On Thu, 5 Feb 2015, Ralph Mowery wrote:


"John Davis" wrote in message
...
On 2/2/2015 6:44 AM, gareth wrote:

I recall when I was in college, The teacher showed us an old movie
produced by the US-Army on FM radio, it eatured SLOPE detection of an FM
signal... I will not say more but you could tell who in the class had a
ham license and/or experience with FM radio right quick when that hit.
(All the students ROFL in case nobody here is old enough to recall Slope
Detection).

I know about the slope detectiion and am 65.

I think it might be more the younger crowd. There was that period, in the
late forties or early fifties were very narrow FM was promoted, no need
for the big modulation transformer, and no being detected by input stages
to audio amplifiers. But slope detection was the usual mode, since
virtually no shortwave receivers had FM detectors. Except one could get
an external FM adapter for a late model HRO (I think, there was an
accessory socket for that, but I'm not sure if there was an FM detector;
people used the socket to add an external product detector).

And then of course, when 2m FM took off, lots of people converted their
existing AM rigs to FM by modulating the oscillator in some way, and using
slope detection for reception. Or there was a project in QST in the
summer of 1971, add a converter to an AM broadcast portable, and use slope
detection, then the next month add an FM IF strip.

I used to use my SP-600 with a converter to listen to 2M FM circa 1972,
and I had to use slope detection.

I don't have any of the very old gear, but every so often I listen to the
hams around 3.85 MHz. that use the old military and AM gear. While I don't
think it is really legal (and don't really care) some of those AM
transmitters sound beter if I switch to FM on my Icom 746 pro. I think when
they were made they seem to put out about as much FM as AM.

That's weird. I thought those hardcore AMers wanted purity, and took
effort to ensure they were putting out a good signal. They sure aren't
using modulated oscillators, and I don't think anything where the
modulation would get back to the oscillator.

When I got my Hallicrafter's S-120A (that's the transistorized model)
shortwave receiver in 1971, that AM bunch was the only voice modulation I
could receive on the ham bands, since the receiver had way too weak a BFO.
(Only later did I learn enough to put a potentiometer between the antenna
and the receiver's antenna terminals to work as an attenuator, at which
point I could receive SSB, but only signals strong enough to overcome the
attentuation.). I gathered at the time that they were youngish, I'm not
sure why, maybe what they were talking about. And they probably are the
same bunch now, 44 years later. But at the time it was great, since I
could listen to hams talk.

Michael