by cfitz on Mon Dec 16, 2002 2:52 am
BuddhaTB asked me a question about the differences between mode 1 discs and mode 2 discs, and if there is any advantage to choosing one versus the other when burning a data disc full of computer files.
I thought others might be interested also, so I am sharing my answer here.
The basic red-book CD-ROM standard for audio CDs divides a CD into logical sectors that each contain 2352 bytes (the actual raw sectors contain additional bytes for error detection and correction and control). The yellow-book standard is an outgrowth of the red-book standard, and provides a standard format for storing computer data on a CD, otherwise known as a CD-ROM. The yellow-book standard defines two modes for storing data, named unimaginatively but practically, mode 1 and mode 2. Both start with the original red-book logical sector size of 2352 bytes. Mode 1 divides those 2352 bytes into 12 synchronization bytes, 4 header bytes, 2048 bytes of user data, and 288 bytes of EDC (error detection code) and ECC (error correcting code). Mode 2 divides the 2352 bytes into 12 synchronization bytes, 4 header bytes, and 2336 bytes of user data.
Mode 1 advantage: more error detection and correction capability built in, so there is less chance of data being corrupted
Mode 2 advantage: more bytes devoted to user data, so you can record more bytes on a disc
Mode 1 is typically used for storing computer data files because even a single bit error can be fatal and the extra ECC information of mode 1 gives extra assurance that no bits are corrupted. Mode 2 is typically used for applications such as video discs and the like because these applications need to store lots of data, and an error here or there isn't very detrimental. It just manifests itself as a bit of snow on the monitor or a small glitch in the soundtrack.
But wait, you exclaim! Why do burning programs like Nero let you choose to burn a computer file data disc in mode 2? Isn't that dangerous if it doesn't include all the ECC information? Well, remember that there is already a fair amount of error correction included in the CD lower-level formats. But, more importantly, mode 2 discs are typically used in conjunction with the XA (extended architecture) format. Nero does just that. If you look at the Nero menu, you will see it doesn't allow you to select just plain mode 2. Instead, you can chose between mode 1 and mode 2 XA.
So, what is XA and why is it important? It is a format that allows mixing of computer data and video/audio data on the same disc. The XA format itself has two forms, 1 and 2 (not to be confused with modes 1 and 2, although, as you will see, there are plenty of reasons to confuse them). Form 1 is used for computer data discs, and it puts those 288 ECC bytes back in for the extra protection, leaving just 2048 bytes for user data. Thus, in terms of both available user data space and ECC capability, it ends up being functionally the same as mode 1 (even though it is really mode 2 XA form 1). Form 2 is used for video and the like, and it leaves 2324 bytes of the mode 2 sector open for user data. So mode 2 XA form 2 ends up being virtually equivalent to just plain mode 2. Confused yet? I am. And there is a lot more to it all too, but this covers the main points.
What does this all mean? In practical terms it doesn't matter whether you choose mode 1 or mode 2 XA when burning a computer data disc. If you do choose mode 2 XA for a computer data disc, your burning software will use form 1, so you will get the equivalent capacity and ECC as if you had chosen mode 1. So, you can freely choose either and not worry about it. (I personally still pick mode 1 for data discs, even though it doesn't matter, just because that is the mode that was specifically and originally designed for data discs - I guess I am just satisfying my sense of order).
If you choose to burn a VCD or SVCD, your burning software will force you to use mode 2 XA, since it is required by the VCD/SVCD standards. Your burning software will put the computer data files that describe the disc in a form 1 section, and the video data itself in a form 2 section.
cfitz
I'm printing your post at this time ...
But I do appreciate the kind words. Thank you and you're welcome to ryus, BuddhaTB and Action Jackson too. 
