|[AMRadio] Physical Reality of Sidebands|
John Coleman, ARS WA5BXO
wa5bxo at pctechref.com
Mon Jan 17 22:12:42 EST 2005
Yep, I know Larry, but I had already gotten deeper than I intended to.
Because most of the video is motionless, the sidebands come out at
multiples of the sweep rate. The color sub carrier frequency which is
only transmitted in burst mode as previously describe, was chosen at the
odd frequency that it is, so that in spectrum, most of the sideband
energy containing the color info, would fall between the energy of the
black and white info. This was in hope of having less "intermodulation"
at the detector. As for I know, it was not until Magnavox produced the
first COMB FILTER that we were able to make use of this bit of spectrum
conservation. It is interesting to note that the color sidebands were
500KHz of upper and lower sideband spectrum except at the phase
difference of around 80-100 deg where the flesh tones are produced and
at that phase difference the band width is much greater but only on one
sideband. The RCA CTC 4 chassis made use of this with the "I and Q"
demodulation system, a very difficult sweep / band pass alignment
This is all getting off the subject, but it was interesting to me that
all this could be kept separate with the fast switching on and off of
the burst and changing bandwidth of the I and Q modulated signal.
I love all this type of discussion. I am afraid I would have to lean
towards the theory, that it is what ever fits the need of the detector.
At what wavelength does Electro-magnetic radiation become a particle?
Is the Universe homogeneous or chaotic? That depends upon how it is
From: amradio-bounces at mailman.qth.net
[mailto:amradio-bounces at mailman.qth.net] On Behalf Of lwill at voicenet.com
Sent: Monday, January 17, 2005 2:40 PM
To: Discussion of AM Radio
Subject: RE: [AMRadio] Physical Reality of Sidebands
You are close. That was the old black and white days. Since color its
down from 3.579454 to 15, 726.xx (approx) and vertical is 59,94 not
On Sun, 16 Jan 2005 22:58:04 -0600, "John Coleman ARS WA5BXO"
<wa5bxo at pctechref.com> wrote :
> Don, I have often pondered the same thing here is another example.
> The Horizontal sweep rate of a TV is 15750 hz. Every 1/15000 of
> a sec there is a sync pedestal, and on the back porch of it is a burst
> of a few cycles (as I remember it was 8 to 10 cycles in length) of the
> color sub carrier (3.579545 MHz). This burst is removed and processed
> by an amplifier that is key on by the horizontal retrace pulse which
> been synced to the horizontal sync pulse that rides atop the sync
> pedestal just in front of the color burst. The 8 cycle color burst is
> phase compared to a crystal oscillator in a phase locked loop. A good
> synchronized scope can look at the full video detected signal and
> the back porch of the sync pedestal out and view the 8-10 cycles of
> burst. I often wondered what a spectrum analyzer would look like when
> monitoring the output of the burst amplifier with the phase detector
> diodes remove.
> The burst amplifier was a simple tetrode whose plate circuit had
> a parallel tank tuned to 3.58 MHZ and where the detected video was
> applied to the grid through a small coupling capacitor that would
> differentiate and pass the frequencies higher than 3 MHz. The grid
> was returned to a circuit where a positive pulse from the fly back was
> present to trigger the tube on. The output tank was link coupled to
> phase detector.
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