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