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About DVD and the Conversion Process


What is DVD?

DVD is the next generation of video storage.  With the capacity for high quality sound and video, DVD has surpassed videotapes as the new industry standard in quality video storage.  A DVD is similar to a CD (compact disc), but has 8 to 10 times the storage capacity.  DVD is the first video format that will truly last a lifetime.  All the major players in video media (Sony, Pioneer, Toshiba, Etc.) have embraced DVD, and together have reaped the benefits of DVD being the fastest selling consumer electronic in history.


Why Go DVD?

Most people are aware that you can purchase Hollywood movies on DVD.  But the real breakthrough of DVD for millions of people is the ability to store video footage accumulated over decades onto this revolutionary new media.  We will cover below some of the reasons to start thinking seriously about going DVD at home.



Tape is a great medium to shoot in but a poor one to store in.  Tape heads wear on the tape during every viewing and chemical processes are at work destroying your tape even while it sits on a shelf.  Tapes as little as 10 years old could be in serious jeopardy from wear, disintegration, and magnetic fields.  A DVD, on the other hand, should last 100 years if properly cared for.  And the image is as good on the 100th year as it is the first time you watch it.



A DVD has many advantages over videotapes.  Instead of fast forwarding to specific spot in your video, a DVD lets you skip right to a specific point, bypassing all of the video in between.


DVD Technical Information

A DVD is similar to a CD.  It is a plastic disc that is microscopically burned or “pitted” by a laser or other method that lays down tracks of different sized indentations just beneath it’s clear surface layer.  These “pits” represent bits of data that when read in sequence can represent words, music, video or just about anything else.  These pits are read by a laser that shines onto the disc (without harming it) and by analyzing the light that is reflected back.  As the disc spins at a high rate of speed, the laser mechanism can read millions of  these pits per second.  A traditional CD-ROM uses “pits” that are at least 0.83 microns long, and uses “tracks” that are spaced 1.6 microns apart.  A standard CD holds 650 Megabytes of information or music.  A DVD optical-disc uses “pits” that are only 0.4 microns long and “tracks” that are spaced only 0.74 microns apart.  Since these pits are smaller and closer together, a DVD can store more data.  A single-sided, single-layer DVD can hold as much as 4,700,000,000 bytes of data.

A DVD player is essentially a computer that reads a DVD, which is an optical (rather than a magnetic) disc.  This is why many DVD terms are the same as computer terms.  DVD size is denoted in Gigabytes and the quality (or density of data) of a DVD is denoted in Megabits per second.  As a general rule, storage of data is generally represented in Bytes (or characters) while the movement or transfer of data is represented by bits (or “on/off” binary data).  Hard disks, floppy disks, and DVDs are measured in bytes while modem, DSL, network traffic, and DVD quality are measured in bits (per second). 

If we think of the video on a DVD as being broken into groups of pictures (about 30 pictures per second), and every picture as being made up of a certain number of dots, it is logical that the amount of data required to store a second of video is a function of the number of dots per picture times the number of pictures per second.                                                                                                                                                

Let’s say that your video is 720 dots wide by 480 dots high.  The total number of dots per picture are then 720x480 or 345,600.  If there are 30 pictures (or frames) per second in your video, the total number of dots needed to hold a second of video is 345,600 x 30 or 10,368,000 (over 10 million dots!).  Now imagine that a number is needed to represent he attributes of each dot (color, brightness, etc.) and that number is 8 bits long and the total number of  data bits needed to store 1 second of video is 10,368,000 x 8 or 82,944,000 bits.  This equates to 82.9 Megabits or 10.4 Megabytes.  If your video is 2 hours long, then the total amount of space needed to store it would be 74,649,600,000 Bytes or around 75 Gigabytes or the size of a small library. 

Unfortunately, the size of a single-sided, single-layer DVD is only 4.7 Gigabytes.  So you would need 16 DVDs to store the raw data for your 2 hour video.  Well, obviously, this simply won’t do.  And so we need to find a way to squeeze or “compress” your video down to fit on the disc.


MPEG-2   The Video Squeeze

Fortunalely for us, there exists (thanks to the Motion Pictures Experts Group “MPEG”) a way to “squeeze” video into a smaller size without losing much in the way of quality.  ALL DVDs use this method, TAKE-ONE Video Productions  DVD’s, as well as all Hollywood movies.  Let’s review below the 2 major ways that a DVD compresses video and why.


Method 1 – Motion Redundancy

One of the main ways that a DVD compresses your video to fit it onto a DVD is by using “Motion Redundancy”.  Essentially, what this method does is to look at every frame of video and try to remember what parts of a scene have not changed and not save those parts of a scene, but simply refer back to the picture of them that it had moments before.  Since NTSC video has 30 frames per second of video, this means that your tape is sending out 30 complete pictures every second that it wants to put on a DVD.  If we were to save every single bit of every picture, we would need many DVDs just to record one 2 hour production.  Instead, by looking backward and forward fromeframe to frame, the MPEG2 Encoder (the device that takes your video and “squeezes” it down for DVD) only saves the parts of a frame that have changed from one frame to the next.

In the case of a video of a person seated and talking, it is very likely that the only movement in the entire picture would be the person’s lips, or at the maximum, their face.  Since most of the scene (walls, desk, fixtures) are still, the Encoder can essentially tell each frame the following:  “Ignore the room and the fixtures, because I’ve got a copy of them and just save the person’s face”.  If the person’s face only takes up 20% of the screen, then you might assume that 80% of the scene could be “compressed”.

Uncompressed Video – In the original video above (your tape), the subject is only moving the red parts their body.  For the sake of this example, assume that all other things in the picture remain motionless.  Since every second contains 30 frames of video, this example covers approximately ¼ of a second.

Compressed Video – When the MPEG2 Encoder reads the stream of video, it grabs an initial frame that contains 100% of the video.  For the next group of frames (generally 14 but only 5 for this example), the Encoder only stores the changes since the last full frame.  Since only arms and legs are moving from one frame to the next and the rest of the scene in motionless, the only data stored for those frames is for the moving parts.


DVD transfer