Commentary: HEVC and AV1—A Behind-the-Scenes Look at the Video Codec Race
There’s a codec race underway between HEVC and AV1, and recent events are making it even more interesting. Today, HEVC, the incumbent codec, is supported by more than 2 billion devices including Samsung Galaxy phones and Apple in its Safari browser, Apple TV, Mac, and iOS devices. This makes HEVC playable on close to 50% of smartphones. Despite its broad support on devices, HEVC has not been widely deployed, mainly due to licensing issues. However, there has been progress with HEVC Advance recently waiving the streaming fee, which is a big step forward for the codec. Streaming Media conducted a survey that looks at deployment scenarios for HEVC in light of Apple’s backing. It found that 24% of operators have already deployed HEVC and by end of 2018, 64% will have deployed a HEVC service.
On the other side of the video codec race is AOM (the Alliance for Open Media), with members that include Apple, Google, Mozilla, Facebook, Hulu, Netflix, Microsoft, and Amazon, among others. AOM just released its AV1 specification, aimed at challenging HEVC for OTT application deployment by offering a royalty-free licensing scheme. The main use case for AV1 is OTT VOD. Live can also be supported, although it might face some encoder scalability issues, as discussed later.
Harmonic and research institutes including HHI, B<>COM, and University of Wiesbaden have conducted research on AV1. In particular, Harmonic and B<>COM compared the AV1 open source codec with the HEVC HM reference model on different reference test sequences provided by ITU-R and EBU, using 1080p resolution across a broad range of bitrates. The results, which will be presented to the public at the 2018 NAB Show, demonstrate that AV1 video quality is pretty much on par with HEVC. The tests found that AV1 performs better than HEVC on the PSNR and VMAF metrics, but not on subjective testing. Looking at the overall codec efficiency, there is not much difference compared with HEVC.
Meanwhile, we have to compare AV1 and HEVC in a more holistic way that includes also the complexity of the encoder and decoder. Table 1 provides a summary of the differences between AV1 and HEVC:
Claims to be
Compression efficiency vs HEVC
On par with HEVC
Not significant before 2020
Encoder is 1000x slower than HEVC real time today and expected to be 100x once optimized
TCO = decoder chip (in volume today) + memory + HEVC license
Need to factor additional decoder + memory cost and compare to HEVC decoder TCO
Deployment environment requirement for live
Can be implemented on SW appliance, bare metal container or native cloud
Cloud is the only technology to enable live UHD
Table 1. A side-by-side comparison of the benefits and limitations of HEVC and AV1
AV1 was designed with unicast delivery in mind, meaning we can foresee a different codec for the difference types of devices, i.e. HEVC-capable and-AV1 capable. At the beginning of AV1’s life, we will see AV1 supported only on web browsers in 2018; note all major web browsers are members of AOM. After 2020, we will see more massive AV1 adoption on connected devices.
From an encoder implementation point of view, adding a new codec will not be a problem, as the encoding is done in software. However, based on current scalability numbers (100x vs HEVC), it will add cost. Currently, the AV1 encoding requirements are high, and a cloud-based architecture is well adapted, as it will enable additional nodes to be instantiated for encoding in real time, especially for high-resolution use cases like UHD. AV1 will force the encoder companies to innovate and implement slice-based processing, where each slice will be distributed to a specific node. Note that this approach will reduce the coding efficiency, as prediction will always be limited to the slice boundaries. On the decode side, to cut the cost, a programmable structure will have to be built to support both HEVC and AV1, as both do not need to be decoded simultaneously.
Another issue that arises in the codec race is market segmentation. Depending on the service and devices, the service provider will need to encode either in HEVC (for the next couple of years) or AV1 (starting in 2020). This will require a transition period of about 36 months (2x the refresh period of mobile devices) as well as a simulcast of HEVC and AV1, meaning dual encoding and CDN storage.
Until there is a strong front-runner in the next-generation video codec race, there is room for enhancement of AVC, which has been used for more than a decade. We have seen a revival of AVC with the advent of content-aware encoding, a new technology that Harmonic was the first to deploy for live applications after Netflix and YouTube did the same for VOD. With content-aware encoding solutions, video content and service providers can reduce the bandwidth for OTT delivery up to 50%, decrease CDN costs, and provide a superior quality of experience without requiring any changes to underlying existing delivery infrastructure and video players. To unseat HEVC, AV1 will have to show real benefits to operators, beyond licensing terms. More specifically, AV1 needs to show a cost-effective solution on the decode side, especially if a dual decode (HEVC-AV1) has to be implemented in the decoder. On the encode side, we see huge difference in encode times between HEVC and AV1 (1000x today), which could be acceptable for VOD but could become a barrier for live deployments. As AV1 continues to put pressure on HEVC licensors, the industry needs to find a resolution. A dual codec scenario is not ideal, but could be likely.
[Editor's Note: This opinion piece is a vendor-submitted article from Harmonic. Streaming Media accepts vendor bylines based solely on their value to our readers.]
When comparing the video quality created by different codecs, consider the companies running the comparisons and the metrics they're offering.
AV1 delivers equivalent quality to HEVC, but with a lower data rate. For now, though, it's slow. A five-second clip took 23 hours and 46 minutes to encode.
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