From: bill@bilver.uucp (Bill Vermillion)
Subject: Re:  Dolby C vs. dbx
Organization: W. J. Vermillion - Winter Park, FL
Date: Tue, 6 Oct 1992 00:31:07 GMT

In article <1992Oct5.101437.12685@alf.uib.no> resou@darcy.mi.uib.no (Odd Arild Urtveit) writes:
>In article <1992Oct1.185829.10974@pilot.dmg.ml.com>, steveh@pilot.dmg.ml.com (Steve Hannoford) writes:
(How about a CR every 60 or so chars - I had to insert them
here)

>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.

dbx MUST be used in playback.  Besides using a straight 2:1
compression on the entire tape, it also does some equalization.
Because of the compression it can put more high end boost in
place - SNs of over 100db are possible in pro machines.

>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.

Wrong on that point.  The Dolby units are most critical in
record and playback adjustment.  There is a 'dolby' level that
is set in record, and MUST be matched on playback for proper
decode.  Only signals below a given level are compressed and if
the playback level is set wrong the levels used to decode are
also wrong.  dbx is relatively insensitive to level changes.

The worst problems I ever had with dbx were caused by 'head
bumps' in low frequency playback causing a disparate increase
in LF response at the bump center.   To compensate we typicaly
notched on record to give linear playback.  We didn't sweat
compatibility too much as there were fewer than 6 (by our
estimates) 32-track 2" analog machines in the states at that
time.  (Between the 2 rooms we had 48 tracks of dbx.  (Only
time that was  a problem was if we needed 32 in A, and 24 in B
- but we worked our schedules to preclude that).

>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)

To be more accurate - the system varies bias current because
excessive HF content cause the signal to be self-biasing. The
bias current is reduced to compensate so the the 'effective
bias' is constant .

Bias can be optimized for several parameters.  The typical bias
points  are set just beyond maximum output.  As the bias
increases past MOL the output decreases.  However the
distortion decreases.  Overbias gives less distortion
sacrificing the high-frequency output.   HX-Pros use is not so
much to give a 'higher treble saturation' but to give a linear
(or perhaps to state it more accurately) a consitant high
frequency response curve.  Without the HX-Pro, the high
frequency response curve would vary depending on the signal
content.       

(I spent most of my life around high-speed reel-reel so I
didn't have to sweat the slow stuff :-)  )
-- 
Bill Vermillion - bill@bilver.oau.org  bill.vermillion@oau.org
                - bill@bilver.uucp 
                - ..!{peora|ge-dab|tous|tarpit}!bilver!bill