Proton SD-1000 Surround Decoder Review

The increased integration of home video and high-fidelity music systems has spurred the development of audio/video (A/V) receivers and surround-sound decoders that can be used with a vcr or videodisc player (VDP) to bring the impact of theater surround sound, or Dolby Stereo, into the home. In general, A/V receivers require only a simple matrix decoder with Dolby B noise reduction to meet the licensing requirements of Dolby Laboratories. Although even this basic configuration can provide much of the subjective benefit of surround sound, its full realization requires a more advanced “logic” decoder. These decoders are not used in typical A/V receivers because of their complexity and cost, but they are a part of most accessory decoders.

Basically, the Dolby system combines four channels of information in a so-called mp (motion-picture) matrix to form two channels that can be recorded on a film or video soundtrack. In playback, the two channels are passed through an inverse matrix decoder to recover the original four channels, consisting of the left and right front signals, a mono center-front channel to fix dialogue in the center of the screen, and a surround channel to supply ambient sounds at the sides or rear of the theater. The center channel, formed by summing the two stereo channels, is heard from a speaker above or below the screen. The surround signal, derived from the difference between the left and right channels, is played through one or more speakers at the sides or rear of the room.

In a licensed Dolby Surround system for consumer use, the surround channel also undergoes Dolby B noise reduction, is delayed by 20 to 30 milliseconds relative to the front signals, and has its frequency response rolled off below 100 Hz and above 7,000 Hz. These modifications to the surround signal are needed to prevent dialogue and vocal sibilance from leaking into the rear channel, where they could destroy the spatial illusion the system attempts to create. Such leakage is inevitable in a basic MP matrix, which provides only 3 dB of separation between adjacent channels.

A variety of so-called “logic,” or steering, circuits have been developed to improve the effective separation between the channels derived from a matrix. They operate by sensing the relative levels and phases of the various signals and rapidly shifting their distribution in the matrix to cancel the leakage into other channels.

As might be expected, these steering systems vary widely in their effectiveness. Dolby Laboratories’ Pro Logic, an advanced version of the basic Dolby matrix system, is widely used in the motion-picture industry. Although no matrix system, even with logic steering, is capable of true discrete performance, in which each of the four input channels is recovered in its original form from the decoding matrix, it is possible to come very close to achieving that result.

One high-performance matrix system was designed by Aphex Systems, a company well known in the professional sound and recording fields. The new Proton SD-1000 Enhanced Surround Decoder employs Aphex circuit designs. Unlike other surround systems, it does not use any time delays or digital signal processing. Its electrical performance has been designed to provide the general signal quality of a high-quality preamplifier without perceptible degradation of the signal. Its logic circuits operate about twice as fast as those of other decoders, and the separation it provides between adjacent channels is claimed to exceed 40 dB (Dolby Pro Logic calls for at least 25 dB separation).

The result is a system whose specifications include a frequency response that is flat far beyond the audible range, a signal-to-noise ratio greater than 80 dB, less than 0.008 percent total harmonic distortion, and typical channel separations ranging from 40 to 58 dB (depending on the locations of the sources). It is designed to work in either the basic four-channel mode or as a six-channel system using additional speakers along the sides of the room. The decoder adapts itself automatically to the type of installation, depending on whether phono plugs are inserted into the extra output jacks, so that the full program content is available in either the four-channel or six-channel mode. There is also a mono subwoofer output with a 150-hz crossover.

The Proton SD-1000's display window has colored, illuminated indications of volume, balance (in four directions, front, back, left, and right), and the status of its several control features. The only front-panel controls are a calibration button and an input-balance knob, with a 6-dB range, used to optimize the cancellation of front signals in the rear outputs. The SD-1000 has no front-panel power switch and is normally left powered at all times. It comes with a wireless remote control that operates all its functions, including on/off switching.

Four balance buttons on the remote control shift the gain along each of the major axes. Other buttons adjust the overall system gain, affecting all channels, and a mute button silences the system. Since the SD-1000 is normally connected into a tape-monitor loop, it has input and output jacks for a tape deck, and monitoring off the tape is switched by a separate button on the controller. A mode button mutes the surround channels, leaving the front channels operating without modification (bypass mode), or selects either the MUSIC or cinema operating mode. In the music mode, the surround channels carry the full audio bandwidth, and the left and right rear outputs are controlled by the front stereo information. In the cinema mode, the SD-1000 emulates a Dolby Surround system, with its restricted rear-channel bandwidth, and is compatible with programs recorded with that system.

The SD-1000 has a separation-enhancement button that adjusts the degree of channel separation, with settings of 3, 6, 18, or 50 dB. There is also a control to deal with the problem of “dialogue scatter,” in which sibilants may come from various parts of the room instead of the center front. The control apparently reduces this effect by rolling off the highest frequencies in certain of the channels (the instruction manual suggests using this feature when playing noisy recordings as well).

