CBR Vs VBR: A Comparison between Constant Bitrate and Variable Bitrate

When it comes to streaming a video online, there is a lot that goes in the background. In order to ensure smooth video playback regardless of network, at times you have to create different bitrate or video qualities to give an optimal experience to viewers. You face several choices here. One set of choices concern the encoder settings. You might have heard about Constant Bitrate (CBR) and Variable Bitrate (VBR) as the two prominent types of encoding. What are they, and how do they differ? Which one should you use? Read on, as we answer these questions.

A Brief Explanation to Bitrate and a Few Other Key Terms

Before we undertake a CBR vs VBR comparison, it’s important to understand a few key terminologies. These are as follows:

1. Video Encoding

The term “video encoding” refers to the process of converting video files from one format to another. Think of what happens during a video stream.When the video file is first recorded, it is in the raw format.  These are too large, therefore, they’re not suitable for streaming.

Different encoding hardware and software products convert this raw video file into a digital format. The converted file can be streamed over the Internet.

2. Video Transcoding

VOD (Video on Demand) enables consumers of video content to access them over different devices. You can’t consider some of these devices as traditional video playback devices. A VOD streaming system needs to create different copies of video files, and these copies are of different sizes.

The common term for these copies is “renditions”. The term “video transcoding” refers to the process of compressing video files to create these renditions. If you want to know more about, what is transcoding, you can visit the blog linked.

3. Video Bitrate

The term “Video Bitrate” refers to the amount of data you transmit over the Internet during a period of time. We sometimes use the term “video bitrate” in the world of online video streaming.

You measure bitrate in “kilobytes per second” (kbps). The quality of the video you broadcast depends on the bitrate. You can broadcast videos of higher quality if you use a higher bitrate.

Broadcasters have to balance multiple factors like quality and the Internet connection. They need to provide a bitrate that suits the Internet connection speed. That enables users to view the optimal quality of video without any notable lagging, skipping, and buffering.

The Internet connection speed varies across geographies, and broadcasters might have users in different geographies. Broadcasters need to cater to different Internet connection speeds. Therefore, they need to make videos available in different bitrates.

Note that bitrate is important for both encoding and transcoding. After all, those processes deal with the transfer of video data.

What is Constant Bitrate (CBR)?

If you choose to transmit data at a constant rate, then you need to use CBR. What does an encoder do when you choose CBR? It produces data outputs at a constant rate. In essence, the encoder doesn’t do anything different for files with different sizes or complexity.

CBR has been the mainstay in the world of video streaming for a considerable period of time. All relevant stakeholders in the streaming industry know about CBR.

The world of online video streaming considers CBR as something of a standard. All hardware and software platforms for video streaming are compatible with CBR. Most broadcast professionals have used CBR over the years.

What if you have complex segments in your video content? Remember that CBR uses one rate only. You can’t change the bitrate for complex segments of your video, therefore, set the bitrate high enough. This will enable the encoder to handle the complex segments of your video.

What happens during the live streaming of video content? The encoding process takes place in real-time. The consistent bitrate of CBR helps here. However, CBR doesn’t do anything to optimize the quality of a video file.

Note that CBR involves encoding a video file in one pass. This is a key aspect in the CBR vs VBR comparison, and we will talk about it shortly.

What Is Variable Bitrate (VBR) ?

Unlike the case of CBR, the rate of data transmission varies in the case of VBR. VBR works as follows:

• A VBR system does a first pass of the video file. During this, it analyzes the complexity of the video file.
• The VBR system then categorizes the segments of the video file by their complexity.
• VBR systems transmit data at a variable rate.
• They use one bitrate for the complex segments of the video file, and they use another bitrate for the simpler segments.
• Since they adjust bitrates based on the complexity, VBR systems produce a better quality of videos.
• VBR encoding systems produce output data at different rates for different segments.
• The system measures the final bitrate by calculating the average bitrate of the video file.

