From: rxv9839@ritvax.isc.rit.edu (Bob Vogel)
Subject: Re: Universal Broadcast Level Theory
Reply-To: rxv9839@ritvax.isc.rit.edu
Organization: Rochester Institute of Technology
Date: Thu, 7 Jan 1993 07:06:05 GMT

In article <1993Jan6.170106.1@vmsb.is.csupomona.edu>, cvafy006@vmsb.is.
csupomona.edu writes:
>I'll tell you what...let's start with a universal output level for all
>television broadcasts.  I hate it when I am watching something on TV and then
>all of a sudden, when a commercial comes on, I get hit with a blast of sound. 
>Once I adjust the volume for the TV, all the programs/commercials should be
>about the same volume.  But NOOOOOOOOOOOOOOOOOOOOOO, the commercial people use
>a signal almost 1 and a half times stronger.  It may get my attention, but I
>don't watch the commercial.  If cable desginated a universal output signal for
>TV broadcast, I would be much happier.

Yes, you and most of the cable television headend technicians in the world.
If you flip from station to station watching off-air TV broadcasts you will
find some sound level variation.  This is partly due to the "enhancements" done
to commercials and partly to the several different methods (standards, though
they are not really) of setting and/or measuring audio levels transmitted.
There is also a phenomenom known as "apparent loudness"--a given sound may
"sound" lounder than another to your ear, yet as measured with most brodcast
quality studio equipment there  is no difference in the actual audio level or
power.  Generally your cannot distinguish a higher level of sound until a power
doubling has occurred (the sound level has increased 3dB).
But on to cable--cable will simply re-broadcast whatever sound variation there
is in program and/or commercial content.  Cable does not inherently add or
subtract from those level variations, however most headend equipment could be
mis-adjusted to cuse greater or lesser sound level variation than what you gett
over the air.  Any well maintained headend will do their best to minimize this
variation.   If you taped programs off the same channel, all set-up to record
precisely the same sound level as the actual program content, and played them
all back simultaneously over multilpe channels, just like a cable system you
would doubtless hear the same variations.
An then there is level control.  Some stations do it manually, some
automatically, some on all programs, some only on certain ones.  Most use a Vu
meter to visually tell how loud the sound is.  There are 2 generally accepted
scales or standards used--some ue one, some use the other.  Some have limiterss
(a sound louder than xdB gets clipped and doesn't get any louder) some do not.
The sound engineer who controls the sound for "Beverly Hills 90210" probably
likes a different tone and level from the one for "Captain Kangaroo".
If you set every channel so the audio sounds identical today at 8AM by 5PM
there will be significant and noticable variations between channels because the
program content has changed and your mix of recorded sound is different.
There is a commercial device (AudioRider2000) which is availablefor cable
systems that samples the sound from each channel and attempts by AGC action
computer controlled with a complex alogrithim using fast Fourier transforms to
keep each channel level "the same".  How fast it reacts and how it handles
transitions from no noise to gunshot (and there are a lot of those on TV these
days) is critical and one of the reasons that cheap and simple AGC's or
limiters either won't work or have very limited application to solve this
Well thank you for letting me vent a bit.  It is a complex  problem which is
about equally frustrating to the cable engineer as it is to the viewing public. 
If the device mentioned above really works, perhaps many will get it.  However
the cost is high, about $25,000 for 35 channes, more channels more $$$
Bob Vogel