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Re: Laserdisc data capacity
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DOSGuy
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Oct 8, 2008, 04:30 PM
 
I recently became interested in how much data a laserdisc could hold, for historical comparison, and found this thread. The thread was too old to bump, but perhaps some helpful admin will merge the threads.

voodoo asked how much data a laserdisc can hold, which is difficult to answer because it was an analog format.

Answer 1: Analog

You could quantify how much data is held on a laserdisc in digital terms by multiplying the resolution of each frame (560x480) by the color depth of the images (technically infinite, so 24-bit true color?) times the number of frames per second (no idea), to get the amount of video on the disc, and then add the audio. Whatever that comes out to is the answer.

MAME recently created a CHD from the laserdisc in Cube Quest, presumably by recording a raw (lossless, uncompressed) video at LaserDisc resolution and framerate, with the highest possible color depth and an appropriate audio frequency and sample rate. The resulting file is a whopping 10.7 GB compressed, 38.27 GB uncompressed. Of course, since the goal is to archive the disc, they're going to record the data stream at a higher quality than they need to in order to ensure that absolutely no quality is lost. At any rate, the laserdisc was only used to create backgrounds for the game, so I doubt that the entire 60 minute capacity of the laserdisc was used. The MAME guys could probably tell us how much video cubeqst.chd represents.

So, we know they hold at least 38 GB, but how much more than that? Since we're talking about an analog medium, you can imagine that the data held on a laserdisc is enormous. 60 minutes of uncompressed video at 560x480 with 24-bit color at 24 fps is 64.89 GB for the video alone, so a laserdisc might hold the equivalent of 130 GB or more on both sides combined. The big question is the frames per second.

Answer 2: Digital

I was personally more interested in the answer to the question, "If a laserdisc was digital, how much data would it hold?" as a matter of historical comparison between LDs, CD, DVDs, etc. This is perhaps a more useful comparison.

Compact discs were based on the technology from laserdiscs, so I'm going to assume that both laserdiscs and CDs use the same laser wavelength. I haven't found any information to confirm or deny that assumption, and the assumption seems reasonable: it's not easy to switch wavelengths. For comparison, CDs use a near infrared 780 nm laser, DVDs use a red 650 nm laser, and Blu-Ray uses a 405 nm blue laser.

So, if the information encoded onto a laserdisc represented digital data (0s and 1s), how much data would it hold?

The diameter of a CD is 12 cm, but it looks like the outermost 2 mm aren't recorded on, so the radius is 5.8 cm. The formula for area is, therefor, Pi x 5.8^2 = 105.68 cm2. The innermost 4.6 cm aren't recorded on, so the central hub has a surface area of Pi x 2.3^2 = 16.62 cm2, so the total recordable surface area is 105.68 - 16.62 = 89.06 cm2. At 650 MB per CD (originally), that works out to an areal density of about 7.298 MB/cm2.

I don't have a laserdisc to do the measurements, but I have a high resolution photo of one. In the photo, the laserdisc is 17 cm, of which 6 cm is the central hub. A laserdisc is 30 cm, so roughly 10.6 cm is the central hub. Let's also assume that the outermost 2 mm aren't recorded on, once again. The radius of a laserdisc becomes 14.8 cm, so Pi x 14.8^2 = 688.13 cm2, minus the central hub radius of 5.3 cm, Pi x 5.3^2 = 88.25 cm2, for a total recordable surface area of about 599.89 cm2. If we assume that the laserdisc used the same laser to read tracks of the same width, then the laserdisc also has an areal density of 7.298 MB/cm2, giving a laserdisc about 4.276 GB per side, or 8.551 GB total.

Although you can't compare analog and digital, the result is nevertheless interesting. A laserdisc requires the equivalent of 4.3 GB of digital data to encode 60 minutes of video in an analog way, whereas a DVD encodes 120 minutes of video in 4.7 GB, and that video is higher resolution (720x480 for DVD, vs. 560x480 for LaserDisc) thanks to a finer laser and MPEG-2 compression, which is made possible by having the data represent digital data instead of analog data.

So, despite being 2.5 times larger than a DVD, and having 6.75 times the surface area, the two sides of a laserdisc have roughly the digital capacity of a dual layer DVD. The surface area result is interesting because a DVD holds about 7 times more data per layer than a CD, so it's possible that the initial goal of the DVD Forum was to create a disc with the same digital capacity as a laserdisc in a disc the size of a CD. That seems like a reasonable mission objective, and a logical starting point for the project. Basically, "We want to create a disc the size of a CD with the capacity of a LaserDisc. A LaserDisc has 7 times the surface area of a CD, so we want to create a disc that has 7 times the areal density of a CD." That gets everyone thinking that they have only to achieve this goal to be able to make laserdiscs the size of CDs, before they even stop to consider that the digital nature of the medium will allow compression, making it possible to create a video disc that doesn't even have to change sides after 60 minutes.

The bottom line

The short answer is that a laserdisc uses the equivalent of about 8.551 GB of digital data to record perhaps 130 GB worth of analog data.
( Last edited by DOSGuy; Oct 8, 2008 at 04:39 PM. )
     
olePigeon
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Oct 8, 2008, 04:53 PM
 
That's pretty interesting. Did you know Laserdisc was invented in the 1958? (According to Wikipedia anyway.) Hit the market in 1969. Technology way ahead of its time, didn't catch on until the 80s.
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starman
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Oct 8, 2008, 04:58 PM
 
I have a laserdisc album. A-ha's Take On Me. 12" LD with the video, and the whole album in CD quality.

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tooki
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Oct 8, 2008, 07:47 PM
 
Originally Posted by olePigeon View Post
That's pretty interesting. Did you know Laserdisc was invented in the 1958? (According to Wikipedia anyway.) Hit the market in 1969. Technology way ahead of its time, didn't catch on until the 80s.
Uhhh... what? No.

Laser disc (2 words) technology began to emerge in the late 50's through 60's, and the earliest prototypes were in the early 70's, but the LaserDisc (one word) product didn't come out for sale until 1978 (under the DiscoVision name) and was later renamed to LaserDisc.

http://en.wikipedia.org/wiki/Laserdisc#History
     
Eriamjh
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Oct 8, 2008, 08:12 PM
 
For potential capacity, just calculate the area and compare it to DVD or blu-ray. Do the math.

Porbably hundreds of GB.

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