Mobile High-Definition Link

Mobile High-Definition Link (MHL) 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 (HDTVs), 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 (CES), based on its transition-minimized differential signaling (TMDS) 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 (CTS) 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. (To deliver an MHL signal to a non-MHL HDMI socket, one can use an adapter device that receives the signal on an MHL-enabled socket, converts it to HDMI, and transmits the HDMI signal to the non-MHL socket). 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 (CEC). 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 (CBUS), 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. (Although MHL ports can be dedicated to MHL alone, the standard is designed to permit port sharing with the most commonly used ports.) The USB port switches from USB to MHL when it recognizes an MHL-qualified sink (e.g., a TV) 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 (or even recharge) while it is being used (although this is dependent on the power available being sufficient e.g., MHL 2 & 3 provide a minimum of 4.5 W / 900 mA, while superMHL can provide up to 40 W). 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 (DDC) (pins 15 & 16) and CEC (pin 13) are instead carried on the bi-directional control bus (CBUS). The CBUS both emulates the function of the DDC bus and also carries an MHL sideband channel (MSC), which emulates the CEC bus function, and allows a TV remote to control the media player on a phone with its Remote Control Protocol (RCP).

Bandwidth MHL uses the same Transition-minimized differential signaling (TMDS) 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 (three data lanes, plus the clock). Therefore these three logical data channels are instead time-division multiplexed into the single physical MHL data lane (i.e., with the logical channels sent sequentially), 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 (24 bit) 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 (i.e., a 24-bit pixel, where each 10-bit TMDS character represents an encoded byte – 8-bits). 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 (YCbCr 4:2:2) pair of adjacent 16-bit pixels (i.e., where two adjacent pixels share chroma values and are represented with only 36-bits), 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 (a pair of 16-bit pixels). 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 (up to four with USB Type-C, or a total of six using a superMHL cable) allowing up to 36 Gbit/s.

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