Using Video-0ver-USB for High Definition recording on mobile handsets - Embedded.com

Using Video-0ver-USB for High Definition recording on mobile handsets

While camera capabilities in mobile handsets have grown from a novelty to mainstream feature, mobile vendors are strategically bringing High-definition (HD) video recording to their high-end products. The dynamic push for HD video in phones would further emphasize its value as a utilitarian device, consolidating not only a digital still camera within a communication device, but a digital video camcorder as well.

Bringing HD video recording into mobile handset sparks off new exploration on how HD videos are played back. While it is common to playback video directly on a phone, screen size and resolution limitations restrain the delivery of the compelling high-definition experience that HD video is intended to display. Similar to the sharing of mobile documents and photos, the challenge for developers is how to share HD video without confining the content to within the mobile handset itself.

Adopting a HD Video Output Interface on Mobile Handset.  

Mobile video output interfaces have not evolved at the same rate as HD video in mobile handsets. In contrast to USB data sideloading which is sufficient for transferring photos, HD video calls for real-time streaming via interconnect to HDTVs, monitors, and other display panels. In the meantime, current HD video-enabled handsets are either equipped with standard-definition analog TV-out or legacy HD video standards used in HDTV which are not optimized specifically for the mobile space (Table 1, below ).


Table 1. Handset HD Video Integration by Vendor

Legacy HD Video Standards

HDMI (High-Definition Multimedia Interface) is undisputedly one of the most well-established high-definition video standards today, with over 850 licensed HDMI adopters as of Q2 2009 (In-Stat).

As forecasted by In-Stat, over a billion HDMI enabled devices will be in the market by 2010, ranging from HDTVs and projectors to handheld devices like media players and mobile phones. HDMI (Figure 1 below ) utilizes three data TMDS (Transition Minimized Differential Signaling) channels and a separate clock channel to transmit uncompressed audiovisual signals at up to 10.2 Gbps (3.4 Gbps per channel). At such bandwidth, HDMI supports beyond the mainstream 1080p and the emerging digital 3D experience.


Figure 1. HDMI Architecture Block Diagram

Meanwhile, another HD standard, DisplayPort is rapidly expanding market penetration with its strong industrial backup from HP, Dell, Apple, and more. At the heart of DisplayPort, Audio, Video, and embedded clock signals are transmitted through scalable 1, 2, or 4 lanes under micro-packet architecture.

As shown in Figure 2 below , the raw data steams are optimally packed and transmitted across lanes through pixel steering and addressing. DisplayPort 1.2a supports a bandwidth of 17.28 Gbps.


Figure 2. DisplayPort Architecture Block Diagram

Issues with Legacy HD Video Standards in Mobile Devices

While both HDMI and DisplayPort are established in desktop devices, their inherited shortcomings limit their adoption in mobile handsets and other portable electronics. Both standards require high pin-counts – HDMI has 19 pins and DisplayPort has 20 pins – which lead to large connector size. In the size-cautious portable electronic market, an additional large form factor connector makes the overall design bulkier and less sleek.

Higher pin-count ties with a higher manufacturing cost, which increases the overall bill-of-material (BOM) for the finished product. Today, the attach rates of legacy video standards for mobile handsets are low. Vendors would be required to reconsider their existing design to accommodate these additional connectors. 

Another downside of legacy HD video standards is that they offer no provision for power through the same cable. This is not a significant drawback for their intended desktop purposes, since desktop electronics, like Blu-ray players, set-top boxes, and PlayStation 3s, are independently powered.

On another hand, the mobile device will be constantly draining power while video streams to an external display panel. This limits the playback duration of mobile video when it is not concurrently charged through a separate wall charger or USB connection. Thus, having legacy HD video standards in mobile devices presents an inevitable tradeoff between playback length, portability, and battery life.     

Miniaturization of HDMI

It is imperative not to undermine the size reduction efforts for legacy HD video standards. HDMI has undergone a series of connector miniaturizations since its original Type A connector. In the HDMI 1.3 specification, a mini connector (Type B) is defined and is intended for portable devices. It reduces the form factor of Type A (13.9 mm x 4.45 mm) to 10.42 mm x 2.42 mm.

More recently, a new HDMI micro connector (Type D) has been defined in the HDMI 1.4 specification. This HDMI micro connector keeps the standard 19 pins of Type A and Type C but shrinks the connector size to 2.8 mm x 6.4 mm, similar to a micro-USB connector (2.94 mm x 7.8 mm). In the hTC EVO 4G, a micro HDMI connector is placed adjacent to the micro-USB connector.

HD Video-over-USB

An alternative to using HDMI or DisplayPort is to transmit audiovisual signals through the highly ubiquitous Universal Serial Bus (USB) interface. USB 1.1/2.0 connectivity has been widely adopted among handsets with a staggering number of 1.28 million USB-enabled mobile phones shipped this year (In-Stat). In addition, USB 3.0 technology introduces expanded bandwidth, enabling high-capacity HD video transmission.

In contrast to legacy HD video standards, USB interface offers two key advantages well-suited for mobile handsets. USB requires low pin-count as its audiovisual signals are transmitted across a single TMDS physical channel. Low pin-count allows smaller form factor connectors which is ideal for mobile handsets.

In addition, low pin-count keeps the BOM of connectors low. More importantly, USB is already attached in majority of mobile handsets as a primarily interface for data transferring and power charging. Thus, the Video-over-USB concept can be easily adopted by vendors without dramatically increasing implementation cost.

Secondly, USB is capable of transmitting high-capacity content over a single cable while simultaneously provisioning power. For instance, when video packets are streamed from a webcam to a PC through a USB interface, power current is supplied by the PC to power the webcam.

Applying the same concept, a mobile handset can stream a full-length movie to a TV through USB interface, while it is simultaneously charged by the TV. After the playback is competed, the mobile handset is fully replenished and can be used for other purposes, such as making calls and sending emails. A similar notion is already witnessed in iPod music docking systems: Using Apple’s proprietary 30-pin dock connector: an iPod is charged as it plays through the external speakers of the docking system.


Figure 3. HD Video over Legacy Standards and Video-over-USB Comparison

A Push for USB Audio Video Class

Several industry players are developing custom USB graphic implementations in effort to fulfill this niche yet blooming market. However, the diversity within these technologies and their need for distinctive custom chipsets and drivers has kept the Video-over-USB concept inches away from mainstream adoption.

 In order to leap toward, a USB Audio Video Class needs to be defined within the Universal Serial Bus specification to standardize Video-over-USB technology. Predefining a class for transmitting audiovisual signals through USB encourages semiconductor vendors to build SoC devices intended for that purpose, cutting out the need for custom drivers.

Tagging along the side of USB Mass Storage Class (MSC) and Human Interface Device (HID) Class, the next time when you plug-in mobile handset to the TV, the HD video streaming can start as easy as pressing the “play” button.

Stephen Harris is Sr. Product Marketing Engineer working in the Data Communications Division at Cypress Semiconductor, where he is responsible for worldwide business development, and product definition.  Stephen holds B.S in Electrical Engineering University from the University of Colorado-Boulder. 

Steven Chen is an associate product manager in the Data Communications Division at Cypress Semiconductor. He is responsible for market research and new product definition. He can be reached at

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