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Data Capacity of CDs [Tutorial] 

Abstract

You can fit on a S/VCD without overburning:

- approx. 735 MB of MPEG data onto a 74min/650MB disc

- approx. 795 MB of MPEG data onto an 80min/700MB disc

You can fit on a CD-ROM without overburning:

- approx. 650 MB of data onto a 74min/650MB disc

- approx. 703 MB of data onto an 80min/700MB disc

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Introduction

Let us ignore for now the terms of megabyte for CD capacity and try to understand how the data is stored on a CD.

As well all know, the data is stored digitally as binary data. This means, however the actual information is actually kept on the disc, this information is in the form of "1"s and "0"s. Physically, the information on a CD is as pits on a thin sheet of metal (aluminium).

An a CD-R disc, the data is physically on an organic dye layer which simulates the metal layer on a real pressed CD.

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How is the information structured

Now, on the CD, the information isn't just organised from beginning to end willy-nilly. Otherwise, it would be really hard to find a useful piece of information on the CD.

Rather, the information is organised in sectors. Consider a sector as like a page in a book. Just like you are able to quickly find something in a book if you know the page number, you can quickly find something on a CD if you know the sector number.

Now, remember that the CD was original made to hold audio data. It was decided, that the CD would would 75 sectors per second of audio. Although I cannot guess where this number comes from, it is quite appropriate for the audio CD. It means that you can "seek" an audio CD accurately to 1/75th of a second -- which is more than enough for consumer purposes.

Now, with this in mind, we can work out the total data capacity of user data for 1 sector.

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The total data capacity of user data of 1 sector on a CD

CD audio uses uncompressed PCM stereo audio, 16-bit resolution sampled at 44.1 kHz.

Thus 1 second of audio contains:

16 bits/channel * 2 channels * 44100 samples/second * 1 second

= 1411200 bits

= 176400 bytes

Since there are 75 sectors per second

1 sector

= 176400 bytes / 75

= 2352 bytes

One sector on a CD contains 2352 bytes max.

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The concept of different MODES and FORMS of burning

Now, audio CD was well and good, but the medium would become much more useful if you could store other data on the disc as well. This became to be know as CD-ROM of course.

Now, the audio-CD uses the ENTIRE sector for audio data.

However, for CD-ROMs this caused a problem. Simply, CDs and the CD reading mechanisms were not 100% faultless. That is, errors (indeed frequent errors) could be made during the reading. For audio CDs, this does not matter as much as you could simply interpolate from the adjacent audio samples. This will obviously NOT DO for data CDs. A single bit error could lead to a program being unexecutable or ruin an achive file.

Thus, for CD-ROMs, part of each sector is devoted to error correction codes and error detection codes. The CD-R FAQ has the details, but in effect, only 2048 bytes out of a total of 2352 bytes in each sector is available for user data on a data CD.

This burning mode is either MODE1 or MODE2 Form1.

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MODE2 Form2 sectors of VCDs and SVCDs

Now, for VCDs and SVCDs, the video tracks do not necessarily require the robust error correction as normal data on a CD-ROM. However, there is still some overhead per sector that is used for something other than video data (e.g., sync headers).

S/VCDs video tracks are burnt in what is called MODE2 Form2 sectors. In this mode, only 2324 bytes out of a total of 2352 bytes in each sector is available for user data.

This is MUCH MORE than for CD-ROMs, but still less per sector than audio CD.

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The disc capacities of CD-ROMs, audio-CDs and VCDs

Now, obviously what ultimately determines the capacity of a disc is the total number of sectors it contains. This is similar to the total number of pages in a blank exercise book (if you recall the book analogy).

The secondary determinant is the burning mode of the disc.

For audio CDs, it is as if you could fill each page from top to bottom with audio data as the entire sector is used for audio data.

For CD-ROMs, it is as if you need to first rule a margin and then leave the bottom part of each page for footnotes (headers + ECC + EDC). The amount of text you can actually write per page is then less due to these other constraints.

For S/VCDs, we still need to rule a margin on the page, but we don't have to worry about the footnotes (headers). We can fit MORE text than a CD-ROM, but less than an audio-CD.

Now remember, 1 second on a CD = 75 sectors.

Thus:

- 74 min CD = 333,000 sectors

- 80 min CD = 360,000 sectors

Data capacity in Mb for an audio-CD

74 min

= 333,000 sectors * 2352 bytes / sector

= 783216000 bytes

= 746.9 Mb

80 min

= 360,000 sectors * 2352 bytes / sector

= 846720000 bytes

= 807.5 Mb

Data capacity in Mb for a CD-ROM

74 min

= 333,000 sectors * 2048 bytes / sector

= 681984000 bytes

= 650.4 Mb

80 min

= 360,000 sectors * 2048 bytes / sector

= 737280000 bytes

= 703.1 Mb

Data capacity in Mb for a S/VCD

74 min

= 333,000 sectors * 2324 bytes / sector

= 773892000 bytes

= 738.0 Mb

80 min

= 360,000 sectors * 2324 bytes / sector

= 836640000 bytes

= 797.9 Mb

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Conclusions

As you can see, the often quoted capacities of 650MB and 700MB refer to CD-ROM capacities.

