The DG-28 not only has the power to adjust and fine-tune frequency response, it also incorporates a signal tone generator and comes with a dedicated measurement microphone of high quality, allowing automated adjustment of overall response characteristics. Ultra-precise digital filters provide 1/6-octave resolution, dividing the audio spectrum into 64 frequency bands, something that invariably involved a quality penalty when implemented in the analog domain. A large liquid-crystal display panel shows response in graphical form, making adjustments immediately evident at a glance. In conjunction with the Digital Preamplifier DC-300, the DG-28 comes really into its own. With this combination, the signal remains entirely in the digital domain, right up to the preamplifier outputs. Signal degradation is totally absent. Optional option boards can be used to accommodate a wide variety of analog equipment as well.

Fully Digital Equalizer
As can be seen from Figure 1, sophisticated digital signal processing technology is used in the DG-28 in order to integrate two main functions, namely sound field measurement and compensation. If analog circuits were employed here, a number of problems were bound to arise, such as component aging, changing circuit characteristics due to fluctuations in ambient temperature, circuit tuning imprecisions, sound quality deterioration depending on adjustment position, etc. The digital approach on the other hand does away with all these problems. It also is highly impervious to externally induced noise. The type of narrow and precise filters that are required for frequency adjustment can be designed very effectively, using DSP (Digital Signal Processor) chips. To assure absolutely top-level performance, the DSP data path in the DG-28 is 24 bit wide for input/output and 48 bit for internal processing. The ultra high-speed DSP chips require no more than 40 nanoseconds for an arithmetic operation. Two such chips are used per stereo channel, resulting in a total complement of four processors.

64 Filter Cells per Channel Connected in Parallel
Filter cells are a vital area of any equalizer, because they determine characteristics such as center frequency and level range. The DG-28 uses a full 64 cells per channel. In the world of automobiles, it might be likened to a 64-cylinder engine, which is why Accuphase likes to call this part the "filter engine". Within this analogy, the DG-28 can be described as having two filter engines, one for each channel.
Internally, the filter cells are connected in parallel, as illustrated by Figure 2. This type of connection has various advantages for a multi-stage filter. Calculation errors will not add up, and any tolerances will be averaged out, thereby reducing noise. Accuphase has developed special algorithms for linking and controlling operation of the filters. Currently unused filter cells are removed entirely from the signal path, to preclude any possibility for signal quality deterioration.

High Precision Adjustment From 16 Hz - 22,400 Hz in 1/6-Octave Steps (64 Frequency Bands) or 1/3-Octave Steps (32 Frequency Bands)
Almost all listening rooms are shaped in rectangular fashion, with walls meeting at right angles. This gives rise to standing waves which are an acoustically highly undesirable phenomenon. Especially in the area of 200 Hz and below, drastic frequency response irregularities are very common. In the medium and high frequency range, often the speaker units themselves have response curves which look like a succession of peaks and dips. The DG-28 can even out such irregularities, bringing the response curve much closer to the ideal shape. This is possible because the audio spectrum is divided into 64 bands, allowing pin-point control that is far beyond the scope of conventional equalizers. If desired, operation of the DG-28 can also be switched to 1/3 octave resolution, resulting in 32 frequency bands.

High-Precision Level Adjustment in Each Range, From +12 dB to -12 dB in 0.5-dB Steps
Gain in each of the frequency bands of the DG-28 can be adjusted over a range from +12 dB to -12 dB, in precise 0.5-dB steps.Ultra Jitter-Free PLL CircuitThe operation of the D/A converter must be synchronized with the digital audio interface (DAI) signal. For this purpose, a phase-locked loop (PLL) circuit generates a master clock which is used as system reference. In the DG-28, as Figure 3 shows, the Ultra Jitter-Free PLL circuit developed by Accuphase is used. This consists of a detector for the preamble component (a kind of start marker for the L/R signal) and a voltage-controlled oscillator (VCO) using a quartz crystal element. The master clock produced by this PLL circuit is totally free from the effects of pulse distortion and jitter.

Outstanding Signal-to-Noise Ratio and Channel Separation
With equalizers using analog circuitry, S/N ratio and channel separation invariably deteriorate as frequency characteristics are adjusted. This kind of degradation is totally absent in the DG-28 , thanks to its all-digital design.

