AES Journal Forum

Thanks for your comments. The Fidelity of the Envelope Shape (FES) is close to what might be a good measure of compressor quality or performance. But it just looks at correlation between the envelope before and after compression. A better measure might be something that looks at the temporal aspects of the envelope, so that a good compressor preserves much of the sustain, decay and release. Something to look at in future.

And yes, the log versus linear domain compressors is one of the biggest differences between compressor designs, and often overlooked in basic designs.

Thanks for your answer. Surprisingly, there is another paper that is stating that Linear domain and Log domain are equivalent. It shows the mathematical proof that it gives the same transfer function. I would like to have your feedback on this. Here is the article: Attack and release time constants in RMS-based feedback compressors by Fred Floru (presented at the 104th convention, may 1998.) There is a pdf containign the preprint.

Regards

Thanks for your comments.

I'm aware of this paper, and in fact cited the revised and extended version,

F. Floru, “Attack and Release Time Constants in RMS-Based Feedback Compressors,” vol. 47, pp. 788–804 (1999 Oct.).

But if all other aspects are equal, then applying attack and release on a linear estimation of input signal level clearly gives different results from applying them on a logarithmic estimation of signal level (or on the logarithmic gain). This is seen in Fig. 11 and Table 1 of our paper, and commented on in Sonnox, “Dynamics Plug-In Manual,” Sonnox Oxford Plug-ins, April 1st 2007.

You can also clearly see the difference between Eq.s 20 and 23 in our paper.

I think there are a few reasons why Floru's result, though very interesting and significant, does not apply;

- He has slightly different definitions of log and linear domain

- He deals only with RMS detectors, and in fact requires that the RMS detection be log domain when applying attack and release in log domain, and be linear when applying attack and release in linear domain

- Technically, transfer function does not apply, since it assumes a linear system. The output of a compressor is, of course, dependent on signal level as well as frequency. Describing functions may be more appropriate, since they describe nonlinear relationships where the input/output relationship is a function of both frequency and amplitude. But even they assume a static nonlinearity, and thus really only model clipping or hard limiters.

Of course, Floru's paper(s) on the subject are very detailed and fairly complex, so without a thorough reading, I can't say for sure why he got that result and what the implications are.
 

Best regards,

Josh