[linux-dvb] Re: dsm-cc format

[Keith Craigie <kcraigie@mediabolic.com>]

> I don't know what you mean by "sync byte".

What I meant was some sort of identity field or semblance thereof which
allowed one to properly scan a bytestream for the start of the DSM-CC
packet/section.

> DSM-CC data is carried in MPEG2 private sections. If the
> section_syntax_indicator bit of the section is set to 1,
> then there is a 32 bit CRC for detecting transmission errors.
> Unfortunately, the DSM-CC via MPEG2 standard makea the use
> of the CRC optional, but encourages it.

The DSM-CC data (specifically, DSM-CC addressable packets) in this case is
being carried in the payload of a transport stream, which in turn
originates from the DTV hardware (PCI card).  Where is this
"section_syntax_indicator" of which you speak?  It's not in the DSM-CC
addressable section or TS headers?  There's a checksum/CRC32 bit in the
DSM-CC header with a 32-bit field terminating the packet, but you're
right, in my experience it's rarely used.

My problem is that if I receive a bad transport stream packet, which for
example, contains the beginning of the next DSM-CC section, and I merrily
continue extracting the TS payload, I'll not know that the beginning of
the DSM-CC packet is missing.  That is to say, there's no field in the
header I can assume to be a given value in all cases (ie. identity field).

Keith



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