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Old 2006-12-18, 19:01   Link #1
zoltan_hellhound
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Does time corupt movie files? (And crc qest)

I was thinking about the longlivety of my personal homemade stuff and downloaded stuff..How long can you expect a file (avi,mpge,mkv etc) to last

Lets say i download/make a video and store it on my Harddrive/dvd
how long will it take before its gets corupt ore messed upp CRC.

And what all this about wrong CRC, and corupted files frome diferent trackers etc
Some torrents dosent match upp whit the original CRC...why is that. and how dose it happen?
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Old 2006-12-18, 19:17   Link #2
emptyeighty
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Your files last forever as long as all the 1s and 0s are left untouched. The only thing that can happen is hardware failure, like a scratched disk or bad RAM in your computer.
If you're paranoid about it use par2 to make patch files. That way you will be able to recover up to a certain percentage of the file should corruption occur.

Quote:
And what all this about wrong CRC, and corupted files frome diferent trackers etc
Some torrents dosent match upp whit the original CRC...why is that. and how dose it happen?
CRC is an easy and quick way to check for corruption. Since torrents are SHA-1 (think CRC on steroids) checked you get exactly what the original torrent creator wanted you to get. If the CRC doesn't match it means that either the CRC itself is wrong or the torrent creator seeded a bad file.
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Old 2006-12-18, 19:27   Link #3
zoltan_hellhound
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is par2 a easy program? and does the patches take upp mutch space

"If the CRC doesn't match it means that either the CRC itself is wrong or the torrent creator seeded a bad file."

How do you mean CRC itself is wrong , you mean that the crc that the makers wright down is wrong?

And how do they gett bad..and how do the crc change
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Old 2006-12-18, 19:44   Link #4
emptyeighty
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Quote:
Originally Posted by zoltan_hellhound View Post
is par2 a easy program? and does the patches take upp mutch space
Quckpar was fairly easy to use iirc.
Quote:
How do you mean CRC itself is wrong , you mean that the crc that the makers wright down it wrong?
Basically yes. Or there was some hardware error on their end.
Quote:
And how do they gett bad..and how do the crc change
CRC is the same when the 1s and 0s are the same. Whenever the 1s and 0s change the CRC changes. That happens most of the time through hardware errors. Sometimes transmission errors. If you aquire the file not through torrents but through IRC bots there is a remote chance of corruption occurring for example.
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Old 2006-12-18, 20:01   Link #5
zoltan_hellhound
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Quote:
Originally Posted by emptyeighty View Post
Quckpar was fairly easy to use iirc.

Basically yes. Or there was some hardware error on their end.

CRC is the same when the 1s and 0s are the same. Whenever the 1s and 0s change the CRC changes. That happens most of the time through hardware errors. Sometimes transmission errors. If you aquire the file not through torrents but through IRC bots there is a remote chance of corruption occurring for example.

Can the crc get bad (file gett corupt) when moved/burned to a dvd
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Old 2006-12-18, 20:31   Link #6
Jinto
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Yes especially when burned to DVD, since those media devices are not known for long lasting data (in terms of decades), it also depends on how often you access the file, the more often you access it, the more likely the hardware will render it errornous. Yet it should be very safe on a hdd that is seldom used or mostly in read operations used. Additionally you can think about a special system that use state/memory scrubbing.

The CRC is only a checksum it rarely occurs that of all files the CRC file gets altered (it is more likely, that the data which the CRC-checksum is based on became altered). CRC and other methods can detect a certain amount of bit errors for sure, but there are limits, if data becomes too errornous it might happen, that bits are changed in a way, that the checksum is correct but the data isn't. There exist bit-error detecting and even bit-error correcting codes (i.e. Hamming codes <= if you are interested in that stuff, wikipedia has a nice article about, Hamming codes are a good entry in the topic for beginners).
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Old 2006-12-19, 04:38   Link #7
zoltan_hellhound
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Quote:
Originally Posted by Jinto Lin View Post
Yes especially when burned to DVD, since those media devices are not known for long lasting data (in terms of decades), it also depends on how often you access the file, the more often you access it, the more likely the hardware will render it errornous. Yet it should be very safe on a hdd that is seldom used or mostly in read operations used. Additionally you can think about a special system that use state/memory scrubbing.

