Two Design Philosophies/Two Differing Results
For almost four decades, there have been two distinct views on how analog compressor/limiters process audio program "peaks" and volume compression, (rms, average).
Most, if not all analog compressor/limiters operate as peak sensing, volume compressors. Hence, the gain of the device is determined by the peak to average ratio of the program material that is to be processed. As an example, a peak to average ratio 10 dB above average audio program material results in approximately 10 dB of gain reduction for the device.
Since the noise of a given audio system is generally fixed, the 10 dB reduction in device gain results in 10 dB increase in system noise. The higher the peak to average ratio, the larger the gain reduction, which results in an increase in system noise and significant loss of system headroom. The Spectra Sonics 610 and 611 Complimiters designs employ independent limiting and compression functions, which may be used separately or combined. The limiter function within the 610/611 eliminates the peak overload associated with short time base transients. As an example, if a 10 dB peak to average ratio is present within the program material, the Spectra Sonics 610 and 611 Complimiters eliminate the 10 dB peak overload component and allows the system gain to be increased 10 dB. The overload recovery occurs by virtue of the sub microsecond attack time exhibited by the 610 and 611. The compressor operates independently of this function is not controlled by peak overload components. In addition, peak overload recovery does not affect compressor output. Results of the instantaneous peak overload recovery are readily apparent:
- Dramatic Increase in System Headroom (Dynamic Range)
- Reduction in Distortion
- Improvement in the System Signal to Noise Ratio