[net.periphs] terminal noise

al@mot.UUCP (Al Filipski) (12/05/84)

[Hic sunt dracones]

I am typing this at a Televideo 950 terminal and am listening to 
a high-pitched continuous sound emanating from the terminal.
It is near the upper limit of my hearing, maybe 15-17 kHz,
and is loud enough to be distracting.  I mentioned this to someone else 
and he said that all terminals do that.  Maybe his hearing is better 
than mine, because I never heard it on any other terminal.  
I seem to remember a letter to the editor of Byte magazine a year 
or two ago complaining about the same thing. Can any of you hardware
types explain this sound? Is it indeed universal? Are the makers of
terminals aware of this and do they try to minimize it?  For me,
this adds a new criterion for the selection of a terminal.


                     Alan Filipski
                     UNIX group
                     Motorola Microsystems, 
                     Tempe, AZ U.S.A

            {allegra | ihnp4 } ! sftig ! mot ! al

[I'd rather have a bottle in front of me than a frontal lobotomy]

tli@uscvax.UUCP (Tony Li) (12/08/84)

> [Hic sunt dracones]
> 
There are NO dragons here.  Even if things do keep draggin' on. (Sorry, I
couldn't resist.)

> I am typing this at a Televideo 950 terminal and am listening to 
> a high-pitched continuous sound emanating from the terminal.
> It is near the upper limit of my hearing, maybe 15-17 kHz,
> and is loud enough to be distracting.  I mentioned this to someone else 
> and he said that all terminals do that.  Maybe his hearing is better 
> than mine, because I never heard it on any other terminal.  
> I seem to remember a letter to the editor of Byte magazine a year 
> or two ago complaining about the same thing. Can any of you hardware
> types explain this sound? Is it indeed universal? Are the makers of
> terminals aware of this and do they try to minimize it?  For me,
> this adds a new criterion for the selection of a terminal.
> 
This is the flyback transformer, and is a necessity for generating the HV
that your screen needs.  Yes, it is universal (for screens - get a plasma
display and you'll never hear it).  Minimization of the noise is usually by
selection of the type of transformer to use.  
> 
>                      Alan Filipski
>             {allegra | ihnp4 } ! sftig ! mot ! al
> 
-- 
Tony Li ;-)		Usc Computer Science
Uucp: {sdcrdcf,randvax}!uscvax!tli
Csnet: tli@usc-cse.csnet
Arpa: tli@usc-ecl

henry@utzoo.UUCP (Henry Spencer) (12/09/84)

It's vibration at the horizontal scan frequency of the display.  Any CRT
terminal will have a fair bit of energy running around at this frequency,
since it's got to bully an electron beam into going from the end of one
scan line to the beginning of the next in a few microseconds.  This means
the horizontal deflection circuitry has to go from "+MAX to -MAX" in that
length of time.  Particularly with magnetic deflection, which is normal
for CRT displays, this is hard work.  So there are quite energetic pulses
present at that frequency.

Given that there are magnetic materials here and there, and that the
deflection yoke itself may well be responding mechanically to some small
extent, there is an inherent noise source at that frequency.  What varies
is how intense the noise source is, how well the surrounding bulk of the
terminal absorbs the noise, and whether anything resonates at that exact
frequency.  We've got Ampex terminals which seem to have something in the
case or innards that can resonate at the horizontal scan frequency -- minor
mechanical disturbances (e.g. giving the terminal a good thump) can flip
the terminal from noisy mode to quiet mode.
-- 
				Henry Spencer @ U of Toronto Zoology
				{allegra,ihnp4,linus,decvax}!utzoo!henry

jaap@haring.UUCP (12/09/84)

A lot of hardware have ``switched power'' supplies. They are based on
the effect that at higher frequency AC less smaller transformers are
needed. Usually they bump the frequency up to 40kHz or more, however
cheaper ones have lower frequencies.
When the transformers are cheap as well, you and up hearing the
noise made by these.

It is the same effect as in cheap/old television sets (In Europe
15.625 kHz.

jack@vu44.UUCP (Jack Jansen) (12/11/84)

Alan Filipski (mot!al) complained about the noise coming from
a TVI950.
Our TVI 950s sometime expose the same problem, and the solution
is probably the oldest way of repairing delicate instruments:

	HIT IT! HARD!!

At least, that seems to work in our case.
By the way, the noise is caused by the deflection transformer.
You could try to take that thing out of the terminal, but this
is likely to produce disappointing results (your screen would
by reduced to a single dot :-)

As an aside, if you want to hear some more noise from your
TVI 950, tune your FM reciever at 95.5Mhz (yep, that's where
my favourite station is :-( ) and you'll find a nice clean
channel which disappears when you turn your terminal off......
-- 
	Jack Jansen, {seismo|philabs|decvax}!mcvax!vu44!jack
	or				       ...!vu44!htsa!jack

chris@umcp-cs.UUCP (Chris Torek) (12/11/84)

The switching power supplies in the back of a 780 also tend to make
whining noises, especially if some of the screws are slightly loose.
In general, anything that switches a lot at high frequencies has to
be very secure mechanically, or it will make noise.
-- 
(This line accidently left nonblank.)

In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (301) 454-7690
UUCP:	{seismo,allegra,brl-bmd}!umcp-cs!chris
CSNet:	chris@umcp-cs		ARPA:	chris@maryland

berry@zinfandel.UUCP (Berry Kercheval) (12/12/84)

In article <522@vu44.UUCP> jack@vu44.UUCP (Jack Jansen) writes:
>... the solution
>is probably the oldest way of repairing delicate instruments:
>
>	HIT IT! HARD!!
>

I did this to my poor Concept 108 once, and was rewarded with a bright
flash and a "puff of greasy black smoke".  Shards of plastic soot drifted
around the office the rest of the day.

I reccomend starting with light pats and working your way up to 
ecky-thumps.

--berry



-- 
Berry Kercheval		Zehntel Inc.	(ihnp4!zehntel!zinfandel!berry)
(415)932-6900

rcd@opus.UUCP (Dick Dunn) (12/19/84)

> >... the solution
> >is probably the oldest way of repairing delicate instruments:
> >
> >	HIT IT! HARD!!
> 
> I did this to my poor Concept 108 once, and was rewarded with a bright
> flash and a "puff of greasy black smoke".  Shards of plastic soot drifted
> around the office the rest of the day.

The lesson to this actually is, "Know your equipment."  I've heard the "Hit
it!" philosophy referred to, in a different domain, as the "RCA treatment."
This is only partly a comment on whatever need RCA equipment might have had
for, ummm, encouragement...it also reflects on the ability of said
equipment to withstand the corrective maintenance procedure.
-- 
Dick Dunn	{hao,ucbvax,allegra}!nbires!rcd		(303)444-5710 x3086
   ...Nothing left to do but smile, smile, smile.