dbx decoder?

igor_levicki · Dec 3, 2003, 04:59 PM

Does anyone know is it possible to implement as DSP code?
It would be very interesting to have it. Does anyone know
any details on dbx?

**Eugene Gavrilov** · Dec 4, 2003, 08:38 PM

you probably could have asked "does anybody know what dbx decoder is"

/Eugene

igor_levicki · Dec 4, 2003, 08:59 PM

Ok, dbx was a noise reduction feature on more expensive tape decks like Technics RSB100 or RSM245 and similar. It was rather good - at least much better than any Dobly (B, C...). I thought that someone here might know more than me about it, sorry.

**Eugene Gavrilov** · Dec 5, 2003, 09:39 PM

the details can be found on the dolby site (www.dolby.com)

>> I thought that someone here might know more than me about it

some people here probably do... who knows?

as for the implementation, dbx seem to be useful for analog =recordings= and I doubt a realtime DSP implementation is really necessary... but any DSP programmers are welcome to investigate this

/Eugene

NightBreed · Dec 6, 2003, 02:33 AM

Couldn't a dbx decoder be used to cut alot of noice coming from a mic source?

Kevin Horden · Dec 6, 2003, 07:14 AM

I can't see what possible use a DBX encoder / decoder would be. It's for tape noise reduction. The encoder is just a 2:1 compressor and the decoder is a 2:1 expander. the idea is just to put as much recording level down on tape as possible. A decoder on it's own wouldn't reduce noise.

marschw · Dec 9, 2003, 05:06 AM

Hmm, I thought dbx was also the name of a stereo audio standard for TV, could that be what he meant? As I understand it, I don't think it's possible to do outside of a TV card, though, since I don't think the stereo data is encoded in the mono audio stream itself.

-=Marcus

HunterZ · Dec 9, 2003, 09:24 AM

Really? TV audio is broadcast on FM on a slightly different band than music radio. I'd be surprised if music and TV use different means of encoding stereo channels.

marschw · Dec 14, 2003, 07:30 AM

As I understand it, dbx is one of several encoding methods used for TV stereo; I think the standard used is country-dependant.

-=Marcus

HunterZ · Dec 14, 2003, 12:18 PM

That's the great thing about standards - there're so many to choose from!

igor_levicki · Dec 17, 2003, 09:16 PM

it is not only compression and expansion - it has preemphasis and deemphasis but I couldn't figure out the exact crossover frequencies and the amount. It used to decrease power of bass and boost trebles while encoding and decoding was just a reverse process. It gave almost 90dB SNR on a plain tape. It was (and is perhaps still used) for radio and as you say probably for a tv. I am just interested in decoder though so I can recover some tapes that are encoded and sound like crap without dbx decoding.

Kevin Horden · Dec 18, 2003, 06:38 AM

DBX is just compression and expansion. There are no frequency dependent components.

See http://hyperphysics.phy-astr.gsu.edu.../tape4.html#c3

igor_levicki · Dec 18, 2003, 07:00 PM

Perhaps you are right, here: http://music.concordia.ca/Programs/E...inciples.html, but I still think that it has something to do with freq. since when you play it back without decoder high freq. sounds louder and bass freq. sounds quieter.

igor_levicki · Dec 18, 2003, 07:04 PM

No, I am right! It was this -- DBX type 2 noise reduction A simplified version of the DBX noise reduction system intended for use with systems with limited bandwidth such as cassette recorders, 8 track recorders and carts. The type I system did not work very well with those types of systems since it works on a fairly linear basis across the spectrum so if the recorder had problems with bandwidth, phase shifts, dropouts or any sort of notches or bumps along the frequency spectrum the type I decoder would have problems. The type II system solves these problems by having a greater pre-emphasis on the detection circuit and by using an RMS detection system that meant that the units are much more immune to tracking errors, a long running problem with DBX type I, Dolby and DNR based systems (and the need behind the Dolby HX Pro system), but for the same reason a type II codec should never be used with a recorder that has a flat response like a high end open reel or a digital unit, the added emphasis will result in distortions and loudness errors in the upper frequencies. The theoretical dynamic range expansion gained by using type II is the same as for type I or about double, although in practice I find it to be somewhat less. While the dbx company tried hard to get type II codecs integrated into consumer equipment and for a time in the 80's archived some success especially in the USA where a few labels issued audiophile cassettes using the system, today however it's almost never seen integrated outside of a few multitrack cassette recorders. But there are still quite a few home users of the system and you can still find the technology used in broadcast situations where type I is not appropriate.

