Production year: 1976
The input signal spectrum is divided into 4 adjacent bands, as illustrated in the following figure:
On each of the 4 paths the compression/expansion process is carried out by processing units each operating on a different frequency band. Let's take a more detailed look at the bands:
Filter A: low-pass filter with a cut-off frequency of 80 Hz
Filter B: band-pass filter with a bandwidth of 80 Hz- 3 KHz
Filter C: high-pass filter with a cut-off frequency of 3 KHz
Filter D: high-pass filter with a cut-off frequency of 9 KHz
Note how there are two units operating on the high frequencies in different ways.
To fully grasp how each unit operates during the compression/expansion stages let's refer to the following diagram:
Encoding: each signal with an amplitude beyond -20 dB (0 dB equals 0 Vu) does not get manipulated. When the signal's dynamic is between -30 dB and -20 dB, a 2:1 compression is carried out. When the dynamic is below -30 dB, the signal is attenuated by 10 dB.
Decoding: as you can guess, the decoding process is carried out inversely. Each signal greater than -20 dB doesn't get processed. When the signal's dynamic is between -30 dB and -20 dB, a 1:2 expansion is applied. When the dynamic is lower than -30 dB, the signal gets attenuated by 10 dB.
This system allows a noise reduction of 10 dB within the 5 KHz zone and up to 15 dB around the 15 KHz zone. It globally allows an average NR of 10 dB over the whole audible spectrum.