MHL: A multimedia connectivity standard

BY MASUD SYED
Technical Marketing Manager
MHL Consortium, www.mhltech.org

Selecting the best multimedia interface for mobile designs depends on technical features, compatibility, and ease of design with lower cost. Mobile High-Definition Link (MHL) technology was created by mobile and consumer electronics industry leaders to support the industry standard CEA-861F for video and for IEC 60958 or IEC 61937 specification audio formats. Targeting popular connectors on mobile devices, MHL requires five pins to transmit audio, video, timing, control, and auxiliary data while concurrently charging the mobile device. MHL designs can be quickly done because of reduced pin count, built-in industry standard encoding support, and flexibility when choosing a shared connector because it’s connector agnostic.

History

With the prevalence of smartphones on the rise, a universal connectivity standard was needed for the viewing of multimedia content from a mobile device to a larger display. Due to inherent size limitations of smartphones, established multimedia connection standards were not practical, especially because they would require a dedicated connector. In 2010, five companies — Nokia, Samsung, Sony, Silicon Image, and Toshiba — introduced the MHL specification to provide an audio/video/data out solution that could run over popular mobile and display connectors. With three major versions, MHL is now found on more than 650 million devices shipped, making it the de facto standard for mobile video-out.

Categories of MHL devices

There are three device categories for MHL technology: sources, sinks, and dongles. MHL-enabled sources include smartphones, tablets, smart watches, and media sticks. MHL-enabled sink devices can receive and display MHL signals and usually have a receptacle-type connector on each input. An MHL sink must be able to charge an MHL source when attached. MHL sinks include DTVs, projectors, A/V receivers (AVRs), and wearable displays. The third category, MHL adapters or dongles, has an MHL input and a non-MHL output, such as HDMI or other video standards. An MHL adapter must have a plug-type connector input.

It’s very important to note that MHL is connector agnostic. This freedom is applied to every MHL device category. Common connectors include USB Type-C, Micro-USB, and HDMI Type A connectors.”

A five-pin solution

MHL specifications can support a variety of features using five pins.

Fig. 1: The MHL five-pin solution.

A differential data pair in MHL carries audio, video, control, timing and auxiliary data without any compression. MHL 1 and 2 specifications achieve resolutions of up to 1920 x 1080p at 60 frames/s. MHL 3 can transmit data up to 6 Gbits/s and can support video modes up to 4K Ultra-HD at 30 frames/s.

 The single CBUS pin is a multipurpose, bidirectional control channel to carry data used for configuration, remote control, and display support information via EDID and HDCP synchronization. MHL 3 can also use this pin to transport additional payloads such as USB and HID data allowing the support of mouse, keyboard and storage devices.

 The VBUS pin enables the 5-V charging function, allowing a sink or adapter to charge the MHL source or alternately allow a source to act as a power supply for the adapter. A single ground provides a return path.

 Video format support

 A wide array of CEA-861F specification resolutions are supported, including:

 720 x 480p @ 59.94/60 Hz

• 720 x 576p @ 50 Hz
• 1920 x 1080i @ 50/59.94/60 Hz
• 1920 x 1080p @ 50/59.94/60 Hz
• 3840 x 2160 @ 24/25/30 Hz (MHL 3)

 Both 2D and 3D formats are supported. Common 3D formats supported include frame packing, top and bottom, and side by side, which can be rendered in high-definition resolutions.

 Since MHL was designed to address the needs of the consumer electronics ecosystem, the universally used pixel encodings and additional color spaces listed below are supported:

 RGB 4:4:4 pixel encoding

• YCbCr 4:4:4 pixel encoding
• YCbC4 4:2:2 pixel encoding
• YCbCr 4:2:0 pixel encoding (MHL 3 only)
• xvYCC
• sYCC601
• AdobeYCC601
• AdobeRGB

 To ensure quick identification of the pixel encoding sent, encode information is included in the MHL data stream.

 MHL fully supports the video quantization feature, where black and white-level component range can be controlled. Video quantization information is sent from the display to the source, which will adjust the component range as required by the display during transmission. Another feature supported by the MHL source is color space encoding conversion.

 Audio support

 All versions of the MHL specification can transmit uncompressed linear pulse-code modulation (LPCM) signals at various sampling rates, bit depths, and multichannel configurations.  

 Sampling rates from 32 kHz to 192 kHz and 1,536 kHz for MHL 3

• Up to 7.1 discrete audio channels
• 16-, 20-, and 24-bit depths
• DTS-HD and Dolby TrueHD for MHL 3

 In MHL 1 and 2, the maximum audio rate can go up to 192 kHz at 24 bits. MHL 3 can transport high-resolution lossless multichannel Blu-ray-quality audio with Dolby TrueHD and DTS-HD master encoding. 

 Content encryption

 Premium content distributed and streamed by the entertainment industry is copyrighted material. High-Bandwidth Digital Content Protection (HDCP) protects this material from duplication. MHL 1 and 2 ensure that each stream is protected, using hardware encryption employing the HDCP 1.4 standard. Content cannot be duplicated by a nonprotected source or display.

 HDCP 2.2 was introduced by the Digital Content Protection LLC in June 2014, and the entertainment industry is driving its adoption. The MHL 3 specification supports HDCP 2.2 using public RSA key authentication and 128-bit AES encryption into the MHL data steam without latency.

 Charging and mandatory remote control support

 MHL specifies a minimum of 500 mA charging current delivered through the VBUS pin for MHL 1 and 900 mA for MHL 2 and 3, with options for 1.5 A or 2.0 A.

 MHL includes a mandatory Remote Control Protocol (RCP) bus of more than 80 unique commands carried on the CBUS pin. MHL source-and-sink devices are required to support a minimum consumer friendly set of remote control codes.

 Character String Transport

 Both MHL 2 and MHL 3 provide a bidirectional protocol for sending not only text but the complete character set between a source and sink. Using the UTF-8 Character Protocol (UCP), characters may be sent from a smartphone to a TV to pop-up messages such as “incoming phone call” or an MHL phone may transmit strings — even Unicode international language strings — to a TV to render as closed captions on the screen.

 HID components, automobile usage, ease of design

 Human Interface Devices (HID) include classes such as mice, keyboards, touchscreen display devices, and game controllers. With MHL 3, consumers will be able to connect their mouse and keyboard to their smartphone and tablet and perform productivity tasks that they commonly use with their personal computer. With the addition of a gaming controller, mobile devices can be transformed into gaming consoles.

 By adding touchscreen support, consumers have the unique ability to use their smartphones in an automobile dock where the car console touchscreen can be used to control the smartphones.

 MHL 3 is fully backward compatible, and MHL can be added into most designs expediently. The global MHL ecosystem includes adapters, automotive consoles, A/V receivers, Blu-ray disc players, set-top boxes, cables, DTVs, monitors, projectors, smartphones, streaming sticks, portable thin clients, tablets, and wearable displays. For further information about MHL technology or to purchase the Abridged Specification, visit www.mhltech.org.