Interlaced vs. Progressive Signals

There are two primary types of signals being used in modern televisions: Interlaced and progressive. Interlaced signals have been used since the invention of CRT televisions. Analog standards are based on the interlaced video signal and is something all television viewers have used at some point in time. Progressive signals scan images from side-to-side, then, top-to-bottom as is interlaced signals, but the latter scans every other line of imagery during the process.

Progressive signals send the entire frame at once whereas the interlaced signal is sent in two intervals. The first half of an image is sent, then, the next half is sent in the next scan completing the single frame. During the first 60th of a single second, every other line of an image is scanned, during the next scan, the rest of the image is scanned and reproduced. Interlaced images like this are called fields. Images tend to flicker more because of the way they are produced due to interlaced signals as opposed to being progressively scanned. Since the viewer believes they are witnessing 60 frames per second, but in reality, they are seeing 60 fields per second.

Your perception is what makes interlaced signals work. If you were able to just look at a single field of interlaced signal, you would only see half the vertical image. Your eyes work in conjunction with the signal being produced and put together the vertical and horizontal images so that they appear as one even when really they do not.

Interlaced signals are used in most SD televisions and 1080i broadcasts. However, it is not used in LCDS, DLPs or plasma displays which utilize the progressive signal. These signals reduce the bandwidth by a factor of two for the line and refresh rate. It can also increase the refresh rate which reduces flicker and allows for a greater range of motion for objects moving on screen. Since the display is updated more often, it will not be interrupted as frequently as lower refresh rates will show.

Since interlaced signals are meant to be captured and reproduced in the same format, they will produce video artifacts if reproduced at a different format. For example, if an interlaced signal is displayed on a progressive signal television, the horizontal resolution will be reduced as well as showcase movement artifacts. Deinterlacing can be used to reduce this output. While it results in a lower resolution, it can help produce the best possibly quality when displaying an interlaced signal on a progressive signal display.

Interline twitter can be caused by interlacing. It is a form of aliasing that occurs when vertical details approach horizontal layers of resolution. Somebody wearing a striped shirt may appear to the viewers as to have a shirt that is moving left and right. This is why you will very rarely see television celebrities and personalities wearing horizontal stripes onscreen.

Progressive scanning was developed in the 1930s in England. It was called sequential scanning and transmits moving images line-by-line in a frame as opposed to horizontally and vertically like its interlaced scan counterpart. Most DVDs use this method to reproduce film and every CCD sensor-based digital camera use this form to capture images.

Some televisions utilize more than one progressive scan mode. The most common is 480p and 720p. 1080p displays are more expensive but allow for a higher resolution. Progressive scan allows for fewer video and movement-based artifacts. It gives video an easier way to compress and decompress and also allows for an increased view in vertical resolution for a clearer and smoother picture. Flickering is non-existent and you do not need higher end DVD players to playback a progressive scan signal. Ghosting effects, noticeable in higher end game consoles, is also reduced. Progressive scan, however, has half the frame rate of interlaced signals. This does reduce movement during sporting events and other fast-changing programs.

Most outputs include 720p or 1080i. The "p" or "i" standing for progressive or interlaced, respectively. 720p signals have a resolution of 1280 by 720 pixels whereas 1080i signal have a resolution of 1920 by 1080 pixels. Since interlacing a signal is a form of compression, it offsets two fields by making up for it during half a frame's timeline instead of leaving the space blank. 24p, a special type of progressive signal, is used to copy the type of features seen on film. Since high-definition does not support 24p in most cases, some cameras do have the ability to use it.

Depending on the television set-up you desire, you may find the either format will work well for you. If you are watching a lot of movies, sports coverage and other programs that are fast moving, progressive scan may be the way to go. If you are avoiding the above or watch very few sporting events, interlaced signals may be the route to go. It all comes down to what type of picture and playback quality you are looking for. As HDTV becomes the standard for most television viewers, both scanning signals will have the advantages and disadvantages spelled out for you by both sides.