Decode tapes using Dolby Noise Reduction

This section is now closed
Forum rules
Audacity 1.2.x is now obsolete. Please use the current Audacity 2.1.x version.

Mac 0S X 10.3 and earlier are no longer supported but you can download legacy versions of Audacity for those systems HERE.
gmushlin
Posts: 11
Joined: Sun Jul 25, 2010 11:23 pm
Operating System: Please select

Decode tapes using Dolby Noise Reduction

Post by gmushlin » Sat Sep 25, 2010 5:15 pm

I have several reel to reel tapes that have been encoded with dolby noise reduction B, but do not have a machine that will decode it. I'd like to make digital copies with Audacity, and am wondering if there are any plug-ins or any other means to achieve this without having to buy a machine with dolby B.

Gale Andrews
Quality Assurance
Posts: 41761
Joined: Fri Jul 27, 2007 12:02 am
Operating System: Windows 10

Re: Decode tapes using Dolby Noise Reduction

Post by Gale Andrews » Sun Sep 26, 2010 12:24 am

gmushlin wrote:I have several reel to reel tapes that have been encoded with dolby noise reduction B, but do not have a machine that will decode it. I'd like to make digital copies with Audacity, and am wondering if there are any plug-ins or any other means to achieve this without having to buy a machine with dolby B.
I only ever knew one plug-in that attempted Dolby de-emphasis but that is no-longer available. So in a nutshell, reduce the volume level of the high frequencies using Effect > Equalization. This is lot less fiddly if you use Audacity Beta.

You can find the Dolby-B decoding curves near the bottom of this page:
http://hyperphysics.phy-astr.gsu.edu/HB ... tape5.html

Something in shape like the middle curve on the decoding graph but a little more pronounced and right-skewed would be reasonable. Once you draw a suitable Dolby-B decoding curve in Audacity Beta you can save it as a preset and reapply it easily without having to redraw it each time.



Gale
________________________________________FOR INSTANT HELP: (Click on Link below)
* * * * * Tips * * * * * Tutorials * * * * * Quick Start Guide * * * * * Audacity Manual

kozikowski
Forum Staff
Posts: 70140
Joined: Thu Aug 02, 2007 5:57 pm
Operating System: macOS 10.13 High Sierra

Re: Decode tapes using Dolby Noise Reduction

Post by kozikowski » Sun Sep 26, 2010 12:25 am

Dolby is a dynamic compander system. Compression and expansion of frequency response depends on the volume of the performance, so you can't just "roll off the highs" and call it good. Or you can call it good, but it won't be.

As you no doubt have already found, there are pages and pages of Googles starting with, "I have Dolby B tapes..." and followed by long threads of sympathy and explanations, but with no links.

I know of no Dolby A, B, or C plugin -- even open source. Dolby AC3 surround, yes. but not straight stereo audio.

Koz

kozikowski
Forum Staff
Posts: 70140
Joined: Thu Aug 02, 2007 5:57 pm
Operating System: macOS 10.13 High Sierra

Re: Decode tapes using Dolby Noise Reduction

Post by kozikowski » Sun Sep 26, 2010 12:28 am

<<<I only ever knew one plug-in that attempted Dolby de-emphasis but that is no-longer available. >>>

Under what circumstances?

Koz

kozikowski
Forum Staff
Posts: 70140
Joined: Thu Aug 02, 2007 5:57 pm
Operating System: macOS 10.13 High Sierra

Re: Decode tapes using Dolby Noise Reduction

Post by kozikowski » Sun Sep 26, 2010 12:34 am

===================================
It's a little worse that you think. Dolby B is the top processor from Dolby A. It has a sliding turnover frequency, IIRC 500 to 2KHz and its a compander in that low level HF was boosted up to 10 dB and loud HF boosted 0 during record encoding and the reverse during playback. Your processing has to change both the gain and the turnover frequency to properly decode it.
===================================


Koz

Gale Andrews
Quality Assurance
Posts: 41761
Joined: Fri Jul 27, 2007 12:02 am
Operating System: Windows 10

Re: Decode tapes using Dolby Noise Reduction

Post by Gale Andrews » Sun Sep 26, 2010 12:49 am

kozikowski wrote:<<<I only ever knew one plug-in that attempted Dolby de-emphasis but that is no-longer available. >>>
Under what circumstances?
Don't know what you mean by "circumstances", and I don't have any details to hand, but the plug-in claimed to do downward expansion of the quieter high frequencies as you suggest. My "in a nutshell" should definitely be read "as a compromise", but far better for gmushlin than doing nothing. That said, you can probably get Dolby-B tape decks fairly cheaply on eBay.



