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PICList Thread
'CMOS video camera modules - quite [OT] really!'
1998\11\04@072544 by paulb

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Sean Breheny wrote, quoting Dan:

>> TV carriers use a vestigial side band.  Only the center + the uppper
>> sideband are transmitted.  There is a small lower side band where the
>> FM audio hangs out.

> Dan, are you sure?
> I thought I had seen Amateur TV transmitters which use full double
> side band video.  In fact, I believe that the reason why vestigial
> side band is used instead of SSB is that some vestige of the other
> sideband (and the full carrier) is required for downward compatibility
> with DSB receivers (not sure of this last assertion).

 Well, Dan may be sure, but I'm sure it isn't!  The part about audio
is, I'm afraid, rubbish.

 TV is vestigial sideband.  This means one sideband, presumably as
suggested, the lower, is attenuated as far as is possible without
distortion.  This is much more difficult than for an audio signal, so
you end up with a small amount of sideband, thus vestigial.  If you are
running a basic ATV system, it may be a lot easier to do without the
tricky filters and just transmit DSB or asymmetric SB, but the unwanted
one is mostly removed by the receiver IF anyway.  You don't need
compatibility for DSB receivers because - there aren't any.

 Sound is "intercarrier".  What this means is that instead of
modulating sound onto a subcarrier as part of the video signal (which is
what you do if you use a simple modulator as was proposed, and what most
Amateur TV systems do), you generate a separate carrier the required
(frequency) distance away from the video carrier, and frequency modulate
it.

 When this is received, it appears as a subcarrier in the IF and is
demodulated accordingly.  The elegance is that transmitted in that
fashion, there is *no* image on the other side of the video carrier *at
all*.  Note that the sound carrier is on the other side of the video
sideband to the main (video) carrier, *not* the low side.  The two
carriers neatly delineate the video channel.  Nowadays, most
broadcasters produce additional sound carriers for stereo, subtitle,
multi-language and so on.
--
 Cheers,
       Paul B.

1998\11\04@122832 by Dan Larson

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On Wed, 4 Nov 1998 23:23:47 +1000, Paul B. Webster VK2BZC wrote:

>Sean Breheny wrote, quoting Dan:
>
>>> TV carriers use a vestigial side band.  Only the center + the uppper
>>> sideband are transmitted.  There is a small lower side band where the
>>> FM audio hangs out.
>
>> Dan, are you sure?
>> I thought I had seen Amateur TV transmitters which use full double
>> side band video.  In fact, I believe that the reason why vestigial
>> side band is used instead of SSB is that some vestige of the other
>> sideband (and the full carrier) is required for downward compatibility
>> with DSB receivers (not sure of this last assertion).
>
>  Well, Dan may be sure, but I'm sure it isn't!  The part about audio
>is, I'm afraid, rubbish.

Yes, the sound carrier part is rubbish!

My memory of classes I took 14 years ago
is somewqhat foggy.  Now I remember that the audio carrier is 4.5Mhz *above* the
main carrier.  The part about using vestigial side band is to keep the bandwidth
used by the channel to a minimum.  The total spacing between most channels is 6M
hz.
If both sidebands were transitted, you would wipe out the audio of the channel b
elow
and probably most of the luminance as well.

Well, now, I am reminded once again why I went into software instead of hardware
!

Sorry for the misinformation.  I should have pulled out my books before speaking
, but
unfortunately they are all packed at the moment, in anticipation of my upcoming
move.

{Quote hidden}

Dan

1998\11\04@125312 by Sean Breheny

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Thanks to everyone for replying.

I guess I should have explained what I was intending. Between my HUGH
amount of school work and my current projects, I won't be attempting this
soon, but I eventually wanted to put a small B+W video camera in my RC
plane. I would then transmit the signal to the ground and record on a VCR
or camcorder in VCR mode.

Being the sort of person I am, I wanted to turn this into a learning
experience about video modulation and implement the modulator/transmitter
myself instead of using a ready-made chip or module. I was only planning on
doing this on a lower channel (like 3 or 4). Since I am so used to cable, I
can't remember which channels broadcast in my area (I don't think there is
a 4). So, I would really only be dealing with VHF, not really UHF. I also
wouldn't need much power, only need to cover about 500 ft or so, and its
totally in free space. I would of course have to avoid the 72 MHz RC signal
<G>

Again, not something I am going to do soon. The thread just got me to
thinking and I though I'd ask while the thread was going.

Oh, one thing, about the DSB receivers. I had the (probably totally wrong)
notion that early B+W TV sets had DSB receivers and that VSB was still used
so that they would still be compatible. Am I totally wrong?

Thanks,

Sean


+-------------------------------+
| Sean Breheny                  |
| Amateur Radio Callsign: KA3YXM|
| Electrical Engineering Student|
+-------------------------------+
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Personal page: http://www.people.cornell.edu/pages/shb7
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1998\11\04@152236 by Gavin Jackson

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part 0 2560 bytes
The only use for a PIC in my system would be to position
the camera with two RC servos. An old 486 PC would have
to analyses the images from the camera.

Does anyone on this list have the Logitech Color QuickCam
as I would like to know how difficult it would be to write my
own capture software before I buy it.

If I used one of these cheap cameras on the market that
had a video output, how would I go about converting the
signal to an bit value for each pixel? What AD rate would
it require and has it been done by anyone on this list?

Regards

Gavin
--------------------------
.....vulcanKILLspamspam@spam@ihug.co.nz
ICQ# 18675389
www.geocities.com/TheTropics/Cabana/2625
--------------------------



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