Par2 Files Explained in Plain English

PAR2 files contain error-correction data that can be used to fix files that become corrupted during the upload/download process on Usenet where there is no built-in error correction. PAR2 files work on both Windows and Mac, and can be used to protect any kind of file.

PAR2 files don’t have a separate life of their own. You don’t decompress them like you do RAR files. PAR’s only exist to fix other files. Sometimes in a newsgroup you will see a set of PAR’s posted with no other files. The poster did that as a favor to somebody who needed to fix a file that he had downloaded previously. So, you don’t need to download PAR files unless you need them to fix a damaged file in your possession. Of course, it is not always obvious if a file is damaged. For example, there could be small flaws in a video that you can’t find unless you watch the whole thing very closely. It is not a bad idea to download the PAR’s to make sure that your files are OK.

PAR2 files are able to magically fix, and even re-create missing files. Suppose you download 9 of the 10 RAR’s from a binary newsgroup that contain a video file, and you can’t find the 10th. If the poster included some PAR files, you can download them and actually re-create the missing file! If the poster did not include any PARs, as newbie posters often do, you could post a message in the group and ask somebody who was able to get all the files to make, and post some PARs for you. Anybody can make PARs as long as they have a good copy of the files in question.

There are two generations of PAR technology in use: PAR and PAR2. PAR filenames look like this:

somefile.p01
somefile.p02
somefile.p03

PAR2 filenames look like this:

somefile.par2
somefile.vol000+01.PAR2
somefile.vol001+02.PAR2
somefile.vol007+05.PAR2

To use PAR files, open the first PAR or PAR2 with the free PAR decoder in SuperNZB. SuperNZB will use the PARs to examine the target files (RAR files, AVI files, etc.) and fix them if needed.

Both PAR and PAR2 do the same thing, but PAR2 is much better. Look at the PAR2 filenames above. The ‘+01′, ‘+02′, and ‘+05′ indicate how many ‘blocks’ each file has, and that can save you download time. To do that, you would first download the associated files whether they are RARs, AVIs, etc. Then you would download the file that ends with ‘.par2.’ Run that file through your un-PARing program, and it will tell you how many more blocks you need, if any. If you needed seven blocks, then you could download the second and third PAR files. If you only needed two blocks, then you could just download the second file.

PAR files cannot contain viruses, so there is no risk in using them.

PAR uses the Reed-Solomon algorithm. Dr. Dobb’s Journal has a good technical explanation here.

21 Responses to “Par2 Files Explained in Plain English”

  1. Lonnie Cross says:

    I have two problems with SuperNZB one that makes the program almost useless:

    At the end of a particular download the DL stops due to a message that informs me the par2 file is not in my downloads folder. Nonsense! Every time I check the file is there.

    1. I setup files for overnight downloading, yet I only get one particular set of files because of this error, when I should get ten or twelve.

    During the day I have to check every half hour or so to see if my download has stopped.

    Is there anyway I can turn the Par Manager off?

    2. When I add a new set of files that I want immediately it is impossible without highlighting the files and dragging them to the top of the queue. Depending on how many files are in the queue this is a long, slow process.

    Toast Titanium has had the “Send to Top or Bottom” feature for at least a half dozen generations. Do you plan to implement this feature anytime soon?

    Thanks for your feedback,

    Lonnie

  2. admin says:

    You can stop SuperNZB from starting the PAR process by going onto the “File” menu, then to “Open Par Job Manager”, and leaving that window open. You don’t have to setup anything; just let it sit there. As long as it is open, SuperNZB understands that you don’t want it to do its default PAR processing.

    The Par program needs to be in the same folder as the SuperNZB program, not the downloads folder. However, even if it is present, there are other things that could prevent the operating system from launching the app. We can look into that further if you want, though the solution above will prevent that error message.

    A “Send to top or Bottom” feature is a good idea. I have added it to the list of possible new features for the next version.

    Matt

  3. adb0rg says:

    I use pars and of course I love them. Even broken PAR2 can be still usable! One thing I don’t understand to this day and can’t seem to find any explanation is as to how PAR2 works exactly.
    The way I look at PAR2 and RAR is like this:
    rar = a wall. Now if that wall has a hole, then
    par = a brick that can patch any hole in the wall, but…
    How the hell that same brick can fit ANY hole!? That’s the real magic here!
    How the hell one block can retain information for the entire set of rars!???

    Let’s use the smallest unit.

    Let’s say I’ve got a set of archive that is 100 rars long with 10% of pars.
    100 users download the same set off the same source, and each of them has an error in a different rar.
    Now just ONE rar is broken and requires only ONE block of par in order to fix it.
    Here is what baffles me. Each of the 100 users can download just 1 block off the entire par2 set (that just 1% of the whole rar set information) and that 1 magical block will fix any damage in a different spot in each of the 100 pars!!!!! ( as long as it’s just 1 block of course)
    they don’t even need to download the whole set, which means par set is not some kind of compressed image of the rar set!
    HOW IS THAT POSSIBLE!?… MAGIC!!!?

    • Glenn says:

      Parity files work the same way that parity memory and raid-5 work. They use AND and OR functions on the complete data to create parity. For example if two bits being compared are both 1′s, then the AND result is 0 (even) and the or result is 1 (odd). So if you only have one of the compared bits and it is a 1, you can use the AND parity to determine that the missing bit, plus the bit you have resulted in even. Therefore, the missing bit is also 1. Similarly with the OR parity. More PAR files means more comparisons, more redundancy, greater ability to extrapolate missing bits.

