What keeps the recording laser 'LOCKED' on track?

Hello all,

I am very curious about what keeps the recording laser locked onto the right position.

I know that there is a very shallow groove on every blank cdr, called the pregroove, and in this pregroove there is a 22khz signal called the ATIP wobble.

Now i’m wandering what keeps the recording laser in its correct position, is it the pregroove itself, which stops the laser beam loosing tracking, or is it the signal inside the pregroove which controls where a laser burns to?

lets pretend I made a cdr drive modification which would allow you to make your own ATIP signal and inject it into the drive, in other words ignore the ATIP signal that is already on the blank cdr and use your own custom ATIP signal instead. Would I then be able to record physicly anywhere onto the disc, even outside the pregroove?

Or would that just mean that I can burn anywhere onto the disc, as long as it’s still in the pregroove?

Well, sorry for the probably false information following. Those are just what I understand of the process, and you shouldnt take this for reliable information.

Anyway, I think there’s a (actually, several, nested) control system (I mean, some closed loop control) dealing with this. So basically, the head follows the pregroove. If you send a fake ATIP signal to the drive unit, this control system may get fooled and think the head is still on track. But then, you have near no information on where the head actually is, or you have to bother for it by yourself.
This control system’s role is to have the burner always write on the pregroove. If it thinks the head has lost track, it will raise a burning failure signal. So your mod system must manage to fool this control system… and basically, I dont know if the area outside the pregroove can physically be burnt…

On a normal CD, or when a burner reads, two systems can come into play:

  • the lateral sensors & associated control system, which would ‘see’ the wobble OR the datatrack itself in case of a big offset of the head relative to the datatrack, and/or when jumping from one radial position to another
  • the central system & associated control system (central relative to the head not meaning main system), responsible for beam focalization and angular speed adjustment. It keeps the beam focused but in normal conditions it may help to stay on track. Anyhow, my guess is that there’s some PLL system recovering the clock : the pits are indeed organised as multiples of a spatial period, which makes a given temporal period when read at a given angular speed. The reading unit has to have a correct angular speed to see correct pit lengths.

Remember that all I said is just my small theory about how this works, and that I cannot assure you any piece of that is true. If anybody has precise facts, I’d be really pleased to see my thought corrected :slight_smile:

fabarbuck, that sounds logical to me. If you try to ‘fool’ the drive and have it burn data somewhere other than the groove…well, then only a ‘fooled’ drive would maybe read it. The entire surface of the blank CD-R is coated with the same chemicals that CAN be burned, but not burning in the groove would probably never work.

I think the pregroove is frequency modulated. Meaning that it is based on 22KHz ± 1KHz (sometimes it is 21 sometimes its 23 etc…) . Kinda like FM radio I guess. In this frequency changing of the groove is where the actual ATIP information is encoded. In otherwords, a blank CD-R does not have ANY pits on it yet. The rate of change in the groove frequency decodes into the data (ATIP). The drive changes it’s rotation speed constantly so it can maintain the proper frequency.

@ Richman
Since the ATIP groove is only used for writing the CD, I do not believe that you would need a chipped drive to actually read the CD written by a chipped writer. The non-chipped drive should just follow the burnt data “groove” and not know the difference as to whether it is written along the ATIP groove or not. That is assuming that the data is still written normally, just that the data is outside the pregroove.

@ Lillster
One problem with not writing in the pre-groove is that you still need a starting place. One idea would be to start the writing in the same location as the pre-groove, and then let the frequency change to pull the writing head out of the pre-groove once the writing process begins. An alternative idea is to delay the start of writing by 1/2 of a rotation. I.e. a normal writer would begin writing at the pregroove, therefore, the chipped writer should delay the start of writing for 1/2 rotation after it would normally begin writing, then follow the same frequency as the ATIP. In this manner you would, in theory, be writing between the grooves, although I believe in practice the laser or the groove is wider than the space between the grooves so there’s some overlap of data into the groove.

Hmm… Thanks for your opinions.

I have another question though, if I was to burn a disc with a chipped writter, would the cd player see it as a blank disc (since there is no data directly on the ATIP) or would it follow the burnt track.

btw the datatrack/burnt track would only be slightly off the pregroove, so that the ATIP can’t be read.

Thanks again.

I think a drive able to read a normal CDR would read this special CDR just normally… at least, I hope so :wink:

FIY, this is all about the psx protection. At the moment, I’m only gathering information and preparing my test setup, but hopefully, when time will be more available here, I should get some results and post them here and there.

