The Difference between Interlaced and Progressive Video (Ⅰ)

There is no denying that the interlaced format is the earliest known form of video compression. As a matter of fact, it is developed in order to address early TV technology challenges and broadcast bandwidth constraints. However, interlaced video was not perfect enough to satisfy people’s various requirements. Therefore, the progressive video was developed as a newer technology which can help meet people’s requirements under specific conditions.

As for interlaced video, each field of a video image displays every other horizontal line of the complete image. For instance, in the first field of the interlaced video, the even-numbered lines will be displayed, and then the odd-numbered lines of the same image will be shown with the second field of the same interlaced video. If this even/odd sequence is repeated frequently enough, for example, 25 to 30 times per second, then the viewer will be able to see the complete moving image with the help of the persistence of human vision.

It is well known that the interlaced video allows more detailed images to be created than would otherwise be possible within a given amount bandwidth, which is its main benefit. As a matter of fact, a doubling of image resolution is also allowed in interlaced video. However, there are also some problems which cannot be ignored. In the first place, interlaced video comes with real-world downside. On the other hand, image softening will take place during fast-motion sequences. In addition, moiré or strobing artifacts are likely to occur in the process of showing striped shirts, plaid jackets, bricks in a building or similar types of objects.

In comparison with interlaced video, progressive video has greatly gotten rid of such problems and thus enjoy more popularity among people. It is possible that progressive video will play a more important role than interlaced video in the future.

Advantages of Progressive Scan

Generally speaking, progressive scanning has its own advantages when compared with interlaced scanning. First of all, the main advantage with progressive scanning is that the motion will be smoother and more realistic. In contrast, the interlaced video of the same rate will have a lot of visual artifacts, such as interline twitter. When capturing video with progressive scan, people have no such worries. It is safe to say that there will be an absence of visual artifacts, which enables the captured image to be used as still photos.

As a result, there is no need in intentional blurring, namely, anti-aliasing, of video in order to reduce interline twitter and eye strain. As a matter of fact, in the case of most media such as DVD movies and video games, the video is automatically blurred during the authoring process itself to subdue interline twitter when played back on interlace displays. Therefore, it is entirely impossible to recover the sharpness of the original video if the vide has been viewed progressively. A feasible solution to this problem is that people are supposed to choose the display hardware and video games come equipped with options to blur the video at will or to keep it at its original sharpness. Under such circumstance, the viewer is able to achieve the desired image sharpness no matter whether he is adopting interlaced or progressive displays.

Actually, progressive video is clearer and faster for scaling to higher resolutions in comparison with its equivalent interlaced video. HDTVs not based on CRT technology cannot natively display interlaced video. Therefore, the interlaced video has to be deinterlaced before it is scaled and displayed on HDTVs. However, in the deinterlacing process, some noticeable artifacts and input lag between the video source and display device are likely to emerge.

About Progressive Scan

Progressive scanning refers to a way of displaying, storing or transmitting moving images in which all the lines of each frame are drawn in sequence. It is also known as noninterlaced scanning alternatively. In comparison with interlaced video, progressive scanning is a way distinguished from interlaced scanning. Interlaced video is usually adopted in traditional analog television systems in which only the odd lines, then the even lines of each frame (each image called a video field) are drawn one after another. As a result, only half of the number of actual image frames are utilized to produce video.

As a matter of fact, progressive scanning is known as “sequential scanning” in the first place when it was adopted in the Baird 240 line television transmissions from Alexandra Palace, United Kingdom in 1936. Nowadays, progressive scanning has been universally adopted in computing.

Progressive scanning is applied in the scanning and storing of film-based material on DVDs. It was agreed that all film transmissions by HDTV would be broadcast with progressive scan in the US. Even though the video signal is sent interlaced, an HDTV will convert it to progressive scan.

Progressive scanning has been applied on most Cathode ray tube (CRT) computer monitors, all LCD computer monitors, and most HDTVs for the display resolutions are progressive by nature. Other CRT-type displays, such as SDTVs, will only display interlaced video.

Generally speaking, some TVs and most video projectors have one or more progressive scan inputs. Before HDTV became well received, some high end displays supported 480p. Under such conditions, these displays were able to be used with devices outputting progressive scan. For instance, some progressive scan DVD players and certain video game consoles are included. HDTVs support the 480p and 720p resolutions which have been progressively scanned. Compared with lower resolution HDTV models, the 1080p displays are usually more expensive. 

Device Driver

Device driver is usually called driver for short. Generally speaking, driver is a more popular name among people. In computing, it refers to a computer program which operates or controls a particular type of device which is attached to the computer. As a matter of fact, a driver provides a software interface to hardware devices, which enables the operating system and other computer programs to have access to hardware functions even without needing to know the hardware being used in great details.

