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Connections and Compression in HDMI Transmission - CE Pro

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The HDMI 2.1 specification introduced compression through VESA’s Display Stream Compression, but what does this mean for integrators looking for the best cable?

Somewhere in the process, "compression" means

Technology has become a bad word. why? Over the years, the promotion of HDMI connections to an uncompressed state may have something to do with it. This part is related to quality, but mainly related to interoperability. But the situation is changing, and integrators should regard compression as their friend, which will increase functionality while reducing cable stress due to the ever-increasing demand for bandwidth.

Remember, the only truly uncompressed video we see comes from the graphics processing unit (GPU), such as video from a game console. All other content is delivered in compressed form. a lot of. Broadcasting, streaming media, optical discs, they are all highly compressed as sources, and then uncompressed to be transmitted through the HDMI system. I think we can all prove the extraordinary performance potential of these resources, so compression itself is not an issue for quality. Well, if it is not done well, but we should avoid doing it badly anyway!

The key is interoperability. The range of video compression methods and processing capabilities is very wide, and the devices must all be on the same page. In addition, many methods take some time to perform compression, which means waiting time. This is also a problem. Since its inception, HDMI transmission has been absolutely decompressed, so the device does not have to deal with this additional complexity, and always remains latency-free. But with the development of video, bandwidth becomes indispensable for every four times the resolution is increased.


This specification introduces compression through VESA's Display Stream Compression (DSC). This is an ultra-fast line code "sandwich" compression codec with a very light variable ratio ranging from 1.3:1 to approximately 3.5:1. I calculated the numbers and found that the delay is lower in two-digit microseconds at 4K/60. This is completely imperceptible and has no effect on image quality. By the way, this is the same codec included in the DisplayPort 1.4 and 2.0 specifications, and can also be used with HDBaseT 2.0 to achieve 18Gbps support.

Two things can be achieved by using DSC in HDMI transmission:

DSC is optional and automatic if supported, but only provided when needed-if the system can send uncompressed files in a given format, it can. The caveat about interoperability is that all devices from the source to the sink need to support DSC to work properly.

The only exception is any bit-accurate throughput without decoding HDMI signals; for example, a fiber extender with direct bit mapping doesn't care what is inside—it has nothing to do with DSC, HDCP, etc. The raw data rate is critical. However, for AVRs, etc., the situation is quite different, because they do decode HDMI signals. However, if such a repeater device does not support DSC, the source will not send it first (EDID will do).

DSC has been standardized for the HDMI specification, which is the first step to achieve interoperability. But for this, integrators need information to make informed decisions. This is where the manufacturers come in and disclose the importance of functionality through appropriate labels. In other words, what the product can and cannot do.

The HDMI Forum has proposed a method to disclose the DSC function-list each supported video format and append "A" (only for uncompressed), "B" (only for compressed) or "AB" ( Indicates that both can be supported at the same time). It is proposed to use it in the manufacturer's specification sheet and marketing to support it in the indicated format, but it depends on whether and how the manufacturer discloses the format. I hope such a list can indicate the available data rate of the device, so this information is only for explanation, not absolute.

For example, a device with 40Gbps capability may declare 4K/120AB and 8K/60AB (despite 4:2:0), but can only support 8K/120B because there is not enough bandwidth for uncompressed 8K/120. Therefore, at 40Gbps, 8K/60 4:2:0 media will be sent in an uncompressed manner.

However, this may cause confusion-assuming the media is 8K/60 4:4:4, it needs to be compressed and sent. From 4:2:0 to 4:4:4 is enough to make it exceed 40Gbps, so some knowledge and explanation are needed. In another example, suppose the media is 4K/120, but there is something (device or cable/extender, etc.) between the source and the sink that limits the bandwidth, and the link training protocol limits the link speed to 24Gbps. As long as all devices support DSC (and 4K/120B), the source must send compressed 4K/120.

This is just my opinion, but I think the marking convention is not complete because it only focuses on resolution and refresh rate, and cannot distinguish between 4:2:0 and 4:4:4/RGB or bit depth. What I really want to see is the obvious difference. For example, a device with 40Gbps capability and DSC is declared as capable of supporting 8K/60AB 10-bit 4:2:0 (may be uncompressed) or 8K/60B 10-bit 4:4:4 (compressed only).

As always, the availability of information and education to interpret the information are the keys to the successful implementation of the HDMI system. I encourage discussion around this topic, and consider the standardization of using DSC in HDMI systems, industry-wide labeling conventions.

And please remember to pay attention to the new CEDIA/CTA-RP28 (previously known as CEB28) HDMI system design and verification recommended practices, which is an important free industry tool that does contain information about compression in HDMI transmission. Watch this space, and thank you for reading!

David Meyer, a senior expert with 23 years of experience in imaging and A/V, is the technical content director of CEDIA. Meyer is a prolific writer, educator and subject matter expert, especially in the field of video and connectivity, having served on the (previously) CEDIA Asia Pacific Board of Directors twice. He is a professional member of SMPTE and IEEE engineering organizations.

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