[78-L] Reversing audio

[Michael Biel]

Jeff Lichtman wrote:
> Did the original author claim that this happens with tape as well as 
> records as the analog source? I really don't see how playing a tape 
> backwards would introduce distortion.
>
>
>   

Actually the paper specifically concerned reversed tape playback, not 
discs.  The initial concern was with establishing proper "absolute 
phase" of the signal so that the positive wave polarity in real life 
results in a positive wave polarity out of the loud speaker.  (Not 
relative phase between the stereo channels, but absolute phase of the 
total recoirding--mono or stereo.)  This was an unpublicized concern in 
the pre-recorded tape duplication industry because most bi-directional 
recordings were routinely duplicated in one pass.  When the presentation 
author started comparing the digital reversals of the reversed analog 
tape recordings to determine absolute phase, they started noticing 
strange things happening when reversed analog recordings were reversed 
digitally and returned to analog for playback.  


> It's not clear to me why playing a grooved record 
> backwards would mess up the signal so badly (unless one does 
> something goofy like play it with the tonearm on the "normal" side of 
> the spindle, so that the direction of travel relative to the stylus 
> is in the wrong direction).
>
>                         -        Jeff Lichtman
>   

I am not sure they have investigated reverse playback of discs because 
the investigation started with examination if it were possible to halve 
their digitalization time by doing bi-directional tapes in one pass.  
But I am puzzled by your tone-arm description.  You MUST leave the tone 
arm in the normal position, on the right side of the spindle when 
playing a record in reverse rotation.  If you put it on the left side 
you are introducing an enormous amount of tracking error by having the 
cartridge at a weird angle in relation to the tangent of the groove.  If 
you are thinking of how they play stampers with a forked stylus and 
counter-clockwise rotation, Stanton produced special tone-arms with the 
bend in the opposite direction to enable the placement of the arm on the 
left side of the spindle with proper tracking angle maintained. 

Mike Biel  mbiel at mbiel.com