[BC] Old FCC Audio Proof rules

[Phil Alexander]

On 4 Jun 2006 at 8:17, DHultsman5 at aol.com wrote:

> I am talking about the 1960's.....I believe you had measure response down  to 
> 30 cycles in those days.  I seem to think that distortion may not have  been 
> required.   My brain may be full of cobwebs but I seem to  remember having to 
> go down to 30 Hz.

Dave,

Your memory hasn't started to fade. 30 cps was required in the 60's 
and before. I remember it because it was such a PITA to get the old
GR pushbutton audio oscillator to kick in at that freq. I also recall
there was a higher level of permissable THD at 30 ... 4% on AM IIRC.
Again, IIRC, 10 kcs was an AM proof requirement, and we routinely 
proofed an AM box out to 12 kcs. 

The early FM requirement was 30 - 15,000 cps, but when phase to freq
became more popular and many exciters couldn't make the required 3.5%
THD at 30, that was dropped. Not sure, but they may have dropped 30
from the AM proof at the same time. The real kicker was making the
noise part of the proof. -60 dB below a -50 dBm at the main mike 
input caused the invention of "Kentucky windage" for broadcast 
proofs. Until the introduction of the EF86 by Mullard, there wasn't
a dependable audio preamp tube. I remember long nights hand selecting
1621's (a/k/a 6J7) and 5879's to get a legit noise test in an FM
chain. Audio proofs had their place in those days, but I for one,
don't miss them.


---------------------------------------------
Phil Alexander, CSRE, AMD
Broadcast Engineering Services and Technology 
(a Div. of Advanced Parts Corporation) 
Ph. (317) 335-2065   FAX (317) 335-9037





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