In video compression, two popular codecs stand out – AVC and HEVC, triggering the AVC vs. HEVC debate. AVC, also known as H.264, has been the industry standard for over a decade, but HEVC, also known as H.265, is the newer and more advanced codec.
Almost 70%-80% of global video streamed uses the H.264/AVC codec standard. But slowly VP9, H.265/HEVC, and AV1 are also picking up as they claim better compression efficiency and improved quality at similar bitrates compared to H.264/AVC standard and also provide a few new additional capabilities. New video compression standards are emerging to provide better video quality, improved compression efficiency, tools, and new capabilities.
This article will explore the key differences between AVC and HEVC and how they impact video quality, bandwidth, and file size. Please note that the FFmpeg implementation of x264 (for AVC) and x265 (for HEVC) is used for this observation.
To compare the quality of AVC vs. HEVC at similar encoding modes, the encoders are run at constant QP mode, and all other encoding options are the same for both encoders. Video clips of varying complexities are used for comparison, where these video clips have different texture information, motion, and scene changes.
HEVC’s Advantages Over AVC
HEVC stands for ‘High-Efficiency Video Coding’ and is also known as H.265. HEVC was developed after H.264/AVC video compression standard pioneered by the Joint Collaborative Team on Video Coding (JCT-VC). This HEVC compression standard was developed to double the compression efficiency of the previous standard, H.264/AVC. More about video codecs can be read here.
HEVC provides several tools that improve coding efficiency over AVC. A few of the effective tools/encoding options are listed below.
|Prediction Unit||16×16 to 4×4||64×64 to 4×4|
|Transform Unit||8×8 to 4×4||32×32 to 4×4|
|Motion Prediction||Spatial Median||Advanced Motion vector Prediction Spatial + Temporal|
|Sub Pixel||6 tap Luma and Bilinear for Chroma||7/8 Tap Luma and 4 Tap for Chroma|
|Filter||Deblock||Deblock and SAO|
The paper “Comparison of the Coding Efficiency of Video Coding Standards – Including High-Efficiency Video Coding (HEVC)” covers the HEVC bitrate advantage over AVC with different HEVC tools/encoding modes, and this paper provides details on an average HEVC delivers around 40% bitrate improvement over AVC for interactive video content.
Bitrate Comparison of AVC vs. HEVC
To analyze HEVC’s advantages over AVC, both encoders are run with various complexity videos of 1080P, and 4K are considered with the same video content. For more details about content complexity, kindly refer to the article “Content Adaptive Encoding To Dramatically Reduce Your CDN Costs by 50%“.
Bitrate comparison for 1080P/2K content
Below is the bar graph for average bitrates AVC and HEVC encoders for the same group of video content. Both the encoders are run in constant QP mode for 1080P videos; the rest of the configurations are the same for both encoders.
The average bitrate gain for HEVC over AVC (AVC_BR/HEVC_BR) is 1.38x. If we check individually for lower complexity content, the bitrate gain is 1.56, and this ratio reduces as content complexity increases; the gains are 1.39x, 1.38x, 1.30x, and 1.28x, respectively.
Bitrate comparison for 2160P/4K content
Below is the bar graph for average bitrates AVC and HEVC encoders for the same group of video content. Both the encoders are run in constant QP mode for 2160P videos; the rest of the configurations are the same for both encoders.
The average bitrate gain for HEVC over AVC (AVC_BR/HEVC_BR) is 1.49x. If we check individually for lower complexity content, the bitrate gain is 1.6, and this ratio reduces as complexity increases, and gain ratios are 1.54x, 1.50x, 1.43x, and 1.36x, respectively.
Bitrate rate for 4K in comparison to 2K
When video content is encoded at 4K, and the same content is encoded at 2K, below is the observation on bitrate for various contents. For AVC average (AVC_4k / AVC_2K) average bitrate ratio is 2.55x, and for HEVC, the same average (HEVC_4K/HEVC_2K) ratio is 2.39x. Hence one can choose appropriate bitrates for 4K videos based on their current 1080P bitrates.
These ratios’ maximum and minimum values are around 3.64x/1.95x for AVC and 4.81x/1.72x for HEVC, and the variation patterns across different videos are similar for AVC and HEVC runs. The max ratio video is content with texture information like a library scene, and the minimum gain video is like outdoor sports close-up scene with a panning camera. One possible explanation for this dynamic range of ratios is if the contents of frames in 4K content fit better in bigger blocks than 2K, which can provide better gain. The videos with less gain might have objects/texture, which may demand smaller subblock sizes for 2K and 4K.
PSNR comparison for AVC vs. HEVC
PSNR values for different video contents run in AVC and HEVC modes with 1080P and 4K are listed below. The average value of PSNRs is considered over a group of videos for easy illustration and analysis.
