Video compression technology has rapidly evolved over the past two decades, driven by the needs of higher resolution video and streaming over bandwidth constrained networks. Two of the most prominent video codecs during this time have been AVC (H.264) and the newer HEVC (H.265) standard. But what exactly is the difference between these widely used formats and when should you use one over the other? This comprehensive guide provides an in-depth technical and practical comparison to help you decide.
A Brief History of AVC (H.264)
First released in 2003, Advanced Video Coding, also known as H.264 or MPEG-4 Part 10, was developed as a successor to older standards like MPEG-2 and H.263. It offered almost double the compression efficiency compared to MPEG-2 and quickly became the codec of choice for a range of applications.
AVC gained widespread adoption for:
- Streaming internet video services like YouTube and Vimeo
- Blu-ray disc standard for high definition video
- Digital video broadcasting and direct broadcast satellite television
- Video surveillance and security systems
- Video conferencing services
The improved compression performance along with reasonable hardware requirements for encoding/decoding made AVC the universal video standard for over a decade.
The Advent of High Efficiency Video Coding (HEVC)
High Efficiency Video Coding, aka H.265 or MPEG-H Part 2, was first introduced in 2013 as the successor to AVC. HEVC was developed with the primary goal of enabling high resolution video like 4K and 8K Ultra HD using existing storage and bandwidth capacities.
It achieves this through improved compression efficiency, reducing bitrate requirements by 50% over AVC for the same video quality. Other enhancements include:
- Support for higher resolutions up to 8K
- Improved lossless compression
- Increased color bit depth up to 14 bits
- In-built support for HDR video
These features have made HEVC the format of choice for:
- 4K/8K streaming services like Netflix and YouTube
- Ultra HD Blu-ray disc format
- ATSC 3.0 next generation terrestrial broadcasting
- Video surveillance systems
- Professional post-production and editing
However, adoption of HEVC has been gradual due to the increased hardware requirements for encoding and decoding compared to AVC. But with newer smartphones, TVs and other devices now supporting HEVC natively, usage continues to grow rapidly.
Technical Differences Between AVC and HEVC
While both AVC and HEVC utilize similar hybrid, block-based coding techniques for video compression, HEVC employs a number of enhancements that improve performance significantly. Let‘s examine some of the key technical differences:
Image Block Sizes
A video frame is divided into macroblocks in AVC – usually 16×16 pixel blocks that can be further split into smaller blocks down to 4×4 pixels. This allows the encoder to isolate regions within each macroblock for better prediction and transformation.
HEVC supports much larger coding tree block sizes up to 64×64 pixels. The quad-tree segmentation of each block can go down to 4×4 pixel sizes. This provides more flexibility in choosing appropriate block partitions for various parts of the image.
Prediction Modes
AVC has 5 intra prediction modes for encoding within a frame and 7 inter prediction modes for temporal predictions between frames. This is expanded to 35 intra and 7 inter modes in HEVC allowing more accurate predictions based on surrounding blocks and frames.
Transform Unit Sizes
In AVC, blocks can use transform sizes of 4×4 or 8×8 pixels for spatial redundancy reduction. HEVC increases this to a range of sizes from 4×4 up to 32×32 pixels allowing the encoder to select the best transforms for each region.
Color Spaces
AVC primarily supports 8 bits per sample for color while HEVC adds profiles for increased bit depths up to 14 bits per sample. This allows preserving fine color and brightness differences especially for HDR video.
Chroma Formats
HEVC defines additional chroma formats beyond the 4:2:0 sampling typically used in AVC. This enables encoding color channels at higher resolutions for improved quality.
Parallel Processing Tools
HEVC includes provisions for parallel processing at multiple levels which can significantly speed up encoding/decoding on multi-core processors. AVC lacks native support for parallelism.
So in summary, the use of larger coding blocks, advanced prediction modes, more transform options and support for higher bit depths enable HEVC to deliver superior compression efficiency compared to AVC. But this comes at the cost of greater hardware demands.
Practical Impact – File Size, Bandwidth, Quality
Beyond the technical improvements, let‘s examine how HEVC compares to AVC in real-world performance metrics like bitrate, file size and subjective video quality.
Bandwidth and Bitrate Savings
HEVC slashes bandwidth requirements by nearly 50% for the same video resolution and frame rate as AVC. Here are some typical bitrate comparisons:
| Video Format | AVC Bitrate | HEVC Bitrate | Savings |
|---|---|---|---|
| 720p @ 30fps | 2 Mbps | 1 Mbps | 50% |
| 1080p @ 60fps | 8 Mbps | 4 Mbps | 50% |
| 4K @ 30fps | 20 Mbps | 10 Mbps | 50% |
This allows streaming or broadcasting more HD and 4K video channels in the same bandwidth using HEVC encoding.
