# Dynamic Range Compression and the Semantic Descriptor Aggressive
## 3 Preliminary Objective Tests
### Distortion Characteristics  
A series of total harmonic distortion (THD) measurements were made on the 1176 at various attack and release configurations to observe how time constant settings affected the distortion characteristics of the compressor. The measurements for release were made by keeping the attack time fixed at its fastest setting, seven, and making a measurement at each release position. The measurements for attack were made by restricting the release time to its quickest setting, seven, and making measurements at each attack position. During testing, the compressor was adjusted to achieve -10 dB of gain reduction and a 1 kHz test tone was used as the input signal. The output of the compressor was recorded on a Digital Audio Workstation (DAW) at 24 bit/44.1 kHz. The results showed distortion artefacts reduced significantly when using release times slower than position five and that the attack control had a smaller effect on the reduction of distortion. Furthermore, higher ratios had the effect of increasing non-linearity, with the all-buttons mode increasing non-linearity significantly more than any other ratio. Plot (a) in Figure 2 illustrates the effect of lengthening the attack and release time on THD. As can be seen, there is a sharp drop off in THD up to release position five and a small reduction in THD with attack times slower than position seven. Plot (b) in Figure 2 shows the same measurements made in all-buttons, which is a so-called \"special\" ratio mode afforded by the 1176 compressor. Although not originally intended for use, it was found by music producers that depressing all the ratio buttons simultaneously produced intriguing sonic behaviour by the compressor. In actual terms, the FET's bias is set outside of calibration, resulting in a significant increase in distortion. Note the much larger amount of THD in this setting, but similar drop off in amount as the release and attack speeds are reduced. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Setting** & **Release Setting Used** & **Attack Setting Used** \\ \hline
1 & 0\% & 46.67\% \\
2 & 0\% & 20\% \\
3 & 0\% & 20\% \\
4 & 18.18\% & 6.67\% \\
5 & 18.18\% & 6.67\% \\
6 & 63.64\% & 0\% \\
7 & 0\% & 0\% \\
1–4 & 0\% & 93.33\% \\
5–7 & 100\% & 6.67\% \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Popularity of 1176 time constant settings. The table illustrates how often a setting was used by professionals. Figure 3a,b are plots created using Matlab's THD function. They represent an FFT display which illustrates the nature of the harmonic distortion. To create the plots, a 1 kHz tone was input to the compressor to achieve -10 dB of gain reduction, with the time constants set for attack at three and release at seven. The output of the compressor using a 4:1 ratio and the all-butttons mode was then recorded into a DAW at 22 bit/44.1 kHz. The figures make clear the differences in distortion characteristics between the 4:1 ratio and the all-butttons mode. As one might expect, the distortion components are integers of the 1 kHz test tone and consist of a mixture of odd and even order harmonics. In Figure 3b, it is worth noting the significant increase in the amplitude of all components and also an increase in higher-level harmonics. Critical listening to an audio example of the compressed test tone reveals the all-butttons mode is highly distorted with increased brightness as a result of the additional high-level and high-order distortion components. The attack and release settings, which were shown to be commonly used in Table 3, are the settings that generate the most distortion. Thus, it appears that professional mix engineers are, perhaps albeit unbeknown to themselves, actively seeking out distortion from the 1176 in their workflow. Therefore, the aggressive sound quality once again appears to pertain to distortion. However, additional perceptual testing was required to substantiate this hypothesis.