There are 2 distinct functional types of noise reduction systems, the single ended system and the codec or dual ended system. The purpose of a single ended device is to remove noise or other artefacts that are already present in a signal while the purpose of codec is to minimise the generation of noise along a signal path, typically along a device that is suspectible to broadband noise like a telephone line or an analogue tape recorder.
We start seeing attempts of noise reduction using simple compander circuits as early as the 1920's although these were not intended to help with recording but rather to be used with phone lines (and vocoders were created to tackle the same problem). By the 1950's we start to see more brain power applied to the problems of reducing tape noise in response to the introduction of stereo and true hi-fi equipment into households, at that time almost all were attempting to apply companding or similar simplistic level altering technologies for tape recording, errors and noise in the recording process became more apparent on playback microgroove records than they had been with the old 78's, also noise is most apparent in the upper frequency ranges and the 78's and their playback equipment seldom really reproduced anything over 7kHz.
What made the problem more pertinent is that the noise characteristics of records and tape are quite different, tape has continuos but fairly low level broadband noise that is at its most noticeable at quieter passages while a record typically has short but loud bursts of white noise due to static, scratches etc. that will be easily heard even in loud passages, what happened however when people started to press LP's from taped masters is that you got the worst of both worlds in a sense, since you could hear both the noise generated by the record and the tape. Ray Dolby came up with the first technically successful system in 1966 or so with his Type A system, although so much money and time was spent on tackling the problem that his system was already outdated as soon as 1970, by that time however it had gained such a foothold in the industry that the technically superior DBX and TelCom systems never made any serious impact outside of the better studios.
DBX type I noise reduction A system introduced by David Blackmer shortly after the Dolby system and known today as the DBX type I, although intrinsically a simple 2:1:2 compander, the system sported a rather fancy VCA that was based on the discovery of the log linear characteristics of silicon transistors that in reality made compression based system a practical reality for the first time, in other words were the Dolby system used a new technique to solve an problem using available components, the DBX system used an old technique but invented an improved component to make it a practical reality. The basic premise of the DBX system is very simple, like with any analogue audio compression the quieter sections of the audio signal are amplified up and then the balance is restored on playback via expansion, this simply leaves less room for noise on the tape so to speak. There is no question that the DBX system is superior to the Dolby A system however it is slightly more difficult to implement as it is more susceptible to problems, both to errors from the medium itself were a loss of information will result in expansion errors, tracking errors result in phase shift and calibration errors result in audible pumping effects. Another factor that gave the DBX system an advantage in the semi-pro and pro markets is that the use of broad band compression has the added bonus of giving you a wider dynamic range and a reduction in tape saturation effects.
Note that type one is very demanding of your recording system, the company only recommends that you use type I if you have a analogue recorder running at 38 cm or higher or a digital recorder with 16bits@40KHz or higher, this is a bit over the top perhaps, if you have a well aligned and maintained recorder, good tape and an old type I codec that you have set up and calibrated for use with your system you can certainly use slower speeds, but with care. It must be noted that I have heard quite horrible type I encoded tapes recorded on badly aligned recorders @ 9,5cm and am constantly perplexed by the letters I get complaining to me about the minimum spec. required for use of type I, first of all this is a recommendation by the dbx company, not us so why write to me, and secondly I happen to mildly agree with them, for consumer devices like open reel recorders running at 9,5 or 19cm and especially considering that most of these machines are at the least 20 years old by now it has to be said that the type II is a much better match, it's slightly less effective as a NR but the RMS circuit makes it close to being foolproof and it is much more tolerant of recording conditions than even the Dolby systems, the end result being that in most cased the type II codec simply sounds much better on those units. Please note that the dbx company discontinued the manufacture of noise reduction units in early 2003, also note that any sort of DBX NR should be turned off if you are recording signals such as time codes or other non audio/machine readable information, and finally the noise reduction is spelled all caps, i.e. DBX, but the company is spelled using lower characters only, i.e. dbx.
Type I Codecs The dbx company recently discontinued the half rack 105X unit, it's a 2 channel codec that has balanced and unbalanced I/O, front panel calibration pots etc., they also had the 911 single channel codec that fits in their modular rack.
