Clarity shines on the murky confusion surrounding the two types of signals.
Clarity shines on the murky confusion surrounding the two types of signals.
Though digital video editing has been around for at least fifteen years now, it remains almost as confusing as programming a VCR; and now it's my turn to take a whack at unconfusing the subject.
Much of the said confusion comes from muddling two pairs of opposites that actually have no inherent connection: analog vs. digital and linear vs. nonlinear.
XX Analog/digital: is the signal recorded by continuous imitation (analog) or by intermittent sampling and encoding (digital)?
XX Linear/nonlinear: is each point in the program material reachable only in the order recorded (linear) or can any point be accessed at random (nonlinear)?
Many people still think that analog video must be linear and digital must be nonlinear.
But it ain't necessarily so! A video laser disk offers random (nonlinear) access to an analog video signal and the tapes in digital camcorders are as linear as you get: to reach the last shot on the roll you gotta crank through every blessed shot ahead of it, one at a tedious time.
Stay tuned as we attack this confusion by looking closely at each pair of opposites, beginning with analog vs. digital recording. (Linear vs. nonlinear is a topic for a later column.)
Amber Waves of Gain
The first thing to remember is that an analog signal copies by imitating. Light from the camcorder lens slams into a sensor on the imaging chip, creating an electrical charge. The stronger the light, the stronger the charge, which is to say that the electrical signal is imitating the intensity of the light that produced it. Multiply this stimulus/response by several hundred thousand sensors covering all three primary colors and you have the entire optical image imitated by an electrical signal of rapidly and continuously varying voltage.
The second thing to remember is that, regardless of the recording system, all video signals start out like this, as analog wave forms. Yes, your expensive DV camcorder is creating old-fashioned analog video signals, just like the humblest old VHS clunker.
It's creating them, but not recording them, because that's where analog and digital video part company. In an analog system the continuously varying voltage magnetizes tape particles in a continuously varying pattern that mirrors the signal. On playback, the tape particles create a continuously varying output signal that (surprise!) continues to mirror the original. Every transfer of the picture information is an imitation--or, more precisely, an imitation of an imitation, with consequences that we'll see shortly.
In a digital system, by contrast, the first thing that happens to the original continuous signal is that it's fed through an analog/digital converter chip. That chip looks at the signal hundreds of thousands of separate times per second and assigns each discrete sampling a numerical value that corresponds to the strength of the signal at that precise moment in time. These numbers, rather than the signal itself, are copied and recopied throughout the rest of the process. This means that digital recording differs from analog in two crucial ways:
--It numerically encodes the information rather than electrically mimicking it, and
--It records only samples of the information rather than all of it.
These differences have consequences of their own, as we're about to explain.
But on the Other Hand...
If you believe all the hype, digital video must be as superior to analog as halogen lighting is to whale oil, right? Maybe, maybe not, because analog video enjoys three powerful advantages over its new rival: compactness, scalability and seamlessness.
Compactness
Above all, the information in analog video can be stored very efficiently and cheaply (up to two and a half hours on one $2 VHS tape at SP speed). High-quality digital video demands gargantuan amounts of storage space. For example, those fancy new video disks, DVDs (Digital Versatile Disks), must squeeze 4.7 gigabytes of data onto a single side of the disk just to fit a feature-length movie, and that's with a hefty dose of compression.
But DVD is not yet available in recordable form, so you have to use computer storage or a digital camcorder instead. Working with computer storage, my students have trouble fitting three five-minute projects on a 4.2 gigabyte hard drive.
Scalability
All video, analog and digital, tends to look sharper and clearer on a smaller screen; it's the natural result of squeezing the same amount of visual information into a smaller space. All but the highest quality digital video, however, suffers greatly from enlargement. When you blow up your digitized image onto a huge home-theater TV screen, for example, all of those invisible digital compression artifacts become quite noticeable--straight lines become jaggy, curves look blocky, etc. Analog video, on the other hand, is much better at filling larger screens with sharp-looking images.
Seamlessness
In the audio world, some purists have returned to analog (vinyl LP) recordings because they can hear the fact that digital recordings only sample the signal at intervals instead of copying the whole thing. To them, CDs sound hollow and brittle in consequence.
It could be argued that digital video suffers a similar artificiality. It's hard to describe, but look at the hyper-crispness of heavily digitized commercials and the oddly stark appearance of digital satellite broadcast. There's no doubt about it: digital video has a certain look that's distinct from analog video.
A Hands-down WinnerWith all that said, we must judge digital video to be the superior recording system for three powerful reasons: copy quality, freedom from noise and computer compatibility.