Large LCD Screen With Graphical Display Makes Adjustments Easy and Intuitive
The display is a large, easy-to-read LCD panel with a resolution of 320 x 240 dots. The equalization curve clearly shows measurement results and compensation settings in the 64 frequency bands. When not needed, the display can be turned off, and a contrast control is also provided. If desired, a beep tone gives audible feedback when controls are operated.

The display of the DG-28 offers three distinct operation modes:

The DG-28 incorporates analyzer and sound field compensation functions. Using a built-in test signal tone generator and a supplied microphone, the acoustic properties of the listening room can be measured and corrected automatically.

Built-In Digital Tone Generator and Analyzer
Adjusting the controls of an equalizer by ear has its limitations. In order to perform precise compensation, a signal tone generator and a frequency analyzer are required. The DG-28 already provides all of these elements.
Figure 4 shows the principle of sound field measurement and compensation as performed by the DG-28 . For measurement, the tone generator output is connected to the filter engine which in turn supplies the signal to the preamplifier and power amplifier, to drive the loudspeakers. The reproduced sound is picked up by the microphone placed at the listening position, and the signal from the microphone is supplied to the frequency analyzer in the DG-28 . This allows measuring the overall response of the reproduction system in its normal state, with all components connected and working as they would normally.
For the measurement, a so-called warble tone (a signal which fluctuates over a very narrow frequency range) is used. By sweeping this signal over the entire audio frequency range, precise acoustic measurements can be made in a short time, without adverse influences from extraneous noise. Because both the tone generator and the frequency analyzer are digital circuits implemented by a DSP, highly sophisticated algorithms developed by Accuphase can be used, assuring precise and stable operation as well as exact reproducibility.

Automatic Compensation of Sound Field Characteristics
Because the DG-28 is a precision equalizer with 64 distinct frequency bands per channel, adjusting the response curve entirely by hand would require a considerable amount of time and expertise. While this is an option that can be chosen if desired, the DG-28 also offers another way, namely automatic adjustment using special algorithms developed by Accuphase. By following the simple steps listed below, the sound field at the listening position can be quickly and effectively optimized.

1. Activate analyzer mode
2. Adjust microphone input level
3. Set desired target response curve (preset or specified by user)
4. Activate automatic measurement and compensation
5. If desired, fine tune response manually
6. Use DG-28 in equalizer mode

The time required for the above process will vary somewhat, depending on the speakers and the listening room conditions. A representative figure is about five to six minutes per channel. For even higher precision, it is possible to repeat the measurement as often as desired and average the results. Figure 5 shows an example for automatic measurement and compensation with "flat" target response. The peaks and dips caused by listening room and speaker irregularities are evened out, resulting in a response curve that is much more linear.

Manual Fine Tuning Possible
After automatic adjustment is completed, the user has the option to fine-tune settings in each 1/6-octave band. It is also possible to perform the entire measurement and adjustment process manually if desired.

Doubles as Sound Field Evaluation Tool
Another use for the DG-28 is as a tool in performing a sound field measurement only, without providing any compensation. This comes in handy for example to evaluate the characteristics of a loudspeaker or the listening room, or to adjust a multi-amped system.

Up to 4 Sets of Measurement Data Can Be Stored
Four response curves can be stored and recalled at the touch of a button, which is handy for example to apply different equalization to different music genres, driving different sets of loudspeakers, making quick comparisons of equalization effects, etc.

Supplied Microphone With Controlled Frequency Response
For sound field measurements, a high-quality microphone with wide and flat frequency response is a must. The DG-28 therefore comes with the 1/4-inch type back-electret condenser microphone AM-28. The ultra-thin diaphragm of this microphone assures linear response over a wide range, allowing accurate measurement and compensation.

For analog output : Line Output Board AO-U2 (Unbalanced high-level output)

For balanced analog input : Balanced Input Board AI-B1 (Balanced high-level input)

For balanced analog output : Balanced Output Board AO-B2 (Balanced high-level output)

For analog input/output : Line Input/Output Board AIO-U1 (Unbalanced input and output)

For playback of analog records : Analog Disc Input Board AI-AD1
(High-performance, high-gain phono equalizer board for use with an analog record player.)

Digital Option Boards

To add digital inputs/outputs : Digital Line Input/Output Board DIO-OC1

For professional standard input/output : AES/EBU Input/Output Board DIO-PRO1

Provides a balanced XLR input and outputs for digital signals corresponding to professional AES/EBU specifications.