DBX type 2 noise reduction A simplified version of the DBX noise reduction system intended for use with systems with limited bandwidth such as cassette recorders, 8 track recorders and carts. The type I system did not work very well with those types of systems since it works on a fairly linear basis across the spectrum so if the recorder had problems with bandwidth, phase shifts, dropouts or any sort of notches or bumps along the frequency spectrum the type I decoder would have problems. The type II system solves these problems by having a greater pre-emphasis on the detection circuit and by using an RMS detection system that meant that the units are much more immune to tracking errors, a long running problem with DBX type I, Dolby and DNR based systems (and the need behind the Dolby HX Pro system), but for the same reason a type II codec should never be used with a recorder that has a flat response like a high end open reel or a digital unit, the added emphasis will result in distortions and loudness errors in the upper frequencies. The theoretical dynamic range expansion gained by using type II is the same as for type I or about double, although in practice I find it to be somewhat less. While the dbx company tried hard to get type II codecs integrated into consumer equipment and for a time in the 80's archived some success especially in the USA were a few labels issued audiophile cassettes using the system, today however it's almost never seen integrated outside of a few multitrack cassette recorders. But there are still quite a few home users of the system and you can still find the technology used in broadcast situations were type I is not appropriate.
DBX type II codecs dbx corp. recently discontinued the 942A 2 channel decoder and the 941A 2 channel encoders, these are modular units designed to fit into a dbx rack, but for the more normal users out there the half rack codec unit that was discontinued at the same time is the 140X unit is a better fit and considerably cheaper to boot in addition to providing balanced and unbalanced I/O. Older models include the Model 222 a 2 channel 19" rack unit that was sold both to semi pro users and to consumers without rack ears, dating from the late 70's and early 80's the model also featured a DBX Record noise decoder (see below), this unit is probably the most common type II codec on the market.
DBX type III noise reduction It's a bit difficult to get hard info on this technology from the company, but it appears to be a low cost codec intended to be integrated into analogue processors etc., the dbx company uses it for instance in its Series 20 graphic EQ's. Claimed extra headroom is 20db, considerably less than with type II.
Dolby A The first commercially successful noise reduction system, provides a 10db noise reduction across most of the audio spectrum. It's basic premise is fairly simple, the audio spectrum is split into four bands and then each is compressed but only the quieter portions and then expanded on playback, the amount/range of expansion is controlled by a modulated high frequency pilot tone recorded onto the tape, this is a controlled narrow band companding rather than the wide band one employed by DBX and due to that there is no real gain in dynamic range when the Dolby Laboratories system is used, it is however more resilient to errors and media quality than DBX type I, but is susceptible to tracking errors. More info on the type A system can be found here. It should be noted that the type A system works fine on low end hardware and can even be used on cassettes, the reason for you not seeing it used there is a question of cost.
Dolby B A simplified version of type A noise reduction targeted towards consumer recording equipment, although the Dolby company originally intended the system to be used with open reel recorders and not cassette devices it later put licensing restrictions in place so that it could only be sold integrated into reel to reel recorders capable of certain speeds etc., i.e. if the recorder had a capability of recording at 38cm the Dolby B system could not be licensed unless it was turned off at that speed. There is only one band and sliding filter technique is used to make it operate down to 1kHz and while you get a 10 dB of noise reduction on anything above 1kHz there is no NR on anything below that and it's actually not as effective in real terms as the type A that used 3 fixed filters to cover the same area (well 2 and a half actually), other advantages and disadvantages are as with type A and the reason for only using 1 band are financial, not technical. The system is fairly ineffective on high quality recorders in comparison with other systems so it did not really get popular until high quality cassette tape recorders arrived on the scene, were its effectiveness was much greater. The B system is normally found built into tape recorders but separate models were common in the 70's. The Dolby B system was initially designed by Nakamichi in the latter half of the 70ís as a cheaper alternative to the Dolby A and was originally intended to be used with cassette recorders, the Dolby Laboratories company however refused to license this simplified form of its system and it was not until the KLH and Advent companies started selling rebadged Naks in and around 1970 that permission was gained from the mother company to release the products.
