depression

n 1: a mental state characterized by a pessimistic sense of

inadequacy and a despondent lack of activity [ant: {elation}]

2: a long-term economic state characterized by unemployment and

low prices and low levels of trade and investment [syn: {slump},

{economic crisis}]

3: a sunken or depressed geological formation [syn: {natural

depression}] [ant: {natural elevation}]

4: sad feelings of gloom and inadequacy

5: an air mass of lower pressure; often brings precipitation;

"a low moved in over night bringing sleet and snow" [syn:

{low}, {low pressure}]

6: a state of depression and anhedonia so severe as to require

clinical intervention [syn: {depressive disorder}, {clinical

depression}]

7: a concavity in a surface produced by pressing; "he left the

impression of his fingers in the soft mud" [syn: {impression},

{imprint}]

8: angular distance below the horizon (especially of a

celestial object)

9: pushing down; "depression of the space bar on the

typewriter"

compression

1. (Or "compaction") The coding of data to save

storage space or transmission time. Although data is already

coded in digital form for computer processing, it can often be

coded more efficiently (using fewer bits). For example,

{run-length encoding} replaces strings of repeated characters

(or other units of data) with a single character and a count.

There are many compression {algorithms} and utilities.

Compressed data must be decompressed before it can be used.

The standard {Unix} compression utilty is called {compress}

though {GNU}'s {gzip} is better. Other compression utilties

include {pack}, {zip} and {PKZIP}.

When compressing several similar files, it is usually better

to join the files together into an {archive} of some kind

(using {tar} for example) and then compress them, rather than

to join together individually compressed files. This is

because some common compression {algorithm}s build up tables

based on the data from their current input which they have

already compressed. They then use this table to compress

subsequent data more efficiently.

See also {TIFF}, {JPEG}, {MPEG}, {Lempel-Ziv Welch}.

There is a compression {FAQ} at

{(ftp://rtfm.mit.edu/pub/usenet/news.answers/compression-faq/)}.

{Usenet} newsgroups: {news:comp.compression},

{news:comp.compression.research}.

2. Reducing the dynamic range of an audio signal,

making quiet sounds louder and loud sounds quieter. Thus,

when discussing digital audio, the preferred term for reducing

the total amount of data is "compaction". Some advocate this

term in all contexts.

(1999-10-22)