What is CBR?
CBR, or Constant Bitrate, is a way of transmitting data at a constant rate, as the name suggests. When it comes to encoding, this refers to using an encoder that outputs data at a constant rate. As a result, it is not responsive to the complexity or size of the files it processes. These variables are irrelevant for CBR, and the output remains constant.
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Because of this reason, CBR is often preferred for streaming situations, and it has also been the standard in online video streaming for a few decades. Further, CBR uses a constant transmission, so the bitrate is set at one rate and is measured as such. The general advice is to set the bitrate relatively high so that the encoder can take care of complex data segments, too.
These consistencies offered by CBR make it perfect for real-time video encoding situations. However, it is not suited for storing data.
What is VBR?
VBR, or Variable Bitrate refers to the transmission of data at a variable rate. This encoding allows segments of data to be packaged in a way that the transmission is optimized. As a result of this, VBR encoding makes it possible to transmit large video files while processing higher bitrates. In terms of encoding, the data from this is output at a fixed rate wrt to a time segment, and the variable bitrate is then measured by averaging the bitrates of the whole file.
Because of this approach, VBR is considered the best for on-demand video transcoding because it can process higher bitrates, too. However, because of these same reasons, VBR encoding is not that suited for video streaming. Another thing to keep in mind is that there are many different types of VBR, and each one is slightly different from the other. For example, there is Quality Variable Bitrate, Average Bitrate, Constrained Variable Bitrate, and Unconstrained Variable Bitrate, all of which are essentially VBRs but work in a different way.
Another thing to note at this point is that VBR does not have such widespread support as CBR.
With that said, let’s look at CBR and VBR head-to-head in a comparison table to get a better understanding of how these two modes differ from one another.
Comparison Table - Difference Between CBR and VBR
For live streaming or streaming of content with a high degree of motion, CBR is the best option, as it provides a more consistent bit rate and image quality. VBR is more suitable for streaming content with a low degree of motion, such as a podcast, as it can provide better image quality at a lower bit rate. Discover which is the right one for you as you read further down.
Here’s a quick comparison table for your reference to understand the differences between CBR and VBR modes of bitrate control.
| CBR | VBR |
|---|---|
| Best for time-sensitive video streaming. | Produces higher-quality video outputs. |
| Produces constant/fixed results based on the bitrate you set it to. | Best in scenarios where there are no time constraints on transcoding. |
| Has been very useful and relevant historically, and is therefore widely supported and compatible. | Not as widely supported and compatible as CBR |
| Better suited for multimedia encoding. | Does not hamper the video quality in any way. |
| Ideal for live streaming encoding. | Ideal for VOD transcoding. |
With the differences in place, let’s look at some use cases of both CBR and VBR and help you get a better idea of when to use which mode.
What is Rate Control?
In simple terms, rate control is the task performed by a video encoder when it has to decide the number of bits to be utilized for a given video frame. Since the goal of lossy encoding is to reduce file size without hampering the quality, rate control happens to be an extremely important step in determining the tradeoff between size and quality.
Different Types of Rate Control Modes
There are different types of rate control modes depending on your requirements. In general, the following rate control modes are commonly used:
- Constant QP (CQP)
- Average Bitrate or Target Bitrate (ABR)
- Constant Bitrate (CBR)
- Variable Bitrate (VBR)
- 2-Pass Average Bitrate (2-Pass ABR)
- Constant Quality (CQ) / Constant Rate Factor (CRF)
- Constrained Encoding (VBV)
All of these modes differ based on how they operate and the final quality of the video that they produce. In this article, we’ll dive deeper into two of these rate control modes - CBR and VBR.
When to Use CBR and VBR
As you must have understood by now, both CBR and VBR are important for different cases. It’s difficult to say that one mode is objectively better or worse than the other since both come with their set of weaknesses and strengths. Let’s look at some common uses of bitrate control and see whether to use CBR or VBR in those scenarios.
Live Stream Encoding
For live stream encoding, CBR is undoubtedly the best because of its consistency, which makes it reliable for time-sensitive data. In live streaming, most live videos are broadcasted at a constant, single bitrate, which makes CBR the best choice for this use case. Note that VBR can be used for live stream encoding, too, but it is quite rare.
VOD Transcoding
VBR is generally considered the most optimal setting for VOD transcoding. The reason behind that is its ability to handle larger files without compromising on quality. Since there is no time sensitivity in on-demand video transcoding, you don't need to worry about the internet speed being exceeded - and that gives you the freedom to easily capitalize on the benefits of VBR for VOD transcoding.
Audio-Only Streaming
Just like for video files, audio files, too, should use CBR for live broadcasting and VBR for on-demand streaming. However, in the case of some audio encoders, you don’t get the option to choose your rate control mode. So in that case you must go with the option that is presented to you. Since audio files are generally smaller and less complex than video files, either of the rate control modes works fine.
Conclusion
Rate control modes are extremely important if you have to get the most out of your media files. You should select a rate control mode based on your requirements. Both CBR and VBR play distinct roles and it is important for you to know which of the two options suits your case the best. The bottom line is that CBR is best for time-sensitive situations whereas VBR is for on-demand video.
FAQs
1. Is VBR faster than CBR?
VBR is generally known to produce higher quality video files as compared to CBR - even though the difference is quite lower than one would expect. CBR, on the other hand, produces files that are much more deliverable than VBR, which makes it better for mobile delivery as compared to desktop viewing.
2. Is VBR better quality?
VBR is typically considered higher quality because it can dynamically adjust the bitrate of the media file to provide better quality.
3. What is a good bitrate for CBR?
The recommended video bitrate for CBR encoding is between 2,000 and 10,000 Kbps. The optimal bitrate for CBR encoding depends on various factors, such as the type of content, video resolution, and the desired quality. Generally, we recommend a bitrate of 128 kbps for MP3 and 256 kbps for AAC.
4. Is CBR better than VBR 2 pass?
Essentially, 2-pass VBR gives better encoding at slightly lower data rates as compared to 1-pass CBR. However, in terms of the quality of encoding, both provide quite similar final results.
