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The Truth About Burn Speeds of Recordable CD/DVD Media

CD and DVD Burn Speeds Explained Compact Discs are just about everywhere these days, from cars and trucks to living rooms and laptops. But how often do you stop and think about how your CD player works? It's fairly common knowledge that the information is read from the disc by a laser beam while the disc rotates, you can even see the laser "eye" in many players. But of course, there is always more than meets the eye. There is an incredible amount of engineering at work here, technology from a century past. So gather round ye children, come and hearken to a tale of yore about the history of recorded music (and a little physics lesson too).

When we talk about burn speeds, we will talk in terms of the ever-present "X" as in 52X or 16X. As always, the differences are in the details. When talking about recordable CDs, "one X" is equivalent to 150Kbps and when talking about recordable DVDs, "one X" is 1350Kbps. Notice I used the "lower case b," this is important because it means bits not bytes, and as we all remember there are 8 bits per byte. These numbers are not just randomly chosen; they are the original intended data read speeds for both formats. So if you want to talk about sheer data rates, 16X on a DVD would be equivalent to 144X on a CD. As if this weren't complicated enough, burners speeds need to be examined for another caveat: their burning method, as in CAV and CLV. Let's break it down together.

Back in the 1900s, there existed a primitive format war between Thomas Edison's phonograph cylinders and gramophone records. Due to their greater capacity, attractive album artwork, and in no small part to the 1929 bankruptcy of Edison's phonograph company, record albums won out. From the 1930s up until the 1980s, records were the popular choice of consumers, outlasting both reel-to-reel, 8-track tapes, and cassette tapes.

The information on records is read from a single long spiral track that looks like "grooves." These "grooves" are molded into the surface of the disc. Since records spin at the same speed all the time (like 33rpm of 45 rpm), the information on the disc must be spread out on the outer part of the disc. The information needs to be spread out because as the needle moves to the outside of the disc and the circumference gets bigger, the information travels by the needle faster and faster. This type of reading used by records is called CAV, or Constant Angular Velocity. So with CAV, the data must be "spread out" because the speed of the motor is constant.

Illustrated Pits and Groves on a DVD

Compact discs were originally developed as a read-only format to be produced through molded replication (almost the same process as with records). Some clever engineers decided to abandon CAV in favor of a new and better method called CLV. CLV, or Constant Linear Velocity uses the motor to change the rotational speed of the disc as the read head (laser) moves out from the center of the disc. This CLV stroke of genius meant the data could be molded on the disc in the same density throughout the disc, which allows full albums to be played non-stop on one side of a single disc. CLV also paved the way for the future of optical media like DVD, and the new HD DVD and Blu-ray Discs. In fact, CLV was the breakthrough that shrunk floppy diskettes from 9" and 5¼" to the omnipresent 3½" versions we all remember so fondly.

So let's sum up this first part. CAV is used for records and has a variable data density with a constant spin speed. CLV is used with CDs and combines a constant data density and with variable spin speed. We'll make this more confusing in a bit when we add a third variable, the "laser rate."

Now, let's consider the CD/DVD burner market of today, where recorders are compared in terms of burn speed, or "X times." Manufacturers today take great pride in advertising their burner's "speed," which is really a measurement of the peak speed at which it can reliably write data. These speeds are important but the burn method is important too. When reading data, you can only read it as fast as the data flies past the reader (the needle on a record player or the laser on a CD player). The same is true when you are writing data, except you can also change the rate at which the laser flashes or "fires." Each time the laser fires, the dye in the recordable disc is changed. The only question is, "how long did the laser fire and how fast was the disc spinning at that moment?" The longer the exposure, the longer the burn mark will be. So if you spin the disc at CAV, you will need to flash the laser more rapidly as you progress from the inside of the disc to the outside. OR, you can flash the laser at the same rate and change the speed at which the disc spins; remember CLV? Now to make this more complex, burners use "zoned" versions of each, appropriately named ZCAV and ZCLV. With these "zoned" versions, the burners will use a predetermined burn speed for a particular "zone" of the disc. (Imagine the zones like rings on a bull's-eye). Each ring allows the burner to "throttle up," kind of like shifting gears in your car. It is this small difference, hidden to most of the world (but not you, my loyal reader) that makes all the difference.

So it is important to mention that the burner is not continuously writing data at top speed, but rather, may peak at that speed along the outermost edge of the disc where linear velocity is the fastest. As the laser moves farther from the inside ring, the burner will shift-up to faster burn speeds. You may have 8X on the inner part of the disc, 10X on the next ring, 12X after that, and perhaps 16X on the outermost ring only. The burner handles all of these complex changes seamlessly, including all of the minor variables like rotational speed, laser speed, laser intensity, laser focus, and head location, ALL of which change throughout the process. Do you have a new respect for your burner?

Most people do not care about the complex engineering or chemistry that is taking place inside their burner, they just have the "need for speed," and of course, the need for "something better" drives the market. A key aspect of burning that is often overlooked in the search for speed is quality. Like everything in life there are trade-offs, and the variables in burning are burn quality and burn speed. Since it is the dye layer that gets the information burned into it, a slower burn will yield a more well-defined disc. The faster the laser moves around the disc, the less time it has to make a good impression on the data layer. Think of it like you would penmanship: the slower you write, the easier it is to read the words. Fast writing is often sloppy writing - just ask your doctor!

Discs that do not have good impressions, or were not burned properly, can encounter compatibility problems almost immediately. These discs do not make good master discs for duplication and are often a trigger for confusion and frustration among consumers. The problem with "speed" is that faster is not always better, and the fact remains that slower burns are easier to read and will cause fewer errors than faster burns.

With this in mind, do not fall prey to the advertising hype that "faster is better." Is it worth the additional cost to switch from 16X to 18X or from 40X to 48X drives? If you're not burning full discs, you may not even notice a difference in performance at all. Take it from the Tech Guy: when it comes to burning, nothing beats a slow and steady burn with high quality blank media.