Companding
Recording to tape is always a balancing act. You need a signal hot enough to be comfortably above the noise floor, but without getting so hot that distortion becomes too obvious.
The difference between the max signal level you can record without unacceptable distortion and the noise floor of the medium gives us the dynamic range of the system. Tape doesn’t do so well here – even pro machines can’t approach the full dynamic range of human hearing.
This was something that needed improving. Enter the ‘compander’ – a compound word combining ‘compressor’ and ‘expander’. Like the record and repro EQ, these were actually two different devices, the former to process the signal to tape and the latter to process the signal off-tape. They were designed such that the expander reversed the effect of the compressor.
Recording a compressed signal with a reduced dynamic range makes it easier to keep signal levels within that acceptable window for longer. When the original dynamic range is restored by the expander on playback, the noise floor level becomes dynamic, dropping lower during quieter places where it may be noticable.
Various different systems were developed. The dbx system uses broadband companding over the whole signal, while Dolby A splits the signal into four bands for multiband companding. The simpler Dolby B compands just the high frequencies, and became popular on domestic cassette decks.
The pro systems can add 10dB or more of extra dynamic range, which is significant. But it also changes the sound: the signal hitting tape is different, so the tape saturation changes in character; and the expander doesn’t quite perfectly restore the original dynamics, especially with regard to transients.
Choosing different companding systems can have quite a significant effect on the sound, especially in a mastering context, where you’re processing a full mix. And perhaps even more so when running multiple instances to emulate multitrack tape: try linking multiple instances of u-he Satin, and experiment with different types of companding for all instances at once. The effect on a mix can be surprisingly profound.
And once again, Satin gives you the option to do it wrong if that’s what you really want. Like the classic trick of recording with Dolby B on but playing back with it turned off, for a more airy high frequency response (at the expense of more hiss!).