[GreenKeys] news wire photos

[David I. Emery]

On Fri, Jun 17, 2011 at 12:46:56PM -0400, Nick England wrote:
> I think the spark-paper method could only be used for black/white
> imagery rather than news-quality photos - the resolution wasn't good
> enough to provide half-tone images as used to print gray-scale photos
> in magazines, etc. For actual gray-scale photos, the receiving paper
> was regular light-sensitive photographic paper and a variable
> intensity lamp was focused to create the spot on the paper. Lamp
> intensity was determined by the incoming AFSK frequency (1500-2300
> cps)
> After reception, the paper was developed using normal photo chemicals
> in a darkroom.

	Transmission over HF radio was FM (FSK), transmission over
leased wire lines was usually DSB AM with a carrier tone of 2400 Hz
being common or sometimes 1800 Hz.   More tone, brighter light usually I
think.

	Early drum and lamp fax machines could use either sheet film or
photographic paper depending on the need...  I believe film was quite
common for newspaper use...

	Some late era photo fax and wirephoto machines used dry silver
paper developed by heat and fed from a roll rather than wrapped around a
drum.   Spinning mirrors provided a scan.    And for lower quality
images there were always the helix-on-drum moist paper electrolytic
machines widely used for weather maps 

	Early machines were manually operated with a speaker to monitor
the line inside the darkroom allowing the operator to load the film or
paper and start the drum spinning and activate the magnet driven clutch
mechanism that would allow the receiving machine to acquire phase
synchronism with the sending machine as the sending operator set up to
start transmission.

	Later machines (especially the roll paper kind) used audio
signaling tones to control start and stop of reception and initiate
phasing.

	From the early days machines generally had internal frequency
standards (tuning fork or later crystal) to generate a highly stable and
accurate motor drive voltage for a synchronous motor that drove the drum.
This was required so the receiving and transmitting drums stayed more or
less exactly in sync during the transmission of the image - if they did
not the image would come out slanted.   60 Hz line power is no where
near stable enough or accurate enough for this.

	Many of the really early machines used a phonic wheel type
synchronous motor to drive the drum that ran off mid audio frequency AC
and made quite a bit of noise... later machines were more likely to just
synthesize an accurate 60 Hz and use that through a power amp to drive
the motor.

	Most machines for wirephotos used a drum speed of either 60 RPM
(early days) or 120 RPM (later on) though there were some 90 RPM systems
apparently and later weather satellite APT was 240 RPM.
	
	DSB AM audio tone wirephoto networks were around into the early
80s at least - used with automated printers, I do not know if there ever
was a digital version developed for wirephotos as there certainly was
for documents and weather maps.    More recently wirephotos are sent
over TCP/IP on Internet or private connections as .jpegs or similar just
as all kinds of other images are.


-- 
  Dave Emery N1PRE/AE, die at dieconsulting.com  DIE Consulting, Weston, Mass 02493
"An empty zombie mind with a forlorn barely readable weatherbeaten
'For Rent' sign still vainly flapping outside on the weed encrusted pole - in 
celebration of what could have been, but wasn't and is not to be now either."