Return To Zero (Part 3)
Noise Reduction Systems
An explanation of the most common noise reduction systems in audio use, including Dolby A, B, C, and dbx.
Until a few years ago only domestic hifi stereo cassette machines were available with a built-in noise reduction system, usually the Dolby B type. At that time, before the Portastudio-style recorder had been released, the serious recordist was faced with the situation of having to buy a separate noise reduction system if any improvement in recording quality was desired. With all the professional studios using either Dolby A or dbx systems the market didn't cater for the home studio and there wasn't the choice of budget systems available nowadays.
Today, the whole budget area of the recording equipment industry has undergone an enormous change which in some ways is exemplified by the fact that our own magazine Home Studio Recording can exist in this healthy environment. The growth has resulted in a new breed of equipment which makes available the possibility of high quality recording at a fraction of the price that was being asked for equipment six years ago.
The revolution started with the Teac M-144 Portastudio which provided a four-track cassette tape recorder with built-in Dolby B noise reduction. Subsequently Fostex took further steps by releasing the A8 eight-track reel-to-reel machine with its built-in Dolby C, and today we have the Fostex B-16 sixteen-track tape recorder which also features built-in Dolby C and brings the track capacity of the big multitrack studios right into the home.
For those embarking on the road of multitrack recording the variety and quality of equipment has never been better. With every four-track cassette offering a built-in noise reduction system as standard, the novice can achieve excellent quality recordings after only a few hours familiarisation. The once complicated use of noise reduction systems has now become so 'user transparent' that it often only manifests itself as a name on the front panel of a tape recorder.
So why is noise reduction used and what are its benefits? The use of noise reduction came about due to the limitations of the recording medium, particularly its small dynamic range. To combat these shortcomings, several companies approached the problem in a number of different ways which resulted in a variety of noise reduction systems, each with its own dedicated followers. Dolby and dbx are certainly the biggest names in this field and to the man in the street, Dolby B is probably the first name that comes to mind as he's likely to have seen it on a domestic stereo cassette deck.
Noise reduction systems are actually misleadingly named as they don't, in fact, remove any noise at all. Rather they make the best available use of the magnetic tape's dynamic range which audibly results in a cleaner sounding recording, hence a reduction in the noise that you would normally hear.
There are two main types of noise reduction known as complementary and non-complementary. Dolby and dbx fall into the first category as both work on a principle of encoding the signal onto tape which is then during playback complemented by a decoding system that restores the sound to normal. A non-complementary system such as the Philips Dynamic Noise Limiter is a playback only system which effectively reduces tape noise in quiet passages of music by filtering out high frequency tape hiss.
The process of recording sound onto tape can, at times, prove a little problematic when the sound you wish to record has a natural dynamic range approaching or in excess of the tape recorder's own dynamic range. For example, an orchestra may have a dynamic range (difference between quietest and loudest sounds) of 100dB but the tape recorder you are using may only have a range of 65dB.
So, having set the recording level to allow the highest possible signal to be recorded onto tape, the quietest passages in the music are going to be masked by the tape hiss and there's still another 35dB of range to deal with but nowhere for it to go. If you compress the natural dynamic range of the orchestra by a factor of 2:1, what was 100dB will now be 50dB and so it will fit quite easily into the 65dB dynamic range offered by the tape recorder with a further 15dB of room to spare before serious noise problems occur (Figure 1). By reversing the process for playback, the sound coming off-tape is expanded by a factor of 1:2, and so the 50dB of signal will be restored to its original 100dB dynamic range.
In general terms, it's on this principle that complementary noise reduction systems are based, however, a detailed look at how different manufacturers have tackled the design can reveal some clever variations on the theme.
At present, three different Dolby systems are in common use: Dolby A, which is the professional system found in top studios; Dolby B, which is usually only found on domestic stereo cassette decks and finally, Dolby C, which is the most recent system, present on many high quality cassette decks as well as some four-track cassette machines like the Yamaha MT-44, and on the Fostex A8 and B-16 reel-to-reel machines. A brief description of how the three Dolby systems work will follow, however, for those who require more information, a list of articles and addresses appears at the end of this feature.
Dolby A (Figure 2) utilises a design which splits the audio bandwidth into four sections and boosts the sound within each band according to its level. In other words, it's a frequency/gain conscious system. This boosting of low level frequencies effectively results in a compression of the original signal's dynamics, which can then be recorded on to tape away from the noise floor. To play back the sound, the reverse principle is used to restore the original relative energy levels of each set of frequencies and so expand the dynamic range.
The advantage of this system is that a signal with a large midrange frequency content will be unaffected in this band while the other three bands will apply varying amounts of noise reduction (up to 10dB) dependent on the energy of the frequencies present. Although Dolby A is favoured by many top studios for its 'sweet' sound, its use requires special calibration of the tape machine. That fact, combined with its high cost make it unsuitable for home studio use unless, of course, you're a qualified sound engineer.
