CBR or VBR: finding the perfect balance between video quality and bandwidth

Different projects stream content that varies greatly in complexity. On one hand, Netflix delivers high-octane blockbusters, filled with dynamic scenes that demand a high bitrate for optimal quality. On the other hand, a national park might offer a serene, static live feed from a bird’s nest on YouTube. In such cases, where the visuals remain largely unchanged—like in surveillance footage—a significantly lower bitrate is often sufficient.

At first glance, Variable Bitrate (VBR) might seem like the obvious choice for all the diverse scenarios, as it can easily adjust to the shifting demands of each frame. However, this isn’t always the best solution. There are many situations where Constant Bitrate (CBR) proves to be the more effective option, and in this article, we’ll explain why.

Understanding CBR and VBR

To make an informed decision between CBR and VBR, it’s important to first understand how each method operates and what codecs are typically associated with them.

Constant Bitrate (CBR) is a method where the bitrate remains consistent throughout the video stream. Regardless of the complexity of the video content, CBR ensures that the data rate does not fluctuate. This predictability makes CBR ideal for scenarios where network bandwidth is limited or fixed, such as live streaming on platforms like YouTube or Twitch.Common protocols used with CBR include RTMP and MPEG-TS, which are widely supported across various devices and platforms.

For instance, a CBR stream might be set to 5 Mbps, maintaining this rate during both low-motion and high-motion scenes. This can lead to underutilization of bandwidth during simple scenes and potential quality issues during complex scenes.

Variable Bitrate (VBR) dynamically adjusts the bitrate based on the complexity of the video content. For example, in a VBR stream, the bitrate might range from 2 Mbps during a simple, low-motion scene to 8 Mbps during a complex, high-motion scene. This adjustment allows VBR to maintain higher video quality during demanding scenes while conserving bandwidth during simpler ones. VBR is commonly used with protocols HLS and DASH. They efficiently handle the dynamic nature of VBR, making them suitable for high-quality on-demand video streaming services like Netflix.

CBR provides a constant and predictable data rate, which is straightforward to manage but can be inefficient. VBR, by contrast, adapts to the content, offering better quality where needed and conserving bandwidth when possible.

Impact on Video Quality and Bandwidth: CBR vs. VBR

Understanding how CBR and VBR function is crucial, particularly in terms of their impact on video quality and bandwidth usage.

CBR maintains a fixed bitrate, which can compromise video quality during complex scenes, potentially leading to artifacts or blurring, especially in high-motion content like sports broadcasts. This issue is more pronounced with older or less efficient codecs, such as MPEG-2. However, CBR simplifies bandwidth management, offering predictability, but it can result in inefficient use, particularly in low-motion scenes.

VBR, on the other hand, dynamically adjusts the bitrate based on content complexity, ensuring high-quality video even in demanding scenes. This makes it ideal for on-demand content where quality is prioritized. VBR is also more bandwidth-efficient, reducing data rates during simpler scenes, which conserves bandwidth and storage—an advantage in environments with limited bandwidth, like mobile streaming. Modern codecs like H.265 (HEVC) and VP9 are particularly effective in VBR mode, balancing quality and bandwidth conservation efficiently.

Average Bitrate (ABR) aims to strike a balance between CBR and VBR by targeting a specific average bitrate over the duration of the video. ABR provides some flexibility in bitrate allocation, adjusting during more complex or simpler scenes while still aiming to maintain a predictable average bitrate. This method can optimize both quality and bandwidth efficiency, making it a practical choice when there is a need for consistent file sizes or bandwidth usage without the rigidity of CBR or the variability of VBR.

CBR vs. VBR

CBR or VBR? Popular Use Cases

Choosing the right bitrate strategy becomes clearer when considering specific use cases where each method’s strengths and weaknesses come into play.

Pushing live video to YouTube and Twitch: CBR

Live streaming platforms prefer CBR for its stability. A constant bitrate helps minimize buffering and latency, which is crucial for maintaining a smooth broadcast. Platforms often impose bitrate limits on incoming video: if the bitrate exceeds, for example, 6 Mbps, the platform may significantly reduce the video quality. Therefore, a gamer streaming on Twitch would choose CBR to ensure consistent quality even during fast-paced gameplay.

Additionally, in live streaming, all viewers receive the same data simultaneously, so a sudden increase in bitrate could overwhelm the delivery infrastructure. H.264 (AVC) is commonly used in these scenarios due to its balance between quality, compression efficiency, and broad compatibility.

Video on Demand (VoD): VBR

In Video on Demand (VoD) services like Netflix or Amazon Prime, Variable Bitrate (VBR) is often preferred due to its ability to optimize video quality while efficiently managing bandwidth. VoD content is typically pre-recorded and stored on servers, allowing the use of efficient VBR encoding. VBR dynamically adjusts the bitrate based on scene complexity, ensuring high-motion scenes get the necessary bandwidth and simpler scenes use less, conserving storage and reducing costs. This flexibility makes VBR ideal for delivering high-quality video across various devices and network conditions.

