Billions of dollars have been invested toward serving up of premium 4K content. Service providers and content owners are expected to make 4K viewing experiences truly superior. But the opportunity to provide audiences an immersive experience brings risk. Much of the premium 4K video on offer is high-motion, high-frame-rate content that presents inherently greater potential for visual artifacts than lower resolution video. A careful, comprehensive approach to 4K content preparation and delivery requires capable video processing and measurement platforms, as well as the knowledge to properly configure these tools for optimal results.
Below are the five most common pitfalls of 4K video quality impairments, their symptoms and root causes, and suggested practices to mitigate or eliminate these issues, so service providers and content owners can offer the highest-quality 4K video possible.
Rules and Thresholds for HD Content No Longer Apply
With 4K video, there is no place to hide visual defects. Any impairment will be much more prevalent in 4K video simply because of the much higher bitrate, greater number of pixels, and larger viewing surface as compared to HD video. 4K content is more likely, at least initially, to be high-motion, high-frame-rate sports content. Not only is this content complicated to properly encode, but the higher frame rate further magnifies the frequency with which errors occur in a 4K versus HD viewing experience.
Symptoms and root cause. 4K video is commonly watched on a larger screen; visual defects that occur in HD, including pixelation, macroblocking, and blurring, occur more frequently and more readily in 4K. The larger the visual display surface, the more obvious the visual artifacts and more magnified the visual defects. The same is true as data rates increase. Rate-based defects, such as pixelation that may be acceptable for 1080p30 content, occur four to twelve times more frequently with 4K delivery.
Suggested practice. Quality levels must be higher for 4K video to offer a viewing experience equivalent to what users are accustomed to with HD. Establish settings and monitoring thresholds tailored for 4K content.
4K video requires greater processing capacity and greater bandwidth than HD to create a satisfactory viewing experience. Squeezing video resources to maximize video processing and network capacity over existing network resources can create highly visible defects. This may be mitigated by shifting to more efficient codecs, such as HEVC, but may only provide partial benefits when delivering high-motion, high-frame-rate 4K content.
Symptoms and root cause. Providing too few network resources inherently degrades video quality. Challenges include limited video-processing resources, limited capacity across CDNs, and limited home network access. This is magnified in 4K and can result in an array of impairments including pixelation, macroblocking, and blurring.
Suggested practice. Properly design and scale network capacity to deliver crisp and life-like 4K video experiences, especially for high-motion sports content.
Excessively Variable Packet and Bitrate Output
For all video content, there is a high degree of variability in encoding complexity and processor utilization, from one frame to the next, especially at scene changes. This is also true for 4K content, especially at higher frame rates such as 100fps or 120fps. Encoding more complex frames can result in bursts of output traffic, while encoding less complex frames causes decreased output traffic.
Symptoms and root cause. Irregular and unpredictable emission of packets can adversely impact video routers and other network elements, and can overwhelm the ability of client devices to render video cleanly. This results in a softening of sharp edges, blurring, blockiness, and stuttering in video playback.
Suggested practice. Monitoring the maximum peak variable bitrate (VBR) of traffic from the encoder, with notification for significant and fluctuating bursts will help. With specific network capabilities in mind, operators must balance VBR settings (both maximum and minimum) in the encoder, though this may impact 4K video quality, especially at high frame rates. Operators can also choose a statistical multiplexer to more efficiently use available network capacity during real-time fluctuations.
Varying Quality of Adaptive Bitrate Output Streams
Adaptive bitrate (ABR) streaming has become increasingly popular for delivering video content to multiscreen devices, particularly over unmanaged or third-party networks for over-the-top (OTT) delivery. This allows client devices to request, on the fly, higher or lower resolutions and bitrates in response to changing network conditions to ensure faster video playback starts and an uninterrupted video flow.
An encoder produces 6–10 variants of each input stream to create ABR output profiles. A single input channel might produce two 1080 output profiles, two 720 output profiles, and two or more SD output profiles, each with different bitrates. This ensures that the client device may request a lower bitrate to spontaneously adapt to network conditions without interrupting the video stream. As network conditions improve, the client device may request a higher bitrate to ensure the highest-possible resolution and quality.
Symptoms and root cause. Improper encoder settings may create noticeable quality differences as a player switches from one ABR stream to another. Group of pictures (GOP) misalignment of the output streams may be perceived by the viewer as jumps or stutters in video playback.
Suggested practice. Fortunately, this pitfall is uncommon. Monitoring the quality of each stream and GOP alignment is important to detect errors or problems with individual ABR output profiles and will eliminate visible video playback stutters.
GOP Misalignment across ABR Output Streams
4K encoding is processor and resource intensive. Even with the most robust 4K encoders, the number of output streams that the encoder is physically able to encode is far more limited than the number of HD or SD streams that the same encoder could generate. To produce a complete, high-quality ABR bouquet – for instance, two 4K profiles, two 1080p profiles, two 720p profiles, and two to four SD profiles – an operator may typically split the encoding across two devices to manage processor load. In this case, each encoder would produce one each of the 4K, 1080p, 720p output streams, and one or two of the SD profiles. This more evenly distributes the processor workload and provides redundancy. If one of the encoders fails, the operator is still able to deliver at least one output stream in each resolution. However, dividing the encoding workload may potentially introduce mismatched output timing.
Symptoms and root cause. When a client device switches from one ABR profile to another, even within the same resolution, GOP misalignment may be visible as jumps or skips forward or backward in playback. This is caused by mistimed output streams across multiple encoders.
Suggested practice. Operators should ensure their encoders are capable of GOP-locking outputs for seamless playback.
4K video has been at the forefront of industry innovation. Yet, as with any new technology, there is a process to fully realizing its potential, so customers can experience it as it was envisioned. 4K video is in a period of refinement, where companies across the video ecosystem have the opportunity to optimize the viewing experience for customers.
To provide the most seamless experience for viewers, operators must understand optimal encoder settings and recognize the unique settings that each network configuration requires for encoding, packaging, and delivery.
This is a key to mitigating and preventing pitfalls, such as those outlined here. Additionally, a critical layer of defense includes a robust automated video-quality monitoring solution to detect impairments in real time. The ability to make real-time corrections will ensure the highest quality 4K experience. Service providers and content owners alike share a vested interest in demanding a higher-quality experience from 4K content to maintain a competitive edge with differentiated product and ensuring an appropriate return on 4K capital investment.
This article is based on Perfecting 4K Video Delivery: Best Practices to Mitigate Common Pitfalls by Elemental