dbx 150 Noise Reduction Unit
Noise reduction for the stereo mastering machine is often one of the neglected areas for the cost-conscious studio, and yet it can be a most significant factor in determining the quality of the final 'product'. When you consider that the ¼" stereo master tape will be the original source of all subsequent copies, it is not difficult to see why the noise performance at this stage is so important.
It is essential that a noise reduction system used for stereo mastering should be one of the standard, professional formats in order to be compatible with the units installed in cutting rooms, duplicating plants and other studios; in practical terms this means that you are limited to a choice between Dolby A or dbx Type I. The Dolby A system is the most widely used for mastering, being reputed to be the system most free from unwanted side effects. However, it is not entirely without problems; Dolby A provides only 10dB of noise reduction (between 30Hz and 5kHz, rising to 15dB at 15kHz) and the system needs to be critically aligned for optimum performance, necessitating the recording of reference tones ('DolbyTone'), but above all, two modern Dolby A units, even second-hand these days, can be prohibitively expensive for the home studio.
The dbx system, developed in the early 1970s, is perhaps slightly less sophisticated than the Dolby process, but in its own way it is still a very elegant solution to the problem of tape noise. It is an accepted professional format which has a number of qualities to recommend it, and in the dbx 150, offers probably the most cost-effective stereo noise reduction unit currently available.
The dbx Model 150 is a 19" rack-mounting device, one 'unit' high (1¾") which can provide more than 30dB of broadband noise reduction.
The unit features two channels of simultaneous encode/decode operation, facilitating off-tape monitoring of a decoded signal if necessary. This is achieved by the use of separate processors for encoding and decoding, which also means that no status switching is required, making the unit very simple to use. In fact, the front panel features just two recessed preset pots and three pushbutton switches, one of which is the power on/off switch. The other two, mounted in the centre of the panel, switch the noise reduction circuitry in and out, and their action is confirmed by a green and a red LED respectively. With noise reduction switched out the unit is actually in 'hard-wired' bypass, allowing the audio signal to pass directly through the unit even when the AC power is turned off.
The two screwdriver adjustable, recessed preset pots, Record and Play, are concerned solely with maintaining a compatible monitor level in record, play and bypass modes, and once set, can be left untouched unless the nominal operating level of the studio is changed. Because of the way in which the dbx system works, accurate level matching is not essential to ensure proper encode/decode operation and no reference tones or time-consuming routine calibrations are required.
On the rear panel of the dbx 150 are the audio connectors, the AC voltage selector and a captive AC power input lead. The excellent audio connectors are all gold-plated phonos and so will exhibit the maximum resistance to tarnishing which can sometimes cause problems in semi-permanent phono connections. The labelling of the inputs and outputs is exemplary, with the source or destination always clearly identified, such as 'From Console Output' and 'To Machine Input', so the connecting up of the unit is very simple even if you don't understand exactly how it works.
The dbx system uses a sophisticated version of the classic compressor/expander (compander) concept, in which compression is applied during the record process, making possible an increase in the average recorded level and so optimising the loading of the tape; on replay a complementary expansion takes place, which restores the full dynamic range of the signal and at the same time forces down the level of the tape noise.
There are however, certain well-known problems associated with this technique; the noise becomes not constant, but modulated by the signal, and the system is dependent on good 'tracking' between the compressor and the expander ie. the action of the expander must exactly mirror the action of the compressor, or unpleasant level-shifting effects will occur.
The classic 'compander side-effect' is best illustrated by the situation where a high level, but predominantly low frequency signal, such as bass drum or bass guitar, when heard on its own, completely fails to mask the tape noise which mainly consists of high frequencies. This effect is made more unpleasant by the noise being modulated by the signal, so that when the sound is heard it is accompanied by a burst of background noise which then dies away along with the natural decay of the instrument.
The Dolby A system tackled modulation effects and the problem of unmasked noise by splitting the audio signal into four separate frequency bands and processing each one independently, so that the higher frequency bands can still be providing full noise reduction in the presence of just a low frequency signal. The system is ingenious and works very well, but it is also quite complex and rather costly.
The dbx process uses the basic compander concept, without band splitting, but also applies a considerable degree of high frequency pre-emphasis (boosting of HF on record). On replay an exactly complementary high frequency roll-off, or de-emphasis, is applied, which returns the frequency content of the programme to normal, but which will also have reduced unmasked modulation noise effects by up to 12dB in the presence of a strong low frequency recorded signal. The system has been cleverly arranged to apply the HF boost before the compressor stage of the encoder, and so avoids the potential problem of tape saturation as the boosted HF is itself compressed in recording and brought within the level that can be safely handled by the tape.
