From owner-freebsd-current Tue Aug 31 14:39:34 1999 Delivered-To: freebsd-current@freebsd.org Received: from alcanet.com.au (border.alcanet.com.au [203.62.196.10]) by hub.freebsd.org (Postfix) with SMTP id 9C9D114CDF for ; Tue, 31 Aug 1999 14:39:20 -0700 (PDT) (envelope-from jeremyp@gsmx07.alcatel.com.au) Received: by border.alcanet.com.au id <40326>; Wed, 1 Sep 1999 07:35:59 +1000 Date: Wed, 1 Sep 1999 07:37:29 +1000 From: Peter Jeremy Subject: Re: HEADS UP! ATA driver (atapi DMA).. In-reply-to: To: dscheidt@tumbolia.com Cc: freebsd-current@FreeBSD.ORG Message-Id: <99Sep1.073559est.40326@border.alcanet.com.au> Sender: owner-freebsd-current@FreeBSD.ORG Precedence: bulk X-Loop: FreeBSD.ORG David Scheidt wrote: >Almost all fast (>12X or so) CD-ROM drives are variable speed. To put this a different way, the data on CD's is stored at a constant lineal density (closely related to the wavelength of the laser). Audio CDs are read using constant-linear-velocity, the angular (rotational) speed reduces from the inside edge to the outside edge, to maintain a constant data rate. CD-ROM drives initially just copied this approach. Almost all fast CD-ROM drives now use constant-angular-velocity (which simplifies the rotational mechanics and should improve seek times, at the expense of slightly more complex read logic). This means the the lineal velocity (and hence data rate) increases from the inside (start) to the outside (end) of the disk. The quoted CD-ROM speeds are always based on the peak data rate (and the better manufacturers actually quote the speed range). Peter To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-current" in the body of the message