Video Conferencing: CBR

In video conferencing (Zoom, Microsoft Teams, Google Meet), where both video quality and low latency are essential, CBR is often the preferred choice. The consistent bitrate ensures that the video feed remains stable, reducing the chances of pixelation or lag during important meetings. H.264 (AVC) is the dominant codec in these platforms because of its low latency and ability to maintain quality in CBR mode.

Security and Surveillance Systems: VBR

In smart home and security setups, VBR can optimize the use of bandwidth and storage by reducing the bitrate during periods of inactivity. For example, a security camera monitoring an empty room might use less bandwidth with VBR, but automatically ramp up the bitrate when movement is detected, ensuring detailed footage is captured when it matters most. H.264 (AVC) and H.265 (HEVC) are popular choices in these systems due to their efficiency and support for both CBR and VBR modes.

VR/AR Streaming and 360-Degree Videos: VBR

Virtual Reality (VR) and Augmented Reality (AR) applications, especially those involving 360-degree videos, benefit from VBR’s ability to adapt to the complexity of the scene. This ensures high-quality visuals during intricate scenes while conserving bandwidth during simpler ones, enhancing the immersive experience. H.265 (HEVC) and VP9 are ideal for these applications due to their ability to handle high-resolution content with variable bitrate effectively.

Technical Considerations for Implementation

When implementing CBR or VBR, several technical factors come into play, including server capabilities, encoding protocols, and the specific requirements of your streaming infrastructure.

CBR is often favored in live streaming due to its predictable data rate. When using protocols like RTMP (Real-Time Messaging Protocol) or HLS (HTTP Live Streaming), maintaining a consistent bitrate ensures stream stability and prevents overwhelming available bandwidth. This is particularly critical when streaming to cloud-based transcoding services, where bandwidth must be tightly controlled. Media servers like Flussonic can leverage CBR to prevent data spikes that might cause packet loss or latency issues, especially under constrained network conditions.

From a hardware perspective, using CBR allows for more predictable load management on both the encoder and the media server, simplifying resource allocation. However, the downside is potential inefficiency during low-motion scenes, where the fixed bitrate may lead to unnecessary data transmission.

VBR, by contrast, introduces more complexity due to its dynamic nature. It requires advanced encoding hardware or software capable of adapting the bitrate in real time, which can increase the computational load. VBR is particularly effective for on-demand content delivered via protocols like DASH (Dynamic Adaptive Streaming over HTTP) or HLS, where it adjusts to the varying complexity of video scenes to optimize quality and bandwidth usage.

Servers and CDNs (Content Delivery Networks) need to handle the potential spikes in data transmission that VBR can generate. This means selecting infrastructure that can scale to manage the variable bitrate without affecting the viewer experience. Additionally, VBR places higher demands on encoding hardware, requiring robust infrastructure to support its dynamic processing needs, especially in large-scale deployments.

In deciding between CBR and VBR, consider your streaming environment’s specific requirements, including server capabilities, network infrastructure, and content nature. For live streaming, where stability is key, CBR is usually preferred. For on-demand content, where quality and bandwidth efficiency are priorities, VBR offers significant advantages if your infrastructure can support it.

Multi-Bitrate and Adaptive Bitrate

Don’t mix CBR and VBR with multi-bitrate and adaptive bitrate, as they serve different purposes. CBR and VBR are encoding strategies that determine how the bitrate is managed during the encoding process, affecting the video quality and bandwidth usage for a single stream. In contrast, MBR and ABR are delivery techniques that involve offering multiple streams at different bitrates or dynamically adjusting the bitrate during playback to match the viewer’s current network conditions, ensuring smooth playback across varying bandwidth environments.

Multi-Bitrate (MBR) involves encoding a video at multiple bitrates and storing these different versions on a server. During playback, the video player selects the most appropriate bitrate based on the viewer’s network conditions. For example, a video might be available in 240p, 480p, 720p, and 1080p resolutions, each with corresponding bitrates. This approach ensures that users with varying internet speeds can still access the content, though they might experience different levels of quality.

Adaptive Bitrate (ABR) takes the concept of MBR a step further by dynamically adjusting the video quality in real-time, depending on the current network conditions. With ABR, the video stream automatically switches between different bitrates as the viewer’s internet speed fluctuates. This minimizes buffering and provides a smoother viewing experience. ABR is commonly used in streaming services like Netflix and YouTube, where maintaining consistent playback is crucial.

Versatile Encoding Solutions with Flussonic

In the world of OTT and IPTV services, choosing between CBR and VBR is a critical decision that can affect both the quality of the video delivered and the efficiency of your network. While CBR offers predictability and simplicity, VBR provides superior quality and bandwidth efficiency, making it a powerful tool for a wide range of applications.

Flussonic Media Server offers a powerful and flexible encoding platform that can handle all your video streaming needs. Whether you’re working with CBR or VBR, Flussonic provides robust support for encoding into any format, ensuring that your content is delivered with optimal quality and efficiency. With Flussonic, you can confidently meet the demands of today’s viewers while staying ahead of tomorrow’s technical challenges.

Ready to see how Flussonic can enhance your streaming capabilities? Request a free trial today and experience the full range of encoding options tailored to your specific needs.