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From: Jonathan Kirwan
Subject: Re: Basic Stamp vs Pic processors
References: <firstname.lastname@example.org> <email@example.com> <3DA184E7.B1F429F3@webaccess.net> <3DA22778.FD66989F@webaccess.net> <3DA2624E.31F292A4@webaccess.net> <3DA2CF48.457919F3@webaccess.net> <3DA37479.2A21C819@webaccess.net>
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NNTP-Posting-Date: Wed, 09 Oct 2002 14:52:03 GMT
Organization: AT&T Broadband
Date: Wed, 09 Oct 2002 14:52:03 GMT
On Wed, 09 Oct 2002 00:12:14 GMT, Chuck Simmons wrote:
>It's getting to be a mixed bag. I have several different optical units
>in the lab and they are all different. Some use the same objective for
>CD and DVD and some don't (it is a little tricky in the single objectice
>types to get both NAs right). The two different focus schemes
>(astigmatic and knife edge) show up in both. The sensors have gone nuts
>with up to 10 photo diodes for each of CD and DVD. Even laser power
>monitor diodes differ. I see some used photovoltaic and some used
>biased. There are lots of different optical configurations for CD and
I'd just assumed (didn't have reason to think otherwise) that DVD focusing requirements would be the same. I don't know how the density is achieved in DVD, but figured that the light wavelength was shorter for one thing. Still, the optical methodology would still be similar, I thought.
Are you saying from above that astigmatics are still used in DVD focusing? Yes? Or no?
>Dave was asking how the analog signals from the detectors are handled.
I was just mystified a bit about what benefits were expected from digitizing and real-time processing signals instead of some kind of modified analog closed-loop control. It just never crossed my mind that there could be enough value in all that extra work needed to digitize and process in the control loop.
>CD used analog servos at one time but I think that all are digital now.
>The servo error is converted from analog to digital and necessary signal
>processing is done then the result is converted back to analog for
>actuator power drivers. The data signal from the sensors also has some
>analog processing before it is finally obtained as channel bits.
I still wish I knew what the problems were with the analog scheme using astigmatics to focus. With the low sales price of CD units, it just doesn't seem reasonable to add cost unless there is a big benefit somewhere. DVD isn't all that much more either, these days.
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