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From: Chuck Simmons
Organization: You jest.
X-Mailer: Mozilla 4.61 [en] (X11; U; Linux 2.0.33 i586)
Subject: Re: Basic Stamp vs Pic processors
References: <email@example.com> <firstname.lastname@example.org> <3DA184E7.B1F429F3@webaccess.net> <3DA22778.FD66989F@webaccess.net> <3DA2624E.31F292A4@webaccess.net> <3DA2CF48.457919F3@webaccess.net> <3DA37479.2A21C819@webaccess.net>
Date: Wed, 09 Oct 2002 15:38:37 GMT
NNTP-Posting-Date: Wed, 09 Oct 2002 08:38:37 PDT
Jonathan Kirwan wrote:
> 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.
Yep. They are very similar.
The density improvement is mostly wavelength. The track pitch and
minimum pit length are half of CD. The DVD NA is 0.65 which helps
because CD is something like 0.45. The NA along with wavelength
determines spot size. Blue is on the way. That is going to be
> Are you saying from above that astigmatics are still used in DVD focusing? Yes? Or no?
Yep. Knife edge is also common.
> >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.
Cost. Once you decide to pull most of your analog process into a single
chip, you see that digital processing has the advantage of controlling
offsets and allowing you to change "components" by typing a few lines.
It is an evelution driven by what is easiest to do in silicon and what
offers the easiest development and support environment.
> >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.
There is no problem with astigmatic focus that I can see. The Newer
optical layouts are trying to reduce optical component count. This has
led to folding the beam back on itself and using a hologram to sort
things out for the sensors. This led to knife edge focus. I can't tell
any important difference in the electrical sensor signals between knife
edge and astigmatic focus. I use exactly the same focus software for
... The times have been,
That, when the brains were out,
the man would die. ... Macbeth
Chuck Simmons email@example.com
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