I used Telecom C4 a couple of times but much preferred the sound of Dolby
A. Dolby A used sided chained compression of signals on record (and
corresponding expansion on replay) so only low level signals where affected
- over the Dolby Level there was essentially no affect on the signals and
they passed straight through. A good deal of psychoacoustic work was done
to ensure that artefacts such as noise pumping were well hidden, including
splitting up the signal into several frequency bands so that signals in one
band didn't affect others and time constants could be optimised. Telecom C4
also split into bands (for the same reasons) but didn't sidechain so
compression was applied at all signal levels. This meant you got more
noise reduction but also more artefacts, though not as much as DBX which
used neither sidechain nor bands. Dolby A, because of its sidechains had to
be aligned to within half a dB of the correct level or the process went
wrong because the compression thresholds went out but neither C4 nor DBX
needed anything other than aligning to optimise machine dynamic range as
they had no thresholds. This would seem to make Dolby A more difficult to
use but as every Dolby A unit incorporated a proper Dolby Tone generator
which produced a unique warbling test tone that all Dolby A tapes should
have on them, this shouldn't be a problem.
Dave
On 30 April 2013 06:20, Kees de Visser <[email protected]> wrote:
> On 30 Apr 2013, at 04:56, David Pickett wrote:
>
> > A standalone Windows app that would decode Dolby-A encoded wavefiles and
> output a restored non-Dolby 24-bit wavefile would be most useful. I have
> several recordings that I have had transfer to hi-res files still in
> Dolby-A format.
> >
> > ... even if such a program were command line only and needed to be left
> overnight to cook!
>
> The "DSP" friend I mentioned before has written a software Telcom C4
> decoder for a client (custom made, not for sale). Telcom was a German
> (Telefunken) tape noise reduction system, equivalent (claimed superior) to
> Dolby. Just to say that it can be done if there's (financial) interest.
> IIRC Telcom was less critical than Dolby regarding playback calibration, so
> decoding Dolby wav files might be a bit more complex.
>
> Kees de Visser
> Galaxy Classics
>
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>
--
As of 1st October 2012, I have retired from the University, so this
disclaimer is redundant....
These are my own views and may or may not be shared by my employer
Dave Malham
Ex-Music Research Centre
Department of Music
The University of York
Heslington
York YO10 5DD
UK
'Ambisonics - Component Imaging for Audio'
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