Dolby B Codecs There have been a lot of separate dolby b codecs available through the years but as far as I know not a single one is being currently produced (but the Dolby 422 4 channel B/C/S codec includes it), note that most models are advertised for use with open reels but they can be used with any sort of recorders just remember to calibrate the unit with the recorder you want to use it with. Some of the more common models include the Teac AN-60 a popular half rack unit from the 70's, it sported independent record levels for each channels but you have to switch between encode and decode and that can be very annoying. The AN-80 full rack model was more like it, while you still had to switch the unit included a calibration tone generator that could help a lot if you had more than one recorder or switched tape types fairly frequently, the rec. and play calibrations were also separate. A slightly earlier and rarer is the Kellar KDB1 it's much more at home in your living room with its brushed aluminium facade and it' wood laminated sides, built in tone generator and calibration was possible for both rec. and play. Sanyo had an interesting variant on the theme available in the N55 and Super D units, they sported both a Dolby B codec AND a compander giving you a unit compatible with most of the recordings out there while also giving you some of the dynamic expansion that you got from a DBX system.
Dolby C An improvement of the B type consumer technology, gives you another band and improvements to the filters result in 20 dB of noise reduction but still confined 1KHz and over, typically found built into mid and high end cassette recorders and I am not aware of any separate Dolby C codecs having been sold except for some combination units.
Dolby S A multiband consumer version of the Dolby system, the first intended for the consumer market from the company that gives you any noise reduction on the lower spectrums of the signal, or 24 db in the high end of the spectrum and 10dB below that (below 1KHz if I remember it correctly, but do not quote me on that). Contrary to reports the Dolby S is more or less compatible with cassette decks that do not have an S NR from the company built in, and it works like a charm with open reel recorders but the company has been unwilling to license it for use in standalone codecs.
Dolby HX Pro Like Dolby HX not a noise reduction system per se, it's a headroom expansion system that also helps the deck to record high frequency sound without distortion when used in combination with a NR system by controlling how high frequencies are recorded onto the tape, thus enabling the noise reduction system to perform much more effectively since there is less danger of tracking errors, slightly less tape saturation effects audible as well when it's used. It can be very effective and is single ended, i.e. it needs only to be used while recording, especially effective when used with cheap ferric tapes that display an uneven HF response and in particular when recording from the radio were MPX tones interfere with the NR tracking. Not developed by Dolby Laboratories at all but by the Danish company Bang & Olufsen and it archived all that the original HX system set out to do, but it's simpler circuitry and the fact that it worked much better with cassette tape recorders than Dolby's system meant that the company gave up on it's own system and licensed the Danish system. Only available as a part of a dolby NR license and built into a recorder, it's not possible to build it into a codec.
Telefunken Tel Com & High Com A noise reduction system developed by the German Telefunken company in the 70's as they were dead unhappy about what they felt was the ineffectiveness of Dolby A & Dolby B systems on one hand and the less than useful application of the DBX system in consumer recorders (dbx had promised a consumer version of its system in the early 70's, it was not delivered in a useable form until 1981 and then only after "tuning" by Matsushita). The result was that the company developed a compander based NR system and the needed IC's to implement them economically, the system was called TelCom in its professional version and that was quite popular with European broadcasters since it solved all of the tracking and calibration problems of the older systems and to boot has a somehow "sweet" character to its sonic imprint. The consumer version was on the other hand referred to as HighCom and it blew anything else available at the time out of the water but was not a great commercial success only about a million and a half cassette recorders were sold using the system, very early HighCom circuits also showed a bit of pumping effects if stressed. There is an interesting German page here that offers a modification to existing HighCom systems similar to the one that Telefunken made to their early decks to eliminate pumping and an Italian page that has some info on the TelCom system.
HighCom Codecs Aiwa had the HR-50 High Com codec, it featured rec. and play vol. along with calibration vol. and a built in MPX filter and is a small neat unit that appears to be well specified, Telefunken had the CN 750 codec which is probably the most common of them all and is very good for its time, Nakamichi had the High-Com and the High-Com II codecs. Rotel had the RN500.
TelCom Codecs The most commonly seen unit is probably C4D from Telefunken itself, it's a card made to fit into a Dolby 361 or compatible frame.
