How to extract part of a file?

[chucky]

I already know, how to get the line I want from a text file. This line I get tells me two numbers: <starting byte> and <length of record>. The thing is, that I have one index file, where each record is one line (so it's simple to find the right line using grep command), but the other file is just a mess, where records simply start at X and are Y bytes long.

Could someone please tell me, which command to use in terminal, when I want to find a sequence of bytes in a file starting at <starting byte>, which is <length of record> long and write it to standard output?

Thanks a lot.