twotoedsloth
Joined: 26/01/08
Posts: 457
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EQ Curve for Dolby B
#972121 - 24/02/12 07:34 PM
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Greetings,
Is there an easy way to remove Dolby B from a cassette recording,
for example, a lowpass filter?
Thanks
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James Perrett
Joined: 10/09/01
Posts: 9645
Loc: The wilds of Hampshire
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Re: EQ Curve for Dolby B
[Re: twotoedsloth]
#972131 - 24/02/12 08:51 PM
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There's no easy way as the effect of Dolby B is level dependent - and it isn't just a
simple high frequency expander either. You've really got to decode it in your playback
cassette deck - and also tweak the level going to the Dolby decoder in order to match the
settings to the tape. James.
-------------------- JRP Music - Audio Mastering and Restoration.
http://www.jrpmusic.net
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Hugh Robjohns
SOS Technical Editor
Joined: 25/07/03
Posts: 18347
Loc: Worcestershire
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Re: EQ Curve for Dolby B
[Re: twotoedsloth]
#972143 - 24/02/12 09:49 PM
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Why would you want to? If it's a recent recording you play it back with the Dolby b
decoder switched in; if it's an old recording then self erasure will more or less counter
it anyway so play back without.
Hugh
--------------------
Technical Editor, Sound On Sound
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twotoedsloth
Joined: 26/01/08
Posts: 457
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Re: EQ Curve for Dolby B
[Re: twotoedsloth]
#972149 - 24/02/12 10:21 PM
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Thanks for your replies. It's an old 1/4" 4 track recording at 3.75ips, and the label on
the tape says "Dolby B", but my Sony tape deck doesn't have Dolby, it just has "noise
suppress" which sounds quite dull when engaged.
Should I copy the open reel
tape to cassette and then play that back with Dolby B engaged on a cassette player?
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Hugh Robjohns
SOS Technical Editor
Joined: 25/07/03
Posts: 18347
Loc: Worcestershire
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Re: EQ Curve for Dolby B
[Re: twotoedsloth]
#972190 - 25/02/12 09:03 AM
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I'd just play it back without Dolby B. That system has relatively subtly dynamic
processing and only a very mild HF lift, both of which quite closely match the effects of
age-related self erasure in most tape formats.
So I'd play the tape back
without Dolby B and tweak the HF balance with conventional EQ if you feel it necessary.
I'm quite certain the results will be entirely acceptable, especially given the likely
quality of a 3.75 ips master.
hugh
--------------------
Technical Editor, Sound On Sound
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twotoedsloth
Joined: 26/01/08
Posts: 457
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Re: EQ Curve for Dolby B
[Re: twotoedsloth]
#972270 - 25/02/12 06:17 PM
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You're correct. Thanks. The recording sounds fine without Dolby, seeing as this is just
an archival job, I can't imagine the guy is going to complain.
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ken long
Joined: 21/01/08
Posts: 4275
Loc: The Orient, East London
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Re: EQ Curve for Dolby B
[Re: twotoedsloth]
#972272 - 25/02/12 06:37 PM
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"just" and archival job?
Sacre Bleu!
--------------------
I'm All Ears.
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twotoedsloth
Joined: 26/01/08
Posts: 457
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Re: EQ Curve for Dolby B
[Re: twotoedsloth]
#972276 - 25/02/12 06:58 PM
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Well, it is for a retired piano professor, so he's not likely to hear the extra high end
tape hiss anyways, is he? The tapes were recorded between 1982 and 1991.
Where
does Dolby B start "enhancing" the high frequency content?
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ken long
Joined: 21/01/08
Posts: 4275
Loc: The Orient, East London
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Re: EQ Curve for Dolby B
[Re: twotoedsloth]
#972282 - 25/02/12 07:40 PM
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I was only joking. I think you've done well so far considering... 
-------------------- I'm All Ears.
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Hugh Robjohns
SOS Technical Editor
Joined: 25/07/03
Posts: 18347
Loc: Worcestershire
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Re: EQ Curve for Dolby B
[Re: twotoedsloth]
#972295 - 25/02/12 08:58 PM
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Quote twotoedsloth:
Where
does Dolby B start "enhancing" the high frequency content?
Above 4kHz for quiet signals, with the
turnover sliding up as the signal gets louder.
Hugh
--------------------
Technical Editor, Sound On Sound
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James Perrett
Joined: 10/09/01
Posts: 9645
Loc: The wilds of Hampshire
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Re: EQ Curve for Dolby B
[Re: twotoedsloth]
#972545 - 27/02/12 10:36 AM
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The Dolby may accentuate the initial attack of the notes but probably won't have much
affect on most of the piano sound. One thing I would do is to keep a copy of the straight
transfer before trying to reduce the noise in any way. If you happen to acquire a Dolby
unit in the future you will then be able to decode the tapes properly. James.
-------------------- JRP Music - Audio Mastering and Restoration.
http://www.jrpmusic.net
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twotoedsloth
Joined: 26/01/08
Posts: 457
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Re: EQ Curve for Dolby B
[Re: twotoedsloth]
#972680 - 27/02/12 09:03 PM
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I guess I can give him copies of the CDs with and without convolution noise reduction (I
use Waves Z-noise). I'm not likely to ever look at these again, even if I do come across
a hardware Dolby decoder. Do you think it's worthwhile to give him 24 bit .wav files on a
DVD instead of a standard 16 bit audio CD for the non-processed files?
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ken long
Joined: 21/01/08
Posts: 4275
Loc: The Orient, East London
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Re: EQ Curve for Dolby B
[Re: twotoedsloth]
#972683 - 27/02/12 09:26 PM
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If he plans on processing them further then give him the raw transfers 24-Bit. I'd also
give him any attempt you've made at noise reduction at 16-Bit 44.1kHz. So 2 of each.
Depends what he wants really.
--------------------
I'm All Ears.
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