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Why are there different offsets for CD Drives, what's does the read and write offsets mean? How can I know the offset for my CD/DVD writer?

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2 Answers 2

CD-audio works like this

Route 66

You drive along Route 66 in your 1985 Chevy, your best friend in the passenger seat points her Leica M1 through a hole she cut in the floor and takes photos of the centre lines. She gets 98 photos on a roll of film. She tosses the exposed roll of film on the back seat and starts another.

Meanwhile your mothers second cousin also drives down Route 66 in a Pontiac, her friend pokes a hasselblad through a hole in the floor and takes photos of the centre lines, he also gets 98 shots a roll.

Both you and your mother's second cousin have a fine photographic record of the exact same road. But due to variations between Chevrolet and Pontiac (etc), the first roll of film each took may start at a slightly different spot on the road. Most of the roll is the same but there is an offset. The offset is only a few milliseconds of driving so doesn't really matter.

You can work out the offset by staring carefully at the two sets of photos and comparing them. There's probably some road kill that appears in your 5th photo and their 56th, so your relative offset is 51.

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I think that explains how offsets work, but why don't they just make all cd drives with same offset? and why they don't mention their offset in spec? thanks. –  Kappri Feb 15 '12 at 13:17
    
@Kappri: I guess because 99.9999% of listeners don't care if Barry Manilow starts to sing 1353 ms after they pressed "play" or 1352 ms after they pressed "play". The Philistines! –  RedGrittyBrick Feb 15 '12 at 13:59

From AccurateRip's site.

Each CD drive reads audio discs slightly out (a number of samples), if your CD drive supports 'Accurate Stream' it will be a constant value, this value tends to be the same for each particular make and model of CD Drive. A small number of drives have [Purged] as the offset, these drives were found not to have a constant drive offset (perhaps different manufacturing batches, or firmwares), as such they have been removed from AccurateRip's drive database (should you have one of these drives, 3 matching key disks will be required to configure AccurateRip).

The page contains a list of known Drive's in AccurateRip's database and their respective offsets.

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