Typically, a driver can only communicate with the device through the computer bus or communications subsystem connected to the hardware. If the routine located in the driver is invoked by a calling program, then the driver will issue commands to the device immediately. As long as the device sends the received data back to the driver, the routines in the original calling program are likely to be invoked as well. As a matter of fact, drivers are hardware-dependent and operating-system-specific. They usually provide the interrupt handling required for any necessary asynchronous time-dependent hardware interface. Therefore, it is safe to say that drivers are of vital significance in computing.

Device drivers act as translators working between a hardware device and the applications or operating systems using it, which can effectively simplify programming. Moreover, programmers can write the higher-level application code regardless of the specific type of hardware being used.

For instance, a high-level application for interlacing can be rather simple. It may have only two simple functions, namely, “send data” and “receive data”. However, it is able to interact with a serial port. In comparison, a device driver at a lower level may only be able to communicate with a particular serial port controller installed on a user’s computer even though it has implemented these functions as well. 

Where Deinterlacing Is Performed

Deinterlacing of the interlaced video signal can be performed at different points in the TV production chain.

Progresive Media

If the broadcast format or media format is progressive, then the interlaced archive programs will need the help of deinterlacing. Generally speaking, there are two ways for people to choose from in order to achieve this.

First of all, people can convert the interlaced video material to progressive scan during program production. In this way, the best possible quality is likely to be produced for videographers are capable of manually choosing the optimal deinterlacing method for each frame since they have access to expensive and powerful deinterlacing equipment and software.

Secondly, people could convert interlaced programs to progressive scan immediately prior to broadcasting with the help of real-time deinterlacing hardware. However, the conversion may be inferior to the pre-production method for the processing time is limited by the frame rate and no human input is available.

Interlaced media

Real-time deinterlacing also plays an important role in converting the interlaced video. If the broadcast format or media format is interlaced, then it can work to help people. However, the quality may be not so satisfactory because the consumer electronics equipment is rather cheap, which means that only less processing power and simpler algorithms are available in comparison with professional deinterlacing equipment.

On the other hand, what can not be ignored is the fact that the majority of users are not trained in video production, which sometimes lead to poor quality for many people have little knowledge about deinterlacing and are no aware that the frame rate is half the field rate. Therefore, deinterlacing is a very demanding work which needs to be done in a careful and patient way. Only with these efforts can people enjoy the video with the best possible quality.

Deinterlacing Methods

Deinterlacing refers to the technique which demands the display to buffer one or more fields and recombine them into full frames. Theoretically, this technique sound as simple as capturing one field and combining it with the next field to be received and then producing a single frame. However, deinterlacing is far more form simple in actual application. Generally speaking, the originally recorded signal was produced as a series of fields. What’s more, any motion of the objects during the short period between the fields will be encoded into the display. As a result, even the slightest differences between the two field due to the motion of the objects may lead to a “combing” effect in the combination process of a single frame. Therefore, deinterlacing is a very demanding technique requiring people to be careful and patient.

Actually, there are a variety of methods for people to choose from in terms of deinterlacing video. In addition, each method will cause different problems or artifacts of its own. On the other hand, some methods are much cleaner in artifacts than other methods.

Most deinterlacing techniques can be divided into three different groups which use their own exact techniques. The first group is called field combination deinterlacers. The name comes from the fact that they are able to take the even and odd fields and combine them into frame displayed later. The second group is called field extension deinterlacers for each field is extended to the entire screen in order to make a frame. The third group is called motion compensation which uses a combination of both.

Modern deinterlacing systems are able to buffer several fields and use techniques like edge detection in order to locate the motion between the fields. Therefore, this is adopted by people to interpolate the missing lines from the original field and reducing the combining effect.

Interlacing Problems

Generally speaking, interlacing is a very useful technique developed by people with the intention of capturing, storing, transmitting and displaying the interlaced video in the exactly same interlaced format. Its wide application in today’s scienece and technology field proves that interlacing is really very helpful and thus widely accepted by people. As a matter of fact, each interlaced video frame is made up of two fields which are captured at two different times. As a result, the interlaced video frame is likely to exhibit motion artifacts. Those artifacts, known as interlaced effects, or combing, will appear when the recorded object moves fast enough to be in different positions when each individual field is captured. If the interlaced video is displayed at a slower speed than it was captured or displayed in still frames, then these artifacts will be more visible. Therefore, people are supposed to try their best to avoid these motion artifacts in order to get the most satisfactory progressive frames. Only under such circumstance can people enjoy the video of the best quality.

Fortunately, there are some both simple and useful methods which can help users to produce satisfactory progressive frames from the interlaced image. For instance, users can make use of the function of doubling the lines of one field and omitting the other, namely, halving vertical resolution, or anti-aliasing the image in the vertical axis, which will be of great use for them to hide some of the interlaced effects or combing. Sometimes, these methods can help users produce progressive frames of more satisfactory quality. Therefore, users are supposed to make the fullest possible use of these functions to help themselves produce better and more satisfactory progressive frames. With these functions and adjustments, people will have the opportunity to enjoy the video of the best possible quality.