The Proton SD-1000 is a compact device, measuring 16-1/2 inches wide, 9-5/8 inches deep, and only 2-3/4 inches high. Price: $1,200.

Lab Tests

In our evaluation of the electrical properties of the SD-1000, the input circuit overloaded at 3.8 volts, well above a preamplifier’s line level or tape-output level. The rear of the SD-1000 contains individual level adjustments for each of its seven available outputs (left and right for front, center, and back speakers and the subwoofer output). At their maximum settings, the gain in each channel was 3.8, and the output of each channel clipped at 7.7 volts. Most other measurements were made with the gain set to unity.

The total harmonic distortion (THD) plus noise at 3 volts output was 0.0055 percent. Spectrum analysis (excluding the noise) reduced this to 0.004 percent, essentially all second-harmonic. The distortion was not significantly greater at 7.7 volts, just below the clipping point.

The A-weigh ted noise was —98.5 dB referred to a 0.5-volt output, — 114 dB referred to a 3-volt level. The front-channel frequency response was flat within 0.01 dB from 10 to 20,000 Hz, falling to —0.5 dB at 90,000 Hz. The rear-channel frequency response was ± 0.2 dB from 20 to 20,000 Hz in the music mode, and in the CINEMA mode it was down by 3 dB at 4,500 Hz and by 13.5 dB at 20,000 Hz. The mute button attenuated all outputs by 67 dB over the full 20- to 20,000-hz range. A square-wave signal was reproduced with no trace of ringing or overshoot. The rear channels were 180 degrees out of phase with the front channels in the music mode; in the cinema mode they were approximately in phase from 20 to 1,000 Hz, changing to 90 degrees out of phase at 20,000 Hz.

Comments

Our measurements indicated that the SD-1000 was very unlikely to degrade even the finest digital signal in respect to frequency response, noise, or distortion, but only listening could show whether the Proton/Aphex approach to surround sound was fully successful.

We connected the SD-1000 to a pair of high-quality stereo amplifiers. These were connected to a pair of moderate-price compact speakers at the front of the room and a pair of smaller speakers at the wall-ceiling junctions near the rear of the room. Our program sources included lp records and fm radio as well as a high-quality VCR and a videodisc player used with a 25-inch monitor. The video material, all Dolby-encoded, included demonstration tapes and discs and commercial movie discs.

Used to enhance ordinary stereo programs, from recordings or off the air, the Proton SD-1000 imparted a healthy ambience, at least as good as what we have experienced from time-delay enhancement systems, but also quite different. The absence of any unnatural echo sounds, which can easily mar the effect of a time-delay system, was immediately apparent. We especially appreciated the SD-1000's performance with stereo fm broadcasts. Voice announcements on fm are normally in mono, and almost all time-delay systems that have been adjusted for a satisfactory effect with stereo music make a mono voice sound as if it were recorded in an echo chamber. The SD-1000 put the announcer front and center, without a trace of leakage in the rear.

It was impractical to make any direct comparisons with a true digital signal processor, but our impression is that while the latter might be more effective in simulating certain specific acoustic environments, it also requires very careful adjustment to avoid unnatural ambience effects. The Proton decoder was much less critical in respect to level setting and other adjustments, and it never sounded artificial.

We brought out some matrixed quadraphonic recordings of more than a decade ago to check the compatibility, if any, of the SD-1000 matrix with the SQ (CBS) and QS (Sansui) matrices. It was totally incompatible with the SQ from the standpoint of directional qualities, but it seemed more usable with the QS recordings. In neither case, however, was the result as satisfying as with ordinary stereo material.

The real test of the Proton SD-1000 was its performance with matrixed audio/video sources. Previous experience with other surround decoders and with this sort of program material had acquainted us with the benefits of even a simple mp matrix decoder, especially with such films as Top Gun and the various Star Wars releases. Well, the performance of the SD-1000 was nothing less than sensational. The ambience was no longer merely a vague sound effect toward the rear of the room, and there were often a number of distinctly placed rear sound sources as well. The subjective separation was total. We heard no trace of front dialogue from the rear speakers, and the sounds they emitted were usually distinctly different from those we heard coming from the front of the room. Moreover, there was no audible noise from any of the speakers.

We can only surmise what these programs would have sounded like with the other two channels and a subwoofer in use. The apparent bass content of the video programs was much greater than one would have expected from the relatively small speakers we used. Of course, the demonstration recordings we used for this evaluation, mostly excerpts from regular commercial releases, were the kind of material that can benefit most from surround enhancement, and not all video releases would produce such spectacular results.

In short, we are unreservedly enthusiastic about Proton’s SD-1000. It is not inexpensive, especially when you add two or three stereo amplifiers, as many pairs of speakers, and perhaps a subwoofer to your present stereo system. But this little box can put you front and center in your own home theater.

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