The short answer: fault tolerance....
The long answer:
First thing to know is that empty discs (no matter of they are CD, DVD, etc) are
made for a specific optimal burning speed, even if to supports multi-speeds. To know this recommended speed, you need to do some advanced testing with
specialized software (example in link), or find some reference tables for a particular brand and type of disc.
Second, a rule of thumb is that
the faster you burn the more errors you WILL burn to the disc. Now, these errors are not visible or audible, you most likely will not notice them when you play the disc. This is because each optical disc player has a build-in error-correction meganism. This is even true for normal CDs and your average CD player. Of course, this error-correction meganism can be more or less advanced depending on brand and type of your device.
The goal of burning is to write a discs with as less errors as possible. It speaks for its own that the less errors you burn on the disc the better. Aka: the less errors which need to be corrected, the better. But there will always be at least some errors.
So, even if your burner can write discs at an extremely fast speed, it does not mean you will burn a perfect disc, even if it can be played without any (visible/audible) errors. Buying a burner because it can write faster than others is not a good thing.
But that isn't the full story, as the above is just a rule of thumb...
Third, the optimum speed, or rather the best burned disc, also
depends on the combination burner / disc. Some combinations of disc brands and burner brands will do better than others. Thus this optimum speed will be different for each burner / disc combination. To make it even further complicated, it also depends on the dye used to make the discs. Thus even when you stick within the same brand of discs, the dye used can make a big difference in burning errors.
eg: Your friend can burn good quality discs using brand A discs with his burner, and thus recommends these discs to you. But it might be very well possible that you wont get the same results with these discs. This has to do with the brand of your burner, which is different than you friend's. It might be possible that the combination of your burner and the brand A discs is actually not that good.
To find the best combination can be very tricky and it is more trial-and-error than science. In PC magazines you often see tables with best combinations. These tests are again done with specialized software. These tables can be used as a guideline. Or look up the compatibility with different disc brands on your manufacture's
website (example in link).
To know more about this I suggest to do a google search on CD/DVD speeds and error-correction, etc. And/or read some articles with this topic in many PC magazines.
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So, to have the best burn quality:
- As a rule of thumb: never burn at a higher speed than required for the media. At least not that much higher. eg: don't burn at x52, it will be very bad quality for sure, burn at x4 or x8 (goes both for CDs and DVDs imo). True, the burn process will take less time if you burn faster, but you will burn a lot more errors. And in the end you will gain more time with burning just 1 good disc, than burning 3 discs of which you can throw away the first two... And you will have less chance of a buffer underrun when you burn at lower speeds....
- Burn at the
optimal speed recommended by the manufacture of a certain type of disc. As a small rule of thumb: the optimum speed for a certain disc type is sometimes written in small print on the label or leaflet in the discbox. Sometimes, it is also good to use the biggest printed speed on the boxlabel, but only if that isn't the highest speed available for that type and brand of disc.
eg:
4x8x16x => 16x is printed the biggest, but that's just commercial eyecatching stuff.
eg:
4x8x16x => Good indication that 8x is the optimal speed for these discs.
- To have the real optimal speed you need to do a lot of testing with specialized software and
test for C1, C2, and CU errors. The speed at which you have the least number of errors is the real optimum speed for that specific burner / disc combination.
Note that it might be well possible that with your burner / disc combination you could burn better quality CDs at 16x speed than your friend who is using another brand of burner and discs and burns at 4x. But you can only know this when you do this extensive testing, in other cases use the other guidelines written above.
In very short: Burning at the highest speed is almost always NOT good.
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