Among the display interfaces on the market, VGA and DVI have gradually dropped out of the stage of history. Type-C is still a minority, while DP (DisplayPort) and HDMI have become the standard configuration of mainstream products. At present, the mainstream graphics card is mainly based on these two output interfaces, and a new problem arises: when both interfaces can be used, which one will be better?
For most ordinary consumers, it is OK for the display to connect with the host normally. It doesn't matter which interface is OK. It can be used normally anyway. However, for DIY players, this problem is very important, but compared with the performance parameters of the display card and display,This is another problem that can easily be ignored.
In order to know how to make the right choice for ourselves, we have to rethink the DP and HDMI interfaces.
The history of DP and HDMI
From the perspective of development history, HDMI is the predecessor. The first standard was published as early as 2002. At present, the latest version of standard protocol is hdmi2.1, while DP interface is a latecomer. The first standard appeared in 2006, and the latest version of standard protocol is dp2.0. These two interfaces mark the full entry of video frequency transmission interface into the digital era. Although the previous DVI interface interface also used several Word signals are transmitted, but cannot carry audio and data streams
In the type-C interface of thunderbolt and usb3. * protocol, they transmit video signal through DP protocol channel, which is also a special interface form of DP protocol strictly speaking. Thunderbolt 2 supports DisplayPort 1.2, while thunderbolt 3 and fully functional type-C conforming to usb3.1 support the latest dp3.0 protocol. However, there are differences in bandwidth between them (40Gb / s for lightning and 10Gb / s for usb3.1). There are some differences between the two in terms of bandwidth. We will have a chance to talk about this later.
After years of development, although the basic form of DP interface and HDMI interface has hardly changed, there are huge differences in the transmission capacity of different protocol versions. Therefore, we must consider the specific version when considering which is more suitable for the two.
Both DP and HDMI standards are backward compatible, and the HDMI 1.0 cable you bought in the last decade can still be used with the latest rtx30 Series graphics cards. However, due to the barrel effect of the connection between the display, wire rod and graphics card, the maximum display sub capacity limit can only be the lower side between the two.
This connection may mean that the 4K game display that supports 144 Hz and HDR with rtx3080 can only run in 4K and 24Hz mode, which can not release the real ability of the two hardware. This example may be extreme, but it directly reflects the key to choose HDMI or DP: whether the performance of hardware can be fully utilized.
The following figure shows the information about the past DP interfaces and HDMI. To avoid the embarrassment mentioned above, you can quickly find the supported display modes in this table.
The most important parameters of these two interfaces in each generation interface are transmission rate and data rate. In the early DP and HDMI tagging, digital signals are mostly transmitted using 8B / 10B bit rate coding. In 8B / 10B encoding mode, it means that every 8-bit data needs 10 bits of transmission bandwidth in actual transmission, and the extra redundancy is used to ensure signal integrity, which means that only 80% of their theoretical bandwidth can be used to transmit data.
Under the latest protocol, DP 2.0 uses 128B / 132b for transmission, and the encoding efficiency increases to 97%, while HDMI 2.1 uses 16b / 18B for transmission, with the encoding efficiency of 88.9%. Although HDMI is equipped with auxiliary channels to transmit other data, the impact on data rate is not great,In addition, the same generation of DP interface generally has higher transmission rate, so the latest generation of DP interface has higher data rate than HDMI,To understand this, we need to better understand the meaning of transmission bandwidth.
The significance of data transmission bandwidth
All the video data transmitted by digital transmission mode (including DP, HDMI and DVI interface) requires a certain data band width. Each pixel on the display screen has three data points: red, green and blue (RGB), or three data points of brightness, blue color difference and red color color color difference (YCbCr / YPbPr) are used for data transmission.
At the output end, no matter what operation is performed inside the graphics card, the final data generated will be converted into signals for display. These real signals generally contain a 16 bit RGBA information (where a is alpha, which means transparency information).
At present, the most common standard is 24 bit color. In this mode, the red, green and blue component data of each pixel occupy 8 bits of data, which is also the source of bit depth data in our computer display data. In HDR and high color depth displays, in order to display richer color data, the data occupation of each color is increased to 10 bits, which means that each pixel using 30 bit color can bring better display effect.
In some top professional displays, the bit depth has even been increased to 12 bits and 16 bits. However, there are still a very small number of them. Most of the products we can contact with in general consumption are 8 bits, and a small number of high color and HDR displays are 10 bits. In this case, the display signal uses 24 or 30 bits per pixel. We multiply this number by the number of pixels and the screen refresh rate, We can quickly calculate the minimum bandwidth needed to realize this transmission picture.
