Resolution

To understand what resolution is and how does it affect the actual image, we have to start with the smallest uniform unit displayed on the screen known as pixel (px). Each pixel can display three RGB colours at once (red, green, blue) with different brightness allowing it to render any colour (Fig. 1). A group of pixels symmetrically arranged on a rectangular grid is a display. Regardless of the different design of display devices, the basic working principle is usually similar. LCD monitors can sometimes have so called “dead pixels”, i.e. faulty pixels that cannot be controlled. The dead pixel does not emit any colour, and can be observed as a black dot.

Fig. 1. Magnified image of 9 pixels

Images at different standard resolutions are presented below. The input image was a 2048x1536 picture. The image is reduced in size, however, the scale is the same for all resolutions (Fig. 2).

Fig. 2. Comparison of several commonly used resolutions

The most common resolutions has been named, e.g. if the specification includes information that the camera records VGA resolution, it means that it records 640 x 480 images. Another key parameter related to the resolution is the aspect ratio defined as a ratio of image length to its height. The most popular is a 4:3 aspect ratio used from the very beginnings of television and a wide screen 16:9 aspect ratio used as standard in high resolution images.

Table 1. Selected resolutions and corresponding names

Image resolution - graphical presentation 

The higher the resolution, the larger the image (more details and more storage space required). Screenshots from two CCTV cameras offered by Delta C86A-2812 and APTI-32C4-2812 are shown below. The differences in colour depth, focus or field of view are due to the cameras being different models using different standards. Overall image quality depends on many other factors. Let’s focus on the resolution offered by both devices. The former is a PAL camera. Although it is an analogue signal measured with a number of lines, its size converted to pixels is 720x576. The latter is an IP camera recording 2048x1536 images. As you can see, the difference in the numbers is significant. How does it look like in practice?

Fig. 3. Comparison of screenshots from APTI-32C4-2812 and C86A-2812 camera

The difference is significant (Fig. 3). It is the best proof that also in CCTV monitoring new technologies are constantly developed, replacing the older standards. To show that the image made up of a larger number of pixels is more accurate, we will crop both screenshots to read the hour on the clock at the back of the test room. This is the result (Fig. 4).

Fig. 4. Comparison of magnified areas in the camera screenshots. Left - screenshot from APTI-32C4-2812, right - screenshot from C86A-2812

Image from APTI-32C4-2812 camera has been cropped and magnified 2x. The dial plate of the clock is not clearly visible, however, we can read the time. We can easily identify the numbers on the binders or the mannequin face. Image on the right recorded by C86A-2812 has been cropped and magnified to the same size as the APTI camera image. Unfortunately, the quality is very low. You can see the general contours of the people, the image lacks details, the objects are distorted and the edges are blurred. The round contours become square - an effect known as “pixellation” due to low image resolution. The image on the left at further magnification would also be pixelated, however, much later than the low resolution image.

Resolution is not the only parameter affecting the image quality, however, it is critical. The higher the resolution, the more storage space and the more efficient device to record the image or video is required. Supported resolutions must be considered when selecting a recorder for CCTV applications. A video seems smooth for the human eye if it is recorded at 25 frames per second. It means, that 25 images are recorder per second of the video sequence. At 1920x1080 and 25 frames per second, the required bit rate is 10 Mb/s (megabits per second). Remember that the recorder can be connected to more than one camera, which means that the amount of data it must support can be significant. There are different methods available to compress the transmitted signal in CCTV cameras that allow to reduce those requirements. The recorders can be easily configured to find a perfect balance between the resolution and the frames per second. All those factors should be considered when selecting the recorder.

To guarantee the best effects, select a high quality device that will meet all the requirements of connected cameras. What are the capabilities offered by a high quality HYBRO-416 recorder available in Delta offer?

The device is compatible with different devices, including AHD (Analog High Definition), PAL and IP cameras. Let’s discuss AHD. At 1280 x 720, the recorder can record 100 frames per second distributed between 4 channels i.e. 25 frames per channel. At 1920x1080, the frame rate drops to 60fps giving just 15 frames per second per channel. The image has a very good quality, however, it is not smooth.

In the specification for the recording equipment, the manufacturers sometimes use the term interpolated resolution. It is a marketing gimmick mercilessly used by the manufacturers of electronic equipment against unaware customers. Interpolation simply means artificial increase in resolution of the recorded images. The devices are usually fitted with lower quality image sensors that can record 640x480 or 800x600 images. The overall cost of the device is in this case lower. How is it possible, that the images are recorded at 1280x720, which would suggest HD image sensor? The resolution is interpolated by the device’s electronics. An extra pixel with colour and brightness calculated using different algorithms based on the adjacent pixels is inserted between two actual pixels to increase the image resolution. However, the quality of the image is not improved in any way. It does not differ from the image recorded by the same device at lower resolution, however it is larger in size.

As mentioned before, the resolution of the image recorded by the camera is not the only factor affecting its quality. Sometimes, the camera recording at low resolution can offer better quality videos depending on the graphics processor or image sensor used. Also, the environment it operates in is critical. Selecting the correct device is not an easy task. It is difficult to say which camera at what resolution would be the best and optimum choice for a specific application. Each application must be thoroughly analysed on a case by case basis and our employees offer support in choosing the right device.