PSNR comparison for 1080P/2K content
The average PSNR for HEVC is 42.47 dB, and for AVC is 41.09 dB. With the content complexity increasing, the PSNR drops as much as up to 5 dB. The PSNR of HEVC is seen with around 1.5 dB better than AVC.
PSNR comparison for 2160/4K content
The average PSNR for HEVC_4K is 45.34 dB, and for AVC is 44 dB. With the content complexity increasing, the PSNR drops as much as up to 5dB. HEVC is around 1 -1.5 dB better than AVC.
Compared to 1080P, the PSNR for 4K is around 2.5 dB better for (same range of contents) for both HEVC_4K and AVC_4K for lower complexity contents and increases to about 3.2 dB for high complexity content for the same compression format.
HEVC High gain contents
During this analysis, we observed a set of videos where HEVC compression gain over AVC is seen as better than the average for both 1080P and 4K modes. Surprisingly this class has all complexity types of content, including Low, Moderate, and High complexities.
The gain for HEVC over AVC is as high as 3.34x for 4K and 2.65x for 2K for specific videos. One of the observations is the higher gain is seen in videos where camera movement is there (Horizontal/Pan motion of handheld camera with slight movements); because of camera movement entire frame will have motion vectors, and HEVC might be doing a better job in terms of Motion Prediction and efficiently coding such inter frames.
HEVC Low Gain Contents
Few video samples are seen where HEVC gain over AVC is lower than the average (lower average level 1.3X)
The gain for HEVC over AVC is as low as 0.93x for 1 or 2 clips, and the average gain for 4K is 1.15x, and for 2K is 1.06x. The major contents in this class are very high texture content with fast motion, like drone cameras moving over a field, full-screen rotating objects, and video with perspective changes (where the size and view of the objects change).
Referring to the low gain of HEVC over AVC, in theory, HEVC should be at least as good as AVC, but the above observation can be seen where AVC performed a little better than HEVC for 1 or 2 clips; the reason might be the x264 implementation of AVC has gone through several quality improvement rounds. x265 might also improve in the coming days and improve the coding efficiency.
Also, another observation is for 0.93x gain in HEVC, the PSNR is 43.2 dB, whereas, for AVC, it is 41.9 dB (i.e., HEVC is 1.3 dB better). This also might explain why the high PSNR output of HEVC might consume more bits than AVC for this class content.
AVC vs. HEVC – Conclusion
The following conclusion can be derived based on the above data and experiments.
- HEVC has provided an average 1.38x bitrate gain over AVC for 1080P content and 1.49x for 4K content.
- An average of 1.5 dB PSNR improvement was observed for HEVC for the same QP values.
- On average, 4K video uses 2.66x more bits compared to 1080P.
- Based on the nature of the video, HEVC’s gain can be as high as 3.34x and as low as 0.93x-1.08x.
- The dynamic range of bitrate for the constant QP mode is enormous due to different content complexities. The table below depicts the range.
Note: The data and analysis are based on x264 and x265 for a set of test vectors identified with varying complexity. These numbers might change with different modes/versions of encoders and with test cases.
|Bitrate in Kbps||Bitrate in Kbps||Bitrate in Kbps|
- Based on average data, if AVC operating bitrates are in the range of 3Mbps-6Mbps; for 4K AVC encoding, the bitrates can be 7.5-8 Mbps to 15-16 Mbps range.
- For HEVC, the bitrate range can be 2.2 Mbps to 4.4 Mbps for 2K and 5 Mbps to 10MBps range for 4K
- The bitrates can be fine-tuned based on the content type (sports/news/indoor/outdoor/EdTech/Meetings, etc.).
- As the dynamic range of bitrates is quite high, of the order of 30x to 80x from above Table 2, a content-adaptive encoder can help in reducing the overall bitrate and hence the delivery/CDN cost and provide a better quality of experience, as higher resolutions can be streamed at lower bandwidths.
Please read this article to learn more about Content Adaptive Encoding.
Ashok Magadum is founder and CEO of Vidarka Technologies. He has a Masters from Manipal University and 21+ years of experience in design and development of various Video codecs to name a few MPEG-2/AVC/SVC/HEVC/Dolby Vision/Content Adaptive, Video Transcoding, Cloud Media Streaming, computer vision, ADAS, DSP, Embedded Systems and various Multimedia Technologies.
He has worked for various Start-ups such as Ittiam System, Zenverge Inc, Saranyu Technologies and MNCs like NXP, Freescale and Samsung for design and development of many solutions.
Website : www.vidarka.com
For any queries or more details, please feel free to reach us at [email protected].
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