Other dual ended systems There were loads of propriety systems manufactured between the late 60's and the early 80's and the last manufacturer of an alternative system disappeared as late as 1990, the reasons for their existence were mostly to do with price and licensing restrictions on gear made by the established manufacturers, most of them probably have no value today but as collector items, except possibly for those that have encoded tapes. Some of the more often seen items include the MXR 119, it's a simple compander that is reasonably effective and gives you some dynamic range expansion, such companders were also very simple to use since they needed no calibration but do not be surprised if you hear some pumping. Another unit was the AEC C39 dynamic processor, it had noise reduction of an unknown type along with a compander.
Generic single ended systems that attempt to remove all or broad band noise from a signal are typically not very successful, produce some unwanted artefacts of their own or reduce quite a bit more from the original information in addition to the unwanted noise, this means that they tend to be mostly used in lo-fi situations such as with PA systems etc. Specialised single systems that address only a certain type of noise can be quite successful however, those are less common however these day's since the rise of digital audio media formats and noise is not generally a problem with digital audio but the linearity's of conversions are etc.
Philips Dynamic Noise Limiter (DNL) A simple but ingenious system based around Psychoacoustic Masking and designed by Philips in the mid 60's, although it was originally conceived as a single ended tape noise reduction it is actually useful for a host of other contraptions. Its basic operation is fairly simple, at the input stage the audio signal passes through a level detector and is then split into 2, signal 1 undergoes a phase transformation of 180° while signal 2 is put through a high pass filter with a cut-off frequency of around 4kHz which is controlled by the aforementioned level sensor, if the signal level is high the filter closes up but with quieter passages it opens up to let through considerable amounts of the signal, the 2 signals are then combined together in an adder and since they are now out of phase what remains of signal 2 cancels out the same portions in signal 1 giving you a effective noise reduction of 10 to 12 db. While it's not quite as effective as dual ended systems that use pre-emphasis like Dolby B or DBX type II it's cheaper to implement, foolproof since it is single ended and very fast acting and thus completely free from tracking or level errors and distortions that are often the by-products of the emphasis process. And why did it disappear from the market? It didn't, it positively thrived, however Philips decided not to patent it and make it available for all to use as a part of their promotion of the Compact Cassette, variants of this system are to be found built into chips for tape recorders, radios and so on and so forth so you probably have at the least 2 or 3 devises in your home that use the system but unlike the licensed products there is nothing to be gained for the manufacturers by it mentioning it on the outside of the product or in the specifications. The system has also seen some further development such as the one offered on this chip from National.
DBX type IV A noise reduction system for digital recorders!? No, not really, ever since the introduction of digital recorders into studios people have been using companders to give the recorders more headroom which is what analogue recorders have in spades but their digital counterparts lack by nature of things, and the most popular units for the job have actually been old DBX type I units. Type 4 is actually a specialised limiter/compander that functions as a part of a A/D stage and expands the available headroom while keeping the integrity of the original signal by only working its magic on the top 4dB of the devices Dynamic Range and is a noise reduction system only in the sense that it will prevent the converter from clipping.
Broadband Noise Reduction Units dbx corp. is manufacturing the 2 channel 929, it's a simple but fairy effective unit that only has filter controls, designed to fit into a dbx modular rack unit but can be used independently from that, it has internally switchable balanced/unbalanced outputs. Earlier models from the company included the SNR-1 a 1U rack mount unit.
DBX Record Noise reduction system A noise reduction and dynamic expansion system for records, basically a record was encoded with a system similar to a type II when mastered and decoded with a separate box hooked up to your hi-fi. The system never really got off the ground for 3 main reasons, first and foremost while the system worked and especially the dynamic expansion part, it had little effect on surface noise and none on cracks and pops and that is what irritated people most about record playback so it seemed less effective to consumers than it really was, secondly the VCA artefacts irritated many audiophiles and finally the system had to be hooked in a amplifier loop (as no one integrated the system into their amps, and the systems the dbx company sold did not have a RIAA de-emphasis circuit), this meant it had to be used in the tape loop with associated signal loss and inconvenience apart from the fact that some amplifiers simply do not have a tape loop option. The 222 unit from dbx corp. had a built in decoder alongside a type II tape NR codec and Technics had the RP-9024 decoder.
SAE Had the Impulse 5000 a system referred to as a noise reductions system for records but actually worked more as a pop reducer, not common but sold reasonably well in the 70's so it's not that hard to get hold of, but not that effective either.