VBR systems make two passes in this process. Naturally, this process takes longer. This is a key difference with CBR systems. Consider the scenario of live streaming of video. You can’t wait for VBR systems to make two passes since the encoding must be in real-time.

This is why you can’t use VBR systems for live streaming. You can use them for VOD scenarios. You need to use CBR for live streaming since it makes one pass only. Also, not all video streaming hardware and software platforms support VBR.

CBR vs VBR: How they differ

CBR vs VBR differences are as follows:

1. Which one among CBR and VBR caters to live streaming?

CBR makes one pass only, which makes it suitable for time-sensitive video streaming. VBR makes two passes. Therefore, you can’t use it for live streaming.

2. Which one is more suitable for VOD?

VOD isn’t the same as live streaming. Viewers consume previously recorded video content, therefore, VOD doesn’t involve real-time encoding.

You as a broadcaster won’t have a problem with the two passes made by VBR. Rather, you would want to take advantage of the better quality produced by VBR. This is why VBR is more suitable than CBR for VOD.

3. Which one produces higher-quality videos?

CBR uses one bitrate consistently for an entire video file. It doesn’t change the bitrate for segments with higher complexity. Therefore, CBR can’t produce the best quality of videos.

On the other hand, VBR first analyzes the complexity of the video file. It adjusts bitrates based on this analysis. VBR uses a higher bitrate for video segments with more complexity, therefore, it produces higher-quality videos.

4. Popularity

All broadcasters know about CBR. Broadcast professionals have been using CBR for quite a while. CBR is very well-known, and it’s popular than VBR.

5. Compatibility

CBR has been the standard encoder setting for a long time. All video streaming hardware and software platforms support CBR. Not all such platforms support VBR though.

A brief note about “Constrained Variable Bitrate” (CVBR)

VBR isn’t a one-flavor-only encoder setting. It has several types, e.g.:

• Quality variable bitrate;
• Unconstrained variable bitrate;
• Constrained variable bitrate.

Many experts in the video streaming industry recommend that broadcasters use 200% CVBR (Constrained Variable Bitrate) for VOD. Experts find this is the best option for VOD in most geographies. This makes it imperative to talk briefly about CVBR.

CVBR is a bitrate encoder setting, and its’ popularity has grown in recent years. It uses two passes to encode video files. You need to set the following parameters:

• Maximum bitrate;
• Maximum buffer window.

The term “200% CVBR” means that the encoder can’t exceed 200% of the target bitrate.

Why do experts suggest using the “200% CVBR” setting for VOD? Think of the matured markets. Users in these countries get high-speed Internet connections. The bitrate of the VBR encoder setting won’t exceed it.

On the other hand, many other countries have a 3G or slower Internet connection. You don’t want the bitrate in the VBR encoder setting to exceed it. This makes it important to use CVBR.

Apple has provided specific guidelines in this respect. The company has published the HLS authoring specifications for Apple devices.

One of the guidelines states the following:

“For VOD content, the peak bitrate SHOULD be no more than 200% of the average bitrate.”

This is why the “200% CVBR” encoder setting is becoming prominent.

CBR Vs VBR: When Should You Use One Over Another?

Choose either CBR or VBR based on the scenario. You can have any one of the following scenarios:

1. You are live streaming video content

You need encoding in real-time. CBR makes one pass while processing video data, whereas, VBR makes two passes. You can’t wait for two passes during live streaming.

This is why you can’t use VBR for live streaming. CBR works the best in this scenario. It provides a consistent bitrate, therefore, you don’t run the risk of exceeding the Internet connection speed.

There might be occasional video quality issues since CBR doesn’t adjust the bitrate for video segments of different complexities. That’s a trade-off you need to accept when you stream video content live.

2. You are catering to VOD (Video on Demand) consumers

This doesn’t need real-time encoding. You can use VBR since the two passes won’t matter. VBR offers better video quality, which is better for consumers of VOD.

Conclusion

We talked about two important video encoder settings, which are CBR and VBR. We explained what they are. After analyzing their differences, we explained when you should each of them. Choose a video encoder setting based on your requirements.