The CRC is only a checksum it rarely occurs that of all files the CRC file gets altered (it is more likely, that the data which the CRC-checksum is based on became altered). CRC and other methods can detect a certain amount of bit errors for sure, but there are limits, if data becomes too errornous it might happen, that bits are changed in a way, that the checksum is correct but the data isn't. There exist bit-error detecting and even bit-error correcting codes (i.e. Hamming codes <= if you are interested in that stuff, wikipedia has a nice article about, Hamming codes are a good entry in the topic for beginners).
Oh. is it in the burn proseces that makes it corupt/defektive, if i burn to a good brand disc an store it safe (changing disc evry 4th year)..would that make a difference,

ore is i time that messes the data upp on the dvds?

i always thought that as long ass the the disc was readable the data was unchanged/safe/coruption free

And i mean data discs..

Last edited by zoltan_hellhound; 2006-12-19 at 05:05.
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Old 2006-12-19, 07:28   Link #8
Jinto
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It can happen in the burn process, thatswhy some burn programs offer an option to check the data that was written. But there are several more things to think about:

1. Using different drives to play back optical media. Each drive is somewhat unique, especially regarding error correction (yes there is error detection and error correction implemented in the DVD sectors - the industry knew there is a need for it because bit errors are more likely). So what works error free on one drive might not work error free on another (Usually the media plays back best on the drive it was created on, provided that the drive did not change over time, i.e. loosing power on the laser diode or dirt and dust related issues).

2. DVD+/-R data layers are very thin, and since they need to be writable (with laser light) they are somewhat damageable to light (reading out a DVD often -with the drives laser- also means that data becoming errornous is more likely)

Imagine a fresh DVD+/-R as zero filled media. The laser burns some 1's between the many zero's (resulting in a bit-sequence). A 1 is basically burining a hole into a thin layer, so light will reflect differently on this position. However if this layer is exposed to intensive light sources over longer time period, it is possible that additional holes are "burned" into this layer, thereby altering the original bitstream. The DVD might still be readable, but contains some biterrors now. A few of the errors can be covered by the error detection/correction methods build in, but if the errors become too many, correction might be impossible.

Real DVD with aluminium layers are pressed with a so called master. The master is basically the negativ of the DVD (in terms of data). By pressing it with high pressure on a thin aluminium foil it will leave an imprint behind. This imprint has nearly the same optical capabilities like the holes in the DVD+/-R media. However such imprint is not damageable to light since it was pressed by mechanical forces not written with light. So a normal DVD should have a much longer lifetime than DVD+/-R media.

3. Copying things often is also not good regarding data consistency. (though lets say a period of 4 years is not often imo, and quite right, because if the period lasts too long, the data might already be altered too much or the devices to read it out behave differently). (There exists a nice analogy in biology regarding copying DNA and its impact on ageing)
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Old 2006-12-19, 08:47   Link #9
zoltan_hellhound
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But that means if you store on a god brand like verbatim and have it in a dark place,And make sure to chek the data after burning,and scan fore degeneration and cheking Crc (every year..in dvdspeed) ...Its a good way to store your files. Ore is ther a better way
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Old 2006-12-19, 10:18   Link #10
Jinto
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Well, that would be indeed kinda safe (however if you encouter data with bad CRC it is already too late. Since CRC uses simple polynomic codes it is not error correcting only error detecting, you wouldn't even know how many errors)

Well considering the prices of hdds nowadays (maybe a hotplugable USB or SATA one) I'ld say with a little expense extra, hdds are the best way to store data permanently (if they are not used on a daily basis). (however, hdds need to be stored far away of anything electrostatic or magnetic)
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Old 2006-12-19, 11:41   Link #11
zoltan_hellhound
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You mean that the crc got bad before ore after..it was put on disc
because you can chek before you burn?


Ok i was into getting a hd..butt i have read in several forums that a externa hd only has a life span of 1-3 years...and they often crash. so changing hardrives every 2-3 years is expensive and the files lose data when being moved..

im i wrong??
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Old 2006-12-19, 12:01   Link #12
Jinto
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Quote:
Originally Posted by zoltan_hellhound View Post
You mean that the crc got bad before ore after..it was put on disc
because you can chek before you burn?
CRC (cyclic redundancy check) is a method to derive checksums. CRC itself cannot become bad. But either the checksum or the data which the checksum was created from can become bad. Now it is very very unlikely, that the tiny CRC checksum file gone bad if you encounter CRC errors. The chances are usually higher that the much bigger data actually was altered. (lets assume the chance for a false bit is equal over the whole disk... now it is only a question of filesize if you want to calculate the probality of a false bit in a certain file...that basically means, the smaller a file, the less likely it contains a false bit).

