Dolby processing in AA
Bert:
The recent discussion about squeaky cassettes proved that still many people are interested in playback of this material. A common problem is Dolby processed tape. In most cases this is Dolby B or C as the professional Dolby A is much more complicated and rare. I would therefore like to have the possibility to do the Dolby playback processing in AA for type B and C.
At the first instance this seems to be stupid idea as most players that have survived have switches you may set accordingly. But Dolby works fine only when adjusted carefully to the brand of tape used and within the machine used for recording AND playback. This calls especially for careful level alignment that has to be equal in recording and playback. In most cases neither the playback machine is the same as for recording, nor is the alignment correct for the actual tape. Even then minor deviations in track width or track position may cause an offset of several dB’s and impair the process.
Processing in AA would therefore allow for different settings of the levels. Although there is no objective indication for the correct level, the possibility of shifting up and down could help to improve by experiment. Does anyone know about such a SW-tool that can be used for that purpose ? If not this might be an urgent wish for a new edition of AA ! Please concentrate the discussion on a technical level first as I know pretty well that this would eventually implicate negotiations with the Dolby labs.
pwhodges:
For playback, only the level needs to be aligned; on most decks it's not that hard to check and adjust the alignment with a test tape. The tape type affects recording in more ways (e.g. bias and equalisation, as well as level).
Paul
SteveG:
Quote from: Bert on February 08, 2011, 06:11:47 PM
Please concentrate the discussion on a technical level first as I know pretty well that this would eventually implicate negotiations with the Dolby labs.
Can't do the technical stuff first, because Dolby beat everybody to it decades ago, by the very simple device of flatly refusing any permission for this at all. It was discussed quite extensively in at least one thread in the forum archives, but nothing's changed. They regard the processing algorithms as proprietary, and would sue anybody successfully implemeting a decode and releasing it in public.
Bert:
Quote
For playback, only the level needs to be aligned; on most decks it's not that hard to check and adjust the alignment with a test tape. The tape type affects recording in more ways (e.g. bias and equalisation, as well as level).
I know pretty well how to align a tape deck for Dolby processing both on the recording as on the playback side. However when I intend to transfer a number of (my own) cassettes to CD it is extremely impractical to run the alignment process for each tape individually. Nobody in practice would take the pain to dismantle the deck and run the test tape again and again. The result is that you rather start tweaking the frequency response in a wild manner.
Quote
They regard the processing algorithms as proprietary, and would sue anybody successfully implemeting a decode and releasing it in public.
I remember processings named DLPF, DNR, NR-Exp or similar to circumvent the Dolby playback patent by a number of companies such as ITT, Philips and others. These algorithms have been published, and I am not sure to posses at least descriptions of the basic parameters governing the Dolby processing as well. Beyond that, Dolby should be aware that they are on a sinking ship in the digital age.
pwhodges:
Quote from: Bert on February 08, 2011, 10:41:14 PM
it is extremely impractical to run the alignment process for each tape individually
But there is only one correct alignment for playback, which should be stable in normal use. Or are you suggesting mis-aligning for each tape played to match a presumed mis-alignment during recording? In which case a test tape is irrelevant, except to provide a reference for the mis-adjustment; and judging the mis-alignment would be wildly difficult, I would expect.
Paul
Navigation
[0] Message Index
[#] Next page