Electronic
Design based on mobile phone USB high-definition video transmission interface technology

Design based on mobile phone USB high-definition video transmission interface technology

In order to meet the challenge of transmitting high-definition video on mobile phones, it is necessary to define a USB audio and video class in the USB standard to standardize the USB video transmission (Video-over-USB) technology.

In order to meet the challenge of transmitting high-definition video on mobile phones, it is necessary to define a USB audio and video class in the USB standard to standardize the USB video transmission (Video-over-USB) technology.

The camera function of mobile phones has evolved from a novelty to a mainstream configuration, and mobile vendors strategically regard high-definition video as their high-end products. The integration of high-definition video in the mobile phone will further reflect its practical value, because it is not only a digital camera, but also a digital video camera.
Putting high-definition video on the mobile phone will bring a new problem: how to make high-definition video playback. It is very common to play back videos directly on mobile phones, but the size and resolution of the screens make it impossible for high-definition video to convey its moving high-definition experience to users. Similar to sharing mobile documents and photos, the challenge for developers is how to share high-definition video without restricting it to the inside of the phone.

Use high-definition video output interface on mobile phones

The development of the current mobile phone video output interface cannot keep up with the transmission demand of mobile phone high-definition video. USB data transfer speed is sufficient to transfer photos, but high-definition video connected to high-definition televisions, monitors and other Display devices requires real-time streaming. At the same time, current high-definition video mobile phones are either equipped with standard-definition analog video output or adopt the high-definition video standard used by high-definition televisions. These are not specifically optimized for mobile phones (see Table 1).

Design based on mobile phone USB high-definition video transmission interface technology

Table 1: High-definition video integration of various mobile phone manufacturers.

Traditional HD video standard

HDMI is one of the best standards for high-definition video today. By the second quarter of 2009, more than 850 companies had obtained HDMI licenses.
In-Stat has predicted that in 2010, there will be more than one billion HDMI-enabled devices on the market. These devices include HDTVs, projectors, and handheld devices such as media players and cell phones. HDMI (Figure 1 bottom) uses 3 TMDS (Minimize Transmission of Differential Signals) data channels and a separate clock channel to transmit uncompressed audiovisual signals, up to 10.2 Gbps (3.4 Gbps per channel). With such a bandwidth, HDMI supports more than 1080p data streams, which gives rise to a digital three-dimensional experience.

Design based on mobile phone USB high-definition video transmission interface technology

At the same time, another high-definition standard, DisplayPort, is rapidly penetrating the market, and it has strong support from manufacturers such as HP, Dell, and Apple. In the DisplayPort core, audio, video and embedded clock signals can be transmitted through scalable 1, 2 or 4 channels under the micro-packet architecture.

As shown in Figure 2, the original data stream is well packed and transmitted via the data channel through pixel control and addressing. DisplayPort1.2a supports a bandwidth of 17.28 Gbps.

Design based on mobile phone USB high-definition video transmission interface technology

Figure 2: DisplayPort structure block diagram.

Problems using traditional high-definition video standards in mobile phones

Both HDMI and DisplayPort are designed for desktop devices, and their inherent shortcomings limit their use in mobile phones and other portable Electronic products. Both of these two standards require more pins. DDHDMI has 19 pins and DisplayPort has 20 pins, which makes the size of the connected device larger. In some portable electronic markets where small size is the advantage, the large size of the connector will affect the volume and shape of the entire design.

More pins also mean higher costs, which increases the bill of materials (BOM) of the finished product. Today, the application rate of traditional video standards in mobile phones is still very low. Mobile phone manufacturers need to reconsider their existing designs to accommodate these additional connectors.

Another disadvantage of traditional high-definition video standards is that they cannot be powered by the same cable. This is not an obvious disadvantage for desktop devices, because desktop devices such as Blu-ray players, set-top boxes, and PlayStation 3 are all independently powered.

In addition, mobile phones will continue to consume power when transmitting video streams to external display devices. If the mobile phone is not charged through a separate charger or USB connection at this time, the playback time of the mobile video will be limited. Therefore, if you use the traditional high-definition video standard, you will inevitably need to trade-off between playback length, portability and battery life.

Miniaturized HDMI

Don’t underestimate the effect of reducing the size of the traditional HD video standard connector. The HDMI connector has undergone a series of reductions. It was originally an A-type connector, but in the HDMI 1.3 specification, a small connector (B-type) was defined for portable devices. It is reduced from 13.9mm x 4.45mm of Type A to 10.42mm x 2.42mm.

Recently, the HDMI 1.4 specification defined a new HDMI micro connector (Type D). The HDMI micro connector retains the standard 19-pin type A and C, but reduces the connector size to 2.8mm x 6.4mm, similar to the micro-USB connector (2.94mm x 7.8mm). In the HTC EVO 4G mobile phone, the HDMI micro connector is placed next to the micro-USB connector.

USB-based HD video

You can transmit HDMI or DisplayPort audiovisual signals through a common USB interface. USB 1.1/2.0 has been widely used in mobile phones, and 1.28 million USB mobile phones have been sold this year. In addition, the USB 3.0 technology increases the bandwidth and can achieve large-capacity high-definition video transmission.

Compared with the traditional high-definition video standard, the USB interface brings two main advantages that are very suitable for mobile phones. USB requires only a small number of pins, and can transmit audiovisual signals through a TMDS (Minimize Transmission of Differential Signals) physical channel. The small number of pins makes the connector size smaller, which is ideal for mobile phone devices.

In addition, the small number of pins can ensure that the connector BOM is small. More importantly, USB is already a standard accessory in most mobile phones, and is the preferred data transmission interface and charging port. Therefore, the Video-over-USB concept can be easily adopted by manufacturers without significantly increasing manufacturing costs.

Secondly, USB can transmit large-capacity content on a single cable while still providing power. For example, when video data packets flow from a webcam to a PC via a USB interface, current can be delivered from the computer to the camera to supply power.

Applying the same concept, the mobile phone can transmit the whole movie and TV to the TV through the USB interface, and at the same time, it can supply power through the TV. After the playback is over, the phone is also charged, and it can be used for other purposes such as making calls and sending emails. A similar concept has been adopted in the iPod docking system. Using Apple’s proprietary 30-pin connector, the iPod can be charged while playing music through the external docking system speakers.

The promotion of USB audio and video

Several industry players are developing customized USB graphics implementations to meet this emerging market. However, the diversity of these technologies and the demand for unique customized chips and drivers have made the Video-over-USB concept gradually approach mainstream applications.

In order to achieve success quickly, it is necessary to define USB audio and video classes in the USB specification to standardize the Video-over-USB technology. Pre-defining a class for audiovisual signal transmission through USB can encourage semiconductor manufacturers to construct SoCs for this purpose without the need to customize driver chips.

Like USB mass storage (MSC) and human interface device (HID), the next time you plug your phone into the TV, you only need to simply click “Play” and the HD video stream can start.

Design based on mobile phone USB high-definition video transmission interface technology

Figure 3: Comparison of HD video transmission between traditional standards and USB.

The Links:   LC171W03-C4K1 LTM150XH-T01