Sorry, Kyle, but I can't believe that this thing is either foolproof or acoustically harmless.
Today's new Radio World (1/3/07) has an article on PPM by the CE of WOR in New York. As it turns out, there are two versions of the enclder available, one that can encode an analog signal, and another that automatically inserts encoding into the digtal stream of a digital STL or inter-studio link.
In order to make the encoding useable through any kind of reproducers, even the two-inch speaker in a pocket portable, the encoding consists of digital data on audio-frequency signals in the 1-to-3-kc range, where the ear is most acutely sensitive. And it's supposed to be effectively masked by stronger signals very close in frequency -- but don't count on that masking effect!
Maybe what we need here is a history lesson. Anybody on this board remember CBS Copycode?
Back when cassettes dominated recorded music sales, vinyl was still significant, and CD's were the province of "early adopters" -- and consumers didn't have DAT, much less computers for audio copying -- CBS, which hadn't yert sold its huge record business to Sony, came up with a technology that was supposed to prevent consumer analog copying. It was called Copycode.
Don't confuse it with the Serial Copy Management System, or SCMS, which put a blocking flag in the the subcode of CD's (and thus could only be u on dial sources) Copycode was supposed to make analog media un-copyable, and it did that by mutulating the analog signal. Here's how it worked -- or was supposed to:
The RIAA wanted Congress to enact a law requiring Copycode chips in all new consumer recorders. The chip would monitor an extremely narrow slice of the audio spectrum between highest B-flat (37.29.3 cps) and the highest B-natural (3951.1) on the piano keyboard, but excluding those notes' fundamentals. The center frequency of this notch was 3840, which is the 64th harmonic of the standard AC line frequency, and is almost exactly a quarter-tone between those to standard-pitch notes.
The encoding consisted of an even narrower notch removing 3840 but leaving other frequencies between those standard-pitch notes and the notch. The consumer chip also had the super-narrow notch circuit, and when it detected energy within the wider notch without any inside the narrower one, it interpreted this as proof that the source material was "Copycoded" and shut down the recording process.
The completely insane rationale for this was the assumption that the only audio energy occuring within the broader notch would be from "unpitched" instruments (e.g., cymbals), which have a white-noise component, and that there would have to be significant energy within the narrower notch whenever there was any in the wider one.
Of course, this view ignored the fact that our musical scale uses equal tempermament, meaning that the octave is divided into 12 equal parts, and adjacent notes on the keyboard have a frequncy ratio of 1 to the twelfth root of 2 (approx. 1.05946). Thus the natural harmonics that give different voices and instruments their distinctive timbres (or tonal colors), being precise multiples of the fundamental, are not exactly equal to their nominal counterparts in the higher octaves (See
http://en.wikipedia.org/wiki/Equal_temperament)
This view also completely ignored the possibilities of (1) original instrument recordings that are not in modern concert pitch, (2) amateur recordings of out-of-tune amateur musicians and (3) recordings of non-Western music. Can you imagine how this thing would react to an Indian sitar?
Well, you don't have to. Sometime back in the Eighties, High Fidelity magazine (before its merger with Stereo Review) ran an article documenting the system's penchant for registering false positives from unencoded recordings, as well as a few false negatives from encoded ones. And yes, it was audible! (I wish I could give you a citation, but you can probably find it if you try. I don't have the time right now.)
After that, we heard no more about Copycode, and the industry focused its attention on SCMS for DAT.
But I can give you another example of how depending on audio cues in an anlog signal can go awry.
Remember the original 1960's "Goldfinger" and its title song sung by Shirley Bassey? That one made the Billboard charts and got a lot of air play on what were then called "M.O.R." stations.
Well, WIP couldn't play it, at least not all the way to the end. WIP was the key station for the Emergency Broadcast System in those days, and every time Shirley's high note on the end of that record went out at a normal modulation level, it would set off false alarms in EBS receivers at stations all over the Delaware Valley.
After WIP was notified that they had to "pot down" the end of the record and talk over it, the jocks made it a running joke of it for a few weeks. Anybody else remember that?
That's just another example of how wrong things can go when you depend on cues from an analog signal located in the middle of the audio range!
So I'm afraid I don't have much faith in PPM -- and even less in those who try to judge its effects on audio quality by listening to it over AM radio!
