Topic: dbx decoding help needed

[Professor chaos]

im transferring some gd cassettes that were made w/ a marantz pmd 430 w/ the dbx on. the original deck is not available. what is the best way to decode the dbx? im thinking that another pmd 430 is the best way to go. is there software for this? anyone have any opinions about this?

[Chilly Brioschi]

There are dbx II hardware boxes that will do it, but you may actually get better results by building out a DSP process chain that decodes each element of dbx for you.

ebay may have a cheap old deck which can get the job done

Here's something from the web that may help:

PLAYBACK RESPONSE
The reference level of the playback curve is +5dB which is confusing to me (I thought it would be 0dB or 3dB). So I will just verbally describe the dbx II overall record playback response curve as I see it on page 35.

It is essentially a flat +5dB between 100Hz and 1kHz. At 1kHz there is a linear rise to +10dB at 7kHz. Above 7kHz, there is what looks like an exponential roll-off from +10dB(7Hz)to 0dB at 20kHz. Below 100 Hz, it appears to exponentially roll -off to -10dB at 20Hz.

The Tape version is the same, except at 100Hz it rolls-off to -3db at 20Hz.

I'm sure there are better specs available somewhere, but hopefully this could get you started.

Those exponential roll-offs are compression, of course, so you will need to expand that range.
The consensus is that you'll use 2 to 1 expanding.
Keep an ear open for any pumping sound in that range, and if so, back it off.

The cassette version is slightly different, I'll hunt around, but an email to dbx may be in order   (was it dbx II?) 

They are still very much in the biz.
http://www.dbxpro.com/contact.htm


more stuff:
http://en.wikipedia.org/wiki/Dbx_(noise_reduction)
http://www.vintagedbx.com/

[Professor chaos]

would you sell the marantz?

[wklitz]

try checking with your boy Jimmy V or Plotnick.....they might have a loaner for you?

[popskull]

It seems like different recorders have different flavors of DBX....kinda like how dolby sounds better on some decks than others

I had a friend who made DBX masters on a Teac 4 track portable, and they would not sound good on my Yamaha kx800u with DBX............ditto with DBX tapes made on the Marantz 430

A friend lent me a DBX 224a noise reduction outboard unit and it was able to dial in the record and playback levels perfectly.....it has meters on the front to calibrate........it really bought out and optimized everything in those tapes

[morst]

I have lots of DBX encoded tapes, some recorded with the PMD430, some with other decks. I picked up a DBX 150 box on ebay some years ago and it works fine, and it was not very expensive. It says it is a Type I decoder. Hope that helps.

[DSatz]

morst, your model 150 is actually not appropriate for decoding those cassette recordings. There were two general categories of dbx noise reduction; the original kind (designed for professional open-reel recorders with flat frequency response across the audio band) was later relabeled as "type I" while the type used in consumer-oriented decks, including cassette decks, was type II. A tape won't sound right if you decode it with the other type of dbx unit from the type it was recorded with.

dbx noise reduction is made up of two circuits per channel. One is the detector circuit, the other is the compander circuit (which is controlled by the output of the detector circuit). The detector is mostly made of a filter and a rectifier; the filters of the type II circuit are far narrower than those of the type I circuit so that the type II circuit won't be affected so severely if there are low- or high-frequency inaccuracies in the recorder.

--best regards

[morst]

Yow! Guess I need a 224a or maybe a DBX home deck then, since my Marantz has always had tape skewing problems.

I didn't notice that they sound weird, but I'm glad to know the difference between I & II now, so I don't have to wind up redoing everything!

As always, thanks for the info DSatz! +T bigtime for this one!

[Professor chaos]

i have found that i get better results if i leave the nr (dolby B, C, DBX) off during the transfer an use an equalizer during playback. if i bump up the lows a little and turn down the highs a bit and the results are far better than if i had decoded the NR. i agree that the best way to decode NR is to do it with the master deck that encoded or at least the same model. if the same model is not available i guess it depends how it sounds. it would be nice if there was software for decoding noise reduction. i have not been able to find anything for noise reduction. i have found i nice plug in for winamp thats pretty cool. it has a dolby B decode and azimuth functions. heres a link. http://www.hansvanzutphen.com/tape_restore_live/download/