Newsgroups: rec.video

In rec.video, kaplanr@shearson.com (Roger Kaplan) writes:

>  Poor tuners (usually on VCR's or old TV's) tend to create an annoying 
>"buzzing" sound whenever text or graphics are superimposed on the screen.
>Examples are scores during sporting events, phone numbers to call during
>mail-order ads, and speaker identification during the news.
>
>  What causes this noise?  None of my equipment exhibits this, so I am
>asking more out of curiosity than need.

This ought to be on the FAQ list.

The problem is due to a design technique called "intercarrier sound".

As you know, a TV signal is made up of two parts--the video, with is 
AM, and the audio which is FM.  The two signals are separated by 4.5 MHz.
Remember that number because it will come up again.

It is more expensive to have two completely separate receivers.  So instead,
to save money (especially on non-stereo TVs and VCRs), both signals are
converted to a fixed frequency (the IF frequency, about 45 MHz) and amplified
and filtered together.  At the end of all this is the video detector.  Not
only does this detector convert the IF signal to a baseband video signal, but
it also produces another signal.  This signal is an FM signal at 4.5 MHz,
which is the difference frequency between the audio and video carrier
frequencies.  This signal is then sent to a simple 4.5 MHz FM receiver.
Note that tuning is not critical, since the frequency of this signal depends
only on the difference in the frequencies between the audio and video signals,
which is set at the transmitter.  This was an inportant point before modern
quartz synthesized tuners.

Now, to the difficulty.  For the 4.5 MHz signal to exist, both signal (video
and audio) carriers must be present.  Note that white is minimum carrier
level, but should be no less than 10% of the black level.  If things are a
bit out of adjustment at the transmitter, what should be 10% carrier white
may just become 0% carrier white.  Since the video carrier goes away, so
does the audio signal (in intercarrier sound systems).  This is one source
of the buzz.  Video titles are usually the worst, as everyone is trying to
make them as bright white as possible.

There are also other possible sources.  All amplitude modulators have a
certain amount of what is called incidental phase modulation, which in
effect puts some FM onto the video carrier.  In an intercarrier sound
system, the 4.5 MHz signal will have FM on it whether it comes from the
desired FM on the audio signal, or undesired FM on the video signal.  
And incidental phase modulation is worst at the white extreem.
Also, if the IF circuitry if the VCR or TV is not well adjusted, there
can be overload effects which could cause the buzzing as well.

Most good stereo TVs and VCRs are now made with seperate IFs at about 45
MHz for the audio and the video.  In this case, the audio doesn't know or
care about what is going on in the video, and so you don't hear the
problem.

Note that some cable TV converter boxes work by demodulating the audio and
video down to base band, and then remodulating.  If the video modulator
is not correctly adjusted, the converter box can take a perfectly good
TV signal and screw it up for intercarrier sound type receivers.


Donald Borowski    WA6OMI    Hewlett-Packard, Spokane Division
"Angels are able to fly because they take themselves so lightly."
                                       -G.K. Chesterton

From: borowski@hpspkla.spk.hp.com (Don T. Borowski)
Date: Thu, 20 Aug 1992 20:46:53 GMT
Subject: Re: Wrong VHS Tape Speed
Organization: Hewlett Packard Company, Spokane, Wa.