During the early1970's the major UK record
companies decided that mono LPs would be
discontinued. However there was still
a very large back catalogue in mono - fine
recordings of excellent performances - for
which there was still a considerable demand.

There had been many efforts to produce
"electronic stereo" by means of EQ and
pan pots in an attempt to "separate"
instruments into frequency bands and pan
them across the stereo spectrum, left to right.
Most of these sounded awful.

At the studio were I worked, a small group
of us discussed an idea to create electronic
stereo soundstage using phase/time shift,
rather than treat the recording itself with
equalisation. Our aim, was to simulate a
recording made with a co-incidental pair
of microphones.

The brief from our production team was to
provide a stereo master which, with both
channels summed (i.e played mono), would
be indistinguishable from the original mono,
with no degradation of the original sound.

This clearly tagged the EQ method as a
complete non-starter.

This was long before the days of digital
processors and computer plug-ins, so
we resorted to three Studer analogue tape
machines - a C37 mono full-track quarter-inch
source machine, an eight track A80 running at 30 ips
with Doldy "A" and a stereo A80 quarter-inch
15ips master recorder with Dolby "A"

The idea, simple in itself, depended on the delay
between the record and playback head on the
multitrack machine to introduce a sense of width
in the sound stage..

To set the delay, a VCSC (variable capstan speed
control) was used on the high-speed multitrack machine.
The audio from the mono source was sent to two tracks
of the multitrack, with the phase of the second track
reversed, so that when they were brought back from
the replay chain to the console, they had 180 degree
phase difference, and cancelled out when switched mono.

Having established the required delay, which was
dependent on both the music content and the location
in which it had been recorded, one could set up
the "stereo sound stage" by bringing up the
original mono signal on the mixing desk, and then
adding to it the out-of-phase pair, at some 10dB
below the mono signal.

We demonstrated the idea by first playing the
"stereo" tape with both tracks panned centre
for mono, and then after about one minute,
opening the pans to L and R. Listeners
were impressed. We wanted to avoid the
word "electronic" and so called it
"reprocessed stereo"

A large number of early classical recordings
were reprocessed using this system for our own
label and also for third party labels. The compatibility
test was perfect, and switching to stereo gave a
spread to the original recording with no other changes.

Here is one of my experiments with a jazz track.

On this audio extract, there are three segments.
The first is mono, the second is reprocessed stereo,
and the third segment is a good example of what happens
when the level difference of mono signal and the
out-of-phase pair is too small.

http://www.kolumbus.fi/iain.churches...StereoDemo.mp3


Iain