Understanding bitrates in video files

It’s important to understand how the bitrate control corresponds to video quality and the file size. In general, a higher bitrate will accommodate higher image quality in the video output. At the same bitrate, video in a newer codec such as H.264 will look substantially better than an older codec like H.263. Another consideration is that variable bitrate (VBR) encoding will produce better image quality than constant bitrate (CBR) in most applications. First, let’s have a look at the units of measure for video encoding, and then we’ll look at the differences between Constant Bitrate (CBR) and Variable Bitrate Encoding (VBR). Finally, we show you how to use Encoding.com to produce CBR and VBR output.

 

What is a video bitrate?

A video bitrate is the number of bits that are processed in a unit of time.Remember that 1 byte consists of 8 bits. Video data rates are given in bits per second. The data rate for a video file is the bitrate. So a data rate specification for video content that runs at 1 megabyte per second would be given as a bitrate of 8 megabits per second (8 mbps). The bitrate for an HD Blu-ray video is typically in the range of 20 mbps, standard-definition DVD is usually 6 mbps, high-quality web video often runs at about 2 mbps, and video for phones is typically given in the kilobits (kbps). For example, these are the targets we usually see for H.264 streaming:

LD 240p 3G Mobile @ H.264 baseline profile 350 kbps (3 MB/minute)
LD 360p 4G Mobile @ H.264 main profile 700 kbps (6 MB/minute)
SD 480p WiFi @ H.264 main profile 1200 kbps (10 MB/minute)
HD 720p @ H.264 high profile 2500 kbps (20 MB/minute)
HD 1080p @ H.264 high profile 5000 kbps (35 MB/minute)

Below we recommend a few tools for detecting bitrate and codecs. 

MediaInfo is a nice basic tool for quickly seeing all the parameters on a video file:

 

 

Winhoros Bitrate Viewer is an excellent tool for scanning your files to get detailed bitrate information:

 

 

For analyzing Blu-ray Discs, see BDInfo for Windows

 

 

How to choose the correct bitrate for your workflow?

When it comes to choosing what bitrate is best for your video workflow, it is important to take into account what device your videos will be played on. As we mentioned above, internet videos, phone videos, and HD blu-ray videos for tV all vary in bitrate. Consider what platform your video will be viewed on prior to selecting your bitrate. Once you have determined  how your users will be viewing your video, you can use our video encoding API to apply VBR bitrate to your media files.

 

What’s the difference between Constant Bitrate (CBR) and Variable Bitrate (VBR) Encoding

Constant bitrate (CBR) encoding maintains a set bitrate over the entire video clip, but limits the image quality in most cases—especially for complex video segments. CBR is often not an optimal choice for streaming since it does not allocate enough data for the complex segments: this results in lower quality overall and unused capacity on the simple segments. Therefore, we recommend you avoid using CBR unless you have a specific requirement.

At Encoding.com, VBR is our default processing mode and will typically produce significantly higher quality at similar bitrates. The primary benefit of VBR encoding is that it allocates a higher bitrate to the more complex segments of media files and lower bitrates to the simple segments. Adding up the bitrates and dividing by the duration (in seconds) gives the average bitrate for the file. This average bitrate usually compares favorably with the equivalent CBR bitrate. Although VBR encoding requires more processing time, for most content it produces superior visual quality.

Using CBR in special cases

Occasionally, compression requirements mandate that you use CBR—especially in the case of live event video streaming. VBR is the default setting for Encoding.com, since it generally provides much higher quality. However, for complex video segments—in which there is a high amount of action or detail—VBR will often spike up to 300% of the target bitrate.

By contrast, CBR is far more restrictive. The actual bitrate often varies only within about 20% of the target. This is important in the case where some of your target audience has legacy hardware or low-buffer decoders that are unable to handle VBR. With H.264 being built into most modern chipsets, this is not an issue for most customers. Another prominent use of CBR is with satellite television broadcasts, in which many channels are multiplexed into one transport stream that has limited bandwidth.

 

XML samples for controlling CBR using Encoding.com

You can chose CBR or VBR in any of your encoding jobs, with either of the following methods.

In the web interface or watch folder:

1. Specify your source video location.

2. Click the Customize button.

3. Expand the Video Settings menu.

4. Set CBR to No or Yes.

Or, if you are using the API, simply set the <cbr>no<cbr/> or <cbr>yes<cbr/> in the <format> section of your API calls.

Have a look at the sample API tags below for both VBR, loose CBR, and hard CBR, along with the bitrate profiles for each. These specifications apply to all three samples: 240 kbps bitrate, 640×360 (16:9) resolution, 29.97 fps, high profile, level 3.0, CABAC, 4 ref frames

 

Sample #1 – VBR

<cbr>no</cbr>
<hard_cbr>no</hard_cbr>
<keyframe>90</keyframe>
<video_codec_parameters>
     <bf>0</bf>
     <level>30</level>
</video_codec_parameters>

 

 

Sample #2 – Loose CBR

<cbr>yes</cbr>
hard_cbr>no</hard_cbr>
keyframe>90</keyframe>
<video_codec_parameters>
     <bf>0</bf>
     <level>30</level>
</video_codec_parameters>

 

 

Sample #3 – Hard CBR

<cbr>yes</cbr>
<hard_cbr>yes</hard_cbr>
<keyframe>90</keyframe>
<video_codec_parameters>
     <bf>0</bf>
     <level>30</level>
</video_codec_parameters>

 

 

If you are having problems getting the bitrate you need, try using a lower value in the bitrate tag. If that doesn’t work, please submit a support ticket with the Media ID # of the job.

 

How our platform can help with your encoding needs

Whether you are looking to use VBR on a daily basis or CBR for a special case, our video encoding API can help your company manage your media workflow. In an ever expanding digital world it is important to make sure your business is staying ahead of the digital curb. Our VBR encoding system will help your company stay in control of your media files and provide high-quality content for your customers. Find out more about our video encoding features by contacting us today.

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