What makes TV picture quality good
In this page we will explain what makes the picture quality in a flat TV good or bad. The most important TV features of a flat TV which makes the TV picture quality good or bad are listed below. Clicking on any of these listed features of a flat TV, it will take you to a detailed explanation of that particular specification further down this page.
What are Pixels in a TV
The picture in a TV is made up of thousands of tiny squares known as Pixels as shown on the left. A pixel is the short form of ‘Picture Element’ where the ‘Pix’ stands for 'picture element'. Pixels are the smallest element which makes a TV image. You will see the pixels in a TV if you go close up to any TV screen. The more number of pixels in a TV image the sharper it becomes.
Meaning of TV Resolution
TV Resolution is defined as the number of pixels arranged horizontally and vertically on a TV screen, for example HD TV screen has 1920x1080 pixels where 1920 pixels are on the horizontal 'X' axis and 1080 pixels in the vertical 'Y' axis.
There are currently four standard resolutions for TV. In the old CRT 4:3 TV the resolution was 640x480 lines. In modern flat TVs with 16:9 aspect ratio, the resolution of the HD Ready TV is 1280x720 pixels, HD TV has 1920x1080 pixel resolution and 4K Ultra HD TV has 3840x2160 pixel resolution.
How TV images are formed on TV screen
To explain how the TV picture is formed on the TV screen we can use the picture shown here.
An image on a TV screen is formed by scanning, by which each TV pixel in the TV is fed with the right digital information from the scanning video signal. The scanning of the TV pixels start from the top left corner and scans from left to right on the first row (1), when the first row scan is over, the scan goes to the left side and starts over from the left side of the second row (2). After the second row, the scanning starts from the third row, again from left to right. This row by row scanning continues till the last row of TV pixels at the bottom row are fed with the video digital signal. When the last pixel on the right side of the bottom most row is scanned, one image frame is completed and the scan goes to the top left corner to start scanning the next image (3). TV needs about 60 frames per second to have smooth motion of movements of images on the screen without any flicker.
What does 'i' and 'p' mean in 1080i and 1080p TV, Progressive and Interlaced Scan
The 'p' and 'i' in the 1080i and 1080p refers to how the 1080 horizontal lines of the HD image is scanned. The 'p' stands for 'Progressive' and the 'i' stands for 'interlaced'
TV transmission companies, in order to reduce Bandwidth required to transmit TV pictures, devised a method called 'interlaced scanning' by which each frame of a picture is split into two parts. In interlaced scanning, alternative rows of the 1080 lines of HD TV are scanned in each frame of the image. What this means that in the first scan, rows 1, 3, 5 and all odd rows of the 1080 lines are scanned to form one frame and in the second scan, rows 2, 4, 6 and all the even rows are scanned to form one image frame. Then the two frames are combined to form the complete picture as shown in the image here. This is how the 1080i interlaced TV image is formed.
In 1080p, where the 'p' stands for Progressive Scanning, the full picture is shown in one go. So there will be 50 full frame pictures per second in a 1080p TV image.
Since each frame in interlaced scanning is only half an image frame, there will only be 25 full image frames per second in 1080i interlaced scanning compared to 50 full frames in 1080p progressive scanning. Thus 1080p progressive scanning produces better TV images than 1080i interlaced scanning.
'Hz' in TV means what?
‘Hz’ is a symbol for 'Hertz' and is a measurement of frequency, the number of cycles per second. For example AC electricity is at 60Hz or cycles per second in the US and it is 50 HZ in the UK and India.
In TV the 'Hz' is the Refresh rate per second or the number of times per second, the electron beam paints a frame on the TV screen. Thus for example 60 Hz vs 120 Hz means 60 frames per second vs 120 frames per second. Normal TV stations transmit their signals at 50 or 60 Hz. Higher refresh rate helps in reducing the motion blur on fast moving scenes, like the football looks blurred on the TV when it is moving fast. Obviously the best refresh rate or Hz for a TV will be the higher number, but there is maximum limit that human eyes can distinguish.
The frame rate is different from the scan rate. The frame rate is the number of time a different new picture appears on the screen per second. The refresh rate is the number of times per second, the scanning electron beam paints from top to bottom. These numbers can be different, since while refreshing, the same picture can be shown again and again. For example if the frame rate is 30 per second and the refresh rate is 90 per second, then each picture of a frame will be shown repeatedly 3 times while refreshing.
Higher refresh rates were introduced to reduce eye strain due to flickering of the picture in the old CRT TV. On the newer TV they are used to make fast action scenes to appear smoother and not to cause blur, 'Judder' or 'flickering’.
Contrast Ratio in TV meaning
The higher the Contrast Ratio for a TV the better it is. As an example a TV with a contrast ratio of 6,000:1 is better than TV with a contrast Ratio of 3000:1. The meaning of Contrast Ratio in TV is explained as the ratio of the screen's maximum brightness in the whitest area to the least brightness in the darkest area or blacks on the screen. If there is absolutely no light in the darkest black of the TV picture then there is zero lumens or brightness there and the value for brightness will be zero. Since any number divided by zero gives an answer of infinity, so the theoretical contrast ratio will be infinity or larger than the largest number. But in actual practice some light bleeds into the darkest area from the bright areas of the screen and the aim of TV manufacturers is to keep it as small as possible. Thus the contrast values quoted by manufacturers are large, sometimes in the millions.