Three advanced digital video interfaces: HDMI, DisplayPort, UDI

With the popularity of digital liquid crystal displays, the video interface and video terminals are also actively digitized. This trend precedes the information technology (IT) field and then extends to the consumer electronics (CE) field. In 1999, the information industry proposed DVI. Interface, it is expected to replace the analog VGA interface used by traditional PCs with digital DVI interface; after that, there is also an HDMI interface in consumer electronics in 2002. It is expected to replace various analog video terminals used in traditional TVs with digital HDMI interfaces, including Composite terminals. (AV terminal), S terminal, Component terminal (color difference terminal), etc.

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Why is DVI for IT/PC? CE/TV with HDMI? Why do you want to set up two interfaces/terminals? Can't it be applied to both industries with a single standard and design? The industry's argument is that there is a need for compatible analog VGA in the IT field, but there is no CE field. The relative CE field needs content protection (copy protection mechanism) in the video transmission process, while IT has no such requirement. In the same CE field, audio and video are used at the same time. Therefore, the video interface can also be used to add audio transmission. However, the IT field is not always needed for audio. Based on the above considerations, two digital video interfaces are required. To meet the needs of the two areas.

Figure 1: DVI's five connectors, DVI-A (analog only), DVI-D (digital only), DVI-I (digital, analog), plus Single Link, Dual Link, the most popular It is DVI-I (Dual Link). Even for Dual Link, most of the actual use only uses one Link.

However, such a statement has not been fully convinced, at least VESA agencies have objections, so in 2005 also proposed a new digital video interface, called DisplayPort, DisplayPort is expected to be used in both IT and CE areas, and opposed "PC for DVI, TV with HDMI" settings, DisplayPort's many functions and specifications are for the weakness of DVI and HDMI, and it is expected to replace DVI and HDMI with DisplayPort.

DisplayPort interface diagram

However, the DVI/HDMI camp has not sat down on this development. The VESA DisplayPort has responded to various questions about DVI/HDMI, and has developed a new specification strategy: abandon the DVI that has been upgraded, and actively strengthen HDMI. Create a new UDI digital video interface as a backup. If HDMI can't beat DisplayPort, use UDI to compete.

HDMI type A connector (19-pin, Single Link) and type B connector (29pin, Dual Link)

The mainstream battle for the new generation of digital video interfaces across IT and CE has already started. Who can win the three standards? The following will be analyzed.

Bandwidth line ratio

The primary focus of the new generation of digital video interfaces is the "bandwidth line ratio", which is to obtain the highest bandwidth with the fewest lines. Therefore, DVI uses 6 sets of transmission pair (Pair) to form 2 sets of links. Each set of Pairs (also known as Lane, but differently defined by Lane) is transmitted by TMDS. The transmission rate is the highest. It is 1.65 Gbps, so each group of links has a bandwidth of 4.95 Gbps, and two sets of links have a bandwidth of 9.9 Gbps (nearly 10 Gbps).

The circuit diagram of the DVI interface has 24-pin (Pin 1~24) on the left and 5-pin (C1~5) on the right. C1~C5 and Pin8 are set up to match the traditional analog VGA.

In terms of HDMI, it is based on DVI and is further developed for CE requirements. Therefore, the overall technology is not much different from DVI. It is also transmitted using TMDS. The same pair of Pairs has 1.65Gbps, but the HDMI connector (connector) has Type A and Type B, Type A has only one set of Link. It is said that the CE field rarely uses the second set of Link to transmit, so the total bandwidth is only half of the DVI, which is 4.95Gbps, but the Type B is still Maintain 2 sets of Link, the total bandwidth is the same as DVI.

