"SventheViking" wrote in message
...
On Apr 2, 2:09 am, "Richard Knoppow"
wrote:
"Scott Dorsey" wrote in message
...
coffelt2 wrote:
D-104C had a "ceramic" crystal element which was far
more tolerant of
moisture, shock, etc.
I can't remember just now what the frequency response
was
(compared to the
original) but when
I used one, I was looked down upon as a traitor to
tradition.
You can no longer get either the original crystal
element
or the ceramic
element, but Astatic will sell you a dynamic
replacement.
The dynamic is a whole lot smoother and less brittle
sounding, but maybe
that's a bad thing in a pileup.
What was good about the D-104 types, was that you
didn't have to speak
directly into it. One
could just walk around the shack, and in some instances,
around the house
with little loss of
readability.
They were very, very omnidirectional compared other
communications mikes
back then. The good part of this was the effect you
note.
The bad part
is that noise sources like fans and people yelling in
the
background were
also very readable on the air.
I am currently using an old Turner microphone and like
the
way it sounds.
Also I am a fan of some of the older EV desk
microphones,
which still turn
up cheaply at hamfests. If you find a low-Z mike that
you like, there is
no reason you can't just stick a step-up transformer in
the base to drive
the Viking.
--scott
There aren't many polar patterns published for cheaper
mics but the directional properties of mics like the D-104
are mostly due to the diffraction around the body. At low
frequencies they are almost perfectly omnidirectional but
at
some frequency begin to have some directionality which
increases with frequency. The same diffraction effect
causes
a rise in the frequency response unless its compensated in
some way. At a frequency where the path around the body
approximates a half wave length the microphone can
approach
a super-carioide pattern, i.e., unidirectional with one or
more lobes toward the back. The shape is important, a flat
pancake shape like the D-104 will have a somewhat
different
pattern than a bullet-shaped mic. The ultimate was the
Western Electric 630A "Eight-Ball". The spherical shape
made
it quite omnidirectional to rather high frequencies but
the
pattern was made even more uniform by the partial baffle
mounted in front of the diaphragm. Metro-Goldwyn-Mayer
made
some microphones using probably standard Western Electric
condenser elements mounted in a spherical case to reduce
the
diffraction rise typical of the older mics. I don't
remember
whether these had baffles on them but the elements were
mounted in a way that also tended to reduce the cavity
resonance typical of both this and other large condenser
type elements. In some respects the rise was useful in
dialogue recording although it could also make some voices
sound harsh. All sorts of mechanical filters and baffles
were tried to aleviate this effect, mostly with limited
success. Electrical filters, which would have been a
better
solution, while known in the telephone industry, were not
very well known outside of it. Much of the early theory of
electrical wave filters was developed by George A.
Campbell,
of Bell Labs, in the mid 'teens. This was cosidered very
advanced stuff at the time.
I am not surprized that no one makes crystal or ceramic
elements any more. For the most part microphones to fill
similar applications now are electrets.
--
Richard Knoppow
Los Angeles
WB6KBL
- Hide quoted text -
- Show quoted text -
Thanks Richard and everyone else here. (For some reason my
email wasn
īt "beeped" when these responses came so I am a bit behind
and have
some heavy reading to do here, but seems to be a wealth of
important
info here that i have to digest, then start the search
engines to find
an appropriate mic(s). Most Vikings I have seen have a
D-104 with
them but the specifics as to their innards of course is a
guess.
Thanks again and best 73s for now. Guess I better keep
checking back
here from time to time. 73s,
Wayne
As mentioned before while the D-104 was an excellent
mic for ham use it was not the only one. _Any_ high
impedance mic will work with the Viking, most have enough
output. For good speech intelegibility in the presense of
noise its desirable to favor the upper-mid frequencies,
i.e., from about 1000 to 3000 hz since that is where the
energy that carries the inteligence is. Most of the power is
below 1000 hz, so a system with a cut off of around 300 hz
and rising characteristic will put more of the modulation
power in the range necessary for understandability. However,
for naturalness you need some lows. Bell Labs, in their
research for minimum bandwidths for telephone service found
that inteligibility needs frequencies up to about 2800 hz
but that, on the low end, it was found that voices were
unnatural sounding unless the low end was extended to 250 or
300 hz. The phone company adopted a channel bandwidth of
2500 hz extending from 250hz to 2750hz. In fact, a wider
bandwidth will sound better. On a noise-free channel
wide-band, uncompressed speech has the best intelligibility
but in the presense of noise band limited speech with
spectrum shaping (fancy for rising frequency response) and
compression or clipping, is more inteligible. A paper by
Licklidder et.al, published in the _Journal of the
Acoustical Society of America_ about 1948 reported
experiments with _infinite_ speech clipping. Licklidder
found that if speech was put through a differentiator (6db
per octave high pass RC network) infinitely clipped (NO
amplitude variations at all left) and then integrated (6db
per octave RC low pass filter) the intelligibility was
perserved and improved where the speech was competing with
noise. In a quiet channel the unprocessed speech was better
but the clipped speech still has something like an 80%
articulation score. When in a very noisey channel the
clipped speech maintained its score where untreated speech
fell off severely.
Lots of mics, both old and new, will work this this
transmitter.
--
--
Richard Knoppow
Los Angeles
WB6KBL