I am (for my sins) the maintainer of Audio::TagLib. There, I said it. My question arises out of constructing code that creates a 32-bit header for MPEG files. The header consists of bit fields of various widths.

This question concerns the ordering of bits in a 8-bit byte, packed into an integer. If I number the bits 7..0, the assignment results in a value when unpacked with 'b*' gives me the expected result: 11110010, with bit #7 on the left. The TagLib C++ library with which I communicate expects the field to look like this: 01001111, which is what one gets when unpacking with B*.

A pointer to doc that explains what's behind b vs B decoding would be greatly appreciated.

The actual code is attached.

Thanks.

#!/usr/bin/perl 
# Reference taglib-1.9.1 doc TagLib::MPEG::Header
# and http://www.mp3-tech.org/programmer/frame_header.html

package MPEG_Header;

my %header = ( 'FrameSync'     => [31,21],    # Frame sync (all bits m
+ust be set)
               'VersionID'     => [20,19],    # MPEG Audio version ID
               'Layer'         => [18,17],    # Layer description
               'Protection'    => [16,16],    # Protection bit
               'BitRate'       => [15,12],    # Bitrate index
               'SamplingRate'  => [11,10],    # Sampling rate frequenc
+y index 
               'Padding'       => [9,9],      # Padding bit
               'Private'       => [8,8],      # Private bit. This one 
+is only informative.
               'ChannelMode'   => [7,6],      # Channel Mode
               'ModeExtension' => [5,4],      # Mode extension (Only u
+sed in Joint stereo)
               'Copyright'     => [3,3],      # Copyright
               'Original'      => [2,2],      # Original
               'Emphasis'      => [1,0],      # Emphasis
             );

my %VersionID = ('2.5' => 0b00,
                 '2'   => 0b10,
                 '1'   => 0b11,
                );

my %Layer = ('III' => 0b01,
             'II'  => 0b10,
             'I'   => 0b11,
            );

my %Padding = ('Pad'   => 0b1,
               'NoPad' => 0b0,
              );

my %Protection = ('Protected'    => 0b0,
                  'NotProtected' => 0b1,
                 );

my %ChannelMode = ('Stereo'        => 0b00,
                   'JointStereo'   => 0b01,
                   'DualChannel'   => 0b10,
                   'SingleChannel' => 0b11,
                  );

my %Copyright = ('No', => 0b0,
                 'Yes' => 0b1,
                );

my %Original = ('Copy'     => 0b0,
                'Original' => 0b1,
               );

my %Emphasis = ('None' => 0b00,
                '5015' => 0b01,
                'CCIT' => 0b11,
               );

sub _set_header_field {
    my ($hdr, $field, $value) = @_;
    warn "Field $field is not defined in the MPEG header\n" unless $he
+ader{$field};
    my $first = $header{$field}->[0];
    my $last = $header{$field}->[1];
    $hdr  = pack("B*", "0"x32) unless defined $hdr;
    my ($pos, $off);

    if ( defined $value ) {
        # Field value assignment
        # Possible symbolic reference
        no strict 'refs'; # For symbolic reference
        $value = $$field{$value} if exists $$field{$value};
        my $width = $first - $last + 1;
        for ($pos = $first; $width; $width --, $pos --) {
            $off = 31 - $pos;
            vec($hdr, $off, 1) = vec($value, $width - 1, 1);
        }
        return $hdr;
    }

    # Fill field with 1s
    for ($pos = $first; $pos >= $last; $pos --) {
        $off = 31 - $pos;
        vec($hdr, $off, 1) = 1;
    }
    return $hdr;
}

sub build_header {
    my %args = @_;
    my $header;
    $header = _set_header_field( $header, 'FrameSync' ); # All headers
+ have this
    foreach my $key ( keys %args ) {
        $header = _set_header_field( $header, $key, $args{$key} );
    }
    return $header;
}

1;

=if 0
#! /usr/bin/perl

use lib '.';
use MPEG_Header;

my $mpeg_header = MPEG_Header::build_header( 'VersionID'    => 2,
                                             'Layer'        => 'III',
                                             'Protection'   => 'NotPro
+tected',
                                             'BitRate'      => 0b1000,
+ # 64 kbps for V2 and L3
                                             'SamplingRate' => 0b10, #
+ 16000 Hz for V2
                                             'Padding'      => 'Pad',
                                             'ChannelMode'  => 'Stereo
+',
                                             'Copyright'    => 'No',
                                             'Original'     => 'Yes',
                                             'Emphasis'     => 'CCIT',
                             );
printf "b %-s\n", unpack("b*", $mpeg_header);
printf "B %-s\n", unpack("B*", $mpeg_header);

Spaces added. Unpack with b vs B. 
b 11111111 11110010 10001000 11000111 00001100
B 11111111 01001111 00010001 11100011 00110000

=cut
[download]

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