dbx Model 228 Dynamic-range Expander Review

The dbx Model 228 (whose full name is the “Type II Tape/Disc Noise Reduction System & Dynamic Range Expander”) is a compact, versatile multifunction signal-processing accessory that operates in the signal path between the preamplifier and power amplifier or in a monitor loop of the preamplifier. It has three distinctly different basic functions. As a dynamic-range expander, it can increase the dynamic range (in decibels) of any program by up to 50 per cent. It is a recording and playback noise-reduction system, dbx Type II, with separate encode/decode circuits that compress the signal by 2:1 during recording and expand it by the same amount during playback. Finally, it can decode the playback of dbx-encoded discs.

The dbx II noise-reduction system (the consumer version of the professional dbx I system) operates over the full audio-frequency range, making its noise reduction equally effective against hum and hiss. It also has internal high-frequency pre-emphasis during recording and a complementary de-emphasis during playback.

A feature of the dbx noise-reduction system is its relative independence of program level. Because of the system’s extremely wide dynamic range, and because the same compression and expansion slopes are maintained over its full dynamic range, precise record-playback level matching is not required. As a tape-recording accessory, the system can theoretically double the dynamic range of a tape recorder. This offers the intriguing possibility of obtaining a 100-dB range from a cassette deck whose inherent dynamic range is only 50 dB. Even with allowance for variations in the placement of the compressed program within the available operating range, a dynamic range of 90 dB is achievable with cassette recorders. As with other encode/decode noise-reduction systems, the dbx II can only reduce the noise added during the recording and playback process and will not affect noise already in the program.

In the dbx disc system, phonograph records are cut with signals processed by a dbx encoder. When played back through a suitable decoder, these discs can deliver roughly the same dynamic range as dbx tapes— up to 90 dB or so. Since the noise reduction is equally effective against rumble, the resuit is a total absence of the usual record noises.

The actual dynamic range of a dbx record is often limited by the master tape from which it was made. Many derive from analog masters, and no matter how good they might be, their background noise will certainly be higher than that of the dbx decoder. Nevertheless, the total silence in the background of these records is uncanny and adds greatly to their realism and impact. A similar limitation exists with respect to making or playing dbx cassette tapes. Under most conditions the limiting “noise floor” will be established by external conditions and prevent the full potential of the system from being realized.

The final mode of the dbx Model 228 is dynamic-range expansion, which is applicable to any program source (including the playback of dbx-encoded discs or tapes— after decoding—although it should not be needed with wide-dynamic-range program material). Virtually every recorded or broadcast program has undergone some degree of compression, limiting, or other dynamic-range restriction before it reaches the listener. The expander makes it possible to restore some of the lost dynamics, making a program more natural-sounding. In the Model 228, the expansion system operates completely independently of the tape-or disc-processing systems.

The dbx Model 228 is furnished with brackets for mounting in a standard 19-inch rack. Wooden sides are available as an option. The front panel contains two horizontal slider controls and a number of pushbuttons. In the center of the panel two mechanically interlocked buttons marked source and tape select the program input for the dynamic-range expander. One slider (expansion) sets the amount of expansion over a calibrated range of 1.0 (no expansion) to 1.5 (maximum). The other (transition level) shifts the range of input-signal levels within the processor’s operating dynamic range. A horizontal row of red and yellow LEDs shows the operating status of the expander at all times. Yellow lights extending to the left of center indicate the degree of gain reduction, and red lights extending to the right of center show the degree of increase. When no lights are lit, the gain of the expander is unity.

The two noise reduction buttons are marked tape and bypass. The latter is normally engaged unless one is recording or playing back a dbx-processed tape. To their right is the dbx disc decode button, which is mechanically interlocked with the others. It is used for playing dbx-encoded discs. The expansion system can be used, to any degree desired, at any time (it affects only the playback signal, after decoding). Setting the expansion slider to 1.0 is all that is needed to remove it from the system.

In the rear of the dbx Model 228 there are standard phono jacks for the signal inputs and outputs, which are normally obtained from the monitor-loop jacks of the associated equipment. Additional tape jacks on the rear of the Model 228 replace the functions of the amplifier jacks. There are three screwdriver-adjusted level controls for matching the output levels from the Model 228 in each of its modes. They are for convenience only, and do not affect the internal operation of its circuits.

Laboratory Measurements

There are few conventional measurements that can be made on a dynamic signal processor such as the dbx Model 228. A valid analysis of its operation requires separate access to its signal-modification and control circuits, which are not available externally. Our measurements consisted principally of verifying the maximum output voltage of the unit (rated at 7 volts and measured at just over that figure), the distortion of the expander section (rated 0.1 per cent and measuring between 0.01 and 0.04 per cent for output voltages between 1 and 5 volts at any expansion slope), and the output noise level of the unit. Since that was well below our 100-microvolt measurement limit under any control conditions, we were unable to verify the specification of —85 dB relative to 1 volt (unweighted), or about 50 microvolts.

