Progressive vs. Interlaced

There are generally two types of scanning methods used in television broadcasting. They are progressive and interlaced scans used to display video. Televisions produce moving images that are broadcast from a studio to an antenna. This was common during the golden age of television, but nowadays cable and OTT streaming are more prevalent. Broadcast television is still widely available and free of charge (with sponsored advertising).

These scanning methods are used to determine the technique for transmitting video frames. These refer to the transmission of signals that represent lines of resolution to a television screen display. Traditionally this used the CRT (Cathode Ray Tube), but now LCD (Liquid Crystal Display) are more common. The signals transmit patterns of how the CRT writes lines onto the television screen. The lines represent the video, and are written many times per second across the screen in the process called scanning.

The scanning rate is the repetition of how many times horizontal lines aka fields are written to the screen to display the video. It uses the same frequency as that of the power grid at 50 or 60 fields per second or Hz. Between 25 to 30 frames per second (fps) are sent. In North America, the monochrome (black and white) system uses 525 scan lines that are transmitted at a rate of 30 Hz, for a horizontal sweep frequency of 15,750 Hz (525 × 30). The color television system, also uses 525 scan lines, but the sweep frequency is adjusted to 15,734 Hz. This was done in order for both systems to remain compatible with each other for years to come.

Interlaced scans transmits the frame as odd (1,3,5 …) and even (2,4,6 …) numbered lines for 1/60 of a second (in reference to 60 Hz). The process is repeated over and over and each series of lines displayed is what is called the field. Only half the frame is actually transmitted at a time, but it happens so quickly (1/60 of second) that it is not noticeable to the human eye. It happens fast enough to make viewers see the complete frame, but there may be some flickering.

The cons of interlaced scanning are the movement within the frame can cause motion artifacts. This happens when the motion is really quick that it causes noticeable differences in the positions of the fields. An example of this is when you shoot sporting events with really fast motion, many artifacts can be generated. Viewers may also notice flickering on the screen of an interlaced display, like when watching TV from a satellite broadcast. This creates a combing effect (jagged edges), which can really affect image quality on screen. This means the frames are not entirely in sync with the actual motion. The interlacing can be so bad, but many systems use deinterlacing techniques to minimize this problem. It removes the combing effect by blurring the motion. The deinterlacing process is not perfect and it depends on how well the system was designed on the display or processing unit (e.g. cable box).

A main reason for using interlaced scanning is to conserve bandwidth. By sending just half the frame at a time, it saves the amount of bandwidth needed for transmitting information on the network. You are not actually using less bandwidth per se. If your transmission channel is 8 MHz, it doesn’t reduce to a lower value, you will still have 8 MHz. Instead think of it like this — doubling the frame rate of a video without consuming extra bandwidth.

The problem with requiring more bandwidth is that the greater the bandwidth, it becomes more expensive and complex to produce and broadcast content. An example is with interlacing on a PAL (Phase Alternating Line) system requires 50 fields per second (25 odd lines, 25 even lines). When interlaced, a half frame is sent every 1/50 of a second with less bandwidth requirements. If the full frame was sent, it could require another 8 MHz, thus increasing the need for more bandwidth.

In a progressive scan, the entire frame is transmitted at once. All the lines in frame are drawn at once to fill the screen. Progressive scans are more ideal for digital transmission compared to older interlaced scanning techniques. It became a technical standard for use with HD (High Definition) TV displays in the early 1990's.

By transmitting the full frame at once, it reduces flicker and artifacts. The video will appear smoother, more realistic and high quality. This allows capturing stills from video without noticeable artifacts in the image. This is great for super slo-motion video that really captures the details. There is also no need to use intentional blurring (anti-aliasing) to minimize problems like combing. This is good for viewers since less flickering means less eye strain. Viewers can watch for much longer hours without catching eye fatigue.

Progressive video is more expensive but desired among independent filmmakers. This is because it has the look like that of film. The scanning technique results in the clearest images without worrying about too many artifacts. It also allows for better viewing of fast motion video like in action sequences in movies and sports.

Interlaced scanning was originally used in traditional analog SD (Standard Definition) broadcasts since it was more efficient in transmitting video. However it is not smooth despite being reliable. For the most part, OTA signals still use interlaced techniques for TV. This requires the use of deinterlacing to convert to progressive scanning when the signal is sent to the display.

Deinterlacing converts the interlaced video into a non-interlaced or progressive form. TV sets and computer monitors support progressive scanning, so they display much better video or digital output. This has been built-in to most modern DVD players, Blu-ray players, LCD/LED HDTV, digital projectors, TV set-top boxes, professional broadcast equipment and computer video players with varying levels of quality (they are not all the same).

The recording, playback and transmission of video used either progressive or interlaced techniques. Interlaced has its roots in the broadcasting industry and is still widely used because of its efficiency and reliability. Progressive is ideal for higher quality displays for smoother video output.

Our eyes are not really aware of the transitions that take place in our TV. On standard displays using interlaced scanning it should be fine, but flicker and artifacts are noticeable. It gets worse on progressive screens like computer monitors, so it requires deinterlacing first before it can be displayed. The overall advantage of progressive is image quality when it comes to video playback. However, interlaced displays are still suitable for video playback at a lower cost.

When shooting interlaced video, known issues are with motion artifacts. This requires more post editing of the content which takes up more time and costs. This is why editors need to deinterlace video. It is also required because most modern displays use progressive scanning.

When choosing displays like a TV, you will see the marketing as 720i, 1080p, 2160p, etc. The letter “i” denotes interlaced while “p” denotes progressive. The trend toward progressive displays is more prevalent now because of OTT streaming video-on-demand content and digital media (e.g. DVD, Blu-ray, etc.). Digital video signals are more attuned to progressive scanning methods. If you compare a progressive scan and interlaced image at 60 Hz, the progressive scan image appears much smoother. While interlaced video signals are still used in broadcasting, progressive displays that have deinterlacing features are the better choice for video output.

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