dbx noise removal

I’m using Audacity to digitize some old cassette tapes that were recorded on a Portastudio with dbx noise removal. Is there a way in Audacity to decode the dbx?

The way I understand it, the dbx noise reduction just compresses the signal. The player is expected to use an audio expander to playback the signal. I’m not aware of an audio expander that Audacity can use. But if you find a Nyquist plugin that will do that, then let us know.

How did you transfer the audio from the tapes ?

If you did it from a deck without dbx than you have essentially applied an EQ curve that will vary though out the material. If you used a dbx capable deck than you don’t anything to decode.

One of the most obvious clues that there is dbx that’s been decoded is the pumping of dynamics, you’ll hear it,

DBX isn’t noise removal–at least the common one isn’t.

DBX (and Dolby) are distortions that are added to the sound to make the show better compete with the noise already on the tape or other transmission medium. The rough idea is to leave the loud sounds alone and boost the volume of the quieter sounds. At the playback, the electronics puts the volume of the lower sounds back where it belongs leaving the volume of the loud sounds alone.

So yes, it does sound like an engineer rapidly turning the volume controls up and down as needed. That’s basically what the electronics do according to a very special formula. So it’s not a simple filter to get the original show back. You do actually need a DBX or Dolby decoder. Then you need to mess with it to make sure it takes out the damage that the encoder put in.


Yup the infamous ‘breathing’ or ‘pumping.’

The original DBX 160 compressors are popular for the distortion they add.

<<<the infamous ‘breathing’ or ‘pumping.’>>>

OK, that’s a little sticky. I usually associate classic “pumping” with a straight-line audio compressor or mis-adjusted limiter rather than DBX. The vocal vanishes in time with the much larger bass notes. Although the effect of a badly adjusted DBX chain can sound like that, it’s usually not so simple. An orchestra swell will cause the effect, not just bass notes–and again, according to a specific formula.

I believe both companies took into account the characteristics of the ear.

Dolby, and a much less significant case DBX cannot be heard while they’re running. That’s one reason you still know who Dolby is after all these years. (Dolby Digital 5.1 Surround) It can be made to work and work very well.

Anybody making a software DBX Decoder? I couldn’t find one.


You don’t get software dbx / Dolby A/B/C/S decoders for exactly the reason you mentioned - they need to be set up right, and most people don’t have the kit to do so. In order for the expansion at playback to match exactly the compression on recording there must be a fixed relationship between the magnetic flux density on the tape (absolute record level) and the compressor threshold (or equivalent level sensing circuit). This is possible within a tape machine where the whole circuit is in one box, but very much harder when you have the tape machine, then gain controls, ADCs and so on to reach the level sensing in software.

In studio kit where Dolby A was often an out-board unit there was always a level meter and a test tone switch on the unit, so that at record time you could put a tone on the tape at the “dolby reference level”, and then on playback you adjusted the gain to get that onto the dolby mark on the meters, so that the whole lot lined up. Older consumer gear often still has the dolby mark on the meters (approximately) but doesn’t have the test tone, and I’ve never found a cassette tape with a reference tone on it.

Thanks for all your replies to my question. Since my original deck with dbx is defunct, I had been hoping I could play my cassettes into Audacity on another deck and simulate the dbx decoding digitally. It looks like I’ll have to get the deck fixed or find another.

Just stopped around to say that I had some success in decoding dBx type II using this plugin:

I used the following settings:
Compress ratio: -0.50
Release speed: 1000ms (you need to enable this control with text editor)
Attack speed: 50ms
Compression hardness: 1.0
Floor: -50 dB

After that I reduced the high frequencies a bit with the equalizer.

Result is acceptable (the volume pumping is gone), but probably not the most accurate decoding.

Very nice tip jpa.
I wonder if you would be able to make a short before/after demo recording and upload it somewhere then post a link here on the forum?
As you say, it won’t be the most accurate decoding, but theoretically you should be able to get significantly better results than just filtering. I’d be very interested to hear how well (or otherwise) this technique works.

(topic moved to “Audio Processing” section)

I realise that this is an old post, but the link to the plugin above is broken. Can anyone tell me the name or author/vendor of the plugin so I can track it down, please? I have the same issue as in this thread - old tapes recorded with dBX noise reduction.

If I recall correctly, that was “Chris’s dynamics compressor”.
Sadly the author Chris Capel died a few years ago, but his plug-in is still available here: Chris's Dynamic Compressor plugin for Audacity

Thank you for the suggestion. I know this forum question is fairly old. I will look for a clean version of that filter, as the link now seems to be a trojan horse. Just a warning to others!

Meanwhile, I have a dbx type II decoder but need to decode tapes that were recorded with Type I encoding. Will report any success that I have had here.

Thank you. I posted a Trojan warning reply to the earlier post mentioning this file, before I read far enough to see that this new link is still good. I am looking forward to using this filter on some dbx encoded recordings. I will definitely share any accounts of success (or failure) worth noting.

Note: It is sad to learn that the developer has passed on. I am glad when others keep the torch lit (as has been done with some of the excellent “Power Tools” app’s by the late Karen Kenworthy).

I think it’s just that the plug-in is no longer there. VirusTotal shows the link to be clean (See: VirusTotal), but it is good to be cautious.

Chris’s final “experimental” beta version of his compressor is available from this forum post: Chris Capel's Dynamic Compressor

The 1.2.6 “release” version is still available via this third party site: Chris's Dynamic Compressor plugin for Audacity