Due to the fact that S/VCDs use a different burning mode where MORE of each sector is available as user data, the relatively capacities are HIGHER.

Now, since S/VCDs are not composed of PURELY video tracks and have some unavoidable overheads, the actually total capacity left for video tracks is a few Mb less for each disc (about 735 Mb for 74min discs and 795 Mb for 80min discs). This is where the often quoted capacities of 740MB and 800MB come from. They are quite accurate.

All these capacities are available BEFORE overburning. Overburning is where you burn MORE sectors than the disc is rated for. If you overburn, you can typically achieve about 1-2 minutes of additional capacity (depending on your drive and media).

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Bit Torrent Tutorials

The first things you need to know about using Bit Torrent:

-- Bit Torrent is aimed at broadband users (or any connection better than dialup).

-- Sharing is highly appreciated, and sharing is what keeps bit torrent alive.

-- A bit torrent file (*.torrent) contains information about the piece structure of the download (more on this later)

-- The method of downloading is not your conventional type of download. Since downloads do not come in as one

big chunk, you are able to download from many people at once, increasing your download speeds. There may be

100 "pieces" to a file, or 20,000+ pieces, all depending on what you're downloading. Pieces are usually small (under 200kb)

-- The speeds are based upon people sharing as they download, and seeders. Seeders are people who constantly

share in order to keep torrents alive. Usually seeders are on fast connections (10mb or higher).

In this tutorial, I will be describing it all using a bit torrent client called Azureus. This client is used to decode the .torrent files into a useable format to download from other peers. From here on out, I will refer to Bit Torrent as BT.

Which BT client you use, is purely up to you. I have tried them all, and my personal favorite is Azureus for many reasons. A big problem with most BT clients out there, is that they are extremely CPU intensive, usually using 100% of your cpu power during the whole process. This is the number one reason I use Azureus. Another, is a recently released plug-in that enables you to browse all current files listed on suprnova.org (the #1 source for torrent downloads).

Before you use the plug-in, take a look at /http://www.suprnova.org, and browse the files. Hold your mouse over the links, and you'll notice every file ends in .torrent. This is the BT file extension. Usually, .torrent files are very small, under 200kb. They contain a wealth of information about the file you want to download. A .torrent file can contain just 1 single file, or a a directory full of files and more directories. But regardless, every download is split up into hundreds or thousands of pieces. The pieces make it much easier to download at higher speeds. Back to suprnova.org. Look at the columns:

Added | Name | Filesize | Seeds | DLs (and a few more which aren't very useful.)

I'll break this down.

Added: Self explanitory, its the date the torrent was added.

Name: Also self explanitory.

Filesize: Duh

Seeds: This is how many people are strictly UPLOADING, or sharing. These people are the ones that keep .torrent files alive. By "alive", I mean, if there's no one sharing the .torrent file, no one can download.

DLs: This is how many people currently downloading that particular torrent. They also help keep the torrent alive as they share while they download.

It's always best to download using a torrent that has a decent amount of seeders and downloaders, this way you can be assured there's a good chance your download will finish. The more the better.

Now that you should understand how torrent files work, and how to use them, on to Azureus!

First, get JAVA! You need this to run Azureus, as java is what powers it. Get Java here: /http://java.sun.com/j2se/1.4.2/download.html

Next, get Azureus at: /http://azureus.sourceforge.net

Next, get the Suprnovalister plugin from /http://s93732957.onlinehome.us/storage/suprnovalister.jar

Install Java JRE before you do ANYTHING.

Install Azureus, and then in the installation folder, create 2 more folders. ./Plugins/suprnovalister (For example, if you installed Azureus to C:\PROGRAM FILES\AZUREUS, create C:\PROGRAM FILES\AZUREUS\PLUGINS\SUPRNOVALISTER). Next, put the suprnovalister.jar file that you downloaded, in that folder.

Load up Azureus, and if you want, go through the settings and personalize it.

The tab labeled "My Torrents" is the section of Azureus you need the most often. That lists all your transfers, uploads and downloads. It shows every bit of information you could possibly want to know about torrents you download.

In the menu bar, go to View > Plugins > Suprnova Lister. This will open up a new tab in Azureus. Click on "Update Mirror". This will get a mirror site of suprnova.org containing all current torrent files available. Once a mirror is grabbed, choose a category from the drop-down box to the left and click "Update". Wah-lah, all the available downloads appear in the main chart above. Just double click a download you want, and bang its starting to download. Open the "My Torrents" tab again to view and make sure your download started.

After your download has finished, be nice, and leave the torrent transferring. So people can get pieces of the file from you, just as you got pieces from other people.

Alternatively, if you don't want to use the plugin... you can just head to suprnova.org and download files to any folder. Then go to File > Open > .torrent File in Azureus.

This should about wrap it up for the Bit Torrent Tutorial. If you guys think of anything I should add, or whatnot, just let me know and I'll check into it.