CBR vs VBR: Pros, cons, metrics and practical examples

Bitrate is a crucial concept in video streaming and encoding that affects both video quality and file size. It represents the amount of data transmitted per second of video playback, much like the width of a pipeline. A higher bitrate allows more data to flow through, resulting in better quality visuals and smoother playback, but also increases file size and bandwidth usage.

In online video streaming, two primary methods manage bitrate: Constant Bitrate (CBR) and Variable Bitrate (VBR). While both aim to deliver high-quality video content, they differ significantly in how they compress and transfer data from the server to the viewer.

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Here in this graph, CBR maintains a fixed bitrate of 500 kbps throughout the video, which ensures consistent quality but may not always be efficient in terms of file size or data usage.

Whereas VBR adjusts the bitrate according to the complexity of the content, ranging between 300 to 900 kbps. This results in better quality for complex scenes and lower data usage for simpler ones, but it can be less predictable in terms of storage and network requirements.

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Understanding bitrate and its influencing factors

Bitrate is a crucial aspect of video quality and streaming performance, impacting everything from visual clarity to storage needs. Here’s a detailed look at the key factors affecting bitrate:

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1. Measurement and implications

Bitrate is measured in kilobits per second (kbps) or megabits per second (Mbps). For instance, a video with a bitrate of 5 Mbps will typically offer better quality than one with 1 Mbps. Higher bitrates accommodate more detailed visuals and smoother motion but require more storage space and bandwidth.
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2. Video resolution

Resolution plays a significant role in determining bitrate requirements. Higher resolutions, such as 1080p or 4K, contain more pixels and thus need more data to maintain image clarity. For example, a 4K video demands a higher bitrate than a 720p video to preserve sharpness and detail. As resolution increases, so does the need for a higher bitrate.
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3. Frame rate

Frame rate, or the number of frames displayed per second, influences bitrate needs. Higher frame rates (e.g., 60 fps vs. 30 fps) provide smoother motion and more fluid playback. To accurately represent these frame rates, a higher bitrate is necessary. Insufficient bitrate at high frame rates can result in choppy or pixelated video, while a higher bitrate ensures fluidity and detail.
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4. Codec efficiency

Codecs, which are algorithms for compressing and decompressing video data, impact bitrate efficiency. Advanced codecs like H.265 (HEVC) can deliver high-quality video at lower bitrates compared to older codecs like H.264. Efficient codecs reduce the amount of data needed without sacrificing quality, allowing for lower bitrates without compromising the viewing experience.

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What is CBR?

Constant Bit Rate (CBR) encoding maintains a fixed bit rate throughout the media file. This means that the amount of data used per second remains constant, regardless of the complexity of the content being encoded. CBR is straightforward and predictable, making it suitable for applications where a consistent data rate is crucial.

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Advantages of CBR:

• Predictability: Consistent data rate allows for easier bandwidth management, especially useful in streaming applications. ‍
• Compatibility: Ensures compatibility with systems that require a fixed bit rate.
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Disadvantages of CBR:

• Inefficiency: Can lead to wasted bandwidth if the content does not require a high bit rate. ‍
• Quality Variability: May result in lower quality for complex scenes if the fixed bit rate is insufficient.

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What is VBR?

Variable Bit Rate (VBR) encoding adjusts the bit rate based on the complexity of the content. During scenes with high complexity or movement, the bit rate increases to preserve quality, while during simpler scenes, it decreases to save space. This method aims to optimize the balance between file size and quality.

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Advantages of VBR

• Efficiency: Provides higher quality for a given file size compared to CBR. ‍
• File Size Reduction: Reduces file size without significantly compromising quality.

Disadvantages of VBR

• Inconsistency: Bit rate fluctuations can lead to variability in streaming performance. ‍
• Complexity: Requires more complex encoding and decoding processes.

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Key metrics and statistics

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Encoding efficiency

A study comparing CBR and VBR encoding for various video resolutions (720p, 1080p) showed that VBR can achieve up to 20% smaller file sizes compared to CBR while maintaining comparable visual quality.

To illustrate this difference, let us consider an example encoding scenario:

• CBR example: Encoding a 90-minute video at 1,000 kbps results in a file size of approximately 675 MB. ‍
• VBR example: Using VBR with an average bit rate of 1,000 kbps, but allowing fluctuations between 500 kbps and 2,000 kbps, can reduce the file size to approximately 550 MB while maintaining similar quality.
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Quality metrics

Quality can be evaluated using metrics such as Peak Signal-to-Noise Ratio (PSNR) and Structural Similarity Index (SSIM). Studies have shown that VBR typically achieves better quality scores compared to CBR for the same average bit rate.

• PSNR for CBR: Ranges from 30 to 40 dB, depending on the chosen bit rate. ‍
• PSNR for VBR: Often surpasses CBR by 2-5 dB for the same average bit rate.

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Video Editing API for developers

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Use cases and practical examples

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Streaming services

CBR use case: Many live streaming services, such as those used for sports broadcasting, prefer CBR encoding due to its predictable bit rate, which ensures consistent quality and performance even with varying network conditions.

VBR use case: On-demand streaming platforms like Netflix and YouTube often use VBR encoding. This method allows them to offer high-quality streams with reduced file sizes, optimizing both storage and bandwidth usage.

