Strange spectral frequency display in Zeppelin's BBC Paris Theatre session

I got the new Led Zeppelin BBC Sessions CD and thought I'd restore the missing parts of "Whole Lotta Love" from the Japanese 1996 FM broadcast bootleg. When I loaded it into Adobe Audition and looked at the spectral view, there was a surprise waiting for me. There is a frequency dropout across the entire track in roughly the 14.5 to 15.1 kHz range. And there is some kind of inaudible signal at 18.99 kHz.

This is the official release, but interestingly, the FM broadcast from 1996 has the exact same spectral fingerprint. Meaning, I suppose, that they are both from the same source. Does anyone know what would cause those kind of frequency characteristics?

 

ALiester Crowley

 

Wouldn't phasing issues possibly cause a loss there? Depending on how the instruments were EQ'd live? Obviously the tech didn't have the ability to analyze frequency dropouts in a live setting back then right?

 

Robert's teleprompter? Oh wait, wrong show

 

No, that's not it. I vaguely remember something about the audio on Betamax or VHS tapes having some kind of signal in that range, but I think it was a little lower than 18.99 kHz. And the dropout between 14.5 to 15.1 doesn't have anything to do with EQ on a live board. You would see a smoother shift than this, which is pretty abrupt. It may be worth noting that the wide-separation mix that circulated before 1996 does not have either of these characteristics.

 

Oh I see. You think it's a physical issue with the tape?

 

if it's sourced from the radio that could be a carrier signal. sounds about the right frequency I think.

 

if it's taped from a FM broadcast, there may be artifacts from the tuner and the broadcast itself:

FM broadcasting - Wikipedia, the free encyclopedia »

The (L+R) Main channel signal is transmitted as baseband audio limited to the range of 30 Hz to 15 kHz. The (L−R) signal is amplitude modulated onto a 38 kHz double-sideband suppressed-carrier (DSB-SC) signal occupying the baseband range of 23 to 53 kHz.

A 19 kHz pilot tone, at exactly half the 38 kHz sub-carrier frequency and with a precise phase relationship to it, as defined by the formula below, is also generated. This is transmitted at 8–10% of overall modulation level and used by the receiver to regenerate the 38 kHz sub-carrier with the correct phase.

 

I think it is there for some technological reason when the tape was transfered sometime before 1996.

An intriguing thought is that it had something to do with the FM broadcast signal in 1996, which would mean that Zeppelin got their current copy of the tape from either the Japanese radio station that broadcast it or from the circulating bootleg of that broadcast.

FWIW, the 1997 original BBC Sessions CD uses the same source.

 

This is fascinating. I was always a guitar player and never that good at tech

Yes. It's an interesting theory to say the least. And if it happened here it would presumably show up on other recordings with the same (similar) transfer history