Quote:
Originally Posted by zoltan_hellhound View Post
Ok i was into getting a hd..butt i have read in several forums that a externa hd only has a life span of 1-3 years...and they often crash. so changing hardrives every 2-3 years is expensive and the files lose data when being moved..

im i wrong??
Well if you handle external drives like a real transportable drive (something in between of playing football with it and handling it like a raw egg) you will encounter a short lifespan of the drive. If you use it just for back-ups and barely touch/move it (or otherwise mechanically stress it, like (un)pluggin it very often at a time), such a drive should survive much longer (and with it the data).
A matter of common sense imo.
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Old 2006-12-19, 12:35   Link #13
zoltan_hellhound
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Quote:
Originally Posted by Jinto Lin View Post
CRC (cyclic redundancy check) is a method to derive checksums. CRC itself cannot become bad. But either the checksum or the data which the checksum was created from can become bad. Now it is very very unlikely, that the tiny CRC checksum file gone bad if you encounter CRC errors. The chances are usually higher that the much bigger data actually was altered. (lets assume the chance for a false bit is equal over the whole disk... now it is only a question of filesize if you want to calculate the probality of a false bit in a certain file...that basically means, the smaller a file, the less likely it contains a false bit).

Butt if i burn to a disc an check the crc after the burn,,and the file cheks out..can it become bad after a time (even though i store i propperly)


Well if you handle external drives like a real transportable drive (something in between of playing football with it and handling it like a raw egg) you will encounter a short lifespan of the drive. If you use it just for back-ups and barely touch/move it (or otherwise mechanically stress it, like (un)pluggin it very often at a time), such a drive should survive much longer (and with it the data).
A matter of common sense imo.

So you wold recomend a external hd instead of dvds? should i buy two and do a back upp on the files frome the first external hd. ore should i make backups on both dvds and a external hd? which external hd should i chose?? what brand?
and should i buy ready made..ore build my self?
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Old 2006-12-19, 13:35   Link #14
Jinto
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Quote:
Originally Posted by zoltan_hellhound View Post
So you wold recomend a external hd instead of dvds? should i buy two and do a back upp on the files frome the first external hd. ore should i make backups on both dvds and a external hd? which external hd should i chose?? what brand?
and should i buy ready made..ore build my self?
Mirroring data is a good method to avoid data loss, but also a very expensive one (so thats up to you). If you want to mirror I'ld suggest this... Have a backup on such an external hdd, and a working copy on DVD. The DVD will likely become bad when often used, but there is always the Backup on the hdd so you can make a new working copy if needed (working copy means, its the copy that you'ld use for normal use).

I'ld buy a cheap one (external hdd), brand doesn't matter much I'ld say, since the drive is meant to be used very rarely, even a relatively bad one should be capable to survive this. (if you use it only for backups). (if you can get a cheap one build by yourself go for it, though I doubt this a cheap approach)
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Old 2006-12-27, 07:42   Link #15
zoltan_hellhound
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Quote:
Originally Posted by Jinto Lin View Post
CRC (cyclic redundancy check) is a method to derive checksums. CRC itself cannot become bad. But either the checksum or the data which the checksum was created from can become bad. Now it is very very unlikely, that the tiny CRC checksum file gone bad if you encounter CRC errors. The chances are usually higher that the much bigger data actually was altered. (lets assume the chance for a false bit is equal over the whole disk... now it is only a question of filesize if you want to calculate the probality of a false bit in a certain file...that basically means, the smaller a file, the less likely it contains a false bit).

but if a file is sucsessfully burned to a disc ,and isent corupted/alterd/false bits
and stored only as backup (not to be used) when i then transfer the file after 1-3 eyears and the transfer went ok and the disc is readable. ther shouldent be any fals bits ore coruptiont etc (if stored propperly). am i right?


