Thanks to BrowserUK I was able to get the following code to work for byte aligned data, however I've ran into a situation where some of the data isn't byte aligned meaning the patterns I'm looking for go across byte boundaries. I'll have to do this at a bit level. Any idea how to tweak the code to do this?? #! perl -sw use strict; use bytes; open IN, '< :raw', $ARGV 0 or die "$ARGV 0 : $!"; open OUT, '> :raw', $ARGV 1 or die "$ARGV 1 : $!"; ## Grab a double buffer load first time so we can check & correct alig +nment local $/ = \768; my $buf = <IN>; ## Read two frames worth ## Check alignment. Assumes the xf4 .191 xf4 is unique per frame? $buf =~ m(\xF4.{191}\xF4); ## Record the offset to the first frame my $offset = $-[0]; ## If there was an offset to the first match if( $offset != 0 ) { ## Chop off the leading junk substr( $buf, 0, $offset, '' ); ## Top up the buffer to two full frames read( IN, $buf, $offset, 768 - $offset ); warn "$offset bytes discarded from front of file."; } ## Process the first two whole frames print OUT unpack 'x2 a190 x2 a58 x132' x 2, $buf ## Now process as before local $/ = \384; ## Read file in 384 byte chunks. while( <IN> ) { print OUT unpack 'x2 a190 x2 a58', $_; } close IN; close OUT;