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Video Over USB

By Ashwini Govindaraman, Cypress Semiconductor Corp.

Not too long ago, high-definition TV changed the world forever by recalibrating the expectations that consumers have from their digital media experience. Content creation and consumption is now at a pivotal point where it is no longer enough to view only television shows and movies in stellar, high-definition quality.

With over one billion users of social media worldwide now connected to some form of social media, user-generated content is a significant source of video traffic, with applications such as YouTube and Facebook becoming universal. As users continue to become increasingly mobile, they expect the same quality, performance and accessibility of content while on the go. They want to be able to watch vacation videos, the family webcam chat, or view a previously downloaded HDTV show with a faster and richer user experience. These multiple trends call for a convergence towards a common interface that is easy to use, portable, and flexible enough to provide a connectivity link for all types of media across a variety of applications. USB immediately comes to mind as one of the most user-friendly and universal connectivity options. With the evolution to USB3.0, this interface now provides a 5 Gbps data rate, which further strengthens its case as the video connectivity interface of choice. This article discusses the role USB3.0 can play in the video transport domain and compares it with two main interfaces that are also well-positioned to succeed: HDMI and DisplayPort. All three technologies have merits that compel their use in the digital living room as well in the PC and mobile realms.

Why Video over USB?

USB3.0 provides for a signaling rate of 5Gbps over a dual-simplex differential signaling interface. This high bandwidth capability makes it an ideal choice for video transport. Accounting for the overhead due to the 8b/10b encoding, the raw throughput over USB3.0 is about 500MBps enabling reliable transport of 1080p video at 120Hz refresh rate. (The bandwidth requirement for a 1920x1080p HDTV at a refresh rate of 120Hz and 10bit data is about 2.5Gbps. The bandwidth requirement for other video applications such as webcam to PC connectivity is typically lower as they typically require only 30fps.) Like USB2.0, the USB3.0 protocol supports bulk and isochronous data transfers. The type of transfer may be selected based on the application requirements. Isochronous transfers offer deterministic bandwidth with a potential accuracy tradeoff and are typically used for video streaming applications such as webcams. The high bandwidth of USB3.0 is further enhanced by the use of the “Streams” model introduced in the new protocol. This model enables multiple streams of data to be transported to and from bulk endpoints over a single physical USB3.0 interface. Different content can be displayed on different devices by sending multiple "streams" of data from a source, which are then routed to the corresponding device by a USB hub.

Since the USB interface is agnostic to the actual data being transferred, USB3.0 can transport both uncompressed and compressed video. This flexibility allows USB3.0 to be the interface of choice for productivity applications, HD video playback, and many other applications such as sideloading videos from a PC to a mobile device or video streaming over a webcam (see Figure 1). Audio can also be supported, which puts USB on par with other interfaces such as HDMI and DisplayPort.

Furthermore, enhanced power management techniques have been introduced to USB3.0, which along with the ability to transport compressed video (see Figure 2), allow for the careful optimization of power consumption in mobile devices. Due to the clear cost advantages of compression, most 2D and 3D content is now distributed in compressed formats over optical media or high speed internet. For example, the very popular Blu-ray is a compressed format. Interesting usage models are arising, such as connecting a cellphone or netbook/tablet to a monitor or several larger displays (see Figure 3). Gaming consoles connecting to the living room TV can all be enabled by the existing USB ecosystem without making any changes.

Adoption of USB 3.0 in these segments would greatly enhance these use cases while maintaining the ease of use.

Figure 1: Webcam connected to HDTV

Adoption and Ease of Use

USB already ranks high in terms of consumer adoption and familiarity. It is easy to imagine the same plug-and-play connectivity interface being extended to media-rich applications such as downloading HD video from a PC to a mobile device, connecting a PC to multiple display monitors, and other scenarios. In addition, the same USB cable can be used to charge the USB device as well.

Despite the advantages that USB has in terms of ease of use, bandwidth for supporting HD formats and a large installed base, it does suffer from the fact that there are incumbent interfaces that are well established in the media room. We will look into two main interfaces that have a stronghold in the living room and what benefits they offer.

Figure 2: Uncompressed/Compressed HD Video Playback

As the name suggests, HDMI is a standard for connecting high-definition products and is an all-digital interface. On a single connection, HDMI supports uncompressed video transfers in any format including HD, up to 8 channels of uncompressed audio, and a CEC (Consumer Electronics Control) connection. HDMI is also backwards compatible with DVI (Digital Visual Interface). It does provide ease of use and quality benefits in the living room.

