From: jeh@cmkrnl.com
Newsgroups: rec.audio
Subject: Re:  Dolby C vs. dbx - correction
Date: 9 Oct 92 20:39:47 GMT
Distribution: na
Organization: Kernel Mode Consulting, San Diego, CA

In article <1992Oct6.163540.15201@oakhill.sps.mot.com>, rvkl60@aus18a25.sps.mot.com (Tom Mathes) writes:
> In article <1992Oct5.150116.24577@oakhill.sps.mot.com> rvkl60@aus18a25.sps.mot.com (Tom Mathes) writes:
>>
>>In article <1992Oct4.021511.709@cmkrnl.com> jeh@cmkrnl.com writes:
>>>Rather, Dolby B and C apply *compression* to the high frequencies on record,
>>>and expansion on playback.  Parts of the program that already have a lot of
>>>high-freq. content won't be much affected by Dolby.  But in parts in which the
>>>highs are of low level, highs are boosted on record and cut on playback. 
>>>During the "cut on playback", the tape hiss, which is also mostly high freqs,
>>>gets cut too.  Voila.  (When there is a lot of high-freq content in the 
>>>program material, you don't notice the hiss nearly so much, as you're paying
>>>attention to the program.  This is a psychoacoustic affect known as "masking".)
>>
>>You've got your theory right, but terminology wrong.  Dolby amplifies, or
>>expands the high frequencies during recording, then compresses them during
>>playback.  That's one reason to critically check the recording levels during
> 
> I'm eating crow on this one.  I realized soon after I posted this that Jamie
> was right and I was wrong.  Dolby does amplify the high-frequencies, and does
> indeed compress the dynamic range of those signals.  I guess my brain was in
> neutral when I wrote this.
> 
> Sorry about the screw-up part folks.

The source of confusion here may be that "compression" can mean either boosting
the signal or cutting it, or some of each, depending on the original signal
level and on compressor's parameters!  

For example, "compressing" the dynamic range of a signal by 10:1 might mean
turning a 0-to-1.0 range into 0.0-0.10 (reducing the level), or into 0.9-1.0
(raising; this is what Dolby NR does to the high freqs on record, though not by
anything like a 10:1 ratio), or 0.45 to 0.55 (some inputs are boosted, others
are cut), or to 0.2 to 0.3 (most signals are cut, but the very lowest-level
ones are boosted)... The term "compression", as in "make smaller", applies not
to the signal level but to the dynamic range.  

Anyway, you brought up a very valid point, which ought not to get lost in the
shuffle:  use of Dolby NR requires that you pay close attention to record
level.  When you're recording with Dolby (and who doesn't on cassette decks,
unless you have and prefer dbx?), it is important not to push the meters into
the red.  Your deck's record level meters probably show the signal *before*
Dolby boosts the highs, and they may not respond well to the high frequencies
anyway!  (A few cassette decks have "meter weighting" switches that cause the
meters to respond more to the highs, so that the "red" range corresponds better
to the hf saturation point of the tape.) 

	--- Jamie Hanrahan, Kernel Mode Consulting, San Diego CA
Internet:  jeh@cmkrnl.com, hanrahan@eisner.decus.org, or jeh@crash.cts.com
Uucp:  ...{crash,eisner,uunet}!cmkrnl!jeh
Newsgroups: rec.audio