Anybody know how to expand an audio file compressed with HighCom compressor by Telefunken?
Oh my goodness no - that's over 40 years old now! And like Dolby-B it's virtually impossible to decode directly. A quote from one of the people who developed it (Ernst F. Schroeder) may begin to explain why:
"But there we had the outstanding principle of linearization by chain-connected amplifiers, a special generator for control signals, high-end Cassette tape recorders like Nakamichi 1000 or the new one under development at Telefunken, what could happen at all? The frequency dependent parts for the sliding-band effect were easily removed. The marginal masking of noise in a simple broad band compander was attacked with an additional pre/deemphasis, and the dreaded noise tails behind abruptly ending signals were attacked with a special generator for control signals that exhibited a dual time constant switchable between "slow" and "fast""
What this means in English is that there's a three-stage process to add the compression in the first place, and they've applied variable time constants to it as well, which will affect the attack times at different frequencies, and a final sting in the tail with this switchable time constant for reverb tails.
I should add that the reason they did all of this is because Telefunken were somewhat worried by potential litigation from Dolby (who have similarly never made a software decoder available). The original idea for telcom c4 worked better than Dolby-A, but it was considered too good by Telefunken for anything other than professional recorders, and so it was a stripped-back and much modified version of this that Nakamichi finally, reluctantly, agreed to use in their top-end N1000.
If you are really desperate, there are usually one or two standalone encoder-decoders available on Ebay, and these are the only devices that tend to work. Of course it would not be impossible for a software decoder to be created, but in the case of Dolby, they threatened to sue anybody who created one, and in the case of HighCom, there was no real market, because it rather rapidly sank under the onslaught of Dolby B.