Subject:Decode Dolby B or C in SF?

Posted by: Bill_Wood

Date:2/15/2003 11:08:53 AM

I am archiving old, one of a kind, cassette stereo recordings to digital media. I would like to do this with Sound Forge 6.0 in the most loss-less way possible. I plan to use a high quality stereo cassette deck without using the deck's Dolby B/C decoder to minimize further hardware losses on the archived copy.

Then I need to decode Dolby B and C when using the archived files in Sound Forge to produce audio CD's for listening purposes. Does anyone know if this can be done using a combination of Sound Forge tools, such as the use of the Graphic Dynamics and Graphic Equalization tools? Or is there a software Dolby emulator written by anyone that would be able to decode Dolby B and C digitally?

bill.wood@mindspring.com

Subject:RE: Decode Dolby B or C in SF?

Reply by: Chienworks

Date:2/15/2003 12:18:06 PM

Dolby B is almost entirely an EQ curve, though i don't know exactly what the curve is off the top of my head. The encoding emphasizes typical tape noise frequencies that generally aren't as loud in source material. The decoding reverses this EQ curve to restore the musical frequencies back to normal and reduce the tape noise. Turning a treble knob down a few dB matches Dolby B decoding to some degree.

Dolby C uses some of Dolby B's method, and also adds a sort of compression that raises the level of the quietest parts of the recording (where noise is the most noticeable), then reduces these sections back to normal on playback, reducing the noise during these sections as well.

To be honest, if you've got a high quality tape deck, the built in hardware decoders are probably just as good as a software solution. Keep in mind that any digital processing you do inside of SoundForge or other editing software introduces distortion as well.

Subject:RE: Decode Dolby B or C in SF?

Reply by: Bill_Wood

Date:2/15/2003 5:47:38 PM

Chienworks

I know I can always play the undecoded Dolby B and C tracks through a hardware decoder, but wanted to see if anyone has worked up a software decoder now. Dolby B and C both use sliding frequency compression and expansion techniques. That is where it gets tricky. I want to archive the undecoded audio tracks in case better software algorithms or hardware decoders are available in the future.

Bill

Subject:RE: Decode Dolby B or C in SF?

Reply by: Geoff_Wood

Date:2/15/2003 7:13:03 PM

Hi Bill,

I've been long seeking the same thing - more for Dobly A and SR, but these seem to not exist in software. The only way (obvious way !) is to transcribe them with a deck that has the approprite decoding, in the first place.

The hassle with the two types I'm after is that in addition to the high speed tape deck, I need to hire two Dobly 361 boxes and sets of *hardware* plugins !

Regards

Bill Wood... sort of (Geoff Wood, but my full name is William Geoffrey Wood !)

Subject:RE: Decode Dolby B or C in SF?

Reply by: jorgensen

Date:2/16/2003 6:50:45 AM

To decode Dolby B/C you must expand specific band(s) where we are sensitive to noise. I cannot remember the exact values for threshold and frequency any more.

Ozone has the capability of multiband expansion, and you may get better result this way than with a Dolby chip. The problem with Dolby encoding is the threshold must match the recording machine - and it rarely does.

You can find Dolby chip manuals on the internet which may help.

Subject:RE: Decode Dolby B or C in SF?

Reply by: Shaun

Date:11/9/2003 5:57:57 PM

I worked up a setting on the Multiband Dynamics plug-in to accomplish some amount of Dolby B decoding. I've had to play with it to get results that were acceptable, and having some idea of the Dolby curves and compression ratios would help. Any pointers would be appreciated!

What I did was use Multiband Dynamics to define an area of about one octave around 7 kHz, with a slow attack and slow release. The compression ratio and needs to be pretty high to cut the noise in softer passages, and the threshold must be low enough to catch the hiss in soft passages. You can try using the parametric EQ bandpass filter to cut out all but the higher frequencies, then use the Multiband Dynamics automatic threshold to find the right spot for the compressor to kick in.

smt

Subject:RE: Decode Dolby B or C in SF?

Reply by: captn_spalding

Date:11/10/2003 10:16:20 AM

I'm afraid jorgensen is correct. Your best shot is to playback on the same deck that did the recording. Failing that, it depends on what your goal is. If its conservation, play them back on the best deck you have with no decoding. If you can locate a Nakamichi (spelling?) Dragon cassette deck, use that. The Dagon will automatically adjust the playback head azimuth allowing you to approximate that of the original recorder. If your goal is restoration, search the web and find the dolby specs. Use them as a starting point and your ears as a guide to get the best sounding results. I urge you to save the original media and digital copies of the undecoded playback.

..spalding

Subject:RE: Decode Dolby B or C in SF?

Reply by: Rednroll

Date:11/10/2003 2:48:15 PM

I haven't used the SF Acoustic Modeler much of creating AM impulses, but was wondering if this could be a good application for it?