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To: Synapsid who wrote (5374)	3/21/1998 12:41:00 PM
From: Grand Poobah	Respond to of 8193

<<I'm still scratching my head about the use of PRML in a DVD drive. CD-ROM drives don't use PRML. I can imagine PRML would allow a higher data density, but the DVD standard obviously doesn't use/require PRML. Maybe the advantage is higher DVD speeds with more margin for error in optical detection technology (i.e. easier/cheaper to manufacture in volume)?>> I don't understand the application of PRML to DVD, either. PRML is used in magnetic media. Before PRML, they used peak detection, which required the bits to be separated far enough on the media that the magnetic fields from each bit did not interfere significantly with the next bit. There was a distinguishable peak from each bit. PRML (Partial Response, Maximum Likelihood) allowed them to be packed closer together such that each peak detected by the head only contained part of a bit (partial response) and then incorporated a detector to determine which sequence of bits most likely corresponded to the the data (maximum likelihood). This all depended on the superposition of magnetic fields from neighboring bits on the media. A CD-ROM can't use PRML (to the best of my knowledge) because it isn't relying on reading fields off the media but rather the reflectance of the media from a laser. I was under the impression that DVD used a similar read/write scheme as a CD-ROM. Does anyone know the details of how the data is written to/read from the media? If it also uses a laser I do not see how PRML technology could be applied. G.P.