A Note on Quality parameters in Video Surveillance – Resolution, Frame Rate, Compression & HD

In this article, let us look at an overview of four important parameters that define quality in a video stream for IP Video Surveillance applications – Resolution, Frame Rate, Compression & High Definition (HDTV).

Resolution & Frame Rate in IP Video Surveillance:

Resolution is defined as the number of pixels (independent picture elements, usually expressed in terms of width x height) that make up each picture frame. This is a one-dimensional definition used to describe an image, which is what an individual picture frame is made up of.

But a video stream consists of multiple picture frames that are continuously displayed one after the other. A term called FPS – Frames Per Second is used to represent the frame rate, which actually represents the motion aspect of a video stream.

So while resolution indicates the clarity of images displayed in each frame, FPS indicates the quality of the video motion.

Two things we need to remember in Video Surveillance:

Higher resolution + Higher Frame rate = Higher Quality Video Stream
But,
Higher resolution + Higher Frame rate = Higher bandwidth & Higher storage requirements for Video Stream

It is always better to balance the two aspects based on the application requirements. Certain applications like face recognition, license plate reading, etc. might require a higher resolution/ higher frame rate. But with certain applications like traffic monitoring, perimeter security, etc. a lower resolution/ low frame rate might be sufficient.

Video Surveillance Cameras used today support the following common resolutions:

QVGA Cameras – 320 x 240 Pixels
VGA Cameras – 640 x 480 Pixels
Megapixel Cameras – 1280 x 1024 Pixels
HDTV Cameras – 1280 x 720 Pixels, 1920 x 1080 Pixels

There might be other resolutions used by certain cameras, as well.

The commonly utilized frame rates might vary from 15 frames per second to 30 frames per second. In many cameras, one can actually select the frame rate they want based on the video image quality/ available bandwidth/ storage space, etc. A frame rate of at least 10 frames per second is recommended for the human eye to be able to comprehend the motion properly.

In certain applications like face recognition, etc. a higher resolution might be more important than a higher frame rate because the images need to be clear enough for people to identify certain individual aspects clearly in order to aid the investigation process.

Some video surveillance applications allow video streams to be transmitted at different frame rates and resolutions. For example, a video can be transmitted to a monitor at a different frame rate/resolution but the same can be recorded at a different frame rate/resolution.

Compression in IP Video Surveillance:

Compression techniques are used to transmit video streams from one location to another in the most efficient manner in order to save as much bandwidth as possible. To do this, compression techniques often eliminate data that is repetitive/ redundant.

Each video frame might contain almost the same information as the previous frame except for minor changes. So, instead of sending entire frames, video compression algorithms send only the changes to each frame and individual frames are reconstructed using this information, at the other end.

MJPEG is a lossless compression format because each frame is individually compressed and decompressed. With this format, there is no data loss but the efficiency / level of compression that can be obtained, is very less.

MPEG-4 & H.264 (MPEG-4 Part 10/ AVC) are two of the most commonly used compression formats in IP Video surveillance. They use complex mathematical algorithms to eliminate the redundant data in each frame. They can achieve much more efficiency/bandwidth reduction (compared to MJPEG method), but some data might be lost during transmission (lossy compression technique).

H.264 compression technique might achieve around 25% better compression rates/ bandwidth reduction when compared to MPEG-4 technique, but it consumes more processing power.

High Definition (HDTV) in IP Video Surveillance:

With the popularity of High Definition Video in Video Conferencing and Consumer electronic devices like HDTV, the High Definition format is sought after in the video surveillance industry as well, as it provides higher quality images.

As mentioned before, there are two common High Definition resolutions – 1280 x 720 & 1920 x 1080. HDTV follows the 16:9 aspect ratio.

There are two types of scanning that are popular with High Definition systems – Interlaced Scanning (denoted by ‘i’) and Progressive Scanning (denoted by ‘p’). Progressive Scanning gives a better video quality than interlaced scanning but it requires more bandwidth to be transmitted.

For example, 720p refers to a video resolution of 1280 x 720 and progressive scanning (and) 1080i refers to a video resolution of 1920 x 1080 and interlaced scanning.

High Definition (HDTV) images can be directly shown on computer monitors without requiring any additional conversion/ de-interlacing techniques to be applied. HDTV Video (High Definition Video) can be compressed using H.264 compression for effective transmission that can achieve both good quality and reduced bandwidth.

Reference:

1. White Paper – Video Surveillance Trade-off’s; A Question of Balance: Finding the right combination of image quality, frame rate and bandwidth by Motorola.
2. White Paper – HDTV (High Definition Television) and Video Surveillance by Axis Communications.

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