Visualize yourself taking a sick day from school or work. In your stupor, you flip the switch for the first time in months. You had forgotten how terrible daytime television is in your time away as an adult. All of these gameshows and soap operas kind of look terrible, don't they?
Behind every disappointing TV special is one historically vital pillar of broadcasting: interlacing. There's a reason why your favorite movies are so much more exciting to watch.
What Is Interlacing?
In the early days of broadcast media, engineers had a brand new problem to solve: figuring out the most economical way of delivering the same thing to a million different homes nationally.
The industry precursor, theater exhibition, used physical, progressive images instead of interlaced video. Many will recognize these images as a reel of discrete film cells. Shipping broadcast media via the same method was not practical, as that would have entailed sending every family in the country an identical, physical media package. This is the opposite of the intention of true broadcast media, especially in its original context.
Paring away some of the bulk of the broadcast signal lightens the load. It also doubles something called the vertical repetition rate of the video feed without compromising resolution. In any other case, those producing the signal would either need to reduce the resolution of their offering significantly or broadcast a much larger and heavier signal out to begin with.
How Does Interlacing Work?
Think about it like this: with progressively-displayed video, each frame consists of exactly one frame’s worth of footage in terms of temporal length. An interlaced video frame, however, does not. An interlaced frame is instead equal to two half-frames; pardon us for mincing words, but the difference is profound.
The first field of the first frame matches up with the second field of the frame shown previously. The second field of the first frame goes along with the first field of the frame that comes immediately after. Both pairs of fields amount to exactly one original frame’s worth of footage.
Each interlaced frame, individually, contains one-half of the two consecutive frames that were in the original, progressive source material. The persistence of vision marries these two asynchronous signals visually to our human eyes, resulting in a quality of video that takes us there while using much less signal bandwidth.
What Are Interlacing Scan Lines?
Signal bandwidth is a term concerned strictly with media as it is being conveyed; the size of the load contends with the width of the tunnel that it is meant to travel through.
A film camera or one that uses magnetic DV tape will naturally produce one full and continuous image per frame. To outfit this image for transit, each broadcast frame must be broken down into smaller and simpler pieces, ones that are easier to convert into an analog signal. Sending each original, aggregate frame in its entirety would have been logistically impossible under the circumstances of the time.
Their solution: horizontal scan lines. Each horizontal scan line of the image was sent to a receiver, where the image would then be reconstructed on the ground.
The NTSC standard calls for each frame to be broken into 525 horizontal scan lines, with 262.5 belonging to each field. Field order determines whether the even field or the odd field arrives first. Usually, the even-numbered field will be the first to be generated at the signal’s destination. This is done sequentially, from the top down.
When transmitting a progressive video signal, the same thing happens. The only difference is that each horizontal scan line is instead part of only one single, continuous field; this field consists of the entirety of the image.
Vertical Repetition Rate
One thing that’s true in a general sense: transmission is not cheap. Transmitting large amounts of data takes proportionally larger amounts of resources as both the amount of data to move grows, and the physical breadth of your scope of transmission widens. Interlacing is one way of mitigating this problem while still allowing for a broadcast image large enough to enjoy.
The flicker effect has plagued engineers since the inception of the industry. Many factors contribute to this aspect of the viewer’s experience, including things like the effective frame rate of the video and even the ambient light conditions in the room as the viewer consumes.
The quality of the video signal is, of course, where one on the other side stands to make the most difference. A flicker-free video signal will usually require anywhere from forty to sixty large-area light flashes per second. These large-area light flashes occur each time a new frame replaces the one that preceded it on-screen.
The vertical repetition rate describes how many of these jarring changes occur over some amount of time. These changes are responsible for triggering the biophysical phi phenomenon that interlaced video relies on.
As mentioned previously, the primordial beginning of television was constrained by the technology of the era. To remain below the limit of what could realistically be broadcast under these rudimentary conditions, television engineers needed to devise a way to refresh the image more frequently without increasing the number of frames being sent over a distance.
Fields Per Second vs. Frames Per Second
Each alternating field signal cascades through the one that follows it. They are displayed in tandem but remain totally separate in a technical sense, instead of two signals first rendered together and then displayed to see. Our eyes perceive these additional large-area flashes, however, even when the presentation rate remains the same.
Those at the helm of this movement understood that at least four hundred scan lines of resolution per frame were necessary to end up with a legible video feed. In North America, NTSC is the only type of analog video signal that our infrastructure will support at full scale. This is because of the way electricity is produced (at a rate of 60hz) as opposed to most of the rest of the world (at a rate of 50hz).
Physically, the data transmission rate relates directly to the rate at which the power used to convey it is consumed. This is where both NTSC and PAL derive their characteristic frame rates.
With this inevitability in mind, an interlaced American signal being transmitted at 60hz will end up with an effective frame rate of approximately 29.97 frames per second after it has been received. On the other hand, an interlaced PAL signal will be perceived by the viewer at 25fps.
The difference between fields per second and frames per second has a lot to do with how these additional large-area light flashes are distinguished from the “real” temporal divisions that separate each video frame at the time of acquisition. As a result, the eye is more thoroughly engaged by a video feed that appears to be much more dynamic than it actually is.
While the true “resolution” of each frame shown on-screen is exactly half of the original image, this loss will not impact the audience unduly under the right circumstances. Thanks to the persistence of vision, the show goes on without skipping a beat.
Common Challenges Associated With Interlaced Video
Scan lines are a cherished hallmark of old-school DV camcorders and archival material from the early days of mass broadcast media. These artifacts occur when interlaced footage has been manipulated after being syndicated or in footage that has degraded naturally to some extent. The same thing can happen when rendering video digitally under certain forms of compression.
This can result in unpleasant “shuddering,” which results in on-screen elements remaining “trapped” visually between two adjacent positions. The effect will usually be much more apparent when the video is evaluated by the frame. Objects moving quickly across the frame are most prone to ending up with artifacts like this. This is especially the case if the moving object stands in contrast to the background behind it.
Reconstituting interlaced video to restore it to its formerly progressive state may result in these artifacts. One reason for this may be that the means of reversion did not match the original signal’s protocol of field order.
When Cutting Corners Is Written Right Into the Book
Interlacing is one of those inspiring stories of mortal victory over the tyranny of nature's iron rule. When the laws of physics tell you to take it easy, it takes a very special type of changemaker to simply shove their show through the pipeline anyway. And, boy howdy, did they ever.
So rarely in life are we given permission to capitalize on shortcuts like this. The many modern applications of interlacing are a testament to the staying power of a truly lateral deviation of thought in any industry.
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