Dolby B (Figure 3) was introduced as a domestic version of Dolby A that didn't require a special calibration procedure and as such has become the norm on stereo cassette decks throughout the world. Similar principles to Dolby A are used except as a budget version of the original, Dolby B only operates across one broad frequency band instead of four narrow ones. As can be seen from Figure 3, frequencies above 500Hz are gradually boosted in level by an amount relative to their energy, resulting in a maximum of 10dB noise reduction from 4kHz upwards. Signals near to the 0dB level are largely unaffected while signals with frequencies at, say, 5kHz and a -30dB level, will be boosted by 10dB in order to record them at a higher level and so avoid the tape noise floor. On playback, the boosted frequencies are reduced to their original level and in doing so push the tape noise down as well.
Dolby C. This recently introduced system takes the basic operation of Dolby B a step further and in doing so achieves a 20dB noise reduction performance. The system is best described as one that operates on a similar principle to Dolby B with one variable band of operation. Frequencies from 100Hz upwards receive an increasing amount of boost to a point where from 1kHz onwards, 20dB of noise reduction is available. In addition, a sliding low pass filter is constantly operating on the signal so that with sound containing little high frequency energy, the filter cuts out the noise that would be apparent were no music present.
Two further elements are also incorporated in Dolby C. Firstly, an anti-saturation network which ensures that any boosted high frequencies which have been pushed up away from the noise floor don't overload the tape. Secondly, a spectral skewing system that improves the transfer characteristics of the encoded signal into its decoded form.
Dbx is the only real competitor to Dolby A, and as a system, offers a greater amount of effective noise reduction at 30dB or more; doesn't require special calibration and is cheaper 'per track' but lacks the smoothness of Dolby A. Devotees of dbx tend to like its very clean and punchy characteristic but it is not without problems.
Dbx works on the principle of a broadband compressor/expander (compander) where the input signal is first processed by adding high frequency pre-emphasis, then compressed across the whole audio bandwith by a ratio of 2:1, so reducing the dynamic range that needs to be encoded onto tape. This enables the signal to be recorded at a much higher level without fear of falling into the tape noise. When the encoded tape is played back, the signal is decoded by first expanding the dynamic range by a ratio of 1:2 and then adding a de-emphasis circuit to restore the high frequencies to their original level.
It's obvious that any anomalies which affect the signal on tape will be enlarged by a factor of two on playback and so signal drop-outs will sound twice as loud. Another side effect of dbx is the 'breathing' phenomenon. This is noticeable, for example, on a recording of a bass drum and manifests itself as a swishing sound that rises in level with the bass sound, then disappears again when the signal is over. This 'breathing' noise is really only obvious on slow passages of music where solo instruments are featured as it is usually masked by the music itself.
The importance of this problem is purely subjective and for many people the other advantages of dbx far outweigh it. As a noise reduction system, dbx would seem to be well suited to the recording of rock music where its inherently punchy sound complements the style of music. In the world of hi-fi, dbx is now being fitted as standard on many stereo cassette decks giving the user a further choice alongside Dolby B and C.
Several manufacturers other than dbx themselves have taken the dbx principle as the basis of their own brand version and so you may come across similar systems from Bel, Tascam and Teac which state that they are compatible with the original dbx.
Designed to operate as a single-ended or non-complementary noise reduction system, the dynamic noise filter is found on many domestic and in-car cassette systems, as well as being used by Philips on several of their reel-to-reel tape recorders. The working principle is straightforward and assumes that noise is most noticeable during quiet passages of music. As the level of the music drops a filter moves down cutting the high frequencies and so removing tape hiss on sections where music is not present (Figure 4).
Unfortunately, unless the system is well designed the music itself can often be affected. However, good systems don't exhibit the problems of 'breathing' or over-compression encountered on Dolby and dbx systems and, of course, being a decode-only system, nothing is recorded on to tape so there are no calibration problems.
There are several high quality devices on the market which use this dynamic noise filter approach, so when faced with the problem of cleaning up precious old recordings or even dealing with noisy live sound sources like guitar amplifiers, they can be extremely useful.
Today, you don't have the luxury of choosing a separate noise reduction system for cassette-based multitrack recorders. For many people, the type of noise reduction used in a machine is of great importance when buying one, which is why you often hear people say that they want the facilities of such a machine but would prefer a different type of noise reduction. A compromise must be made.
Fortunately, some might say, reel-to reel tape machine manufacturers haven't taken to this idea of built-in noise reduction, except Fostex with their A8 and B-16 machines but even these allow a different external system to be used if Dolby C isn't your preference. With 4, 8, 16 and 24 track tape recorders largely untouched by built-in systems, there is still the opportunity to choose the type of noise reduction that fits your requirements, whether influenced by the style of music you're recording or the budget you have available.
Further information on noise reduction has appeared in the following HSR articles:
Summary of Dolby A and dbx Sept 83.
Review of the dbx 150 May 84.
Review of the Symetrix 511 (single ended type) Oct 84.
Tantek Dynamic Noise Filter project Feb 85.
Manufacturers' information available from:
Dolby Laboratories Inc, (Contact Details).
Dbx is distributed by Scenic Sounds Equipment Ltd, (Contact Details).
Symetrix distributed by Atlantex, (Contact Details).
Tascam/Teac distributed by Harman UK Ltd, (Contact Details).
Fostex distributed by Bandive Ltd, (Contact Details).
Bel, (Contact Details).
Feature by Paul Gilby
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