On Wed, May 01, 2013 at 08:02:20PM +0300, Sampo Syreeni wrote: > On 2013-05-01, Dave Hunt wrote: > > >Dolby A (etc.) decode. Would it not be possible to to do this with > >convolution ?? Find a working unit, record its impulse response, > >use that in one of the many convolution reverb/filter plug-ins. > > Unfortunately this is not an option. Convolution can be used to > model any system that is linear and time invariant
While I agree with the 'not an option', the motivation is not entirely to the point. Convolution processors *can* be used to model non-linear systems, e.g. guitar amps producing lots of distortion, or an analog telephone channel. Angelo Farina wrote a very interesting paper about that some years ago. The point is that companders such as Dolby-A and Telcom are linear at least over a short time span - they do not introduce distortion. But they are certainly not time-invariant - they would be useless if they were. The compander gains (and hence the IR) depend on the current input signal and its short term history. Now even time-variant systems can be emulated using convolution, but the two issues are orthogonal: using convolution processing of the audio signals does not help in any way to simplify the implementation of the dynamics. The difficult part in writing any software emulation of the Dolby-A or similar systems is modelling the dynamic behaviour of the compander, not the actual audio processing. Such systems will have 'designed' and documented attack/release times, but analog electronics being what they are, the dynamic behaviour of the compressors and expanders will very probably not fit to any simple equations. Nor does it have to: the way Dolby-A and Telcom switch between encoding and decoding (by making the decoding algorithm the mirror image of the encoding) guarantees that the two will cancel each other, whatever they are. Ciao, -- FA A world of exhaustive, reliable metadata would be an utopia. It's also a pipe-dream, founded on self-delusion, nerd hubris and hysterically inflated market opportunities. (Cory Doctorow) _______________________________________________ Sursound mailing list Sursound@music.vt.edu https://mail.music.vt.edu/mailman/listinfo/sursound