Gale
________________________________________FOR INSTANT HELP: (Click on Link below)
* * * * * Tips * * * * * Tutorials * * * * * Quick Start Guide * * * * * Audacity Manual

billw58
Forum Staff
Posts: 5675
Joined: Wed Aug 12, 2009 2:10 am
Operating System: macOS 10.15 Catalina or later

Re: Decode tapes using Dolby Noise Reduction

Post by billw58 » Sun Sep 26, 2010 1:46 am

Gale Andrews wrote:That said, you can probably get Dolby-B tape decks fairly cheaply on eBay.
Gale
Dolby B reel-to-reel decks?

A google search turned up the TEAC AN-60 external Dolby B decoder unit. None available on eBay worldwide.

-- Bill

billw58
Forum Staff
Posts: 5675
Joined: Wed Aug 12, 2009 2:10 am
Operating System: macOS 10.15 Catalina or later

Re: Decode tapes using Dolby Noise Reduction

Post by billw58 » Sun Sep 26, 2010 1:56 am

kozikowski wrote: Dolby B is the top processor from Dolby A. It has a sliding turnover frequency, IIRC 500 to 2KHz and its a compander in that low level HF was boosted up to 10 dB and loud HF boosted 0 during record encoding and the reverse during playback. Your processing has to change both the gain and the turnover frequency to properly decode it.
Don't know about the sliding turnover frequency, but the high-frequency band in Dolby C was above 9 kHz.

From wikipedia:
Dolby B was developed after Dolby A as a single sliding band system providing about 9 dB noise reduction (A-weighted), primarily for cassettes. It was much simpler than Dolby A and therefore much less expensive to implement in consumer products. Dolby B recordings are acceptable when played back on equipment that does not possess a Dolby B decoder
This is my recollection. IIRC Dolby B was deliberately designed so that if you have a Dolby B encoded cassette and no Dolby B deck to play it on, you can get "acceptable" results by turning down the treble control on your stereo.

This is about as good as it is likely to get for the OP unless he can lay his hands on a TEAC AN60 external Dolby B encoder/decoder.

There is one other (remote) possibility. If the OP knows an electronics hacker, it should be possible to hack up a cassette deck and inject the signal from the R2R into the appropriate point in the playback signal path and decode the Dolby B that way.

-- Bill

Gale Andrews
Quality Assurance
Posts: 41761
Joined: Fri Jul 27, 2007 12:02 am
Operating System: Windows 10

Re: Decode tapes using Dolby Noise Reduction

Post by Gale Andrews » Sun Sep 26, 2010 2:23 am

billw58 wrote:
Gale Andrews wrote:That said, you can probably get Dolby-B tape decks fairly cheaply on eBay.
Gale
Dolby B reel-to-reel decks?
Yes I've definitely seen them from time to time and they were (when I saw them) pretty inexpensive (under $100). There is a (Dolby version not specified) reel-to-reel tape recorder on eBay here, but certainly not cheap...



Gale
________________________________________FOR INSTANT HELP: (Click on Link below)
* * * * * Tips * * * * * Tutorials * * * * * Quick Start Guide * * * * * Audacity Manual

kozikowski
Forum Staff
Posts: 70140
Joined: Thu Aug 02, 2007 5:57 pm
Operating System: macOS 10.13 High Sierra

Re: Decode tapes using Dolby Noise Reduction

Post by kozikowski » Sun Sep 26, 2010 6:29 am

<<<Don't know what you mean by "circumstances">>>

Circumstances. Like: "I was Director of Engineering for IBM Content Management around 2002 and one of the participants in the "Skunkworks" outside of Poughkeepsie NY, wrote a Dolby B decoder module on the back of a Piggly-Wiggly napkin and brought it to work.

Richness of detail like we're always trying to get our posters to give us.

Koz

Locked