As for DisplayPort, there are 4 pairs of pairs, and 4 pairs of lines form a set of Link. However, the transmission code does not use the exclusive technology of TMDS, but uses the industry standard ANSI 8B10B, so each group of transmissions can be 1.62Gbps or 2.7Gbps, 4 groups get 10.8Gbps, and the advantage of using ANSI 8B10B technology is that the clock signal is embedded in the data line. In contrast, each group of DVI/HDMI links requires an additional set of clocks to match the transmission. As a result of this calculation, DisplayPort only needs 8 lines (each pair of Pair is equal to 2 lines) to break through 10Gbps, while DVI/HDMI requires 16 lines to be nearly 10Gbps. In fact, DisplayPort allows flexible configuration of the number of Pairs in the Link. One Link can be a group of Pairs or two groups of Pairs. It is not mandatory to use 4 groups of Pairs at the same time. However, DVI/HDMI must use 1 each time. Or 2 sets of Link, each group of links is forced to 3 sets of Pair.

HDMI mini connector: C connector (pictured right) and A type connector volume comparison.
DisplayPort is used for connectors that are connected inside the machine.

DisplayPort connector for external connection

Since DisplayPort emphasizes fewer lines, it has to achieve high bandwidth, which is superior to DVI/HDMI. Since DVI has been released from the 1.0 version of the specification in 1999, there has been almost no change. Even if it is more difficult to move the DisplayPort proposed by the 2005 advanced technology, the DVI/HDMI camp strategy is completely focused on HDMI. In 2006, HDMI proposed 1.3. Version, the clock is increased from 165MHz to 340MHz, such a Link can have a bandwidth of 10.2Gbps, which closes the gap with DisplayPort, which can break through 10Gbps in 8 lines.

The UDI proposed in 2006 is expected to replace DVI. The technology still inherits the DVI/HDMI design. That is, the three groups of Pairs constitute one Link, and the code also maintains the use of TMDS. However, the transmission rate of each group of Pairs is 2.7Gbps, 1 group. Link has 8.1Gbps, but currently UDI does not seem to have a second set of Link, so it is weak compared to DisplayPort and HDMI 1.3.

In addition to these three, in fact, you need to mention another video interface, that is, the LVDS used in the notebook computer, LVDS bandwidth line ratio is the lowest, each group of Pair is 945Mbps, 4 groups constitute 1 pass (Channel ), in addition to each channel with an additional set of clocks, so even if the dual channel is only 7.56Gbps, but used 20 lines.

DisplayPort link layer architecture

Why are the new standards emphasizing the bandwidth ratio? Mainly based on cost and design considerations, the future digital display will continue to grow, and the long and wide pixels will continue to increase, so the amount of video transmission will increase, if the number of lines must be increased in order to cope with higher transmission, In the R&D design, the added effort will be added, and the line cost will also increase in manufacturing.

The newly released Apple TV video box has adopted HDMI.
Internal use

After the introduction of the DisplayPort standard, many questions were raised about DVI and HDMI. One of them is that DVI and HDMI are only suitable for external connection, but there is no internal connection design, so equipment manufacturers must choose another internal solution, notebook computer manufacturer. The LVDS interface is used in the machine, but as mentioned above: the bandwidth ratio of the LVDS interface is low, and it is difficult to cope with the higher picture size and bandwidth in the future. DisplayPort believes that DVI and HDMI do not bring a complete and consistent connection solution for digital video. The relative DisplayPort has both internal and external versions, and the internal version can replace LVDS.

UDI output connector (left) and receiver connector (right)

There is no solution for this HDMI, and the improvement is expected to be transferred to UDI. UDI has an internal solution and is expected to replace LVDS and compete with DisplayPort.

More elaborate connector

The continuous ingenuity of connectors is an inevitable trend of hardware components. At the same time, it is the second fine project outside the line number and wire cost. The smaller the connector, the more efficient the material cost. This corresponds to the video interface. In the past, the connector of DVI was quite large, and it was necessary to fix it with screws at both ends to prevent the wiring from falling off due to accidental pulling. When HDMI was used, the connector was greatly reduced and the screw was removed. It is directly fixed by the mechanical friction between the joint and the joint (similar to the USB and 1934 joints), but it is more susceptible to external force than the screw method.

