or Right arrow keys will cycle through the menus as shown in Figure 2.
- Pressing the Help button while in this Aux Data Status display can show a brief summary of the EDID Syntax as shown in Figure 1.
- The user can also download this EDID information via the web interface of the instrument.
- The EDID information is stored within the Sink (Input) device and provided to the Source (Output) when the connection is made.
- The EDID information is then used by the source to configure the appropriate video and audio signals to provide to the Sink.
- When the HDMI loop through output is connected from the WFM5250 the EDID information can be obtained from the connected device.