Frequency response, as such, cannot be measured while compression or expansion is taking place, since the response of the sensing and control circuits determines the total response in a steady-state measurement. With the expansion set to 1.0, the response of the Model 228 was flat from below 100 to 20,000 Hz and rolled off slightly at lower frequencies to — 2 dB at 20 Hz. This satisfies the manufacturer’s specification of 20 to 20,000 Hz ± 1 dB.

We also measured the transfer characteristic (output voltage vs. input voltage) of the expander at expansion settings of 1.0, 1.2, and 1.5. The 1.0 setting, of course, gave a linear (1:1) relationship between input and output, and with the others the output varied more rapidly than did the input. The three curves intersected at one point (determined by the setting of the transition level control). Below that input, the expanded output curves fell much more rapidly than did the input, and above it the expanded output exceeded the unexpanded output by approximately the amount indicated by the expansion control setting.

Comment

We used the dbx Model 228 as part of a music system, recording and playing back cassette tapes and expanding FM and phono programs. We also auditioned some recent dbx disc releases that had been made from digitally taped masters.

When we reported on the original dbx disc decoder we were favorably impressed with its potential, but we noted that the only discs available at that time with dbx encoding were made from analog tape masters and that the master-tape noise, which might have been masked in a conventional record, was all too audible when the dbx process eliminated the record noise.

When we listened to digitally mastered records (from M&K and Chalfont) with the Model 228, the effect was astonishing. For all practical purposes, the sound quality was nearly equal to some digital tapes we have heard. At no time was any background noise audible, including the set-down of the stylus on the record. The average level on these records is not particularly high (in fact, the undecoded cartridge output seemed considerably lower than that we normally find in other “audiophile” discs), and this no doubt reduces the normal playback tracing distortion.

Best of all, the dynamic range of these discs is unsurpassed by anything so far available for home use. As expected (but still coming as a surprise) the peak power requirements of a system to play these discs are far beyond what one would have expected in the past. At moderate listening levels (which would not interfere with conversation or using a telephone in the same room) the average power required with typical moderate-to low-efficiency speakers is 1 watt or less. The program peaks, however, often clipped a 200-watt-per-channel amplifier and lit the protection lights at the 200-watt level on a pair of KEF 105.2 speakers. With the peak hold meter indications on a McIntosh MC 2255 amplifier we could tell that the program peaks reached levels between 250 and 500 watts per channel with a moderate average listening level.

Although this may affect people’s amplifier buying habits in time, the benefits are immense. This kind of reproduction must be heard to be appreciated—words are completely inadequate to describe it. It does not yet duplicate “live” sound (probably an unattainable goal), but it is a giant step closer to bringing the realism of a live performance into the home.

The dbx 228 is very easy to use with a cassette deck (if the deck has three heads, one can monitor the decoded output while making a recording). The improvement in the signal-to-noise ratio is dramatic. Under certain conditions, depending on the program material and in-put-noise spectrum, one may hear traces of noise modulation (the level of the program background noise being modulated by high-level transient signals). This was never obtrusive enough to be noticed without specifically listening for it (which, of course, we were doing). Although dbx suggests recording at a nearly normal (0 dB) level on the recorder’s meters, we suggest keeping the maximum average level under —5 dB if possible unless the recorder has exceptional high-frequency headroom. It is normally possible to sacrifice a considerable amount of noise reduction (when the noise in the final tape will still be well below that in the program) and gain advantages in high-frequency linearity and clarity by keeping the levels lower than usual.

We spent some time using the Model 228 as an expander. There have been many improvements in expanders in recent years, and the Model 228 shows the results of that progress. While it is possible to create an unnatural surging of the program if maximum expansion is used, especially with such material as solo instruments where the gain changes are not masked by a more complex program structure, we found the dbx recommendations of an expansion slope of 1.2 to 1.4 with classical music and 1.3 to 1.5 with popular music to give the best results. When the transition level is set according to instructions (so that the average program level tends to balance the “on” time of the yellow and red lights), there is a minimum change of overall perceived level, yet during quiet or low-level passages the yellow lights will be lit and the background noise greatly reduced (usually eliminated entirely). Used properly, the expansion of the Model 228 is never apparent to the listener—at least until it is switched off. This is a sine qua non for any signal processor, whose operation must never be perceived directly.

The dbx Model 228 is one of the most useful single accessories one can add to a reasonably well-equipped music system. Each of its operating modes is likely to be useful in some phase of the system operation, and if one does not already have a dbx disc decoder or one of the other dbx units, the Model 228 represents one of the neatest ways to realize their advantages.

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