As for DisplayPort, the external connector design is similar to HDMI, but an optional alternative is to design a button that can be pressed by the thumb in the plastic part of the connector to increase the resistance of the pull. However, in addition to this, DisplayPort takes into account the trend of thin and light notebook computers, and believes that the existing video connectors and jacks are still too large, which is not suitable for thin and light bodies, so a new mini-connector has been added. In addition to being used for notebook computers (including other handheld or portable products), this type of connector is also suitable for multi-valued output of desktop computers. Multiple DisplayPorts can be placed simultaneously in the backplane area of ​​a single interface card. Video output connector.

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The mini connector is also a point of the DisplayPort camp DVI/HDMI. In addition, the HDMI is also updated and replenished in the 1.3 version. In addition to the original A-type and B-type connectors, a C-type mini connector is also established, and the new UDI interface is also available. At the beginning of the development, a more compact joint design will be adopted to meet the challenges of DisplayPort. Under the mini-sound of the connector, the DVI has already been eliminated, and the HDMI type B (larger than the A-type) connector is no longer mentioned.

AC coupling

Another question of DisplayPort is AC Couple. It is believed that as the semiconductor process continues to advance, the voltage level of video signals transmitted from the chip is constantly decreasing. However, from the perspective of today, the HDMI proposed in 2002 is The transmission voltage level is too high, so it will be difficult to respond to the future use of the new process of the chip, but must increase the circuit design of the voltage level conversion, not only increase the design effort and manufacturing costs, but also the transmission efficiency. Once again, HDMI is under pressure from DisplayPort, and the specification of the transmission voltage is reduced in HDMI version 1.3.

Analog VGA compatible

The reason why the DVI connector is large is partly because it is compatible with the analog VGA signal in the past, so it is difficult to reduce the number of lines. However, the initial setting of HDMI is only for CE use, so the compatibility with analog VGA is abandoned.

As for DisplayPort, since it is set to be applicable in both IT and CE fields, it must also be compatible with analog VGA. In this case, DisplayPort adopts an additional adapter (Adapter) in the middle of the transmission connection. The video conversion of the connector is compatible with analog VGA, but this method will be degraded in signal energy and transmission efficiency, so the design of the compatible adapter must be quite strict. In contrast, HDMI also emphasizes that it is suitable for both IT and CE. However, the specifications of the previous connectors have been fixed, and it is no longer possible to add a line compatible with the analog VGA. Therefore, it is only possible to adopt the interface conversion.

What's more, the newly created UDI also adopts a similar conversion, but further considers the requirement of “possibly adding the adapter during the transmission process”, so an auxiliary power supply line is added in the interface definition (similar to USB). The 1394 method allows the installed adapter to obtain a relatively large operating power supply. Providing power for the video conversion adapter during transmission is a point that is less of a consideration for DisplayPort, but as analog displays fade away, the importance of compatible VGAs continues to decrease.

Color format

Originally, HDMI only supported the YCrCb format in the CE field. However, after the emergence of DisplayPort, HDMI was also positioned as both IT and CE. Therefore, it also began to support the RGB format used in the IT field.

In addition, DisplayPort supports Color Depth of 30-bit (10-bit RGB three primary colors) in response to demand for high-definition TVs, and challenges HDMI to only 24-bit (8-bit RGB three primary colors). Forcing HDMI to improve, not only supports 30bit since version 1.3, but also supports 36-bit (three-color each 12-bit), 48-bit (three-color each 16-bit), and adds the latest color format standard xvYCC Support, HDMI is now higher than DisplayPort in color format support.

With audio transmission

Because it was originally set to be used in the CE field, and CE has always been used for video and audio, HDMI is equipped with audio transmission function at the beginning of design, and can transmit up to 8 channels of audio. Conversely, the IT field does not have to use audio. Many commercial PCs or workstations only need simple two-channel, and do not use multi-channel surround sound. Therefore, the video interface for IT does not have to be accompanied by audio transmission. .

In order to be applicable to both IT and CE, DisplayPort can also transmit multi-channel audio. In addition, the “auxiliary channel” for communication coordination and control in its interface has a bidirectional transmission bandwidth of 1Mbps, which can also be used for Two-way audio transmission, in order to support video conferencing, Internet telephony and other information applications, and this is not possible with HDMI, HDMI can only carry out one-way audio transmission, not two-way. UDI is the same as DVI because it is preset for the IT field, and it does not have audio transmission at all.

