Digital Video Compression 101August 9, 2012 in Security Cameras
Digital video compression may seem like a difficult concept to grasp, unless you know why it’s essential, what to look for and how it works.
Why Video Compression Is Essential
When digital video was first introduced in the 80’s, it was stored in its pure, uncompressed format, and storage was difficult because the uncompressed video took up so much storage space.
Today, video compression is necessary for capturing, transferring, storing, and viewing video footage, because if the video wasn’t compressed, download and viewing times would take too long, and storage capacity would continue to be an issue.
How Video Compression Works
During the video compression process, a video stream is analyzed and unnecessary parts of the data are discarded in order to make a large video file smaller in size.
There are essentially two ways to compress data in a video file: intraframe and interframe.
Intraframe (I-frame) compression compresses each individual frame of the video (similar to JPEG compression of a still image). Every frame of the image is considered as a still image. With intraframe compression, the complete frame is only slightly compressed, so the file size isn’t that much smaller because each individual frame is included in the newly-compressed version.
Interframe compression takes a look at each frame in a video file, compares it to the previous frame and stores only the changed data frame from frame so the file size is much smaller than intraframe compression. Interframe compression uses a key frame solution.
Video Quality After Compression
It’s almost impossible to not lose video quality after it’s been compressed, but compression technology has improved tremendously since its introduction. Today, you can compress video without sacrificing too much of the video image quality.
After the compressed video has been decompressed, some of the video quality will be lost, and may or may not be noticeable to the human eye, depending on how much of the original video has been compressed.
In order to ensure video quality during compression, it’s important to identify the key components of the video that you want to preserve. You may want to consider your video viewing application. Will you be watching the video on a small or large screen? Is file size is important?
Is the compressed or uncompressed file size acceptable to be emailed or downloaded? What will you be watching and what is the complexity of the scene? Will you be watching moving traffic shots or a static scene? Are there a lot of contrasting colors or movement in the scene that you will be recording?
Another important consideration for ensuring video compression quality is choosing an appropriate codec and compression format to reduce the size of your video.
A video file must be encoded and compressed and decoded and decompressed when played, and the two types must be compatible, or there will be problems viewing the video file. A codec (coder-decoder) is a compression standard for encoding the video and translating it into a digital form.
In order to view the encoded video, you need to decode it with the same codec that was used to encode it. For example, if you encoded video in MPEG-4, you need to decode it with an MPEG-4 decoder.
M-JPEG (Motion JPEG) is an unlicensed standard that uses a lossy form of intraframe compression. It’s a digital video sequence made up of individual JPEG (Joint Photographic Experts Group) images. With M-JPEG compression, the data is a series of complete still images and there’s no dependence between video frames.
M-JPEG provides a range of capabilities where individual frames in a video are required, but is an older format from the nineties, so the image quality is not the highest.
MPEG-4 compression is a licensed standard that delivers object-based compression. Each object in a scene is tracked individually and compressed to create an MPEG-4 file. MPEG-4 compression is very scalable from low to high bit rates. It is versatile compression format designed for streaming and real-time viewing.
H.264 compression can greatly reduce the size of digital video without compromising image quality compared to M-JPEG and MPEG-4. This compression standard can also record for longer periods of time as it requires less network bandwidth and storage.
H.264 compression delivers higher video quality, so it’s a fantastic choice when high frame rates and higher resolution are needed, yet H.264 requires more processing power to view and decompress than M-JPEG and MPEG-4 formats.
Video Compression Formats and Your DVR
When digital video passes from a security camera into a DVR, the file is compressed for storage. The compression occurs inside the DVR and the level of compression and file size depends on the compression format supported by your DVR. Choosing a DVR compression format that’s right for you depends on how you need to share and view your recorded video.
For example, for sending video footage to slower processing computers, M-JPEG compression formatting may be better. For sending video to standard faster-processing computers and mobile devices, MPEG-4 may be an acceptable choice.
H.264 is a good choice when preserving image quality and keeping file size small is desired, especially in regards to high definition videos. A DVR that features H.264 compression technology allows for optimized image quality and lower bandwidth and storage requirements.
Most embedded DVRs come with equipped with codec H.264, because this compression format helps maximize bandwidth and image quality and lower storage requirements and costs. H.264 is the latest video compression technology format and is ideal for most security applications today.