caf (01/02/83)
Some recent articles concerning "Quad" sound using the Hafler circuit
have prompted me to share my experiences with such arrangements.
At various times in the past, I have experimented with the Hafler
circuit for playing stereo (classical) discs without any "substantial"
effects. My recent experiences with Laserdiscs of "Dolby Stero"
encoded movies have been much more interesting.
The most important feature of Dolby Stereo seems to be the encoding of
the rear channel(s) as a L-R difference signal. The Dolby Stereo
decoding used in theatres uses a fairly complex gain-riding circuit for
improved directionality over the most of the auditorium. For home use
with a fixed listening position, the Hafler circuit works quite well.
In my system, I use a Pioneer VP-1000 videodisc player with an outboard
MXR "Discoder" CX dedocer modified with for the slightly different LV
CX characteristics. The modification consists of switch selectable 100
Hz or 500 Hz high pass filtering of the input to the full wave
rectifier. I recommend making this mod switchable for two reasons:
There is still a slight possibility that some CX LP's may yet appear on
the market, and the 100 Hz postition adds more "punch" to some discs
without excessive "pumping".
SInce I use a Carver 400 amp, I summed the output channels with 47k
resistors to yield a difference signal that I feed to the aux input of
my old stereo system driving the rear speakers (Heath AR15 & AR3's).
The balance and tone controls on the rear channel are a significant
help in getting the effect to work right.
I also sometimes use a 10-band equalizer and/or the "peak unlimiter"
function on my Carver preamp. Do not use the hologram as it will
disrupt the rear channel signal.
The discs to hear with this setup are "Star Trek The Motion Picture",
"Black Stallion", "Apocalypse Now", "Alien", "Close Encounters", "Black
Hole", and "Dragonslayer".
With Dragonslayer, full boost on the 30Hz equalizer band, 100Hz high
pass for the full CX full wave rectifier, and the Carver's peak
unlimiting results in a most impressive dragon. (If I ever hear a real
live fire-breathing dragon, I'll know if the sound was "realistic"!)
Don't boost the bass when playing Close Encounters or you may need new
woofers.
Best results are often obtained when the rear channel has full bass
response.
The equalization in movie soundtracks is quite variable. As a result,
equalizers, expanders and fancy filters ("auto-correlation") are much
more useful than is generally the case with LP's where low and high
frequency noise limit the amount of equalization that can be tastefully
applied.
Since the CED versions of these movies are in mono, you can't get by
with the cheaper RCA system; Laser is the only way to go. You'll need
the full LV video bandwidth anyway when you opt for a large screen
projection set. I use a Kloss Novabeam 1; from what I've seen so far,
it it still the one to beat.
Chuck Forsberg, Chief Engr, Computer Development Inc.
6700 S. W. 105th, Beaverton OR 97005 (503) 646-1599