From: resou@darcy.mi.uib.no (Odd Arild Urtveit)
Subject: Re:  Dolby C vs. dbx
Organization: University of Bergen, Norway
Date: Mon, 5 Oct 92 10:14:37 GMT

In article <1992Oct1.185829.10974@pilot.dmg.ml.com>, steveh@pilot.dmg.ml.com (Steve Hannoford) writes:
> I can tell you how Dolby works, but I can't really say how dbx
> works, but my understanding (explained by more than one audio
> engineer) is that Dolby is a reduction system which requires
> the same implementation during playback that was used during
> recording, where-as dbx will improve the quailty of the tape
> even if you do not have dbx at playback (in fact I think that
> dbx is only effective during recording).
> 
> What does that mean?
> 
> Dolby has two classes of NR:  Type B and the newer Type C.
> Manufacturers can buy Dolby NR at several price levels, so
> no all Dolby C are the same.  The type refers to how it reduces
> noise, the level determines the quality of circuitry used, etc.
> 
> Dolby labs researched tape noise and isolated the frequencies
> where the noise typically occurrs.  When you make a recording
> with Dolby, the signal is boosted in that range, so that the 
> signal to noise ratio is increased. (The signal is boosted so
> that it 'covers up' the noise.)  This results in a top heavy
> (increased treble range) recording.  During playback, the 
> signal is reduced by the same amount, restoring the proper
> treble levels, but the noise isn't there (well not as much
> of it).  
> 
> If you record with Dolby and play back without, the tape will
> sound brighter - obviously, because the treble was boosted.
> It's like turing the treble all the way up and the bass all the
> way down on the pre-amp...  Some people 'prefer' this sound 
> (the brightness of dolby record without dolby playback), but
> the signal is not like the original signal - the sound is altered.
> 
> My understanding with dbx is that its noise reduction is during
> record only, and is not required during playback... Not only
> is it a better reduction system (results in less noise than
> either Dolby B/C), but it is also preferential because it
> works without the requirement that the implementation at
> playback match the implementation used to record....
> 
> 
> -- 
> 
> +-------------------------+--------------------------------------+
> | steveh@pilot.dmg.ml.com | My employer has no opinions and has  |
> | Steveh Hannaford        |  nothing to do with anything I say.  |

Dolby B boosts the treble, but not in a linear way.  The weaker the signals are the more they are boosted.  Strong signals are not increased at all. This results in a 'compression' of the treble.  During playback the inverse process takes place: The Dolby system 'uncomresses' the treble and 'hide' the tape-hiss away.   The improvement in S/N-ratio can be up to 10dB.  Because of this non-linear boost, this is not equivalent to increaseing the treble.

Dolby C works in a similar way as Dolby C by boosting the treble.  However, Dolby C also works in the mid-range (>500Hz).  The boost is even more nonlinear for Dolby C than for Dolby B resulting in a bigger improvement in S/N-ratio.  Up to 20dB (or 15dB depending on the frequency that are measured).  Dolby C also increases the saturation-level of the tape by a few dB. (= higher recording-levels).

dbx works by 'compressing' the entire signal by boosting the weak signal more than the strong ones.  Put into other words:  The dynamic range of the signal is squeezed together so that it would fit into the much narrower dynamic range of the tape.  This results in a recording that would sound very unpleasent without 'uncompression'.  Therefore, dbx should also be used during playback.  The result is an improvement in the S/N-ratio of aprox. 30dB.

Because of the way these NR systems works properly, it is vital that the tapedeck is correctly adjusted.  (Bias & tape sensitivity level).  The heavier the alterations to the signal the NR system performs, the more important is this matter.  dbx is the most effective, followed by Dolby C and Dolby B, but because ov the reason mentioned above dbx is also most critical to the tapedeck adjustment and it's therefore easier to ruin a dbx recording than a Dolby B/C recording.

Dolby HX Pro is not a NR-system, but a system to obtain constant bias.  Without Dolby HX Pro the treble will work together with the bias, resulting in a varying bias.  The more treble the more bias, and more bias results in a reduction in the tapes treble saturation level.  By keeping the bias constant, Dolby HX Pro tries to elliminate this effect resulting in a higher treble saturation level and therefore better treble.  (<=> better frequency range)

O.A.Urtveit
resou@mi.uib.no
Newsgroups: rec.audio