Dolby B/C software?
"Chris Hornbeck" wrote in message
...
On Mon, 27 Oct 2008 17:06:49 +0000, Don Pearce
wrote:
Arny Krueger wrote:
What about decoding existing Dolby-encoded recordings?
How would you perform the calibration?
I'll have to disagree with several august posters about this.
Dolby B and C levels are referenced to magnetic levels, so any
properly recorded tape will be properly de-encoded if the
playback machine has been calibrated with a reference-level
tape, 400Hz at Dolby reference level of 200nW/m.
You've got to calibrate the entire transcription process including both the
playback machine and the recording process on the computer. I presume that's
what you mean by calibrating the playback machine. In this case the
"playback machine" is two interconnected boxes and 2 pieces of software.
Assuming that everything else is up to snuff, natch. A pretty
big assumption these decades later... but fundamentally right.
Dolby B and C do not require tones on every tape for de-encode
tracking.
After testing the software Dolby B decoder, and also examining the Dolby B
specs on their web site, here's what I would do:
(1) Play the calibration tape and digitize it, making sure that I also knew
the settings of the various level controls that would be involved in the
recording. In my case there would be two - one the output level on the
cassette machine, and the other the record input gain on the digital audio
interface.
(2) It appears that the software Dolby B decoder is set up so that Dolby
level = digital full scale. So, I would find out how much amplification I
had to apply in the digital domain to get the tone I recorded off the
calibration tape to be FS. I would then apply that much gain to any tapes I
transcribe, before passing them through the software Dolby digital decoder.
This whole process strikes me as being the Y2K analog for how one used an
outboard Dolby Digital decoder in the day of. Been there, done that. ;-)
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