ArpeggiatorThe distinction is that a compressor takes a sound wave and compresses it. An expander expands the wave to a certain loudness (say, a compressor can compress a wave down to 24 dB average dynamics at -16 dB; an expander then can take this wave and boost it to a loudness of -6 dB, increasing loudness by 10 dB).
Original compressors were analogue electronic devices that processed electric analogue waves. Nowadays the commonly used kind of compressors is digital compressor plugins. The big difference between digital-domain compressors and electronic compressors is how waveforms are handled in the digital domain. Behold, the...
16-bit Resolution Table of Doom
| Bit Depth | Amplitude | Levels available |
|---|---|---|
| 16 | 0dB | 65536 |
| 15 | -6dB | 32768 |
| 14 | -12dB | 16384 |
| 13 | -18dB | 8192 |
| 12 | -24dB | 4096 |
| 11 | -30dB | 2048 |
| 10 | -36dB | 1024 |
| 9 | -42dB | 512 |
| 8 | -48dB | 256 |
| 7 | -54dB | 128 |
| 6 | -60dB | 64 |
| 5 | -66dB | 32 |
| 4 | -72dB | 16 |
| 3 | -78dB | 8 |
| 2 | -84dB | 4 |
| 1 | -90dB | 2 |
| 0 | -96dB | 1 |
16-bit samples are still fairly common. Now think about what a compressor plugin does... It takes a waveform past a threshold level (say, -16 dB) and then boosts it by a certain level. The issue is, of course, that certain samples will fall into the low-bit amplitude levels. Which are cold/hollow/blurry (more so than high-bit amplitude ranges). The effect is that digital compressors produce a "glassy/hollow" tint to the sound.
Analogue, hardware compressors working with real, unquantised current, don't suffer from the same problem, however running 16-bit samples through analogue compressors will still produce the same effect. With added analogue colour and possibly cable distortion, which will make things a bit more varied, but still.
The general issue with digital compressors is their boosting of low-detail amplitudes to higher loudness. Their main flaw is the root flaw of all digital PCM wave formats, irregular detail levels depending on amplitude power. This is why digital compressors (and digital waveform processing) will always be inferior to properly implemented analogue devices.