Video Questions and Answers

People usually pose one of the four following questions about connecting video displays to host devices:

• What cable do I need? Let me start with this one, because that answer is in DisplayPort and HDMI Capabilities. It’s very straightforward once you know what display and resolution you’re using, as determined from the next questions and the rest of this chapter.

• How can I connect a display with a particular resolution and refresh rate to my computer or other device? And…

• Why doesn’t my host device support higher resolutions or refresh rates when connected to my display? And…

• How many displays can I attach to my host device?

Those last three questions are linked together. The general answer is that you need to know the capabilities of your host device and display or displays—ones you own or might buy—to see where they intersect. Every piece of hardware has a limit as to how many and what kind of displays you can connect to it; every display has a set of built-in resolutions and refresh rates that it can negotiate with a host controller.

Let’s consider a scenario. Suppose Neha owns a desktop computer that predates Thunderbolt 3 and USB-C. It includes just an HDMI port for external video. Should she purchase a 6K display? No: HDMI is limited to at most 4K. See HDMI and HDMI Standards as Deployed later in this chapter. Could she add a 6K display?

If her computer has card slots, she might be able to buy a video card with a version of DisplayPort and DisplayPort jacks that handles 6K. See DisplayPort and DisplayPort Standards as Deployed. What if she purchased an HDMI-to-DisplayPort adapter? She would still be limited to the maximum resolution and refresh rate supported by the controller driving her HDMI jack. See Adapt Among Video Standards.

Let’s look at another common situation. Phil has an Apple M1 Mac mini and wants to add three external displays. He looks up specs and sees that he can add up to a 6K display at 60 Hz via a USB-C DisplayPort adapter and plug in a 4K display via the Mac mini’s HDMI port. But what about a third? He could use a DisplayLink adapter, which pushes video over USB 3.x and requires a software driver; see DisplayLink Isn’t Part of DisplayPort. Otherwise, he’s limited to two.

As I noted in Identify Jacks and Plugs, you can Read Technical Specifications and Find Operating System Information for some details. For display-related information:

• Find the technical specifications for your host device or display. Most manufacturers list those on their website even for hardware they no longer sell.

• Turn to third-party resources. Websites and apps can help you track down the specifics. For Apple hardware, I turn to the free Mactracker app, which provides a thorough explanation of the number and type of displays you can add (Figure 32).

  

• Plug in and see what happens: macOS and Windows provide details when you connect a computer jack and a display. This can reveal what the operating system has negotiated (Figure 33).

Uncompressed Video Eats Bandwidth

Nearly all video you stream online is compressed. Using clever algorithms, the audio and video components of an original recorded video or audio file are reduced in size by recognizing repetitive elements and ones that aren’t perceived by eye or ear when removed. Compression makes internet-streamed video possible.

You may notice compression at times when you see odd artifacts. YouTube and other services sometimes stall and restart or have strange jitters when you go back 15 or 30 seconds. You’ll see what seems like a combination of a previous frame and only portions that are new. That’s showing some of the underlying aspects of the compression, in which a keyframe captures a scene and then only the differences are recorded while the video remains mostly fixed in place.

HDMI and DisplayPort come from an earlier era in which compression was expensive: it required a lot of computational power or pricey specialized chips. As a result, the standards were built around uncompressed data streams. It’s also the case that when using apps on a computer or mobile display, a user wants 1:1 pixel accuracy, refreshed at the highest available frequency. Some lossless compression algorithms that preserve every bit can reduce the data stream somewhat, but at times it might be nearly as broad as uncompressed video.

These days, compression is affordable and no big deal. Nearly everybody who isn’t an audio or video professional captures all the sound and movement on their mobile devices in high-quality compressed form; most non-professional photographers do the same on cameras.

The difference is huge. An uncompressed 4K video stream at 30 Hz consumes 6 Gbps; at 60 Hz, it eats up 12.5 Gbps. The latest best video compression algorithm, H.265, would require only about 15 Mbps and 50–60 Mbps for the same 4K streams.

But because display standards are built around uncompressed video for home A/V, and because app users want full fidelity, HDMI and DisplayPort remained uncompressed until relatively recently.

HDMI started using a selective data omission called chroma subsampling that using less data to achieve a similar picture quality. Our eyes perceive most colors less well than we perceive differences in tone, or luminance, which is where we observe most of the fine detail in a still or moving image. Chroma subsampling in HDMI essentially streams color information at a lower effective resolution than luminance. It can be noticeable when using a computer, but it’s nearly invisible or imperceptible for filmed content. This is typically used in HDMI for higher refresh rates, providing smoother motion.

DisplayPort separately added support for Display Stream Compression (DSC), which reduces both color and luminance data in a way that’s designed to not be noticed, either. It can achieve up to 3:1 compression and enables higher refresh rates and much higher resolution within the constraints of bandwidth that wouldn’t otherwise support it. HDMI adopted DSC for very high resolution and refresh rates.

Even with compression, HDMI and DisplayPort have to reserve space on a given bus for video. Depending on the version of each standard, the space required can range from about 4 Gbps all the way up to 80 Gbps. The 10 to 25 Gbps range is more typical for 4K and 5K displays.

Video Standards

In every case, though, no matter the connector, DisplayPort and HDMI rule the video roost in the way that USB and Thunderbolt now dominate data standards. Also like those two standards, DisplayPort and HDMI have matured to be fairly inter-compatible, reducing frustration when attempting to connect displays or devices.