File Size Reductions
HEVC encoded video results in much smaller file sizes owing to the substantially lower bitrate. Assuming 1 hour of video, a typical size comparison is:
| Format | AVC File Size | HEVC File Size | Reduction |
|---|---|---|---|
| 720p | 900 MB | 450 MB | 50% |
| 1080p | 3.6 GB | 1.8 GB | 50% |
| 4K | 18 GB | 9 GB | 50% |
The smaller file sizes are especially beneficial for media storage and distribution. Video-on-demand libraries can store over 2 times more titles using HEVC compared to AVC at the same resolution.
Subjective Video Quality
Numerous subjective comparisons have demonstrated equivalent or better video quality with HEVC at just half the bitrate of AVC. Viewers often prefer the visual sharpness and clarity achieved by HEVC encoding even at lower bitrates.
This allows streaming services to improve user experience by offering higher resolutions like 1080p and 4K while using no additional bandwidth. For applications like video surveillance that have fixed storage and network capacities, the superior compression performance translates directly into improved video quality.
HEVC offers improved video quality even at lower bitrates compared to AVC
Backward Compatibility Challenges
While HEVC offers impressive performance gains, it suffers from limited backward compatibility compared to the ubiquitous AVC standard. Most modern consumer devices like phones, PCs and smart TVs now support hardware accelerated HEVC decoding.
However, older devices may lack native playback support. This means AVC is often still used to provide wider compatibility across all devices.
On the content creation side as well, HEVC encoding has substantially higher hardware requirements. This has slowed adoption since upgrading encoding equipment can be costly for media companies. Hardware accelerated AVC encoding and decoding offers a more budget-friendly option in many cases.
Use Cases Where HEVC Shines
While backward compatibility issues remain, HEVC brings tangible quality and efficiency benefits to several applications:
4K/8K Streaming Video
HEVC‘s compression improvements are indispensable to deliver ultra high definition video over broadband networks and limited capacity mobile connections. Top streaming services like YouTube, Netflix and Amazon Prime use HEVC for their 4K content libraries. The bandwidth savings also make live streaming 4K sports and events viable.
Broadcast TV and OTT Delivery
Over-the-air broadcasters like the BBC are switching to HEVC for HD channels to free up capacity for additional content. Pay TV operators are moving to HEVC for their IP based multi-screen services. The smaller size of HEVC streams reduces costs for content distribution networks.
Video Surveillance Systems
The substantial storage savings from HEVC are extremely beneficial for surveillance systems having to record continuously from multiple cameras. This allows storing 2-3 times more footage using existing storage capacity. HEVC also enables higher resolution security cameras while keeping storage needs reasonable.
Professional Editing and Post Production
HEVC‘s advanced compression tools like increased color bit depths and chroma sampling formats make it the preferred delivery format for HDR video content. The smaller file sizes also accelerate workflow for editing, backing up and transferring UHD footage.
When AVC Still Works Best
Despite HEVC‘s improvements, AVC remains a good choice in some scenarios:
Standard Definition Video
For applications like video conferencing that only need to work with SD or HD resolution, HEVC offers negligible benefits. The computational overhead versus performance gains make AVC more suitable.
Latency Sensitive Applications
Real-time communications like video calls and live event streaming require minimizing encoding latency. HEVC‘s sophisticated algorithms add playback delay, making AVC faster for low-latency video.
Cost and Hardware Limitations
For developers working within tight budgets or limited processing power, AVC‘s lower encoding/decoding requirements provide a pragmatic choice. This is especially true when targeting older or low-end devices.
Interoperability Needs
AVC ensures wider compatibility across the complete range of legacy devices and playback software that rely on the older standard. When interoperability is a priority, AVC is less likely to have issues.
The Road Ahead for Video Codecs
HEVC represented a giant leap over AVC, but work is already underway for a successor called Versatile Video Coding (VVC). Expected to be finalized in 2020, VVC promises another 50% bitrate reduction over HEVC. However integration into ecosystem standards and hardware will likely take over 5 years.
On the streaming media front, the AV1 royalty-free codec developed by the Alliance for Open Media (AOMedia) aims to challenge HEVC‘s licensing model. AV1 adopters include Google, Amazon, Netflix, Intel, Facebook, Hulu, and more. But so far, AV1 has yet to match HEVC‘s widespread hardware acceleration support.
So while HEVC appears poised for continued growth in the mid-term, expect a wave of further innovation in video compression technology over the next decade!
Key Takeaways on Using AVC vs HEVC
- For SD and HD video, AVC enjoys broader hardware support and provides a nice quality/complexity sweet spot.
- HEVC offers major benefits for streaming and archiving 4K and 8K ultra high definition content.
- Balancing quality gains, storage needs and hardware costs is key to choosing the right codec.
- Legacy device compatibility should be evaluated when selecting between AVC and HEVC.
- New standards like VVC and AV1 on the horizon may disrupt the codec landscape going forward.
In summary, while HEVC represents the state-of-the-art in video compression, AVC continues to enjoy wider compatibility and hardware support. For many applications, AVC provides a good quality/complexity trade-off. But content owners future-proofing for 4K/8K and video streaming services stand to benefit tremendously by transitioning to HEVC. Hopefully this detailed AVC vs HEVC comparison helps provide guidance on the best option for your specific use case!