dbx (noise reduction)
[1][2] A miniature dbx Type II decoder on an integrated circuit was created in 1982 for use in portable and car audio, although only a few devices took advantage of it, such as certain Panasonic portable cassette players and Sanyo car stereos.
The size of the particles and the speed of the tape transport defines the maximum frequency that the media can record.
[9] During the 1970s, several new types of magnetic recording films were introduced, notably "chrome" and "metal", that used smaller particles and thereby pushed the tape hiss to much higher frequencies.
Companding is useful even outside the field of noise reduction; a cassette might have 40 decibels of dynamic range before the media saturates, while the original signal might use 70 for, say, a live recording of a concert.
[9] The reason this technique works for noise reduction is that the tape hiss manifests itself as a constant low-volume signal.
When the signal is recorded in its original form, without compression, the amount of hiss may be the same volume as softer sounds, masking them entirely.
Instead, one can simply roll off all the higher frequencies in a low-pass filter, and the hiss will largely disappear.
On playback, the louder section has little or no muting applied, so the tape hiss is also left alone at its natural volume.
When the softer section plays, having been amplified during recording, the expander mutes it down its original level.
dbx Type-II is for more noisy media that have a lower S/N and much more restricted frequency response.
In the control signal path, the dbx Type II process rolls off the high and low-frequency response to desensitize the system to frequency response errors – since the roll-off is only in the control path, it does not affect the audible sound.
This protects the system from audible mistracking due to record warps and low-frequency rumble.
Since tape hiss is primarily a problem for high-frequency sounds, Dolby uses much stronger pre-emphasis at high frequencies than low.
[11] Although it brought a wider dynamic range, and therefore diminished noise, to the cassette tape medium, dbx noise reduction did not achieve widespread popularity in the consumer marketplace, as compressed recordings did not sound acceptable when played back on non-dbx equipment.
Although Dolby noise reduction also used some companding, the level of compression and expansion was very mild, so that the sound of Dolby-encoded tapes was acceptable to consumers when played back on non-Dolby equipment.
[13] Billboard noted in August 1981 that the total number of releases with dbx encoding was expected to approach 200 albums.
[15] In addition, dbx LPs were produced from only the original master tapes, with no copies being used, and pressed only on heavy, virgin vinyl.
MCA's Sensurround+Plus, used on the film Zoot Suit, employed dbx Type-II with the 4-track magnetic sound format on 35mm film prints, providing the motion picture with a stereo soundtrack capable of wide dynamic range and freedom from noise.
The first generation Public Radio Satellite System (PRSS), introduced in 1979 and used by the American National Public Radio for delivery of network programming to their member stations via satellite, was a single channel per carrier (SCPC) system that had about 40 dB of analog (recovered) signal to noise.
