04 January 2021, 20:44 | #601 |
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Since the miss list on the first post hasn't been updated since October, can someone list what actually is still missing?
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04 January 2021, 20:57 | #602 | |
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From reading haynor666's first post in the thread; and then looking at the "[not in TOSEC] IPFs - Official" + "[not in TOSEC] IPFs - Unofficial" directories... these can now be crossed off:
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04 January 2021, 21:53 | #603 | |
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Anyone know how Paula precompensation actually work? We're dealing with a CAV drive. The big problem with magnetic surfaces is that in the inner (shorter) tracks of the disk the 'repulsive' force are stronger because polarity changes can be closer. With certain bit patterns the attraction/repulsions of several successive bits combine is such a way that it can result in 'bit shifting' (a 'bit' could moves slightly ahead, or behind, the positions it should occupy on the surface). This is where the possibility of using precompensation on write comes into play. Track 79.1 of this game is a perfect example where you can see the effect described: two close polarity changes (where the interaction and the displacement is very strong) separated by a large distance without polarity change (and therefore almost no interaction with the previous change). Therefore can be detected that the two adjacent 1 are now spaced 4ns+0.5, and the two far 12ns-0.5. My idea is that Paula applies precompensation every other 1 bit. But how? One 1 regular and the other -PRECOMP or 1 as +PRECOMP and the other -PRECOMP? or something still different? Doing some real tests (changing the precomp value in writing) with this peculiar track can help understand? Maybe someone already has the answer or a better idea @Keir: what is the precomp value that gave you the best results? Last edited by ross; 04 January 2021 at 22:19. Reason: @ |
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05 January 2021, 00:09 | #604 | |
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Quote:
0817 - Stoppt den Calippo Fresser (unofficial IPF released not official) 1891 - Dungeons of Avalon II - The Island of Darkness & Tetrix 2287 - Amber 2529 - Icerunner 2535 - Hexuma Demo 2545 - Fears Preview 2758 - Flowers of Crystal 2764 - Ratco DrBong said he had some queued up ready to send to Denis but didn't mention exactly which ones! |
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05 January 2021, 13:05 | #605 | |
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Code:
10[1]00 -> Current bit is written early (-PRECOMP) 00[1]01 -> Current bit is written late (+PRECOMP) The extremely long 0 sequence in the Methane Bros track I've found works well with 250-500ns of precomp (both work, but 500ns is probably best in my tests). Note that the sequence ...00001010000... actually applies precomp to both 1s, and they end up being written 2xPRECOMP closer together. |
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05 January 2021, 14:37 | #606 | |
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I suppose that only a pair of neighboring bits are considered for the sliding windows check between 'current' 1s. In a standard MFM stream there can be three consecutive zero but probably even if distant bits are not considered for the "current bit", is a breeze for the PLL to correct little discrepancies. Different for the big hole in polary changes for Methane Bros track where probably there is not any interaction at all for the 'far' 1's position (and the big PRECOMP make really sense). All these inquiries because I wanted to understand how it was better to use precompensation for an RLL(1,4) encoding where in fact the presence of 4 zeros could make the 1 shift more. Probably it would be better to divide the disk into 3 zones (0~26->0, 27~53->125, 54~79/80->250) instead of the natural (2 or none). Another question: how MFMPREC=0 (i.e. GCR precomp) affect early/late polarity changes? (and is it disabled/ineffective in the case of 2ns cells?) Cheers! |
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05 January 2021, 16:16 | #607 | ||
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Quote:
Quote:
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05 January 2021, 18:31 | #608 |
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Thanks
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07 January 2021, 20:33 | #609 |
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Do you know if it is possible to use TD_RAWWRITE on a real Amiga ( http://amigadev.elowar.com/read/ADCD.../node00F8.html )
to write a long series of 0 bits (violating the MFM timing rules)? In order to create fuzzy bits when reading that area, because of the AGC? I would say yes but I'm not 100% sure... Maybe there are copy programs on Amiga able to reproduce disks that would use this trick? |
07 January 2021, 21:13 | #610 | |
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But I highly doubt you can generate fuzzy bits with a normal floppy head.. if for any reason the polarity changes, I suppose it changes sharply, even if not explicitly requested by the flux. Take this with a grain of salt, I've no hardware to test it. The more experienced however will be able to answer better |
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07 January 2021, 22:46 | #611 |
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If that works, such a "no flux area" should create fuzzy bits (or weak bits, use the name you prefer) because of AGC (Automatic Gain Control):
Read an explanation in the "Magnetic Flux" section at https://diskpreservation.com/dp.php?pg=floppyhow (Fuzzy bits are not created with actually "weak" signal, this would not be reliable) |
07 January 2021, 23:12 | #612 |
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Hi guys, Am I the only one having issues with the ipf of Super Methane Bros?
When I try to install the game with WHDLoad the installation fails reading the last track of disk "B" for a "checksum error" (actually something similar, maybe "no sync" or so, I don't remember exactly what DIC says but it says that the installation has failed). |
08 January 2021, 09:14 | #613 | |
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08 January 2021, 09:16 | #614 | |
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EDIT: It's a shame the installer even bothers to read the protection track. Is that a standard choice on WHDLoad installers? In this case there's no data to read. No protection key. Nothing but a repeating pattern. |
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08 January 2021, 10:54 | #615 | |
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I created an ADF from the IPF by clearing the last track and using it together with the existing slave and the game starts. Therefore only a small update is needed in the WHDLoad game package. |
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08 January 2021, 11:07 | #616 | |
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The case was different for the unformatted tracks, where the data were actually totally random at each different reading. This test however is quite simple to do with a real floppy |
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08 January 2021, 11:10 | #617 |
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08 January 2021, 11:45 | #618 |
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Could a GMod please move all posts discussing disk formats / flux / weak bits etc. to a new thread?
This one is about tracking down / obtaining the last missing .IPF releases. |
08 January 2021, 12:02 | #619 |
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Sorry DamienD I didn't want to start a discussion about copy protections, etc... I was just asking if the ipf of disk "B" of Super Methane Bros was defective in some way and, as far as I understand, it isn't defective but it is the WHDLoad installer that does a check that it should skip...
Thanks again for uploading the ipf files... |
08 January 2021, 12:46 | #620 |
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My post wasn't directed at you jurassicman; just in the way in which this thread has gone off topic and taken a different direction.
Easy to have a new thread for this disk formats / flux / weak bits discussion |
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