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Topic: how to apply Dolby on a wave file? (Read 6113 times) previous topic - next topic
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how to apply Dolby on a wave file?

hello, I need to apply "Dolby NR" (type "B")  to a wave file I have on my hdd, that was recorded from a tape. For some reason the dolby was not switched on when recording from the tape. Is there any software that can apply Dolby NR to a wave file?
thanks, J.M.

how to apply Dolby on a wave file?

Reply #1 –
I've seen this question many times on a number of forums. There appears to be nothing available, there probably are copyright issues that prevent it, and the BBC technical types seem to believe there are technical difficulties with doing the process in software.

how to apply Dolby on a wave file?

Reply #2 –
Shouldn't you be able to use some simple filter in an audio editing program? Dolby NR B does AFAIK a mild (compared to Dolby C) amplification of the higher frequencies before recording, and lower them at playback in order to reduce the higher frequency tape hiss. Exactly which frequencies, and how strongly they are amplified is another question.

Edit: Tip: If you have an old tape deck with Dolby B, record a sine wave (generated with an audio editor) with and without Dolby B, transfer it to PC, and compare them in a wave editor. The difference is your filter.

how to apply Dolby on a wave file?

Reply #3 –
Quote from: tycho on
Shouldn't you be able to use some simple filter in an audio editing program? Dolby NR B does AFAIK a mild (compared to Dolby C) amplification of the higher frequencies before recording, and lower them at playback in order to reduce the higher frequency tape hiss. Exactly which frequencies, and how strongly they are amplified is another question.

As far as I remember (long time since I used tapedecks) there is a certain level above which Dolby kicks in. I believe it constantly switches the filter on and off or something like that. It is a little bit more complex than a static filter. Probably nothing you couldn't let a computer do but you'd need to know the mechanism.
Every night with my star friends / We eat caviar and drink champagne
Sniffing in the VIP area / We talk about Frank Sinatra
Do you know Frank Sinatra? / He's dead

how to apply Dolby on a wave file?

Reply #4 –
You are right, I just checked the wiki. Actually I begin to remember how it sounded when not turning on Dolby B during playback. Loud parts was not affected, while quiet parts sounded strange.
Quote
Dolby B (and C which is similar) is a form of dynamic preemphasis. The background hiss of a tape (white noise) is unnoticeable if it is masked by a stronger audio signal, especially at higher frequencies. This is called psychoacoustic masking.

When the tape is recorded, the amplitude of the signal above 1 kHz is used to determine how much pre-emphasis to apply - a low level signal is boosted by 10 dB (Dolby B) or 20 dB (Dolby C). As the signal rises in amplitude, less and less pre-emphasis is applied until at the "Dolby level" (+3 VU), no signal modification is performed.

On playback, the opposite process is applied (deemphasis), based on the signal level. Thus as the signal level drops, the higher frequencies are progressively more strongly filtered, which also filters the constant background noise level.
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how to apply Dolby on a wave file?

Reply #5 –
Dolby B in software may indeed be 'not easy'  but it's not impossible either.  You need MAGIX Audio Cleaning Lab 2005.  The 2005 edition had this capability.  Previous and subsequent editions did not.  I can only guess why.  Their documentation is careful to describe it as 'Dobly B simulation.'  Here's a link to get you started:

http://www.floridamusicco.com/proddetail~p...MAG32600-wc.htm

You should also try a Google search for it.

I purchased this software explicitly for this one feature.  It does work and I hope you're able to find a copy.

how to apply Dolby on a wave file?

Reply #6 –
Quote from: woody_woodward on
Dolby B in software may indeed be 'not easy'  but it's not impossible either.  You need MAGIX Audio Cleaning Lab 2005.  The 2005 edition had this capability.  Previous and subsequent editions did not.  I can only guess why.


Mmm, patents.

-brendan

how to apply Dolby on a wave file?

Reply #7 –
The Dolby noise reduction stuff is "simply" a multiband compressor before recording to an analogue medium ("noisy channel") and a multiband expander after reading the signal from the analogue storage medium. So, it's a non-linear process (=> You can't test what exactly the DolbyNR black box does by sending sines of different frequencies to it).

I'd like to mention the similarity to perceptual coding for anyone who's interested:
multiband companding (Dolby NR) <-> PsyModel driven dynamic EQing via scalefactors or floor curves (MP3/AAC/Vorbis)
In both cases noise is introduced "in the middle" of the processing chain (noise due to analogue storage medium which Dolby NR tries to hide / intentional quantization noise in perceptual coding to reduce data rates). The audibility of this noise is reduced by exploiting masking effects via multiband compander / PsyModel-controlled noise weighting.

Re: how to apply Dolby on a wave file?

Reply #8 –
There is a free Dolby B encoder and decoder dolbybcsoftwaredecode.sf.net

Despite the name, it doesn't do Dolby C (yet) and is also an encoder.

It's written in Pascal but the author provides Windows EXEs

If you need it on Unix, I just translated it to C
released at codeberg.org/martinwguy/libdolbyb

Re: how to apply Dolby on a wave file?

Reply #9 –
Quote
As far as I remember (long time since I used tapedecks) there is a certain level above which Dolby kicks in.
And that's the problem.   There was a Dolby calibration level. There were calibration tapes with a "Dolby tone" for calibrating the decoding.   I think some studio tapes had a Dolby tone at the beginning (presumably Dolby C) so that level would have to be known so the studio could put it on the tapes.    (I don't know how the recording calibration was done.)   

Since there is no fixed calibration between tape-level and dBFS (digital) you'd need to record the Dolby calibration tape from the particular tape machine being used to get a dBFS calibration level.

The patents are expired but I don't know if the specs/details were ever published. 

Re: how to apply Dolby on a wave file?

Reply #10 –
Though I am not so sure I agree with the criticism on necroposting lately - if a thread is the source of information, then new audiences should likely read it - but the author of that eighteen-year old reply hasn't been logged in for a couple of years.

(But I have to admit that when I see such old threads brought up, I often suppose it is a bot.)

Re: how to apply Dolby on a wave file?

Reply #11 –
Speaking of which, someone made an "software-based" Dolby-B/C "compatible" "compander"
www.anaxwaves.com/DDiCodec/

Closed-source, is only available on like two digital storefronts (Apple App Store and Microsoft Store) and little else, and costs US$20 to boot, but for quality playback from your Dolby-B/C tapes this may be your only option.

Re: how to apply Dolby on a wave file?

Reply #12 –
Quote from: Lampion on
Speaking of which, someone made an "software-based" Dolby-B/C "compatible" "compander"
www.anaxwaves.com/DDiCodec/

Closed-source, is only available on like two digital storefronts (Apple App Store and Microsoft Store) and little else, and costs US$20 to boot, but for quality playback from your Dolby-B/C tapes this may be your only option.

Really specific and detailed, thank you