CRC codes

On Sat, 4 Aug 2012 20:21:36 -0500, "Youdidnt buildthat"
<invalid@invalid.com> wrote:

  [Snipped by Lyons]
>
>not so.
>the crc is too short,
> out of a codeword set 2^128 (128 bits) and a crc of 16 bits 2^16 , you will 
>have the same crc generated for 128/16 or 8 codewords
>
>> -- 
>> poncho

Hello Poncho,
  I haven't read this complete thread.  
But remember, if someone does build a reliable 
CRC system, they didn't build that.  Some one 
else made that happen.

[-Rick-]

On 08/05/2012 10:58 AM, Rick Lyons wrote:
> On Sat, 4 Aug 2012 20:21:36 -0500, "Youdidnt buildthat"
> <invalid@invalid.com>  wrote:
>
>    [Snipped by Lyons]
>>
>> not so.
>> the crc is too short,
>> out of a codeword set 2^128 (128 bits) and a crc of 16 bits 2^16 , you will
>> have the same crc generated for 128/16 or 8 codewords
>>
>>> --
>>> poncho
>
> Hello Poncho,
>    I haven't read this complete thread.
> But remember, if someone does build a reliable
> CRC system, they didn't build that.  Some one
> else made that happen.
>
> [-Rick-]
>
>

I was under the impression that CRCs are generally designed and
used to detect errors, for example in packets sent over
the Internet ...


I was looking for strong evidence that advanced error-correcting codes
were used in hard drives.

Hitachi produces enterprise-class hard-drives, which tend to
pack more GB per square inch, spin faster and have lower
error rates.

 From this piece from 2008, one learns that they produced dense
packing based in part on LDPC codes (low-density parity check codes):

< http://techon.nikkeibp.co.jp/english/NEWS_EN/20080804/155945/ > .

---

More recently, they've done production od drives that use
LDPC codes:

 From the spec. document for Deskstar 5K3000 & Ultrastar 5K3000
hard disk drives:

"576 bit LDPC in 512 byte format"
-->
"Deskstar 5K3000 (3TB) and Ultrastar 5K3000 OEM Specification"
-->
http://www.hgst.com/tech/techlib.nsf/products/Ultrastar_5K3000

For the Ultrastar 5K3000, they give a mean time before failure
of ~= 2.0 million hours.

But enterprise-class disk drives are more expensive ...

David Bernier

In <jvffng$rpf$1@speranza.aioe.org>, on 08/03/2012
   at 03:14 AM, glen herrmannsfeldt <gah@ugcs.caltech.edu> said:

>As far as I know, they originated as part of tape coding, and then
>carried over to disk drives. 

Early tape drives used longitudinal and vertical parity bits for error
checking. AFAIK the first use of a CRC was in data communications
during the mid 1960's.

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