Mobile High-Definition Link
Mobile High-Definition Link is an industry standard for a mobile audio/video interface that allows the connection of smartphones, tablets, and other portable consumer electronics devices to high-definition televisions, audio receivers, and projectors. The standard was designed to share existing mobile device connectors, such as Micro-USB, and avoid the need to add video connectors on devices with limited space for them.
MHL connects to display devices either directly through special HDMI inputs that are MHL-enabled, or indirectly through standard HDMI inputs using MHL-to-HDMI adapters. MHL was developed by a consortium of five companies: Nokia, Samsung, Silicon Image, Sony and Toshiba.
History
Silicon Image, one of the founding companies of the HDMI standard, originally demonstrated a mobile interconnect at the January 2008 Consumer Electronics Show, based on its transition-minimized differential signaling technology. This interface was termed "Mobile High Definition Link" at the time of the demonstration, and is a direct precursor of the implementation announced by the MHL Consortium. The company is quoted as saying it did not ship that original technology in any volume, but used it as a way to get a working group started.The working group was announced in September 2009, and the MHL Consortium founded in April 2010 by Nokia, Samsung, Silicon Image, Sony and Toshiba. The MHL specification version 1.0 was released in June 2010, and the Compliance Test Specification was released in December 2010. May 2011 marked the first retail availability of MHL-enabled products.
The first mobile device to feature the MHL standard was the Samsung Galaxy S II, announced at the 2011 Mobile World Congress.
MHL announced in 2014 that more than half a billion MHL-capable products had been shipped since the standard was created.
Overview
MHL is an adaptation of HDMI intended for mobile devices such as smartphones and tablets. Unlike DVI, which is compatible with HDMI using only passive cables and adapters, MHL requires that the HDMI socket be MHL-enabled.. It has several aspects in common with HDMI, such as the ability to carry uncompressed HDCP encrypted high-definition video, eight-channel surround sound, and control remote devices with Consumer Electronics Control.There are a total of five pins used in MHL rather than the 19 used in HDMI, namely: a differential pair for data, a bi-directional control channel, power charging supply, and ground. This permits a much lighter cable and a much smaller connector on the mobile device, as a typical MHL source will be shared with USB 2.0 on a standard 5-pin Micro-USB receptacle. The USB port switches from USB to MHL when it recognizes an MHL-qualified sink detected on the control wire. A typical MHL sink will be shared with HDMI on a standard 19-pin HDMI receptacle.
Because the same five-pin Micro-USB port is also typically used for charging the device, the sink is required to provide power to maintain the state of charge while it is being used. The use of the power line in this way differs from HDMI, which expects the source to provide 55 mA for the purpose of reading the EDID of a display.
Because of to the low pin count of MHL versus HDMI, the functions that are carried on separate dedicated pins on HDMI, namely: the Display Data Channel and CEC are instead carried on the bi-directional control bus. The CBUS both emulates the function of the DDC bus and also carries an MHL sideband channel, which emulates the CEC bus function, and allows a TV remote to control the media player on a phone with its Remote Control Protocol.
Bandwidth
MHL uses the same Transition-minimized differential signaling as HDMI to carry video, audio, and auxiliary data. However, MHL differs from HDMI in that there is only one differential pair to carry the TMDS data lane, compared to HDMI's four. Therefore these three logical data channels are instead time-division multiplexed into the single physical MHL data lane, and the clock signal carried as a common mode signal of this pair. From MHL 3 onwards, the method for carrying the clock signal changed to being carried separately on the MHL CBUS pin instead.The normal mode operates at 2.25 Gbit/s, and multiplexes the same three channel, 24 bit color signal as HDMI, at a pixel clock rate of up to 75 MHz, sufficient for 1080i and 720p at 60 Hz. One period of the MHL clock equals one period of the pixel clock, and each period of the MHL clock transmits three 10-bit TMDS characters.
