[Ardour-Users] Automation from analog mix?

[Giso Grimm]

Am 12/18/2010 02:56 PM, schrieb Jörn Nettingsmeier:
> On 12/18/2010 10:35 AM, David Kastrup wrote:
>> Arnold Krille <arnold at arnoldarts.de> writes:
>>> No, the 16 channel converter is already there to record the main
>>> signal directly after the pre-amps. You need another 16 channels for
>>> the signal after the fader to get all the information you want for
>>> automation of eq, gate, compressor and volume.
>>
>> Nope.  Just the stereo mix at the end.  It has all the required info
>> after decorrelation ("echo compensation techniques") if you take into
>> account that changes are confined to small bursts of activity.
> 
> i don't quite grasp what decorrelation or echo compensation techniques
> have to do with it.

It is simply an adaptive filter technique, used in echo compensation (in
nearly any modern phone or VoIP software), used for feedback
cancellation in hearing aids or PA systems, used for noise cancellation
in cars and active noise cancellation headphones for planes. Adaptive
filters are even used to control heating systems and many more simple
tasks - they are designed to estimate quasi-linear black box systems.
They can involve estimation of time-depending complex filters (e.g.,
feedback cancellation, echo compensation), but they also can be used to
estimate a single gain only (the more constraints can be applied to the
resulting filter, the more stable is the estimate, and it can be adapted
faster and with less pre-conditions to the input signal (e.g., low
auto-correlation). This is everyday technology. And the stereo mix would
be sufficient (in combination with the inputs): If you see your mixing
console as a time dependent matrix operation, X(t) is your input signal
(with many channels, lets say N), H(t) is your time dependent mixing
matrix with size Nx2, then your stereo mix Y is Y=H*X, and adaptive
filtering is nothing more than estimating H. The method provides an
error estimate. And if it does not have to be real-time, then the
estimation definitely can be improved, especially to find the initial
states.

> 
> in any case, i really doubt this is feasible. even less so if you start
> using interesting spatialisation techniques - just imagine the amount of

I am sure that this method is definitely feasible, as long as the
applied filters are linear and quasi-stationary, and if we have the
input signal. If the mixing is ambiguous, then it doesn't matter if the
estimated mixer is not correct - the result will be the same. Estimating
non-linear operations is another topic, not impossible either, but far
more complex.

- Giso


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