Copy QualityOut of old habit, we talk about "copying" a floppy disk or a digital video file, but we are not actually making a copy at all. Instead, we're making a transcription: rewriting the information rather than duplicating it.
Instead of copying the video signal, digital duplication transcribes the numerical code that describes that signal. If you transcribe it accurately (and computers are outstanding at chores like that), you can decode the result into a daughter signal that is essentially indistinguishable from the parent.
Freedom from Noise
Noise is any disturbance in an electrical current that is not part of the signal, and every current carries a certain amount of this electrical garbage.
Since an analog dupe is an imitation, it happily copies the noise right along with the parent signal, while adding new noise in the process. That means that in each generation, the noise level relative to the signal (signal-to-noise ratio) increases and the quality decreases proportionately.
In digital recording, noise is not a problem because the signal consists entirely of current pulses carrying information like Morse code: power on = 1; power off = 0. If the voltage level of the "power on" part of the signal is well above the noise level, then the transcribing (copying) system can be set to respond only to current at that level and ignore the noise entirely. So even if the process adds a small amount of its own noise, it never copies the parental noise--nor does it pass on its own noise to the grandchildren.
The result is that digital video can be copied through many generations without appreciable quality loss. This is a massive improvement over analog video (and even over cinematic film, which is another analog medium).
Computer Compatibility
By far the biggest advantage of digital video is that a computer can process and store it. Computers are astonishingly powerful but they cannot work with pictures, or more accurately, with the continuously varying wave forms that record them. Before you can get your computer to handle or even recognize video input, you have to digitize the video.
For many years, professionals have digitized video, not only to take advantage of loss-free duplicating, but also to perform image processing. Image processing means superimposing titles, compositing multiple images, and adding effects like dissolves and wipes.
In image processing, digital is an ephemeral state: an analog signal is digitized, massaged for a few microseconds at most, and immediately reconverted to analog.
But as hard drives got bigger and faster, and as image compression techniques improved, it became possible to digitize the signal and then keep it in that form indefinitely by storing it in the computer.
Digital storage also saw the birth of nonlinear editing, with almost instant access to any footage anywhere in the computer. This advantage is so great that digital video would probably prevail over analog due to random (nonlinear) access alone.
But linear/nonlinear is that other pair of opposites, the pair that's often muddled with analog/digital. Linear and nonlinear are such important concepts that they deserve their own discussion; and, in a later Edit Suite, they'll get it.
As we said much earlier, stay tuned!
Though digital video editing has been around for at least fifteen years now, it remains almost as confusing as programming a VCR; and now it's my turn to take a whack at unconfusing the subject.
Much of the said confusion comes from muddling two pairs of opposites that actually have no inherent connection: analog vs. digital and linear vs. nonlinear.
XX Analog/digital: is the signal recorded by continuous imitation (analog) or by intermittent sampling and encoding (digital)?
XX Linear/nonlinear: is each point in the program material reachable only in the order recorded (linear) or can any point be accessed at random (nonlinear)?
Many people still think that analog video must be linear and digital must be nonlinear.
But it ain't necessarily so! A video laser disk offers random (nonlinear) access to an analog video signal and the tapes in digital camcorders are as linear as you get: to reach the last shot on the roll you gotta crank through every blessed shot ahead of it, one at a tedious time.
Stay tuned as we attack this confusion by looking closely at each pair of opposites, beginning with analog vs. digital recording. (Linear vs. nonlinear is a topic for a later column.)
Amber Waves of Gain
The first thing to remember is that an analog signal copies by imitating. Light from the camcorder lens slams into a sensor on the imaging chip, creating an electrical charge. The stronger the light, the stronger the charge, which is to say that the electrical signal is imitating the intensity of the light that produced it. Multiply this stimulus/response by several hundred thousand sensors covering all three primary colors and you have the entire optical image imitated by an electrical signal of rapidly and continuously varying voltage.
The second thing to remember is that, regardless of the recording system, all video signals start out like this, as analog wave forms. Yes, your expensive DV camcorder is creating old-fashioned analog video signals, just like the humblest old VHS clunker.
It's creating them, but not recording them, because that's where analog and digital video part company. In an analog system the continuously varying voltage magnetizes tape particles in a continuously varying pattern that mirrors the signal. On playback, the tape particles create a continuously varying output signal that (surprise!) continues to mirror the original. Every transfer of the picture information is an imitation--or, more precisely, an imitation of an imitation, with consequences that we'll see shortly.
In a digital system, by contrast, the first thing that happens to the original continuous signal is that it's fed through an analog/digital converter chip. That chip looks at the signal hundreds of thousands of separate times per second and assigns each discrete sampling a numerical value that corresponds to the strength of the signal at that precise moment in time. These numbers, rather than the signal itself, are copied and recopied throughout the rest of the process. This means that digital recording differs from analog in two crucial ways:
--It numerically encodes the information rather than electrically mimicking it, and
--It records only samples of the information rather than all of it.
