The expansion-compression system in the dbx 117 is virtually self-adaptive. This self-adaptive feature means that users don’t need to worry about level matching or pilot tone requirements, providing greater flexibility for interconnection. When used as a noise reduction system for tape recording, the dbx 117 can enhance the dynamic range of any reasonably good tape deck by 15 to 20 decibels. It accomplishes this without introducing any audible harmonic distortion, intermodulation distortion, or transient distortion.
Modulation Noise and the dbx 117
Tape noise can be heard not only as a steady background hiss due to the granular nature of the oxide layer but also as modulation or sideband noise. This type of noise is caused by irregularities in the magnetic domains on the tape surface. Unlike steady background hiss, modulation noise varies with the level of the recorded signal and is most noticeable in recordings of solo voice or piano. In these recordings, modulation noise presents itself as a fuzziness or “edge” on isolated notes.
At high sound levels, modulation noise is masked by room reverberation effects and the ear itself, while at low levels, it is masked by background hiss. However, modulation noise becomes a significant issue in the pursuit of a wide dynamic range, particularly at intermediate levels. The dbx 117 noise reduction system offers a significant improvement in the instantaneous signal-to-noise ratio at these intermediate levels, where modulation noise is most apparent.
Advanced Tape Surfaces
In response to the issue of modulation noise, new and improved tape surfaces, such as Scotch 206 and 207, are being developed. These new surfaces offer a sharp reduction in modulation noise, making them highly effective when used with the Model 117. These advancements in tape technology will be available soon and will complement the noise reduction capabilities of the dbx 117.
Using the dbx 117
When using the dbx 117 as a noise reduction system, the background noise of the tape is present at all times, but in a modified form. This modified tape noise is lower at all low signal levels compared to what it would be without the compression and expansion process. However, the level of tape noise still varies depending on the instantaneous signal level.
This variation in tape noise levels is generally not objectionable when copying records or FM broadcasts, as these sources already have a poor signal-to-noise ratio. However, this effect is most noticeable with live recordings and when using extreme compression-expansion factors. To minimise these effects, it is recommended to use the lowest compression-expansion factor necessary to capture the dynamic range of the material being recorded.
Recommendations for Live Recordings
For critical live recordings, using the dbx 117 requires careful attention to the settings to ensure optimal performance. Detailed instructions on how to use the Model 117 for such recordings can be found in the instruction manual. This includes recommendations on setting the appropriate compression-expansion factors and other settings to achieve the best possible sound quality while minimising noise.
In summary, the dbx 117 expansion-compression system offers significant improvements in dynamic range and noise reduction for tape recordings. By understanding and utilising its features effectively, users can achieve high-quality recordings with minimal distortion and noise. For further details, refer to the comprehensive instruction manual provided with the dbx 117.
