[LAU] Pro Audio? OT rant.

Gene Heskett gheskett at wdtv.com
Mon Dec 24 16:13:46 UTC 2012

On Monday 24 December 2012 10:37:40 Fons Adriaensen did opine:

> On Mon, Dec 24, 2012 at 04:06:32PM +1300, Chris Bannister wrote:
> [ ringing caused by filters ]
> > Also I remember that he
> > said that if the peak was on the leading edge it was filtering on the
> > input stage of the circuitry and if it was on the trailing edge it was
> > caused by filtering on the output stage of the circuitry.
> Let's hope, for that engineeer's name and fame, that you don't
> remember that very well. For it is utter nonsense.

Absolutely.  However, the response of a passive filter is 100% predictable.  
Enough math applied to that distorted waveform can tell you very accurately 
what the bandwidth and group delay characteristics of that path the square 
wave followed from the input jack to the output terminals.

There are many other things involved, and there can be, in some amplifiers, 
such as the high powered klystron used in tv broadcasting back in the day 
of 2 cents a kilowatt electricity, a delay that varies with the power level 
precisely because of the relativistic effects imposed on the electron beam 
that does the amplifying.  It is a beam velocity modulation and because of 
the e-mv2 relationship, the positive swing of the input cannot accelerate 
the beam quite as much as it can slow it down on the equal negative swing.  
So that device is electrically longer at the high power levels of the sync 
tip portion of the old ntsc signal.  It is also called incidental carrier 
phase modulation or ICPM in shorthand speak.  And it is not easily 
corrected, so the sync waveforms were horribly distorted by having the high 
frequency edges moved as much as 200 nanoseconds to the left.

It was also the cause of about a 20 db loss in the aural SNR in the viewers 
receiver because the receivers used the intercarrier frequency only in 
detecting the sound, that imposed that ICPM as a background buzz in the 
audio.  I didn't fully understand just how bad it was till a circuit 
breaker running the coolant pump failed, single phasing that 3 phase 10hp 
motor, and the electron beam burned a hole in the klystrons collector, 
filling it with water.  That hole was a $150,000 hole, and 2 months running 
at low power because I wheeled the 2200 lb aural dolly with an old tired 
klystron in it into the visual cabinet, tuned it even more broadband than 
usual and, using an N 't", fed some of the aural drive into the visual 
tube.  So we had about 25% of our normal power on the air.  Because with 
both signals being subjected to identical ICPM effects, and the receivers 
operating on the frequency difference only, the normally 45 db down audio 
'sync' buzz was gone, and the on air aural SNR was suddenly close to 72 db!

When the new tube arrived, I almost didn't want to go back to full power 
because I knew that &*^*& buzz would be back.  I did get comments on it 
from the viewers too, they noticed it right away.

> > That test alone made me realise that a simple listening test is not
> > sufficient. I'm guessing that the Sharp model would produce "listening
> > fatigue" long before the other model which had minimal distortion on
> > the output squarewave.
> You can't conclude anything relevant to audio quality from this test.

Yes you can, particularly from the aspect of how good the anti-aliasing 
filters are. 

> Ciao,

Cheers, Gene
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