It would be very precious to hear from a CD reader expert about how those control systems are designed. I’m preparing two identical drives for tests but it’d be great to have some more hints about read modes, from the electronical point of view. At the moment, I can only guess and think to tests able to prove things.

Yep, it’s true.

I’m also preparing a nice website for all of this, but it won’t be available for quite awhile yet.

At the moment I need to:

  1. Design the website (almost finished).

  2. Gather information, make it presentable and then add it to the website.

  3. find a nice host (e-mail me ‘thelillster@gmail.com’ if you don’t mind hosting the site for us. - Very much appreciate that.

  4. Possibly install a CMS (content management system) so that news and updates to the site can be made fast and easy. - Everything is HTML at the moment.

Thanks again.

Tracking is obtained by the pregroove only. A simple explanation is given
at http://home.mira.net/~gnb/mac-cdis/cd5.html, for real-life use you
can read http://www.semiconductors.philips.com/acrobat/datasheets/TZA1038HW_1.pdf

No, you would not be able to write anywhere at all. Things are more complicated than that.

Tracking is obtained by the pregroove only.

Ok, that’s when writing. The datasheet is interesting, thanks !

No, you would not be able to write anywhere at all. Things are more complicated than that.

You mean… more complicated than what? And why it wouldnt work ?

Suppose you insert your mod system inside the control loop, by interceipting the tracking diodes signals, and generate your own, according to what you want to do. Then I guess the control unit would get fooled and pilot the focus, radial and angular speed according to what it receives, that is, exactly what you fed it with.
That make me think one has to study a burner’s tracking servo control, to have its focus almost unchanged, but angular and radial speed modified. I’m not saying the modchip would have an easy task to adapt those signal, but that it isnt totally doomed to try to fool the control system.

Suppose then that you have tested enough to know how many radial offset and angular stretch gives you another response from the psx reader than '1’s, then you’d be able to switch between diodes signal interceiption/modification and through pass, to achieve '0’s and '1’s. If the psx track following system only considers the datatrack itself, it shall be okay, right?

Then… well, I suppose it’d be almost done.

Tell me what you think, I saw you got really involved in this thing and I’m pretty aware I may have forgotten some details, some bad results from former tests, and I would really appreciate feedback from people who have constructive remarks to add :slight_smile:

I’m just wandering, and this is only theory, lets pretend I burned over the entire disc, including inside and outside the pregroove. Im not saying to have a track inside and outside the track. What i’m trying to say is what if the whole of the material on the disc was burned, would the cd-r reader still pickup the ATIP signal, even though it’s been burned over.

-Sorry if that didn’t make sense.

I guess no… but it wouldnt get anything… actually, that’s not that simple.
Have a look at the datasheet. The focus/tracking diodes generate signals which in turn serve to generate differential ones. So everything the controller sees is based upon those diodes signals and their differential interpretation (note that for each purpose, it uses different differential interpretations…).

In most drives the same diodes are used for tracking, focus, wobble and data,
so you will not be able to just change tracking without messing up the others.
For instance, the laser will only start if an expected ATIP address is decoded,
so if you want to burn anything you will also need to encode ATIP into whatever
diodes signal you want to send. There are many overlapping hardware and software
control loops in a drive, so I wish you best of luck to play with them.

In fact I have not been involved in this psx thing, I got bored very quickly.
I spend enough time with drives during the day to start modding things at
home the evening. Actually I think que tu perds ton temps et que tu vas juste reussir
a niquer ton drive :slight_smile:

In most drives the same diodes are used for tracking, focus, wobble and data

yeah, that’s what I saw. :slight_smile:

There are many overlapping hardware and software
control loops in a drive, so I wish you best of luck to play with them.

I will surely need it, many thanks :smiley:

In fact I have not been involved in this psx thing, I got bored very quickly.
I spend enough time with drives during the day to start modding things at
home the evening.

Ok… I read the old threads about this and saw you posted there. I guess you work in this field, then? great, I might have some questions to ask later :wink:

Actually I think que tu perds ton temps et que tu vas juste reussir
a niquer ton drive

Yeah thank for the advice, et pas de problème :wink: Actually, I have two old identical drives which have nothing else to do than being destroyed, so everything’s alright :smiley:

To come back to our (my? :wink: problem, that wont be that easy to characterize everything I need to know, but IMO it deserves to be tried, so… I’m ready to waste some time practicing my electronics skills.

Comme dirait l’autre, en route pour la joie ;D