Taking the above display as an example, the bandwidth required to achieve the display effect of 3440 * 1440 @ 100 Hz: 24 * 3440 * 1440 * 100Hz = 118886450000bps ≈ 13.8gbps in 8bit mode, and 30 * 3440 * 1440 * 100Hz = 14860800000bps ≈ 17.3gbps in 10bit mode.
When the wire or output interface can not meet the demand, we can only choose under the conditions of reducing the resolution, reducing the refresh rate and turning off the HDR. When I use the laptop to connect, the 3440 can be opened smoothly when using the hdmi2.0 interface (14.4gbps)* 1440@100 However, when the full function type-C (usb3.1 protocol) is used for connection, the display mode of 3440 * 1440 @ 60Hz can only be turned on due to the limitation of interface bandwidth (10Gbps)
Moreover, in practical applications, it is not rigorous enough to meet the minimum theoretical transmission bandwidth. In practical applications, a certain amount of redundancy needs to be reserved, and the required bandwidth can be determined through more complex calculation. In order to intuitively reflect the bandwidth requirements, the Video Electronics Standards Association, VESA) introduced a set of intuitive reference standards.
If the required data bandwidth is less than the maximum data rate supported by the standard, the resolution and frame rate can be used normally.
It should be noted that the standard set in the above figure is based on the uncompressed signal. In order to output higher resolution pictures under limited hardware bandwidth, HDMI and DP interfaces also add support for display stream compression (DSC), which can help devices better overcome the physical limitations brought by interface bandwidth. DP has added support for dsc1.2 as early as version 1.4, and HDMI has added support for this technology in the latest version 2.1.
When 8 K and 60 Hz ,49.65 Gbps of data bandwidth is required in basic 8 bit mode ,62.06 Gbps bandwidth is required when 10 bit of HDR mode is turned on, and 127.75 Gbps of transmission bandwidth is required in uncompressed state when 8 is turned on.
Then by converting to 4:2:2 or 4:2:0 YCgCo and using incremental PCM coding, DSC can provide up to 3:1 compression ratio, Claiming to provide a "visual lossless" effect, Use DSC to make the required bandwidth of 8 K 120Hz HDR only 42.58 Gbps, This extreme display effect can be realized under the existing HDMI2.1 and DP2.0 interfaces.
In addition to video transmission, HDMI and DP also need to reserve bandwidth to transmit digital audio data. Under the current standard, they currently use up to 36.86 Mbps (0.037 Gbps) bandwidth audio for audio. Although it will occupy the overall bandwidth, the overall impact is not significant.
From the above introduction, it is not difficult to see that the simple bandwidth calculation method of resolution * refresh rate * color depth is obviously not comprehensive, because the timing standard, encoding method and audio bandwidth occupation should be taken into account in the specific performance performance,Although the bandwidth occupied is still the larger the better, it is obviously not the only reference standard that affects the transmission capacity of video signal.
Advantages and disadvantages of DP and HDMI interface
In the current mainstream market, dp1.4 is the most powerful and popular version of DisplayPort standard. Although the dp2.0 specification was released in June last year (2019), there is still no consumer grade graphics card and display supporting this interface.
However, the rtx30 Series graphics card launched a few days ago still carries dp1.4 interface. Although the available bandwidth is not as good as HDMI 2.1, it is up to 8K@65Hz Has been able to meet the current demand.
One of the advantages of DP is that it supports variable refresh rate (VRR) natively. This technology has become a part of DP standard after dp1.2a. This is also the basis for the large-scale popularization of freesync Technology (compatible with g-sync) in recent years. To use this technology, we must use DP interface.
In addition, due to the addition of fixed snap switch, the stability of DP interface and wire rod is better than that of HDMI, and the risk of "Breakage" will hardly exist if HDMI is pulled carelessly.
Moreover, DP can also connect multiple screens to a single port through MST. Some displays also support DP series function based on this technology (monitors are directly connected through DP interface), which will be more excellent in expansibility.
Because HDMI needs authentication standard protocol, DP is an open standard protocol,Many innovations in display technology (such as DSC, g-sync and freesync) will take precedence over DP, and then gradually appear on HDMI.
The DP cable also has a key parameter that limits its use scenarios: its maximum length is limited to less than 3m under the current standard, which makes it unlikely to be used in home theater, long-distance signal transmission and other application scenarios. 99% of the time, it can only be used between the console host and the display, which indirectly limits the interface in TV and projector and other equipment Application of.
