How Dbx Works
dbx Type I and Type II are types of "companding noise reduction". Companding noise reduction works by first compressing the source material's dynamic range (in this case by a factor of two) in anticipation of being recorded on a relatively noisy medium (magnetic tape, for example).
- Upon playback, the encoded material, now contaminated with noise, is passed through an expander which restores the original dynamic range of the source material.
- The contaminating signal (tape hiss) is heavily attenuated and/or "masked" by the dynamic expansion process, resulting in a significant reduction in perceived noise.
Because dbx Type I and Type II are broadband (single-band) companders (unlike Dolby-A's four bands), they are susceptible to audible noise modulation and other artifacts. To deal with this, both Type I and II use very strong high-frequency pre-emphasis of the audio signal in both the recording path and the control signal path. This causes the compander to 'back off' the gain in certain circumstances and reduce the audibility of noise modulation – even with this pre-emphasis, noise modulation can become audible when using very noisy media to begin with, such as the Compact Cassette format. 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.
The dbx Type-II "disc" setting on consumer dbx decoders adds an additional 1–3 dB of low-frequency roll-off in both the audio path and control path. This protects the system from audible mistracking due to record warps and low-frequency rumble.
The dbx Type I system is meant to be used with recording media that have a S/N, before noise reduction, of at least 60 dB and a -3 dB frequency response of at least 30 Hz to 15 kHz. dbx Type-II is for more noisy media that have a lower S/N and much more restricted frequency response. Both systems use 2:1 companding and provide exactly the same amount of NR and dynamic range improvement – in other words, they provide the same end results, but are not at all compatible with each other.
Read more about this topic: Dbx (noise Reduction)
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