Figure 3: PC Connected to Multiple Display Devices

Key features and benefits that HDMI provides include:

• Support for the majority of video/audio formats
  • HDMI transmits all formats of digital audio and video through a single cable, replacing as many as thirteen older cables and eliminating cable clutter.
  • HDMI simplifies hooking up other devices to a home theater system: PCs, gaming consoles, and video cameras can all be connected with the same one-plug convenience.
  • The HDMI architecture makes it easy to install or upgrade an all-digital home entertainment system as all DMI versions are backward-compatible with previous versions.
• Bandwidth
  • HDMI provides a bandwidth capacity of up to 10.2 Gbps, more than twice the bandwidth needed to transmit an uncompressed 1080p signal. This means better looking movies, faster gaming, and a richer audio for consumers.
  • HDMI’s bandwidth also means it is future-ready for emerging technologies such as 3D movies, higher resolution gaming, and support for higher resolutions (higher than 1080p), like 1440p or Quad HD. It would also allow for faster refresh rates such as 120Hz and deeper color, taking the HDTV palette from millions to trillions of colors.
• Other Auxiliary Functions
  • HDMI is a "smart" two-way connection which allows devices to communicate and interact with each other to improve the overall home theater experience.
  • Devices connected using HDMI have the ability to scan each other’s capabilities and automatically configure certain settings. For example, an HDTV and Blu Ray player can auto-negotiate settings like resolution and aspect ratio to correctly match the format of incoming content to the highest capabilities of the HDTV.
  • Consumer Electronics Control (CEC) provides for integrated, "one-touch" commands across multiple linked components. When enabled by the manufacturer, CEC allows system-wide behaviors like one-touch play or one-touch record where pressing a single button on the remote launches a series of coordinated commands.
  • HDMI 1.3 provides for “Lip Sync” capability, whereby synchronization of audio with the corresponding video is completed with very high accuracy.

HDMI is well-suited for its application in the living room, but HDMI falls short for broad application to high-performance PC displays due to limitations in terms of scalability and the fact that its primary market focus is on consumer electronics box-to-box connectivity.

DisplayPort

DisplayPort is a display interface and is governed by VESA (Video Electronics Standards Association). It was primarily designed for use between a PC and its display, leaving HDMI to holds its position with consumer electronics devices and in the living room. DisplayPort was designed to replace VGA and DVI. Since DisplayPort technology is now integrated into all main-stream general-purpose processors (GPU) and integrated GPU chip sets, DisplayPort receptacles are beginning to appear on most new PC’s and notebooks. DisplayPort overcomes some of the limitations faced by HDMI by providing an open standard that consolidates internal and external display signaling. It also has enough bandwidth for future performance scalability. With the latest standard DisplayPort 1.2, bandwidth has gone all the way up to 17.28 Gbps (up to 4 lanes with 5,4Gbit per second, per lane). Embedded DisplayPort (eDPTM) is the new standard for internal display panels and replaces LVDS as the panel interface. The battle for the mobile display interface is still on, and DisplayPort is enabled in hand-held applications with the introduction of the mini DisplayPort interface. Mini DisplayPort (mDP) was introduced by Apple in 2008 and is now competing with USB 3.0 in laptops and small form factor compute devices.

Key advantages of DisplayPort include:

• Performance
  • Higher display bandwidth of up to 17.28Gbps supports higher resolutions – up to 4K x 2K at 60 FPS and 24 bpp. Refresh rates of up to 240 FPS for 1080p at 24 bpp are also supported, as well as rich a color depth up to 48 bpp even at 2560 x 1600 at 60 FPS. 5.4 Gbps link rate increases video data bandwidth to 2160Mbytes/sec of stream bandwidth.
  • Supports high frame rates for 3D gaming PC display applications. DisplayPort 1.2 can support 120 frames per second on up to a WQXGA (2560P X 1600P) display.
  • Can be used to increase TV display capabilities for PC gaming applications, especially for 3D applications.
• Multiple Monitors
  • DisplayPort enables daisy chaining of monitors or supports multiple displays (up to 63 separate A/V streams supported across a single connection).
  • The number of monitors supported depends upon the resolution of the monitors and varies from 10 WXGA (1280x768) monitors to 2 WQXGA (2560x1600) monitors.
  • 63 AV streams are supported by a time division multiplexing mechanism and virtual source to sink connections are established.
• Auxiliary Functions
  • A fast auxiliary channel of 720Mbps is available with DisplayPort 1.2. This enables bulk data transfers over a single DisplayPort cable. Example applications include USB mass storage transfers, audio, and camera video transfers.

Comparison of Display/Video standards

Figure 4: USB serving as the universal connectivity interface.