Gonzo · Dec 23, 2003, 03:41 AM

Igor:
Count with me too, on this subject!
I'm currently transfering all my old cassettes to CD and I''ve found exactly the same problem.
Half of the tapes were encoded with DBX, and half with Dolby B an C.
I would give all I have just to find some kind of software decoder for these noise reduction systems.
The problem is that long years ago I sold my Yamaha cassette multitracker, the one with DBX codec (don't know if it was type I or II though), and my (Yamaha too) cassette deck has worned out the playback head, so it causes problems with the alignment and this in turn makes that al my Dolby encoded cassettes doesn't decode in a proper way.
I'm really worried because I have thousand of hours of my own music recorded there, thing that I love from the past...so if we could find some kind soul that would be interested in programming a decoder for all this ancient technology...
Let's keep searching...

Cheers
GONZO

Dec 3, 2003, 04:59 PM	#1
igor_levicki Intel Black Belt Join Date: Dec 2003 Location: Belgrade, Serbia Posts: 128 Rep Power: 0 System Specs	dbx decoder? Does anyone know is it possible to implement as DSP code? It would be very interesting to have it. Does anyone know any details on dbx? __________________ Regards, Igor Levicki -- Creative Audigy 2 ZS Retail (SB0350) kX 3538h

Dec 4, 2003, 08:59 PM	Thread Starter #3
igor_levicki Intel Black Belt Join Date: Dec 2003 Location: Belgrade, Serbia Posts: 128 Rep Power: 0 System Specs	Ok, dbx was a noise reduction feature on more expensive tape decks like Technics RSB100 or RSM245 and similar. It was rather good - at least much better than any Dobly (B, C...). I thought that someone here might know more than me about it, sorry. __________________ Regards, Igor Levicki -- Creative Audigy 2 ZS Retail (SB0350) kX 3538h

Dec 9, 2003, 09:24 AM	#8
HunterZ DriverHeaven Addict Join Date: May 2003 Location: Seattle, USA Posts: 257 Rep Power: 0	Really? TV audio is broadcast on FM on a slightly different band than music radio. I'd be surprised if music and TV use different means of encoding stereo channels. __________________ Athlon XP 3200+, Gigabyte GA7N-400Pro2 Rev 2.0 (nVidia nForce2 chipset), 1 GB DDR400 (CAS 2.0, dual-channel), ATI Radeon 9800 Pro 256MB AGP 8x, WinXP Pro SP2, DX9.0c., Logitech Z-5500 5.1 speakers

Dec 14, 2003, 12:18 PM	#10
HunterZ DriverHeaven Addict Join Date: May 2003 Location: Seattle, USA Posts: 257 Rep Power: 0	That's the great thing about standards - there're so many to choose from! __________________ Athlon XP 3200+, Gigabyte GA7N-400Pro2 Rev 2.0 (nVidia nForce2 chipset), 1 GB DDR400 (CAS 2.0, dual-channel), ATI Radeon 9800 Pro 256MB AGP 8x, WinXP Pro SP2, DX9.0c., Logitech Z-5500 5.1 speakers

Dec 17, 2003, 09:16 PM	Thread Starter #11
igor_levicki Intel Black Belt Join Date: Dec 2003 Location: Belgrade, Serbia Posts: 128 Rep Power: 0 System Specs	it is not only compression and expansion - it has preemphasis and deemphasis but I couldn't figure out the exact crossover frequencies and the amount. It used to decrease power of bass and boost trebles while encoding and decoding was just a reverse process. It gave almost 90dB SNR on a plain tape. It was (and is perhaps still used) for radio and as you say probably for a tv. I am just interested in decoder though so I can recover some tapes that are encoded and sound like crap without dbx decoding. __________________ Regards, Igor Levicki -- Creative Audigy 2 ZS Retail (SB0350) kX 3538h

Dec 4, 2003, 08:38 PM	#2
Eugene Gavrilov kX Project Lead Programmer and Coordinator Join Date: Dec 2002 Posts: 3,119 Rep Power: 75	you probably could have asked "does anybody know what dbx decoder is" /Eugene

Dec 5, 2003, 09:39 PM	#4
Eugene Gavrilov kX Project Lead Programmer and Coordinator Join Date: Dec 2002 Posts: 3,119 Rep Power: 75	the details can be found on the dolby site (www.dolby.com) >> I thought that someone here might know more than me about it some people here probably do... who knows? as for the implementation, dbx seem to be useful for analog =recordings= and I doubt a realtime DSP implementation is really necessary... but any DSP programmers are welcome to investigate this /Eugene

Dec 6, 2003, 02:33 AM	#5
NightBreed DriverHeaven Addict Join Date: May 2003 Location: Philadelphia, PA Posts: 380 Rep Power: 0	Couldn't a dbx decoder be used to cut alot of noice coming from a mic source?