Audio encoding

CBR use case: Music streaming services that prioritize a constant quality of playback and predictable bandwidth usage often employ CBR encoding. For example, traditional radio streaming services and some older music streaming platforms use CBR to ensure a stable listening experience.

VBR use case: Modern music streaming services, such as Spotify and Apple Music, use VBR to enhance audio quality and reduce file sizes. For instance, Spotify’s Ogg Vorbis format uses VBR to provide high-quality audio with efficient compression.

Archiving and storage

CBR use case: When archiving video content for long-term storage, CBR encoding might be preferred to ensure that each file is of a consistent size and quality, making it easier to manage and catalog.

VBR use case: For archival purposes where space is a concern, but quality is still important, VBR encoding can be advantageous. For example, a digital film archive may use VBR to reduce storage costs while preserving high visual fidelity.

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Advancements in video compression with FastPix’s Video API

At FastPix, our goal with the Video API is to significantly reduce video file sizes while maintaining top-notch quality. We’re thrilled to say that our technology achieves on average 30% more compression and delivers 35% superior quality compared to industry standards.

A holy grail for folks when working with video is “how much you can compress, without losing quality.”

- because compression = costs of storage and streaming.
- because quality = viewing experience and satisfaction.

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Hence our aim to reduce file sizes (MB) but keep quality (VMAF score) greater than 90. And do this better than our competitors (if not, why bother). As our product’s v1.0 comes together, we checked how we’re tracking.

In this exercise we measured four different videos (of varying complexities) which are relevant for this type of workout. A lot more cool things are in the works, but this right here feels good.

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Click here to explore this more in detail about we reduced file sizes.

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But for now, will talk about how constant and variable bitrate factored in this exercise. For the four different types of videos, we also measured the bitrate shifts. Below are graphs which show these with estimated numbers.

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1. Anime video: VBR dynamically adjusts the bitrate, increasing during fast-paced action sequences and decreasing during static scenes, while CBR remains constant at 5 Mbps.
2. ‍Water video: VBR peaks during highly detailed water scenes (e.g., splashing waves), allowing for better quality during complex moments. CBR remains constant, which may result in loss of detail during high-motion segments. ‍
3. Slow-motion video: VBR gradually increases to accommodate the detailed frames in slow-motion sequences, while CBR stays flat, potentially causing a lack of clarity in more intricate scenes. ‍
4. Fast-motion video: VBR shows sharp increases during periods of rapid movement (e.g., sports or action scenes), maintaining visual quality, whereas CBR struggles to allocate sufficient bitrate during these moments.
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Summarizing CBR vs VBR across all aspects in streaming

                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   

Aspect	Constant Bitrate (CBR)	Variable Bitrate (VBR)
Bitrate allocation	Allocates a fixed bitrate for the entire video.	Dynamically adjusts the bitrate based on content complexity.
Video quality	Provides consistent quality throughout the entire stream or call.	Offers higher quality for complex scenes and potentially lower quality for simpler scenes
File size	Predictable and stable file size.	Varies based on scene complexity; larger file size for complex scenes and smaller for less complex ones.
Adaptive streaming	Limited compatibility with adaptive streaming.	Well-suited for adaptive streaming algorithms, ensuring smooth video delivery across varying conditions.
Bandwidth efficiency	Less efficient; may result in wasted bandwidth, especially in less complex scenes.	Optimizes bandwidth usage to maintain high-quality video in varying network conditions.
Storage efficiency	Consistent file size can be advantageous for storage management.	Varying file sizes may pose challenges for content distribution and storage planning.
Editing flexibility	Fixed bitrate simplifies post-processing and editing of video content.	Dynamic bitrate requires more careful post-production adjustments to accommodate varying data rates.
Ideal for	Video calls or conferences with stable and limited bandwidth.	Video conferencing over varying network conditions and changing bandwidth.
Content types	Best suited for video conferencing with constant scene complexity.	Ideal for dynamic video conferences with varying content and complexity.
Visual experience	Potential for compression artifacts in complex scenes.	Enhanced visual experience in high-motion and complex scenes due to higher bitrate allocation where needed.
Encoding efficiency	Efficient for static and straightforward video content.	Efficient for dynamic or visually complex content due to adaptable bitrate allocation.
Compatibility with devices	Well-suited for devices with limited processing capabilities.	Compatible with a wide range of devices and varying network speeds due to its adaptability.
File delivery	Suitable for streaming in environments with consistent bandwidth	Effective for on-demand streaming with varying network conditions, allowing for better adaptability.
Real-time streaming	Ideal for low-latency real-time streaming, maintaining a consistent bitrate throughout the stream.	Suitable for both live and on-demand streaming services, balancing video quality, bandwidth efficiency, and scalability.
Overall flexibility	Offers limited flexibility in adjusting to varying content complexity.	Provides flexibility for optimizing bitrate based on content complexity, adapting dynamically to conditions.
Complexity of settings	Simple to set up with fixed bitrate settings, making it easier to manage.	Requires understanding of content complexity and appropriate settings for dynamic adjustment.

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Wrapping up…

Choosing between Constant Bitrate (CBR) and Variable Bitrate (VBR) can greatly impact video quality and efficiency. VBR, with its dynamic bitrate adjustments, provides superior quality and better bandwidth management compared to CBR, especially for complex scenes.

At FastPix, our Video API leverages VBR to deliver 30% more compression and 35% higher quality than industry standards. Explore how FastPix can enhance your video streaming experience.

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