The dbx 128 combines two distinct signal processing systems in a single unit. One section functions as a dynamic range enhancer, which boosts music dynamics and reduces audible noise from records, tapes, or radio broadcasts. The second section features a dbx II tape noise reduction system, allowing wide dynamic range material to be recorded on tape without introducing any additional noise. The unit includes switching facilities for various applications, making it ideal for listeners, live recordists, and tape copyists. Additionally, the dbx 128 can decode dbx encoded discs.
Dynamic Range Enhancer Section
Stereo Expander
The dynamic range enhancer section includes a stereo expander with a continuously adjustable expansion factor from 1.0 to 2.0, effectively increasing the dynamic range by up to 100%. In expansion mode, the system makes loud passages louder (indicated by a red LED) and quiet passages quieter (indicated by an amber LED), enhancing the overall dynamic range. A key benefit of expansion is the significant reduction of record surface noise, tape hiss, and broadcast noise.
Stereo Compressor
This section also features a stereo compressor, which operates opposite to the expander by making loud passages quieter and quiet passages louder. The compression factor is continuously variable from 1:1 to infinity, making it useful for reducing dynamic range when creating tapes for background music or for use in automobiles.
Adjustable Expansion
The amount of expansion needed varies based on the original compression or limiting applied to the recorded material and individual listening preferences. For classical music, a suggested expansion factor is 1.2 to 1.3, while for pop or rock music, 1.4 to 1.5 is recommended. Over-expansion can lead to audible variations in background noise, known as “breathing” or “pumping.”
Customising Expansion for Different Recordings
Not all records or tapes require the same degree of expansion, as there is no industry standard for compression or limiting in the recording process. Some record companies use minimal compression but apply peak limiting to manage only the loudest signals. For such recordings, particularly classical ones, the “Above Threshold” mode may be more appropriate than the linear mode.
Setting Expansion Levels
Users can adjust the level control to activate expansion during loud passages, indicated by the illumination of the “Above Threshold” front panel LED. In this mode, loud passages are enhanced while medium and low-level signals remain unaffected. Higher expansion ratios, such as 1.5 and above, can be employed in the “Above Threshold” mode without causing pumping or breathing during quiet passages, an issue that could occur with high expansion ratios in the linear mode. The “Above Threshold” LED indicator lights up when the dynamic range enhancer section is active, while the “Below Threshold” LED illuminates when no expansion is taking place.
How to Use the Compression Section
The dbx 128 provides two types of compression: linear compression and above threshold compression. Linear compression reduces the entire dynamic range of the program, making loud passages quieter and quiet passages louder, while medium levels remain relatively unchanged. This mode is ideal for background music and making tapes for automobile use.
Above threshold compression specifically targets loud passages, making them quieter while leaving medium and low levels unchanged. LED indicators on the front panel show whether the unit is operating in above or below threshold mode. Compression ratios in the range of 1.0 to 1.0 are typically referred to as “compression.” Higher ratios, such as 10:1 or higher, are used in the above threshold mode, where the selected ratio determines whether compression or limiting is achieved.