Since HDMI and UDI both use the I2C interface for communication coordination and control on the display, although the highest speed I2C transmission rate is 3.4Mbps, HDMI and UDI use 100kbps/400kbps standard/fast transmission rate, 100kbps/400kbps in communication. In addition to coordination, it is difficult to have more bandwidth to support audio transmission.

However, if the audio standard is supported, it is currently dominated by HDMI. HDMI has established a standard to support 8-channel compressed audio. In version 1.2, it supports Sony's SACD audio. Version 1.3 adds support for Dolby. Non-distorting audio such as TrueHD and DTS-HD Master Audio. DisplayPort has no movement in the actual audio standard support.

Line length

For IT applications, video connectors are rarely pulled long, but there is a need for CE applications, especially when building home theaters. Since HDMI is set to CE, it must support longer connections, claiming a length of up to 15m. Similarly, DisplayPort claims to be up to 15m. As for DVI and UDI, DVI can only have a few meters of wiring, and UDI has not released relevant information.

It should be noted that 15m is only an existing implementation claim, but it must be counted through the actual verification test. Take DisplayPort as an example. At this stage, the full-speed transmission can only be within 3m, but there can still be HDTV 1080p within 15m. The level of transmission capability, that is, the transmission efficiency, has been attenuated by the increase in length.

Similarly, HDMI must have the same practical test. The 15m is a declared value. The maximum transmission bandwidth can be maintained for a certain length. There is no specific details, and it will be strengthened with the advancement of technology.

Content protection

In the field of CE, it pays great attention to the copy protection of video content. In particular, the creators of video, distribution channels, broadcast operators and so on have been stunned by the network P2P software, so HDMI, DisplayPort, etc. all take the anti-copy mechanism into consideration. .

HDMI supports Intel's HDCP copy-protection mechanism, while DisplayPort only supports Philips' DPCP copy-protection mechanism in version 1.0, but DPCP is a new standard. Really widely used HDCP, DisplayPort is not as good as HDMI at the beginning, but Display Port Version 1.1 also supports HDCP, but goes beyond HDMI. As for UDI, only HDCP is currently supported.

Technology licensing fee, royalties

Technology licensing fees and premiums are also a point of DisplayPort's aggressive attack on HDMI. Manufacturers must pay an annual fee of $15,000 to use HDMI technology. At the same time, each HDMI connection is set to charge 4 cents. The premium, which is a big load for manufacturers, has been published since 2002, but the implementation and penetration rate has been quite slow for several years.

In contrast, DisplayPort adopts a license-free and royalty-free policy. Only the DPCP's copy-protection mechanism requires additional licensing costs, and is optional. There is no need to pay for this mechanism. Due to the pressure on DisplayPort's licensing fees, HDMI has now lowered its annual fee to US$10,000 per year, and pointed out that DisplayPort's support members still charge relevant technology fees to downstream industry players, but this has not been confirmed. .

Actual adoption

This point is currently leading HDMI, the well-known Sony PS3 TV game, Apple's Apple TV digital media adapter, etc., all have HDMI interface, DisplayPort has not seen the actual product adoption; UDI has not seen the actual use. However, this lead may also be caused by the difference in implementation time. HDMI was proposed in 2002, DisplayPort is in 2005, UDI is in 2006, and the subsequent growth and decline remains to be seen.

With the competition to make progress, HDMI has been released from the 1.0 version in 2002, and until the introduction of DisplayPort in 2005, it can be said that there has been no progress in the past three years. In the process, there have been only minor adjustments and revisions, but in the competition of DisplayPort. After the standard appeared, it was strengthened and upgraded in less than two years, and the new UDI standard was provoked. Therefore, although the competition of the three standards has caused hesitation and disorder to the designers, manufacturers and consumers, it has forced the standard makers to progress and increase the number of users, thus accelerating the pace of technological progress. Will bring more benefits and convenience to consumers.

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