This time we are publishing a blog post about VBR vs CBR which has been originally published at Linkedin on November 9, 2024 written by one of our active community member Sudeep Kumar who is an experienced product head and business hunter with a strong background in video streaming, health tech, and edtech.
In the world of video streaming, especially live streaming, choosing the right bitrate strategy is essential to delivering high-quality, seamless content to viewers. Two main encoding techniques dominate the streaming landscape: Variable Bitrate (VBR) and Constant Bitrate (CBR). While both serve the same purpose of encoding video, they do so in distinct ways that impact the streaming quality, network usage, and viewer experience.
This guide explores the core differences between VBR and CBR, their pros and cons, and why CBR often stands out as the better choice for live streaming.
Understanding VBR vs CBR
Variable Bitrate (VBR) and Constant Bitrate (CBR) are two fundamental encoding approaches. Let’s dive into what they mean and how they differ.
- Variable Bitrate (VBR): Adjusts bitrate based on content complexity. In simpler scenes, VBR lowers bitrate to save bandwidth, while in complex scenes, it increases bitrate to preserve quality.
- Constant Bitrate (CBR): Maintains a fixed bitrate regardless of content complexity, ensuring steady network usage and a predictable quality across the entire video.
These methods play a pivotal role in how efficiently data is streamed and can significantly influence a viewer’s experience based on content type, network stability, and other factors.
Variable Bitrate (VBR): Optimizing Bandwidth, Adjusting Quality
Variable Bitrate, or VBR, is an encoding method that dynamically adjusts the bitrate based on the complexity of the video content. VBR encoding increases the bitrate for complex scenes and reduces it for simpler scenes. This allows VBR to make efficient use of available bandwidth and achieve higher quality levels where necessary.
Key Benefits of VBR
Efficient Bandwidth Usage: VBR is intelligent in managing bitrate based on scene requirements. Simple scenes with minimal motion or fewer details require less data to maintain visual quality, allowing VBR to lower the bitrate during those times. This saves bandwidth, especially in environments where network usage is high or cost-saving measures are crucial.
High-Quality Output for Complex Scenes: When video scenes become more complex — think of fast action, intense motion, or detailed graphics — VBR automatically increases the bitrate to avoid quality degradation. This ensures that high-motion scenes are rendered sharply and smoothly.
Drawbacks of VBR
While VBR has its advantages, it also has some limitations, particularly when it comes to live streaming:
Potential Buffering Issues: VBR can fluctuate significantly in terms of bandwidth requirements. If a viewer’s internet connection isn’t stable, they may experience buffering or playback issues, as the player will struggle to adapt to the varying bitrate.
Higher Encoding Complexity: Encoding video in VBR mode is often more complex than in CBR. It requires more processing power and sophisticated encoding techniques to continuously adapt the bitrate, which could impact overall system performance.
Constant Bitrate (CBR): Stability and Predictability for Live Streaming
Constant Bitrate, or CBR, is an encoding technique that keeps the bitrate consistent throughout the video, regardless of scene complexity. Every second of video is delivered with the same bitrate, providing a steady network load and avoiding the peaks and valleys of VBR.
Key Benefits of CBR
Predictable Network Usage: With CBR, bandwidth usage remains steady, making it easier for network managers to allocate resources and avoid unexpected traffic spikes. This is especially beneficial for live streaming, where predictable bandwidth usage ensures that the network can support multiple concurrent viewers without congestion.
Reduced Buffering and Consistent Quality: By maintaining a consistent bitrate, CBR minimizes the risk of buffering, particularly in environments where bandwidth availability is uncertain. This helps deliver a smoother playback experience and keeps viewers engaged without interruptions.
Drawbacks of CBR
Less Efficient Bandwidth Usage: CBR doesn’t differentiate between simple and complex scenes. This means that simple scenes may consume more bandwidth than necessary, potentially wasting resources, especially on lower-capacity networks.
Why CBR Might Be Better for Live Streaming
For live streaming, consistency and reliability are paramount. Viewers expect a seamless experience, and any interruptions or quality fluctuations can lead to dissatisfaction. Here’s why CBR is often the preferred choice for live streaming:
Predictable Network Usage: Since CBR maintains a steady bitrate, it’s easier to estimate and manage network resources. Network operators can plan bandwidth allocation more effectively, helping to avoid sudden bandwidth spikes or dips that might interfere with other network traffic or services.
Reduced Risk of Buffering: Because CBR offers a stable stream, there’s less risk of buffering or lag, especially in environments with limited bandwidth. This steady flow of data is particularly beneficial during peak hours or large-scale broadcasts, where buffering can quickly lead to viewer dissatisfaction.
Enhanced Quality of Experience (QoE): By maintaining a consistent bitrate, CBR helps provide a smoother viewing experience. Without bitrate fluctuations, viewers can enjoy a consistent quality level, minimizing the chance of resolution drops or stuttering during high-action scenes.
Streamlined Encoder and Player Compatibility: Many encoders and streaming platforms are optimized for CBR, making it easier to maintain compatibility across diverse devices and players. This simplifies the technical requirements and can improve reliability in large-scale streaming setups.
When to Consider VBR for Live Streaming
While CBR is typically recommended for live streaming, there are some scenarios where VBR might be viable. For instance, if your live stream is directed at a smaller audience with reliable, high-speed internet connections, VBR could potentially offer a higher-quality experience without the usual buffering risks. However, these conditions are uncommon in large-scale live streaming environments.
For on-demand video or pre-recorded content, VBR is often preferred, as the bandwidth fluctuations don’t impact the viewing experience as critically. In these cases, VBR allows for efficient bandwidth usage while maintaining high quality, which can lower storage and delivery costs in the long run.
A Hybrid Approach: Combining VBR and CBR
Some streaming services are experimenting with hybrid encoding approaches to balance the advantages of both VBR and CBR. Here’s how this works in practice:
VBR for Pre-Recorded Content: For movies, shows, or other pre-recorded videos, VBR can optimize storage and streaming quality. The viewer isn’t impacted by bitrate shifts since they aren’t viewing in real-time, and the higher quality in complex scenes can enhance the experience.
CBR for Live Streams: For live streaming, the consistency and predictability of CBR make it the better choice. This ensures a steady and uninterrupted stream, even for users with varying connection speeds.
In certain advanced setups, adaptive streaming protocols, such as Dynamic Adaptive Streaming over HTTP (DASH) or Apple’s HTTP Live Streaming (HLS), can be employed to dynamically switch between different bitrates and resolutions based on viewer connection quality, providing a seamless experience even in challenging network conditions.
However, it’s important to note that the optimal encoding method depends on various factors, such as network conditions, viewer expectations, and content complexity.
In some cases, a hybrid approach combining VBR and CBR can be used to balance quality and efficiency. For example, you could use VBR for recorded content and CBR for live streams.
