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question about dd reading of CD

I've purchased a combo CD/DVD reader. I'm checking it. I find a

With my old CD only reader, I was able to check on my CD writing by
using dd to read back a recently burnt CD and comparing the read back
file to the original iso using diff.

But when I use the new reader with dd and using a recent copy of
sarge-netinstall for which I know the true file length of the iso, I
get a file from dd that is 4096 bytes too long.

I've also checked a Knoppix 3.3 CD, and there, also, the readback is
4096 bytes too long.

Is the reason for this error known to anyone on the list?
Is there anything I can adjust in my setup to stop it?
I'm pretty sure its junk at the end of the file because
I can truncate the readback file to the same lenth as the 
original and the they compare equal.

But is it always 4096 bytes?  Or is it sometimes half or twice times

Paul E Condon           

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