I started this thread a couple of months ago when I decided that all Lonely Few material would be released in mono - or dual mono to be factual. The band would be recorded on the two track Revox in stereo, mastered down to the mono Nagra then dubbed off the Nagra to dual track mono cassettes on an Akai GX-95.
At the time, when I was dubbing the cassettes off the Nagra with a splitter close monitoring the Akai's playback head through headphones, I kept noticing tiny little incidents of flutter where the signal would suddenly phase left to right, or vica versa. It was totally random.
After a lot of investigation, opinions and trials on lesser tape decks (where it was much worse) we concluded that it was caused by small variations in the cassette tape stock, tape path sticktion and pad pressures as it passed through the cassette deck's transport that was being magnified into audibility by the fact that it was dual mono and that it was closely monitored through headphones. It is not noticible through speakers nor is it noticible in a true stereo signal where there is sufficient difference between the left and right program material so that it doesn't phase when it hits a variation causal incident.
We concluded that it will always be a possibility - even in a reference grade cassette deck such as the GX-95 - and is a function of the very narrow track width, cassette body / reel mechanical imperfections, miniaturised transport and slow tape speed of cassettes. We have been able to minimise it - but not eliminate it - by making sure that only new cassettes are used (it gets worse if the tapes have been used and bulk erased) and to put the split mono signal through a 15 band graphic with minor mirror differences between the left and right (eg -2db down on on band on one side and +2db up on the corresponding band on the other side). This creates a sort of false stereo dual mono signal that can reduce the potential for a variation to cause an audible azimuth drift / flutter incident.
It doesn't happen when you do a dual mono in digital off a mono 1/4" 15 ips source and I have no idea whether it would happen with a dual mono lathe cut.
But I'd like to know as that is where we're heading with it all in the end.
I'll soon find out I guess.
Jed