Editing Dbx (noise reduction) (section)

==How dbx works==
===Tape hiss===
Magnetic tape consists of microscopic particles that can be magnetically charged to record signals. The size of the particles and the speed of the tape transport defines the maximum frequency that the media can record. For [[high fidelity]] recordings, [[reel-to-reel audio tape recording]] typically works at tape speeds of 15 or 7.5 inches-per-second (38 or 19&nbsp;cm/s), but this requires a lot of tape for a given amount of recording. Lower fidelity recordings can be made at 3.75 or even 1.875 ips, which allows more recording time on a given tape, but at the cost of adding more high-frequency noise.<ref name="Nave_2001_4"/>

The [[cassette tape]] was designed for convenience, not audio quality, and ran at 1.875 ips (4.75&nbsp;cm/s) to maximize recording time in the relatively small (compared to open-reel) tapes. This resulted in significant [[tape hiss]]. Combined with their limited width, which limits the [[dynamic range]] of the signals, the hiss tended to overwhelm any high frequencies in the signal, especially low-volume ones.<ref name="Nave_2001_4"/>

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. During the same period, noise reduction systems like dbx and Dolby attempted to do the same using conventional media and actively addressing the tape noise through electronics.<ref name="Nave_2001_4"/>

===Companding===
[[File:Dbx_compression_overview.svg|thumb|dbx companding compresses the original source (left) into a version with less dynamic range (middle), and then re-expands it (right). The tape hiss (pink) is also expanded by this process, but is overwhelmed by the now-expanded original signal.]]
dbx Type I and Type II are types of "companding noise reduction". These systems work by first compressing the dynamic range of the signal into a range that can be safely recorded on the tape. This type of compression, [[dynamic range compression]], mutes down loud sounds and amplifies soft ones, making the volume of the recording much more even. On playback, the dynamic range is expanded by the same amount, causing the low-volume sounds to become low-volume again and vice versa. The combination of compression and re-expansion gives rise to the name [[companding]]. 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. In this case, companding at 2-to-1 will result in a signal with 35 decibels of range, which can be recorded without clipping.<ref name="Nave_2001_4"/>

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. However, when the signal is compressed before recording, those soft sounds are recorded at a louder volume, so now even the soft sounds are louder than the noise. This improves the [[signal-to-noise ratio]].<ref name="Nave_2001_4"/>

When the signal is re-expanded, the tape hiss is expanded along with it, making it louder as well. However, the ''ratio'' of the signal to noise remains (close to) constant through this process, so the resulting output retains this higher signal-to-noise ratio. Ultimately, it means that while tape hiss does get louder during "soft" portions of the recording, the recording itself is (hopefully) always greater in volume and renders the hiss much less noticeable.<ref name="Nave_2001_4"/>

===Pre-emphasis===
Note that the tape hiss is limited to higher frequencies. That means a signal that is primarily low-frequency does not necessarily require noise reduction. Instead, one can simply roll off all the higher frequencies in a low-pass filter, and the hiss will largely disappear.<ref name="Nave_2001_4"/>

Consider a signal that contains a high-volume section and then low-volume. During recording, these signals are compressed to be much closer together in level, so that the high-volume section does not saturate the tape and the low-volume section is louder than the tape hiss. 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. This also mutes down the tape hiss.<ref name="Nave_2001_4"/>

This causes the volume of tape hiss to change during playback. This is not really noticeable when the original signal contains high frequencies that play over the hiss, but for lower frequencies, this can be easily heard. The rise and fall of the tape hiss was known as "breathing" because it sounded like something breathing into a microphone.<ref name="Nave_2001_4"/>

To address this, dbx uses strong high-frequency "pre-emphasis" of the original signal. This amplifies high-frequency sounds before they are sent into the compressor. This causes the compressor 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 cassette format.

===dbx I and II<span class="anchor" id="I"></span><span class="anchor" id="II"></span>===
dbx Type I system is meant to be used with professional recording media that have a signal-to-noise (S/N), before noise reduction, of at least 60&nbsp;dB and a -3&nbsp;dB frequency response of at least 30&nbsp;Hz to 15&nbsp;kHz. The system relies on the medium being fairly linear in volume and frequency response.

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. The dbx Type-II "disc" setting on consumer dbx decoders adds an additional 1–3&nbsp;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.

Both systems use 2:1 companding and provide exactly the same amount of noise reduction and dynamic range improvement – in other words, they provide the same end results, but are not compatible with each other.

===dbx vs. Dolby===
Both dbx and the [[Dolby noise-reduction system]] use companding to control noise. They differ in the way they address the frequency response of the companding process. dbx uses a single frequency pre-emphasis system, whereas Dolby uses four separate pre-emphasis amplifiers, each for a different frequency band. Since tape hiss is primarily a problem for high-frequency sounds, Dolby uses much stronger pre-emphasis at high frequencies than low. This means that a low-volume, low-frequency signal may see little or no companding, whereas the same volume at high-frequencies will have been strongly pre-emphasized to a higher volume level before compression.<ref name="Nave_2001_5"/>

The use of separate pre-emphasizing "encoding curves" allows the overall compression to be much less than it would be on dbx, where it is always 2 to 1. For lower frequency signals, like a conversation, Dolby may apply no compression at all. In contrast, dbx would continue to compand these signals, in which case the tape hiss is also re-expanded on playback, continually varying as the volume changes.<ref name="Roberts_2004"/>