wil816 said:
I am familiar with spectrum analyzers. Using pink noise and a spectrum analyzer does not give distortion information. And I should have clarified that I did not mean the people doing the testing reduced the bias, but that apparently this one of the things Dolby C does, constantly adjusting the bias based on the program material. That's not all bad, but I'm very suspicious of anything that appears to give you something for nothing. But as I said earlier, the Dolby designers may have concluded that such extended frequency response at high record levels is seldom needed for more than a fraction of a second at a time for something like a cymbal crash, and that more distortion under those circumstances is more acceptable than a cymbal that sounds more like a toilet flushing because its sizzle is gone.
Another thing Dolby C apparently does is some companding, and not just around a lower threshold like Dolby B does. I used dbx for awhile on cassettes, and although the improved dynamic range is wonderful, the fact that it accentuates the cassettes' output inconsistency drove me nuts. Any little variation, even if not really enough to call it a dropout, became objectionable.
You have got a number of your facts rather mixed up here. Repeat after me: Dolby C never, Ever,
EVER adjusts the bias! Period! You are just plain 100% incorrect about this. It is just a dynamically applies an
equalization curve to adjusts the frequency spectrum of the signal in a manner extremy similar to how Dolby B does. At 0dB, and above, dolby B theoretically does nothing. At around -20db or so, it provides you 10 db of boost above about 1kHz durring recording, and 10dB of mirrored cut durring playback. In-between, it processes something less than 10dB of boost/cut.
Dobly C merely does more of the same, except that is does a 20dB boost/cut instead. Furthermore it "skews" its boost to the very HF in the exact opposite direction of the direction of the rest of the signal. So at 0dB, where dolby B is doing nothing, dolby C actually cuts the extreme highs (above about 15kHz) durring recording, and then boosts then again durring playback (the exact opposite of what it normally does). This "skewing" efffect keeps the tape from saturating at the very high frequencies, and provides a little more headroom. This is one of the reasons why, when properly calibrated, a Dolby C recording doesn't tend to sound "dull" the same way that Dolby B does. Now if improperly calibrated (as unfortunately a great many decks are), Dolby C can cause dulling too. But that is due to an effect known as "Dolby mistracking".
Technically, all noise reduction schemes are considered to be "companders" of one sort or another, and this includes Dolby B. But neither Dolby B nor C is a compander in the same sense of the word as is DBX. DBX is a 2:1 linear compander which applies the same amoun of compresion and de-compression at all frequencies and levels. Dolby is a highly non-linear compander, because it boosts/cuts only certain frequency ranges, and it does so to differing degrees at different levels.
So while the Dolby NRs (this includes Dolby A, B C, S, and SR) can techically be consider a companders becuase they do non-linearly compress/decompress certain frequency ranges, they are all COMPLETELY different animals than are any of the versions of DBX! DBX by contrast, can certainly be considered a compander in every sense of the word.
Where I suspect you amy have gotten thinking that Dolby effects bias is by a completely unrelated technology, which is called Dolby HXPro. HXPro does dynamically vary the bias to create more HF headroom.
But HXPro is NOT Dolby NR, it is not part of Dolby NR, and it has no relationship to Dolby NR, other than that this technology was also licenced by Dolby Laboratorties! Infact a great many Dolby C tape decks were made which completely lacked HXPro.
Now, let met state yet again, since you seem to have trouble grasping this fact:
Dolby HXPro has never been included inside of any Nakamichi ever built! Therefore, no dynamic bias tweaking is happening with these machines, no matter how suspicious you may be about this. Have I explained this well enough this time? I hope that I'm typing slowly enough for you...
But hey, don't listen to me!
I'm just some guy who merely has a degree in Electrical Engineering, has serviced and calibrated tape decks as a hobby for 20 years, and who has been an avid tape recording enthusiast for over 25 years (durring which time I have followed with great interest all of these technologies closely) So what the heck should I know about this stuff anyway?!? :stupid:
Now if your opinion is that all NR is bad because of getting something for nothing, then you are free to have that opinion. Many here will infact completely agree with those sentiments. But please get your facts straight about how these things work, ok?
