On Tue, 14 Feb 2017 17:57:47 +0200, Iain Churches wrote:

"Johnny B Good" wrote in message
...
On Sun, 12 Feb 2017 23:35:04 +0200, Iain Churches wrote:

"Jim Lesurf" wrote in message
...
In article , Iain Churches
wrote:
Prerecorded cassettes were always something of a compromise
(high-speed loop-bin duplication) but towards the end of that era,
chrome tape with Dolby B was starting to sound pretty good.

That reminds me that there was at least one company who did 'real
time'
Cassette duplications for the sake of sound quality. I can't now
recall their name(s), though. Something like "White(something"
perhaps was one of them.

There were probably many. One I know of in North London was called
"SuperCassette" (original, eh?) They had a large room with dexion
shelves floor to ceiling with two or three realtime high-end cassette
recorders (Nakamichi or something similar) on each shelf. Each
"aisle"
was fed by its own "master recorder" (Revox A77) with a studio copy of
the master running at 15 ips. The cassettes sounded quite good!


I wonder, seeing as they were going to such an extreme, whether they
also ran the master and slaves in reverse to mitigate the phase delay
'distortion' effect on low frequency square wave test signals which
made such test signals look like triangle waves on playback when viewed
on an oscilloscope, or did they just accept that despite this very
visible departure from the original waveshape, no one could distinguish
the direct versus the phase distorted magnetic recording playback by
ear alone anyway? :-)

The system was a simple one, and only needed two young girls operators
to keep it running. As soon as the EOT switch clicked on the Revox one
of them walked along the row, ejecting the cassettes and putting in new
ones.
The other dabbed the Revox replay head with a spot of Isoprop, rewound
the"master" and then helped with the casette replacement.

I would hope the other YL would have done a *lot* more than just dab the
replay head alone with a "spot of isopropyl alcohol". Never mind the
replay head, you also have to decontaminate the rest of the tape path
components (guides, capstain(s), pinchwheel(s), erase and record heads,
*everything* the master tape has to negotiate in its transit between the
supply and take up reels) even when using the modern Japanese Maxell and
TDK master tape formulations which showed up the deficiencies of EMI,
Ampex and Scotch tape formulations that had once been regarded as leaders
in magnetic tape technology.

Even the cassette recording slaves needed similar levels of TLC, even if
it was administered more sparingly (perhaps every second or third
cassette's worth of duty).

In a matter of minutes they were ready to go again. There was a pair of
headphones connected to the Revox (QC ?) I did not see them used.

They probably relied on spot checks of the cassette dubs, rarely needing
to audition the master playback quality unless they were attempting to
copy from a master made using Philips tape which had a tendency to clog
ordinary Mu-metal heads in less than 1800 feet of tape ime (the only tape
decks capable of getting a clean playback in a single pass of 1800/2400
foot reels of Philips tape were those blessed with glass crystal heads
such as the Akai GX630DB and the GX747 decks I used (and still own).

BTW, when I mentioned "low frequency square wave test signals", I meant
frequencies in the range 300 to 900 Hz, which I think might be better
described as 'mid frequency' rather than low - my bad.

The point being that they'd be safely above the low frequency cut off
point of any decent amplifier or tape deck whilst still allowing odd
harmonics up to at least the 7th to be reproduced below the typical 16 to
22KHz high frequency cut off point of a high quality cassette or reel to
reel deck.

Passing a 1KHz square wave through a typical amplifier with a 20 to 20K
Hz frequency response would retain the original waveshape as examined on
an oscilloscope, albeit with ripples and rounded shoulders due to the
limited bandwidth. Playing back such a test wave from a magnetic
recording boasting a similar overall frequency response to that of the
reference amplifier would show the classic triangularisation distortion
effect typical of magnetic recordings.

"Distortion" that was distressingly visible on an oscilloscope trace yet
a seemingly inaudible form of distortion. I guess there was some concern
over allowing this form of distortion to 'multiply' when creating second
generation dubs, hence the trick of running the master and the slaves in
reverse to successfully largely cancel this "visible on a 'scope trace"
effect on the 2nd gen copies themselves.

The last words I recall reading in regard of this phenomena was the
phrase, "Thankfully, our ears seem totally insensitive to phase delay
distortion." so I guess such 'phase distortion" correction measures as
dubbing in reverse were never (widely, if ever) adopted. I only mentioned
it in case anyone had had any experience of using this technique to
achieve the "best possible" quality dubs from analogue master tapes.

These days, any such phase distortions in original tape recordings can
now be dealt with using DSP technology, a technology that was becoming
available to the music industry with the advent of the Compact Disk in
the early 80s. I'm guessing the technique of reverse dubbing would have
had only a narrow window of ascendency of less than a decade between the
advent of the music-cassette and that of the Compact Disk.

--
Johnny B Good