The dbx encode processor compresses the input signal at a 2:1 ratio; if the signal has a dynamic range of 90dB, the compression will reduce it to 45dB. The encoded signal can now be recorded much more easily within the usable dynamic range of the tape recorder (typically 60dB), and even with the peak levels still comfortably below the tape saturation point, the quietest parts are now recorded well above the level of the tape noise.
When the encoded signal is replayed through the decode processor it is subject to expansion at a 1:2 ratio and the recorded 45 dB dynamic range is thus returned to its original 90dB; in effect, the loud bits get louder and the quiet bits get quieter again. Because the compression and expansion exactly mirror each other, the original programme dynamics are precisely restored, but the tape noise, which was of course not present in the original input signal, has been subjected only to the decoding process, and being below the level of the quietest recorded signal component it is forced down in level by the action of the expander.
Apart from the reduction of tape noise, the dbx process has a beneficial side-effect, as the compression applied in recording also reduces distortion due to tape saturation on high level peaks; effectively you gain a very useful extra 10dB of headroom in addition to the 30dB of noise reduction. In practice the dbx 150 will effectively eliminate tape noise from the machine with which it is used, but, like all compander based systems, it can of course do nothing to reduce the level of any noise already present in the source, so if your original multitrack tape is noisy, mixing down through a dbx unit won't make it any quieter.
Because the dbx system uses the same 2:1 compressor/expander relationship over its entire operational range, there is no need for accurate matching of record and replay levels. It is very important, however, that the tape machine should be 'lined up' as accurately as possible, as large errors in the frequency response can cause mistracking in the decoder, and the magnitude of any error will be doubled on replay by the expander. But this can not really be regarded as a drawback to the dbx system, as, even without dbx, the machine's frequency response should be as accurately maintained as possible.
The dbx 150 really is very simple to use, and once the console and the tape machine's meters are aligned, with dbx in bypass (NR out), you can simply forget that it is there. Then, in operation (with NR in), you use only the mixing console meters to set the recording level. The meters on the tape machine will appear to be reading much lower than those on the desk, due to the action of the dbx unit, but this is normal and on no account should the output of the console be increased to try and compensate for this effect; simply ignore the meters on the tape machine and rely solely on the console's meters to verify the recording level.
If you do overdrive the system, you will run into tape saturation on high level signals and decoder mistracking will almost certainly occur, resulting in some peculiar level-shifting effects. It is also possible to record at such a really low level that even dbx cannot prevent the quietest signals from almost disappearing into the tape noise, resulting in the replayed sound being accompanied by noise 'pumping' or 'breathing'. But, between the two extremes, you have in fact a much larger working range than you would have without dbx. The dramatic reduction in tape noise means that you no longer need to strive to get as much level onto tape as possible, so you can 'play safe' with levels and still be certain of getting a quiet recording, with the added advantage of the reduced risk of distortion from tape saturation.
It is perhaps slightly ironic that the dbx system, which offers the greatest reduction (30dB) of tape noise of all noise reduction systems currently in use, seems to work best with tape machines that already have a fairly reasonable noise performance. When it is used with inherently very noisy machines the compander 'breathing' effects become much more apparent, and in extreme cases can prove more objectionable than the constant tape noise level without noise reduction! But, used with a tape machine of the quality of a Revox or similar, I have found the dbx 150 to be entirely without adverse side-effects, and given a suitably quiet source, it is capable of producing 'master' tapes of quite stunning dynamic range.
The internal design of this model uses just one large circuit board running almost the whole length of the casing, the audio electronics being well screened and physically separated from the AC power components. The board is not very densely packed, and the quality of construction and soldering is good, although the layout does seem to involve rather a lot of wiring external to the board. The unit never gets particularly hot, so stacking for multitrack use or housing in a full rack should have no adverse effects on longterm reliability.
The Model 150 is specified as using dbx Type I noise reduction, but similar models are available, like the 140 series, which use the dbx Type II system. Type II was specifically developed for use with equipment with restricted bandwidth and limited headroom, and although the basic principle of operation remains the same, there are a number of significant differences in the level detection circuitry and the pre-emphasis curve which make dbx Type I and Type II incompatible. Type I was formulated to take advantage of the headroom and wider frequency response of professional quality tape machines operating at high speeds (15 or 30ips) and it is therefore Type I that is recognised as a professional mastering format.
The dbx Model 150 has much to recommend it; a rugged, compact unit, offering two channels of simultaneous encode/decode noise reduction, which is simple to use, and, when used under the correct conditions, is without any serious undesirable side-effects. But in addition to its operational attributes, the dbx 150 is very affordable and almost certainly lies within the budget of anyone who may already have the necessary equipment to make use of it. I would recommend this unit to anyone who is considering investing in a noise reduction system for mastering. The operational simplicity, the excellent audio performance and the competitive price make the dbx 150 the most cost-effective two channel noise reduction unit currently available.
Review by Dave Lockwood
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