Jeff Liebermann wrote in message
...
On Sun, 4 Apr 2010 12:03:59 +1000, "Phil Allison"
wrote:

"Jeff Liebermann"
"Phil Allison"
** DO NOT BUY A DIGITAL SCOPE !!!!!!!!!!!!!

Why?

Almost any ANALOGUE scope with bandwidth of 5 MHz or more is OK -
DC
coupled or not.

Digital scopes absolutely SUCK for audio work.

Why duz it suck?

** There is no way to prove the point to fools like YOU by posting
messages on a newsgroup.

Got it. You can't explain why a digital scope is not usable for
analog work. Perhaps I can help jog your memory. See below.

Have you ever actually used a digital scope? A sound card based
scope? You really should try it some time. I think you'll be
pleasantly surprised.

Drivel: iPod Touch based sound analysis instruments:
http://www.faberacoustical.com/products/iphone/signalscope/
I'm really tempted.

Buy anyone familiar with the use of analogue scopes for audio test and
repair work will find using a DSO to be mighty irritating and tedious -
at
best.

Long ago, in my mis-spent youth, I worked in a repair shop that did
mostly audio. Not audiophile, but production line warranty repair for
various manufacturers of various audio related equipment. The lead
tech never used a scope. He would just listen to whatever was coming
out of the speakers, scribble down what stage or device was blown, and
move on to the next machine. My job was to do the unsoldering and
replacement. His batting average was about 80% correct. I couldn't
even come close to that level of accuracy, especially without a scope.
One day, I saw him try to use a scope, and fail. He didn't know how.

40+ years later, I still can't do it with audio equipment. I gotta
have my test equipment, white noise, pink noise, sweeper, distortion
analyzer, and all important oscilloscope. However, I can do something
like that with 2way radios. I've heard enough of them on the air to
be able to diagnose problems by simply listening to the audio.

Moral: Use your ears first, then use the scope.

The displayed traces on a DSO are often very misleading and hence useless
for many test procedures that analogue scopes do just perfectly.

Oh? Misleading in what way? What measurements are misleading? How
will using a digital scope produce a misleading diagnosis?

I use a digital storage scope for doing the all important square wave
test. Instead of the fuzzy blur of high frequency ringing and
oscillations seen on the analog scope, I see the digital equivalent,
which looks like a jitter infested trace in the same area (top of
leading edges). By superimposing multiple stored traces on top of
each other, ringing and oscillations are fairly obvious, even if they
exceed the frequency response and resolution accuracy of the A/D
converter.

DC related phenomenon are a problem with a PC sound card based scope.
There's no DC response, and the lower limit is about 20Hz. Low
frequency display during the square wave test will show up as a "sag"
in the horizontal part of the waveform even with a DC coupled audio
amplifier. Scope probe compensation also shows the same "sag". I
partly compensate with the scope probe compensation, and just remember
what the "sag" looks like when the scope is directly connected to the
square wave generator. In other words, I ignore the low freq sag.

High frequencies are more of a problem.
http://en.wikipedia.org/wiki/Slew_rate
Slew rate testing is difficult with a digital scope, unless the scopes
usable bandwidth is more than 5 times the highest frequency of
interest. For a 2MHz bandwidth digital scope, that limits the maximum
frequency to about 400KHz, which should be more than adequate for any
slew rate testing. That's NOT the case with bottom of the line sound
card based scopes, which are bandwidth limited to about 22KHz. The
96KHz 24bit sound cards are much better.

There are probably other areas where an analog scope is better than
digital. Tuning and tweaking in real time is much easier with a fast
responding analog scope than on a more slothish digital equivalent.
Seeing oscillations and ringing at tiny points during a frequency
sweep is somewhat easier to see on an analog scope.

Did I miss anything on why an analog scope is superior to digital?


--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

After a DVM the next most useful piece of test equipment is a crystal
earpiece with a high voltage cap in tow, for audio repair work . Next comes
a sig gen and then a scope (analogue not DSO and stand alone, not tied to a
pc). Other useful pieces of test kit is a nose, eyes (with good magnifying
inspection lamp) and ears.

--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://diverse.4mg.com/index.htm