On the most important transmission bandwidth, because the DP protocol has the advantage of latecomer, HDMI has been slightly inferior to the bandwidth performance in the same period DP,HDMI2.1 this time the market is a city back. While the paper parameters have been "backward ", in the case of non-extreme high resolution refresh rate, the current most mainstream DP1.4 and HDMI2.0 will not have a perceptible difference.
Since the release of HDMI 2.0 in 2013, HDMI interface has been implemented in 8 bit color 4K@60 Hz transmission effect, and 4:2:0 YCbCr output mode to achieve the highest 8K@30Hz But there will be edges that look blurry.
So far, only some NVIDIA's rtx20 series and the latest rtx30 Series graphics cards will support hdmi2.1 interface (some non-public graphics cards are only equipped with hdmi2.0 interface), and most monitors are still equipped with hdmi2.0 interface.Therefore, considering the comprehensive function, the best way to use the desktop computer to connect the monitor is still DP.
Although it is not dominant in theoretical performance for a long time, the HDMI specification has a high popularity and compatibility. When the standard was just launched in 2004, millions of devices with HDMI were shipped. By 2020, devices with HDMI will be everywhere, and mainstream real devices will be equipped with this interface.
Although there are no products supporting full specification hdmi2.1 interface on the graphics card, a large number of HDMI 2.1 devices have been put on sale in consumer class devices such as TVs, Blu ray players and home cinemas. The popularity prospect and versatility of HDMI 2.1 devices will still be stronger than that of DP in the near future.
The biggest advantage of HDMI over DP in practicability is that the longest cable length can reach 15m, which is five times of DP cable. This feature is very important for players who use desktop computers to connect to monitors, but it is very important for scenes requiring long-distance connection such as home theater, so that this kind of cable can have more abundant application scenarios.
DP and HDMI: how do gamers choose
We have introduced the technical parameters of DisplayPort and HDMI, and we have a full understanding of these two transmission protocols. So, let's go back to the original question, but which connection method is more suitable for gamers in fact?
Some depend on the hardware you already own or intend to buy. Both standard protocols offer a great gaming experience, but if you want a great gaming experience, HDMI 2.1 among the products you can buy on the market today
In some high-brush high-resolution application scenarios, gamers need to choose according to their own platform.
For gamers using N cards, the best solution is still to use the DP 1.4 interface to connect to the display equipped with g-sync function,Only when connected to the TV can the new HDMI be of practical significance, because at present, only TV can be compatible with g-sync through HDMI 2.1, while on the ordinary display, using DP can fully play the characteristics of N card.
The a-card game player's choice is relatively loose, because the current display with freesync function can be turned on through the HDMI interface,Therefore, there is no significant difference in function and practicability between a card and DP 1.4 and HDMI 2.0.However, DP will still be the preferred standard for PC displays, as many HDMI freesync displays can only operate at lower resolutions or refresh rates, and there are fewer products above 144hz.
If you already have a refresh rate below 144 Hz, does not support g-sync or freesync functions, and the display supports both HDMI and DP input, and the video card is also equipped with these two interfaces, the choice of connection mode is not so important.
At 2560 * 1440 (1440p) @ 144 Hz resolution and 8 bit color depth, protocols above dp1.2 and HDMI 2.0 can work normally. Any connection type below this value will work normally. There is no actual difference between the two interfaces in daily use (the signals that can be transmitted in theory are the same).
In the scenario of desktop computer with monitor, DP interface is obviously the best choiceAt present, there will be more than one HDMI interface in the HDMI series, but it will only support two full-new HDMI interfaces.
However, HDMI is still the best choice for a long time in the scenarios of connecting the host computer to the TV, projector and other large screens, because it has longer transmission distance and more convenient wiring. The most important thing is that HDMI has far more device compatibility than DP interface,At present, the devices with screen size more than 50 inches are rarely equipped with DP interface, and HDMI will still be the preferred interface for a long time in the future.
Finally, despite the specification advantages of DP, HDMI's excellent compatibility and convenience can help it to have a richer application scenario, and the two standards overlap in many fields and technologies.
VESA, which is responsible for the development of DisplayPort standard, mainly considers the application scenarios in the PC field. However, when the consumer electronics alliance formulates the HDMI protocol, it will definitely give priority to consumer class devices such as TV and projector. Their different concerns will eventually lead to different application scenarios of the two protocols.
For high score masters, high color gamut and comprehensive functional experience, use DP, and HDMI if you need large screen and strong compatibility. Users of fever games can choose according to the use scenarios. For more than 90% of ordinary users, the bottleneck affecting the display effect may be in the graphics card and display, so don't worry about the choice between the two.