These differences have consequences of their own, as we're about to explain.
But on the Other Hand...
If you believe all the hype, digital video must be as superior to analog as halogen lighting is to whale oil, right? Maybe, maybe not, because analog video enjoys three powerful advantages over its new rival: compactness, scalability and seamlessness.
Compactness
Above all, the information in analog video can be stored very efficiently and cheaply (up to two and a half hours on one $2 VHS tape at SP speed). High-quality digital video demands gargantuan amounts of storage space. For example, those fancy new video disks, DVDs (Digital Versatile Disks), must squeeze 4.7 gigabytes of data onto a single side of the disk just to fit a feature-length movie, and that's with a hefty dose of compression.
But DVD is not yet available in recordable form, so you have to use computer storage or a digital camcorder instead. Working with computer storage, my students have trouble fitting three five-minute projects on a 4.2 gigabyte hard drive.
Scalability
All video, analog and digital, tends to look sharper and clearer on a smaller screen; it's the natural result of squeezing the same amount of visual information into a smaller space. All but the highest quality digital video, however, suffers greatly from enlargement. When you blow up your digitized image onto a huge home-theater TV screen, for example, all of those invisible digital compression artifacts become quite noticeable--straight lines become jaggy, curves look blocky, etc. Analog video, on the other hand, is much better at filling larger screens with sharp-looking images.
Seamlessness
In the audio world, some purists have returned to analog (vinyl LP) recordings because they can hear the fact that digital recordings only sample the signal at intervals instead of copying the whole thing. To them, CDs sound hollow and brittle in consequence.
It could be argued that digital video suffers a similar artificiality. It's hard to describe, but look at the hyper-crispness of heavily digitized commercials and the oddly stark appearance of digital satellite broadcast. There's no doubt about it: digital video has a certain look that's distinct from analog video.
A Hands-down WinnerWith all that said, we must judge digital video to be the superior recording system for three powerful reasons: copy quality, freedom from noise and computer compatibility.
Copy QualityOut of old habit, we talk about "copying" a floppy disk or a digital video file, but we are not actually making a copy at all. Instead, we're making a transcription: rewriting the information rather than duplicating it.
Instead of copying the video signal, digital duplication transcribes the numerical code that describes that signal. If you transcribe it accurately (and computers are outstanding at chores like that), you can decode the result into a daughter signal that is essentially indistinguishable from the parent.
Freedom from Noise
Noise is any disturbance in an electrical current that is not part of the signal, and every current carries a certain amount of this electrical garbage.
Since an analog dupe is an imitation, it happily copies the noise right along with the parent signal, while adding new noise in the process. That means that in each generation, the noise level relative to the signal (signal-to-noise ratio) increases and the quality decreases proportionately.
In digital recording, noise is not a problem because the signal consists entirely of current pulses carrying information like Morse code: power on = 1; power off = 0. If the voltage level of the "power on" part of the signal is well above the noise level, then the transcribing (copying) system can be set to respond only to current at that level and ignore the noise entirely. So even if the process adds a small amount of its own noise, it never copies the parental noise--nor does it pass on its own noise to the grandchildren.
The result is that digital video can be copied through many generations without appreciable quality loss. This is a massive improvement over analog video (and even over cinematic film, which is another analog medium).
Computer Compatibility
By far the biggest advantage of digital video is that a computer can process and store it. Computers are astonishingly powerful but they cannot work with pictures, or more accurately, with the continuously varying wave forms that record them. Before you can get your computer to handle or even recognize video input, you have to digitize the video.
For many years, professionals have digitized video, not only to take advantage of loss-free duplicating, but also to perform image processing. Image processing means superimposing titles, compositing multiple images, and adding effects like dissolves and wipes.
In image processing, digital is an ephemeral state: an analog signal is digitized, massaged for a few microseconds at most, and immediately reconverted to analog.
But as hard drives got bigger and faster, and as image compression techniques improved, it became possible to digitize the signal and then keep it in that form indefinitely by storing it in the computer.
Digital storage also saw the birth of nonlinear editing, with almost instant access to any footage anywhere in the computer. This advantage is so great that digital video would probably prevail over analog due to random (nonlinear) access alone.
But linear/nonlinear is that other pair of opposites, the pair that's often muddled with analog/digital. Linear and nonlinear are such important concepts that they deserve their own discussion; and, in a later Edit Suite, they'll get it.
As we said much earlier, stay tuned!