Dec 6, 2003, 07:14 AM	#6
Kevin Horden DriverHeaven Junior Member Join Date: Nov 2003 Location: Shropshire, England Posts: 21 Rep Power: 0	I can't see what possible use a DBX encoder / decoder would be. It's for tape noise reduction. The encoder is just a 2:1 compressor and the decoder is a 2:1 expander. the idea is just to put as much recording level down on tape as possible. A decoder on it's own wouldn't reduce noise.

Dec 9, 2003, 05:06 AM	#7
marschw DriverHeaven Newbie Join Date: Aug 2003 Posts: 19 Rep Power: 0	Hmm, I thought dbx was also the name of a stereo audio standard for TV, could that be what he meant? As I understand it, I don't think it's possible to do outside of a TV card, though, since I don't think the stereo data is encoded in the mono audio stream itself. -=Marcus

Dec 14, 2003, 07:30 AM	#9
marschw DriverHeaven Newbie Join Date: Aug 2003 Posts: 19 Rep Power: 0	As I understand it, dbx is one of several encoding methods used for TV stereo; I think the standard used is country-dependant. -=Marcus

Dec 18, 2003, 06:38 AM	#12
Kevin Horden DriverHeaven Junior Member Join Date: Nov 2003 Location: Shropshire, England Posts: 21 Rep Power: 0	DBX is just compression and expansion. There are no frequency dependent components. See http://hyperphysics.phy-astr.gsu.edu.../tape4.html#c3

Dec 18, 2003, 07:00 PM	Thread Starter #13
igor_levicki Intel Black Belt Join Date: Dec 2003 Location: Belgrade, Serbia Posts: 128 Rep Power: 0 System Specs	Perhaps you are right, here: http://music.concordia.ca/Programs/E...inciples.html, but I still think that it has something to do with freq. since when you play it back without decoder high freq. sounds louder and bass freq. sounds quieter. __________________ Regards, Igor Levicki -- Creative Audigy 2 ZS Retail (SB0350) kX 3538h

Dec 18, 2003, 07:04 PM	Thread Starter #14
igor_levicki Intel Black Belt Join Date: Dec 2003 Location: Belgrade, Serbia Posts: 128 Rep Power: 0 System Specs	No, I am right! It was this -- DBX type 2 noise reduction A simplified version of the DBX noise reduction system intended for use with systems with limited bandwidth such as cassette recorders, 8 track recorders and carts. The type I system did not work very well with those types of systems since it works on a fairly linear basis across the spectrum so if the recorder had problems with bandwidth, phase shifts, dropouts or any sort of notches or bumps along the frequency spectrum the type I decoder would have problems. The type II system solves these problems by having a greater pre-emphasis on the detection circuit and by using an RMS detection system that meant that the units are much more immune to tracking errors, a long running problem with DBX type I, Dolby and DNR based systems (and the need behind the Dolby HX Pro system), but for the same reason a type II codec should never be used with a recorder that has a flat response like a high end open reel or a digital unit, the added emphasis will result in distortions and loudness errors in the upper frequencies. The theoretical dynamic range expansion gained by using type II is the same as for type I or about double, although in practice I find it to be somewhat less. While the dbx company tried hard to get type II codecs integrated into consumer equipment and for a time in the 80's archived some success especially in the USA where a few labels issued audiophile cassettes using the system, today however it's almost never seen integrated outside of a few multitrack cassette recorders. But there are still quite a few home users of the system and you can still find the technology used in broadcast situations where type I is not appropriate. __________________ Regards, Igor Levicki -- Creative Audigy 2 ZS Retail (SB0350) kX 3538h

Dec 23, 2003, 03:41 AM	#15
Gonzo DriverHeaven Junior Member Join Date: Feb 2003 Location: Uruguay Posts: 57 Rep Power: 0	Igor: Count with me too, on this subject! I'm currently transfering all my old cassettes to CD and I''ve found exactly the same problem. Half of the tapes were encoded with DBX, and half with Dolby B an C. I would give all I have just to find some kind of software decoder for these noise reduction systems. The problem is that long years ago I sold my Yamaha cassette multitracker, the one with DBX codec (don't know if it was type I or II though), and my (Yamaha too) cassette deck has worned out the playback head, so it causes problems with the alignment and this in turn makes that al my Dolby encoded cassettes doesn't decode in a proper way. I'm really worried because I have thousand of hours of my own music recorded there, thing that I love from the past...so if we could find some kind soul that would be interested in programming a decoder for all this ancient technology... Let's keep searching... Cheers GONZO