Well if you handle external drives like a real transportable drive (something in between of playing football with it and handling it like a raw egg) you will encounter a short lifespan of the drive. If you use it just for back-ups and barely touch/move it (or otherwise mechanically stress it, like (un)pluggin it very often at a time), such a drive should survive much longer (and with it the data).
A matter of common sense imo.
yeah i know. but ive read that external hdds are inferior to internal ones
and the lifespan is much shorter than internal (even when handeld whit care)
and in many forums people complaine about bad batches etc. and some say the are only built to last 2-3years so the companys can gett more money

and if i mirror my files (putt them on ext hd and dvd) i still have a problem whit the probebility of burning defective files whit bits missing etc and then they are useless.Wouldent it be better to get to harddrive instead
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Old 2006-12-27, 09:20   Link #16
kivine
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think of it this way, if u copyinto a hdd, you will have more data losses if the whole thing is wreck. the best case scenario is to backup in a hdd and into a dvd.


What I do: when I burn into a dvd, every now and then during my free time, i would make a backup of that dvd. effectively having 2 copies.
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Old 2006-12-27, 10:05   Link #17
Jinto
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Quote:
Originally Posted by zoltan_hellhound View Post
yeah i know. but ive read that external hdds are inferior to internal ones
and the lifespan is much shorter than internal (even when handeld whit care)
and in many forums people complaine about bad batches etc. and some say the are only built to last 2-3years so the companys can gett more money

and if i mirror my files (putt them on ext hd and dvd) i still have a problem whit the probebility of burning defective files whit bits missing etc and then they are useless.Wouldent it be better to get to harddrive instead
Maybe... I don't believe it is that bad a conspiration of the external hd manufacturers. But if you are unsure you can always buy hot plugable SATA drives or normal hdds. You just have to open the PC everytime you need data of it (because I would not recommend to run these drives all the time, so pluging them in and out for the occasional use should be a necessity then). With every hour a hdd runs it dies a little bit (natural ageing of the devices).
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Old 2006-12-27, 10:56   Link #18
zoltan_hellhound
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but if a file is sucsessfully burned to a disc ,and isent corupted/alterd/false bits
and stored only as backup (not to be used) when i then transfer the file after 1-3 eyears and the transfer went ok and the disc is readable. ther shouldent be any fals bits ore coruptiont etc (if stored propperly). am i right?

im just paranoide that the files will be alterd/korupt when i burn it to a data disc
i just dont get how the data gets korupt/loses bits when i burn to a dvd disc.
and how can i se that the file is compleat when burned.
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Old 2006-12-27, 11:34   Link #19
Jinto
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Quote:
Originally Posted by zoltan_hellhound View Post
but if a file is sucsessfully burned to a disc ,and isent corupted/alterd/false bits
and stored only as backup (not to be used) when i then transfer the file after 1-3 eyears and the transfer went ok and the disc is readable. ther shouldent be any fals bits ore coruptiont etc (if stored propperly). am i right?

im just paranoide that the files will be alterd/korupt when i burn it to a data disc
i just dont get how the data gets korupt/loses bits when i burn to a dvd disc.
and how can i se that the file is compleat when burned.

No, if it was only about readability, then there would not be such a thing like "error correction" for Audio-CDs. The point is, optical media like any other media will alter stored data when used often and given enough time (or depending on the device that reads the data out). The question word regarding altered data isn't if, but when.
Comparing sensitive optical media like DVDs with magnetic storage devices like hdds I can say (from my experience), that hdd's data lasts longer error free (if only used as backup drive - not running all the time, just occasional).
Some of my early CD-Rs (burnt 7 or so years ago) contain very bad data now. Since I know DVD-Rs are more damageable, I suspect they won't last much longer.
This becomes even worse with blue ray disks.

btw. reading data on a hdd is less data damaging then writing stuff on hdd. So if you really use it for backup purposes only, chances should be low that data becomes altered just by reading them out occasionally. There is no 100% guarantee however (no media can store data 100% secure/consistent/reliable).
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Old 2006-12-27, 17:38   Link #20
zoltan_hellhound
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hdd's data lasts longer error free

how will the files get errors on the hdd?
and whats the point in saving on dvds, the best way would be to have to hdds ans make par2 files ans store the files on boath the hdds.that way the file could last forever...ore at least 50 years

and another thing what wil hapen if my video files do get corupt in time..is ther a way to fix corupt files like avi,mkv,ogm
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