NAME

bin2ecm - encoder and decoder for the error code modeler format

SYNOPSIS

bin2ecm cdimagefile [ecmfile]

ecm2bin ecmfile [cdimagefile]

DESCRIPTION

ECM is a compression format for raw CD images (those with 2352-byte sectors) which removes ECC/EDC data where it is possible to do so losslessly. Compressing a CD image with bin2ecm first then compressing the ECM file with a general-purpose compressor such as gzip(1) or xz(1) can result in better compression than gzip or xz alone.

This works because raw CD-ROM images contain a lot of redundant data that, if constructed fully to the standard specification, can be regenerated without data loss. Some discs contain invalid ECC data normally, usually as copy protection means. ECM will preserve this invalid data as-is.

ecm2bin reverses the process and recreates the original CD-ROM image from an ECM file.

bin2ecm followed by ecm2bin should be lossless for any kind of file, but it is only intended for and works properly with 2352-byte sector CD images.

TECHNICAL BACKGROUND

Raw CD-ROM sectors, 2352 bytes each, contain five main segments in them:

1.Sync - a special code used by the drive firmware to tell where the sector begins.

2.Address - informs the drive firmware of which sector on the disc this is.

3.Data - the 2048-byte block of data returned to software on normal reads. This is usually a file system (such as ISO-9660 or UDF) block.

4.EDC - Error Detection Code, a checksum to detect if the data is corrupt.

5.ECC - Error Correction Code, parity data used in an attempt to repair a damaged data sector.

The EDC and ECC segments sector are effectively random noise to a general-purpose compressor and will make it difficult to gain much in the compression process.

When the sync, EDC, and ECC data are verifiably reproducable by standard means, bin2ecm will remove them and leave only the address and data portions, potentially providing better compression results on that sector. If these segments deviate from the standard, which is usually a result of the disc having copy protection employed on it, ECM preserves it as-is. Copy protection schemes usually leave only a few sectors with invalid data, such as at the very beginning or end of the disc, so that the bulk of the disc can properly take advantage of the CD-ROM format’s capability for self-repair on read. Copy protection and preserving this invalid data is also one reason why backing up the entire 2352-byte sector, instead of the 2048-byte data segments, can be useful.

ecm2bin reverses the process, recalculating the sync, EDC, and ECC segments for all the sectors that bin2ecm had trimmed.