| Subject: | "read" fails for >16 bits per sample |
I tried to read sample-values from a 24 bit/sample .wav-file and got
very strange results but no error message. When I look at the
sourcecode of Audio::Wav::Read my impression is that it can only work
for <=8 bit/sample and for 16 bit/sample. Below is a proposed
replacement for _init_read_sub that I've tested with 8, 16, 24 and 32
bits/sample and 1, 2 and 4 channel wav files generated by SoX. Might
still contain errors, so good review and testing should be done.
sub _init_read_sub {
my $self = shift;
my $details = $self -> {'data'};
my $channels = $details -> {'channels'};
my $bits_sample = $details -> {'bits_sample'};
my $sub;
if ( $bits_sample <= 8 ) { # Data in
.wav-files with <= 8 bits is
my $offset = 1 << ($bits_sample - 1); #
unsigned, for >8 bits it's signed
$sub = sub { return map $_ - $offset, unpack( 'C'.$channels,
shift() ) };
} elsif ($bits_sample == 16 ) { # 16 bits could be handled by he
general case below but might be faster like this
if ( $self -> {'tools'} -> is_big_endian() ) {
$sub = sub
{ return unpack 's' . $channels, # 3.
unpack native as signed short
pack 'S' . $channels, # 2. pack
native unsigned short
unpack 'v' . $channels, shift() # 1.
unpack little-endian unsigned short
};
} else {
$sub = sub { return unpack( 's' . $channels, shift() ) };
}
} elsif ($bits_sample <= 32 ) {
my $bytes = $details -> {'block_align'} / $channels;
my $fill = 4 - $bytes;
my $limit = 2 ** ($bits_sample-1);
my $offset = 2 ** $bits_sample;
warn "b: $bytes, f: $fill";
$sub = sub
{ return map {$_ & $limit ? # 4. If sign
bit is set
$_ - $offset : $_} # convert
to negative number
unpack 'V*', # 3. unpack as
little-endian unsigned long
pack "(a${block}x${fill})*", # 2. fill with
\0 to 4-byte-blocks and repack
unpack "(a$bytes)*", shift() # 1. unpack to
elements sized "$bytes"-bytes
};
} else {
$sub = sub {$self->_error("Elements have $bits_sample bits per
sample. Unpacking elements containing more than 32 bits per sample is
not supported!")}
}
$self -> {'read_sub'} = $sub;
}
I didn't check but a similar patch for writing might be required.
Regards,
Wolfram