Plug into Video (and Audio)

As with any other peripheral, there are three parts to consider: the host device, the cable that connects them, and the peripheral. You can check the host and display’s specifications to make sure the host can output at the highest resolution you want to use on the display. But you also need to make sure you have the correct generation of HDMI or DisplayPort cable or the right adapter.

HDMI Standards as Deployed

Interestingly, native HDMI ports tend to be deployed using older standards for lower resolutions than DisplayPort over USB-C. For instance, the latest version of HDMI supported by Apple on its most recently shipped computers remains version 1.4: 4K at up to 30 Hz on consumer-level devices and up to 60 Hz on professional-level devices.

HDMI can be used over USB-C, which includes an alternative mode for HDMI 1.4b. USB-C natively, and Thunderbolt 3 and 4 within USB-C, can encapsulate DisplayPort over USB-C. If you insert HDMI into that environment, such as using a USB-C to HDMI adapter or cable, you’re limiting your maximum display capability to the lowest compatible level between the display and the version of DisplayPort coming out of the USB-C port.

HDMI connectors come in three varieties: Micro HDMI, Mini HDMI, and standard HDMI (Figure 34). Each fits a niche: digital cameras often use Micro HDMI; cameras more commonly now opt for Mini HDMI, as do video cameras and some mobile devices; and standard HDMI is found on computers, TV sets, and home-entertainment gear.

If the display has both HDMI and DisplayPort jacks, you need to check that its HDMI port can deliver the same resolution and refresh rate as its DisplayPort side so you’re not using the best output possible.

To use later HDMI standards, you typically need to install an add-on video card in a pro-level computer that supports it.

DisplayPort Standards as Deployed

The Mini DisplayPort connector that’s also used for Thunderbolt and Thunderbolt 2 hasn’t appeared on new devices for years. You’re almost certain to either have a standard DisplayPort jack on a display and standard DisplayPort on your host device, like a computer, or require a USB-C adapter for DisplayPort on a host device (Figure 35).

Audio Output

This chapter has absolutely been dominated by video because choosing your display’s resolution and refresh rate are often the most complicated choices you make. Both DisplayPort and HDMI support various kinds of audio incorporated in the same data stream as video. In typical use, you use your operating system to select the display as audio output (Figure 36).

If you’re using a computer with a separately installed graphics card or one that you configure at the time of purchase, you might have to use the audio outputs of the card—the card might not carry audio over a DisplayPort or HDMI connection.

Most computing devices include built-in speakers, which can range from poor and tinny to super high-fidelity that you paid a lot extra for or are a selling point of the device. In most cases, the only audio output option from a computer or mobile device is a standard 3.5 mm (⅛-inch) stereo or hybrid mic/stereo output cable. (See Audio Capabilities.)

Cope with Copy Protection

One of the most frequent problems with computer-to-display connections, particularly with video projectors, is the inability to watch downloaded and streaming video content. That’s due to HDCP (High-bandwidth Digital Content Protection), a form of DRM (digital rights management) came to be because the film industry didn’t want to allow high-fidelity digital video to be copied just by plugging in a cable.

Of course, HDCP doesn’t really prevent an effective way to reduce piracy. Instead, it just makes it harder for all of us with legitimate purposes to do what we want while not deterring or derailing those who make unauthorized copies.

Nevertheless, it’s an HDCP world and we live in it. HDCP was built into HDMI from the start and added to DisplayPort in version 1.1.

Any hardware or software you use that requires HDCP—like a standalone Blu-ray player, Netflix web player, or Apple’s QuickTime Player app—has to engage in a handshake with the display on which its content plays. This handshake typically takes place as a negotiation over a cable, though for built-in displays, like with a laptop, it’s entirely invisible.

If the handshake fails for some reason, you will see an irritating message in varying forms. Sometimes it will read, “Content not authorized,” but it might have a more technical messages like “HDCP handshake failed” or “HDCP unauthorized.”

Here are some of the potential causes:

• Bad cable or incorrect cable tier: Surprisingly, you can plug in an HDMI cable and have some video work while HDCP-based content fails. This can happen if the HDMI cable is poorly made, has an internal fault, or doesn’t meet the spec for your resolution and refresh rate. See DisplayPort and HDMI Capabilities.

• Lack of support in display: An external display sold in the last 15 years should fully support HDCP, but if you’re using a perfectly good older display, it may lack necessary code. It’s also possible the HDCP software was poorly implemented by the display’s maker and is incompatible with what you’re playing back. The way to rule that problem out is to try the same content using the same cable with another display to see if that works.

• Lack of support in projector: Older video projectors often lacked HDCP compliance, in part because it costs money to deploy HDCP and not all projector makers felt that was worthwhile. Newer projectors—those of the last decade, certainly—should be labeled as HDCP compliant or not.

• Lack of external display support for HDCP: A complaint I commonly hear—in my confessor, er, public support role as a long-time writer of how-to material for Apple users—is that someone has an Apple laptop and HDCP has stymied them. The formulation is usually that they have connected a 1080p or higher external display, and—whether mirroring or using an extended desktop—cannot get the content to play at a resolution above 480p (standard definition) or they see an HDCP error message. This problem appears to have disappeared around 2017 or so, potentially overlapping the rise of Thunderbolt 3. It’s plausible that HDCP over DisplayPort directly from a USB-C port works where output via a Mini DisplayPort or HDMI adapter didn’t.