    • James says:

      Glenn is correct. The only thing I would add is that nor only does PAR help determine what missing properties sould be (either another one or zero added) but it also helps to determine where and at what level in the ‘data layer’ the property must be inserted. Remember that data occurs in digital streams and in layers in the case of CD/DVD’s in data layering. And sometimes if the data cannot properly attach additional software must be used to analyze and ensure proper placement of the digital signal. If you have ever had the privalege to use electronic equipment to visualize the different levels of digital sound replication it is very interesting indeed.

  4. admin says:

    adb0rg,

    It sure does look like magic, however the Reed–Solomon algorithm has been around for a long time. I haven’t studied it myself, but it looks pretty complicated on the Wikipedia page. If you come across a simple explanation please let me know.

    Matt

    • James says:

      Matt, the Reed-Solomon algorithm is only part of the digital sound equation on the hard science side. There are other elements to consider in detail. But to make it easy for most to understand I offer the following wiki link that puts it nicely in a small article complete with an interesting diagram of sound from source to digital recording at;
      http://en.wikipedia.org/wiki/Digital_audio

      Cheers

      James

  5. Brian says:

    To continue your wall analogy, let’s say that you have 10 different looking bricks in your wall. For the PAR files you might take some extra bricks and cut them up into 8 even pieces. If you put a few pieces in one sack, that would be a 1 block PAR file. It may or may not have the pieces to fix your damaged brick. If it does, great. If not, you get a “2 block” sack that has more pieces and a different mix. Now maybe between the two sacks you can fix your brick. Lather, rinse and repeat as necessary. There is a finite number of different pieces and through external data (shape, pattern, etc.) you can figure out which piece or pieces are correct to fix your wall.
    Does that help at all?

  6. ken says:

    i have downloaded a 12gb file through sabnzbd with a couple hundred blocks missing and it won’t unpack, how can i correct this error through quickpar? help please

    • admin says:

      Ken,

      You will probably need to download more blocks. Load your NZB into SuperNZB:

      http://www.techsono.com/supernzb/index.html

      …and click the “Check” button. That will show you which files are available on your Usenet server. Compare that to the list of damaged/missing files that you see in QuickPAR, and if they are available, download/re-download them.

      Matt

  7. KansasCitySmitty says:

    Thanx for your excellent “plain English” post. Some of us geezers want to use this technology but haven’t a clue how to deal with an endless array of files, formats and protocols. And most explanations, though well-intended, are posted by very experienced and well-informed individuals who likely don’t even remember when they too were “clueless”. Thanx again.

  8. Brad says:

    Up until this year Par files had always worked. Then I got a new computer with Windows 7 64 bit and downloaded QuickPar. I have yet to get it to work.

    All the Par2 files are there in the DL folder but if I am missing one rar file in a sequence after running Quickpar it says I am missing a certain number of files but will not extract them from the available Par files in the same folder like it used to. I’m not talking about the 1st file either. That is there.

    Is there something different in the newer version that keeps it from automatically searching for and finding the necessary Pars? Is there a preference I am not aware of?

    • admin says:

      Brad,

      I’m not aware of any differences. However, Quickpar will sometimes fail to locate files if there are funny characters in the filename. If Quickpar doesn’t list all of your files, try using the “Add” button to add them manually.

      If you are missing a lot of RAR files, then your Usenet provider might be having a “completion” problem.

      Admin

  9. RJM_2013 says:

    Came here to get an explanation ‘cos it’s pretty amazing stuff… I have used this in the past to fix mp3 files that had segments missing on the server, but today I wanted a file that couldn’t download at all; I suspect because first segment was missing/corrupted. So, didn’t even have a filename to work with. Downloaded everything else ( mp3 and par files), ran Quickpar 0.9.1, and voila – 9.6 MB created from thin air!! God bless the bit-crunchers…

  10. Anthony G. says:

    I’ve been using the newsgroups for about 16 years. There was a day when you had to assemble downloads manually. QuickPar works great just in an instance where the source files (ex: source.001, source.002) are all complete but Quick Par won’t run because of missing PAR2 files to create the entire re-assemble of the original source file.

    My question is this: I have all the source files, is there a way to manually assemble the 001, 002, etc to its original file name without the PAR2 files?

    Anthony G. – I can be contacted at revup67 at yahoooooooooo . calm – thanks much

  11. mixmaster says:

    when joining of pars begin I get a promt to go to the snzb folder and unpar.text. what is this

    • admin says:

      Sometimes the operating system locks the unpar.txt log file and won’t allow SuperNZB to update it. So, see if you can delete the file manually. SuperNZB will recreate it.

  12. Scott Baker says:

    Are you sure it’s not magic?

    Specifically, is it not quite likely that Satan created par2 files and that this so-called “Reed–Solomon algorithm” is just a PC euphemism for “Demonic Math”?

    I say this because, if a set of files 1/10 the size of the entire file it fixes can replace ANY 1/10 of the original file, filling in data for images, video, audio, programming language, then why can’t that 1/10 of the original file simply be “unpacked” and recreate the entire file?

    The only logical answer is that these fellows, Reed & Solomon or whomever fashioned this piece of deep dark evil code must have sacrificed a small child, likely a baptized Christian child, free from all sin, to the Dark One.

    I can think of no other explanation that makes sense.

  13. Nigel Mackay says:

    If I managed to get all the parts of the file, albeit some damaged/incomplete, and a whole set of par2 files EXCEPT the first one, is there any way I can do a repair? The first par2 file on the server is empty. I have Qickpar.

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