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MHL vs. Miracast
By Manmeet Walia, Sr. Product Marketing Manager
In recent years, cell phone technology has evolved to offer very powerful devices that handle a lot more than simple voice communication. They act as multimedia players, set-top boxes, game consoles, cameras, and even PCs. However, the screen size of a device that fits into the palm of your hand can never be satisfactory for a rich multimedia experience. Hence there is a strong need for interface technology that can connect mobile devices to a bigger screen, like a digital television (DTV) or a monitor.
Two protocols have emerged to address consumers' need for big-screen connectivity: the wired mobile high-definition link (MHL) protocol and the wireless Miracast protocol. MHL provides high-definition (HD) audio/video connectivity between a mobile device and a larger display while simultaneously charging the mobile device's battery. While MHL offers HD multimedia connectivity between a mobile device and display device, it will need to prove its value to overcome the convenience of wireless Wi-Fi-based technologies. Miracast, created by the Wi-Fi Alliance, allows a mobile device to share HD audio/video with a compatible display over Wi-Fi by using data compression. An alternative to Apple's AirPlay Mirroring, Miracast works as a peer-to-peer screencast.
With the advent of these two protocols, the "wired versus wireless" debate has yet again come to the forefront, this time for mobile multimedia connectivity. This article discusses how MHL and Miracast can both compete and co-exist in next-generation consumer mobile multimedia electronic devices. Each of these technologies provides an interface to the larger screen but offer different approaches to connectivity, performance, latency, power delivery, quality, security, and convenience.
The MHL specification has not defined any particular connector but maps itself to existing and popular connectors: USB 2.0 micro connectors (MicroUSB) on cell phones and HDMI connectors on DTVs. A typical MHL cable has a MicroUSB connector on one side and an HMDI connector on the other side. Both of these connectors are prevalent in existing consumer electronics devices. In addition, as new smart phones are introduced with MHL support, they provide low-cost MHL-to-HDMI dongles to circumvent the problem that most legacy DTVs do not support the MHL protocol. Likewise, Miracast leverages the Wi-Fi networks that are available in almost every home. Since both technologies use existing infrastructures to provide mobile multimedia connectivity, neither holds a distinct advantage over the other.
The batteries that power mobile phones have not kept up with the phones' functional demands, making power delivery and charging a key consideration. To address this limitation, MHL includes VBUS functionality which allows charging via the power pin and provides up to 900mA at 5V. This is sufficient to power a broad range of mobile devices including smart phones, tablets, mini projectors and even media players. On the other hand, there is no existing technology to deliver power over wireless networks. Hence, while connected to displays through Miracast, mobile devices may also need to be connected to a wall charger for power.
Per the specification, MHL performance is defined up to 2.25 Gbps in normal mode of operation that can support a 1080i/720p, 60Hz, 24 bit color signal. MHL also defines a packed pixel mode of operation up to 3 Gbps that can support up to a 1080p, 60Hz, 16 bit color signal. In other words, MHL can deliver up to 1080p signal without any need for compression. This means that consumers can transfer and enjoy stunning quality video from a phone to the big screen with no lag for hours at a time. In addition, MHL’s high performance and battery charging capabilities have given rise to ultra-mobile set top boxes in the form of USB thumb drives, such as the Roku set-top box stick.
Miracast, on the other hand, is limited by the bandwidth of Wi-Fi technologies. Even today, most Wi-Fi devices use the 802.11g standard, which is theoretically capable of transferring up to 54 Mbps, though real-world performance is far less. The arrival of 802.11n standard promises speeds up to 600 Mbps, although most residents will be limited to 150 Mbps or less, depending on the system's reliability, signal range, and distance from the access point. Furthermore, interference and obstructions in the path from point of access can degrade performance. Regardless, Wi-Fi-based technologies require compression to transfer HD or even standard definition video, and as the content gets richer (3D and 4K), the demand for bandwidth will increase. Hence, MHL has a distinct advantage over Miracast in terms of bandwidth it can support.
Latency is the time between the display of the picture on the source (e.g., mobile phone) and the display of the mirrored image on the sink (e.g., DTV). This is another area where MHL presents a distinct advantage over Miracast. MHL latency, through the entire system, is less than 1 ms. MHL outperforms Miracast by a huge margin, making it much better suited for gaming applications. Currently Miracast certification does not mandate a maximum latency.
In general, non-encrypted content has a greater probability of being hacked over wireless networks. However, content over both MHL and Miracast can be protected by HDCP technology, offering a secure mechanism to transfer data from mobile devices to the larger display devices.
Convenience and Cost
Convenience is one of the greatest limiting factors of any wired technology, like MHL, since it involves cables and connectors. This also makes the wired connectivity more expensive compared to wireless connectivity. However, the cost of cables and dongles has come down significantly in the past few years. New cables also typically offer auto detection to provide a plug-and-play experience for the end user, removing any need for driver downloads or intensive configuration.
On the other hand, Miracast, with its wireless connectivity, is more convenient and offers the freedom of working anywhere. No cables or connectors means that more source devices can connect to a single sink device. Miracast offers a distinct advantage over MHL in overall convenience.
Systems designers must remember that wireless connectivity is not available everywhere. For example, the automotive sector, including Hyundai, JVC-Kenwood and Pioneer, view MHL as the compelling solution to integrate the smart phone into the car infotainment experience. Mobile devices can be conveniently connected to the display screen in the automobile for easy access to maps, traffic and other infotainment features.
Both MHL and Miracast come with distinct advantages and limitations. The MHL protocol's high performance and battery charging capability is well suited to transfer rich multimedia content from mobile to display devices. On the other hand, the convenience factor of wireless technologies can never be discounted. Each protocol will find its application niche, but in the end, both technologies are likely to become device requirements and will co-exist to enrich the mobile life.
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