[linux-dvb] Re: Which CRC-code is used in TS of DVB?

[Ralph Metzler <rjkm@metzlerbros.de>]

Gregor Lawatscheck writes:
 > At 23:26 26/05/2003, you wrote:
 > >The decoder does not get the TS.
 > 
 > AFAIK there is no CRC in the TS but in the PES header? I haven't checked 
 > this, so I could be wrong.

No CRC in either.

 
 > Nevertheless: 
 > http://www.linuxtv.org/mailinglists/linux-dvb/2002/11-2002/msg00176.html is 
 > interesting, from Oct Nov 2002 where Holger speaks about CRC32 checking 
 > being broken in the av7110 firmware. Is this still the case and would that 

It is not broken, just not implemented.



 > explain, why broken streams reliably manage to confuse and crash the decoder?

No.


 > 
 > >There are also other instances between
 > >tuner and decoder which handle the TS or demuxed stream and can fail
 > >because of a broken stream. And think of a soft-player. It increases
 > >CPU-load dramatically if a soft-player has to handle bitstream error.
 > >
 > >Holger told that CRC32 is implemented in the driver, it just has to be
 > >activated.
 > >
 > >And before you talk about dropping packets, again, please read my
 > >explanation of CRC on VDR-ML.
 > 
 > Didn't see it in time - there's a lot of traffic on the vdr list. Yes, 
 > correcting via CRC32 sounds good  -- too good to be true?
 > 
 > Because as Holger writes:
 >  >It's implemented. Set the DMX_CHECK_CRC flag in the 
 > dmx_sct_filter_params >struct if you want only receive sections that passed 
 > the CRC32 check.
 > 
 > Now that sounds like it'll drop sections that didn't pass CRC32, doesn't 
 > it? I'm not aware CRC32 corrects anything. I personally just fear CRC32 

Correct.



 > errors, for example in archives etc. To me it's a checksum: According to 
 > http://www.cyberdyne-software.com/crc32.html CRC = "cyclic redundancy 
 > check(sum)". What do you do when the checksum fails? "Invert[ing] the wrong 
 > bits" is certainly beyond me.
 > 
 > If what you write be true - then it'd actually be grand but it's contrary 
 > to what Holger says about "receiv[ing] sections that passed the CRC32 check":
 >  >>Smart CRC-codes like CRC32 can detect nearly every bit-error AND it's
 > POSITION in the bitstream. This way the decoder can simply invert the
 > wrong bits and you get the original, valid bitstream. This is of course
 > better than giving a broken bitstream to a hardware and say "Hey, this
 > is broken data, just see how you can handle".<< [quote from Rene on vdr ML]

Confused with FECs.


 
 > I actually thought the CRC32 was good exactly for the latter reason - error 
 > detection - passing it to the decoder with the notice: "hey, by the way, 
 > this data is broken, handle with care". I'd like to believe otherwise. It'd 
 > be a great benefit to everyone if CRC32 from a budget card could be used to 
 > correct bitstream errors.  I thought FEC (forward error correction) was for 
 > error correction of the bitstream - and surely that's done even for the 
 > budget cards... 

Yup, it is part of the demodulator hardware. 
(but it can also be done in software if you want, see e.g. GNU radio)


Ralph


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