Hamburg  - Most customers find themselves overwhelmed by the variety when buying blank CDs or DVDs.

But they can only tell whether they have bought the right one after burning the CD. If it skips when played or if the data stored on it becomes unreadable, then they realize what a bad choice they have made.

Simple CDs and DVDs for burning usually sell for about 20 to 40 cents a piece, in packs of 100. Rewritable discs usually cost about twice that.

"Generic products are usually no worse than brand names," says Michael Schmidt of the Hamburg-based magazine Computer Bild.

But discount products can vary dramatically in quality and some discs in the same package show major differences in quality.

A recent test showed that 41 of 45 blank CDs were ranked "satisfactory" as most discs were prone to scratches. However, few of the discs showed any problems during burning.

Some of the pricier versions offer extras like scratch resistance, which should increase durability. "That kind of special coating can make sense, especially in CDs for the car, since they often lie around," said Schmidt.

A whole slew of specially formatted discs are on sale alongside standard blank discs. But experts advise against those. Audio-CD discs are a waste of money if they're burned in a PC, says Juan Garcia from Nero, a company which specializes in software for burning discs.

Those discs are intended for HiFI CD burners and cost more because of copyright issues. "And just because they're called audio discs, it doesn't mean that the music sounds any better."

Extended play discs are rarely worth the bother. Some CDs now offer 100 minutes of play time, as opposed to the standard 80. But these long-play discs often cause problems for CD players. " As the tracks are pushed closer together, reading errors occur more often," says Garcia.

The equivalent of the DVD is the dual-layer (DL) model which offers extra storage space - 8.5 gigabytes versus the standard 4.7 GB.

"The extra charge is only worth it for extra long films when you don't want to get up in the middle to change discs," says Garcia. Older drives often won't support the DL discs, adds Bernd Schwenke of Stiftung Warentest, a Berlin-based product testing group.

Customers should be wary when purchasing CDs for printing purposes such as Lightscribe discs. They can be laser-etched when burned, but are often susceptible to ultraviolet light rays, says Schmidt. So customers should protect them from sunlight.

Different burning speeds are usually only an issue for DVDs. "With CDs, it has practically become a marketing gag," says Schmidt. "Whether a CD supports 48x or 52x speed usually only means a difference of seconds." (dpa)