User's Manual

Data compression

Hardware Compression

If data compression is used in software on the host computer rather than in the

hardware of the drive, you can slow down the transfer rate of the host because it

must perform compression computations in addition to its regular computations.

Also, any other host that wants to retrieve (decompress) the data must have the

same software.

Hardware data compression (HDC) refers to the implementation of the DCLZ

algorithm in the data compression engine, with the compression processing activity

transparent to the host computer and the user.

Seagate’s data compression engine is designed to provide a complete data

compression system using the DCLZ algorithm. This IC provides support circuitry as

well as the core DCLZ compression machine.

A more detailed description of the data compression engine is given later in this

chapter.

Data Integrity

There are various types of data-compression algorithms, but in this document they

are divided into two basic types:

lossless

algorithms, such as DCLZ or ALDC, and

lossy

algorithms, such as those used in some consumer audio products.

Lossy algorithms drop out or lose some portion of repetitious data during the

compression process to reduce the actual data bytes that are recorded to tape. The

data lost during this process is lost forever and cannot be recovered. In consumer

audio, this is not a problem because this method reduces required storage space

and still provides better-than-analog recording and playback quality.

As you would expect, lossy algorithms are inappropriate for computer data storage of

any type; hence the choice of lossless algorithms for computer data storage use.

Lossless algorithms are designed to compress data using a complex algorithm,

ensuring that all data is compressed and recorded to tape and that all data can be

decompressed and returned in the identical format as before. No bits are lost, and no

data is compromised.

The DDS standards specify the use of the DCLZ algorithm, a lossless algorithm for

data compression.