Mobile high-definition link
Mobile high-definition link is a new standard closely related to HDMI. This new interface technology is most common in smartphones. It allows content such as Internet-streamed media, movies and user-created video to be output to TV screens in HD resolution. MHL runs on the micro-USB port now included in many mobile devices. MHL consortium founders Nokia, Samsung, Silicon Image, Sony and Toshiba point to the new technology’s following benefits for mobile applications:
• The micro-USB port can be used to transmit uncompressed HD video and audio.
• Mobile devices can charge while playing media.
• Eliminating the need for a separate audio/video port – combined with micro-USB’s small size – enables manufacturers to produce even smaller smartphones.
The USB port is used to exchange data with a personal computer as usual. However, if the built-in MHL transmitter chip detects that it is connected to an MHL enabled sink or MHL to HDMI converter, it switches automatically to MHL transmission mode. In this case, the MHL transmitter sends the audiovisual data across the micro-USB connection.
Like HDMI, MHL has an additional pin for transmitting control signals. This is for the MHL control bus (CBUS), which performs various tasks: It detects whether an MHL enabled sink or source is connected, and it transmits data relating to the encryption of audiovisual data. The MHL voltage bus (VBUS) has a 5 volt line carrying a maximum of 0.5A, which is powered by the MHL sink. The most recent version of MHL is V1.2. Work is currently in progress on V2.0, which will extend the standard. Future versions will add new capabilities, such as support for 3D video formats and higher charging currents.
Interface conformity at the protocol level
Both HDMI and MHL transmit the visible video signal in TMDS frames and the audio and meta data (e.g. InfoFrames) in the blanking intervals (HSync and VSync). The purpose of the meta data is to tell a connected device about the properties of the data stream (resolution, color range, frame rate, etc.).
Consumer electronics devices’ digital interfaces are tested for protocol conformance during development and certification to ensure the devices’ interoperability with other equipment. During development in particular, the ability to view the following key protocol parameters in real time is crucial:
• Video timing parameters such as pixel clock and resolution in line with CEA-861.
• Audio clock regeneration (N/CTS) and audio sample packets
• High-bandwidth digital content protection (HDCP) status and the keys used.
• Auxiliary video information (AVI) InfoFrame, audio InfoFrame, source product description (SPD) and MPEG InfoFrame.
Figure 2 is an example showing how the new Rohde & Schwarz video tester family uses MHL protocol analysis to display TMDS data. The screen shows an analysis of AVI InfoFrame data. A typical test scenario in which this capability is used is when troubleshooting an MHL enabled device that fails to switch to an expected state.
Figure 2: Auxiliary video information (AVI) InfoFrames
A special application is conformance testing in line with the standard’s compliance test specification (CTS) as a final interoperability test. Before a device fitted with an HDMI or MHL port is introduced to the market, it must first be checked by an authorized test center (ATC) to verify that its functionality conforms to the CTS. Tests are not just conducted at the protocol level but also on the physical layer, with separate tests for sinks (TVs), sources (set-top boxes) and cables.
The tests carried out during the certification process are both time-consuming and costly. If errors are found during certification, the entire test has to be repeated. It therefore makes sense to conduct tests using suitable T&M equipment capable of assuring devices’ quality prior to the certification process.
To certify consumer electronics equipment in a lab or to conduct advance assessments, protocol tests must be carried out as described in detail in the standards under specific test IDs. The section headings in the CTS correspond to the test ID numbers. Figure 3
shows the system tests specified in MHL.
Figure 3: Raw TMDS data is analyzed for system tests conducted in accordance with the compliance test specification (CTS)