MHL can also operate in PackedPixel mode at 3 Gbit/s, catering for 1080p, in this case only two channels are multiplexed, as the color signal is changed to a chroma subsampled pair of adjacent 16-bit pixels, and the pixel clock is doubled to 150 MHz. In this mode, one clock period of the MHL clock now equals two periods of the pixel clock, so each period of the MHL clock transmits twice the number of channels i.e., four 10-bit TMDS characters.
Version 3 of MHL changed from being frame-based to a packet-based technology, and operates at 6 Gbit/s. superMHL extends this by carrying the data signal over more than one differential pair allowing up to 36 Gbit/s.
Versions
All MHL specifications are backward compatible to previous versions of the standard. MHL is connection agnostic. The first implementations used the 5-pin MHL-USB connector described below, and all are supported over USB Type-C MHL Alternate Mode. Other proprietary and custom connections are also allowed.MHL 1
Version 1.0 was introduced in June 2010, supporting uncompressed HD video up to 720p/1080i 60 Hz. Support for 1080p 60 Hz was introduced in version 1.3. The specification supports standard SD and HD color spaces, as well as those introduced in HDMI 1.3 and 1.4. Other features include 192 kHz 24-bit LPCM 8-channel surround sound audio, HDCP 1.4 content protection, and a minimum of 2.5 W power between sink and source for charging. The MHL sideband channel includes a built-in Remote Control Protocol function allowing the remote control of the TV to operate the MHL mobile device through TV's Consumer Electronics Control function, or allowing a mobile device to manage the playback of its content on the TV.MHL 2
Version 2.0 was introduced in April 2012, and raised the minimum charging supply to 4.5 W, with an optional 7.5 W maximum allowed. Support for 3D video was also introduced, permitting 720p/1080i 60 Hz, and 1080p 24 Hz 3D video modes. The specification also included additional MHL sideband channel commands.MHL 3
Version 3.0 was introduced in August 2013, and added support for 4K Ultra HD 30 Hz video, increasing the maximum bandwidth from 3 Gbit/s to 6 Gbit/s. An additional YCbCr 4:2:0 pixel encoding for 4K resolution was also introduced, while the maximum charging supply was increased to 10 W. Support for compressed lossless audio formats was added with support for Dolby TrueHD and DTS-HD Master Audio.The specification increased the speed of the bi-directional data channel from 1 Mbit/s to 75 Mbit/s to enable concurrent 4K video and human interface device support, such as mice, keyboards, touchscreens, and game controllers. Other features include support for simultaneous multiple displays, improved Remote Control Protocol with new commands, and HDCP 2.2 content protection.
superMHL
superMHL 1.0 was introduced in January 2015, supporting 8K Ultra HD 120 Hz High Dynamic Range video with wide color gamut and 48-bit deep color. Support for object-based audio formats were added, such as Dolby Atmos and DTS:X, with an audio-only mode also available. The Remote Control Protocol was also extended to link multiple MHL devices together and control them via one remote.The specification introduces a reversible 32-pin superMHL connector, which supports a higher charging power of up to 40 W, and is designed for future bandwidth expansion. The increase in bandwidth over previous MHL versions is achieved by using multiple A/V lanes, each operating at 6 Gbit/s, with a maximum of six A/V lanes supported depending on device and connector type. For example, Micro-USB and HDMI Type-A support one A/V lane, USB Type-C supports up to four A/V lanes, and the superMHL connector supports up to six A/V lanes.
In addition to supporting a variable number of lanes, the specification supports VESA Display Stream Compression 1.1, a "visually lossless" video compression standard. In cases when the bandwidth of the available lane is unable to meet the rate of the uncompressed video stream, bandwidth savings of up to 3:1 can be achieved with a DSC compression rate of 3.0×. For example, 4K 60 Hz is possible using a single lane with a DSC rate of 3.0×.
superMHL can use a variety of source and sink connectors with certain limitations: micro-USB or proprietary connectors can be used for the source only, HDMI Type-A for the sink only, while the USB Type-C and the superMHL connectors can be used for the source or sink.