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'[EE] Analog broadcast TV and Composite TV'
2010\04\14@144303 by Jonathan Hallameyer

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Hello all,
I have a question about composite (CVBS) video and analog broadcast TV in
NTSC land.  Going from a composite video source to an RF input on a TV
usualy would be done with a "modulator'  but the real question is,  does the
'modulator' actually modulate anything, given that for NTSC you are going to
have to Amplitude modulate the luma,  frequency modulate the audio, and use
QAM for the chroma signals, I would expect that the commercial 'modulators'
wouldn't be doing this, for how cheap they are.   Is the composite signal in
actuality just upmixed with a local oscillator, filtered to the standard
bandwidths and sent down the coax, for the reverse to be done in the TV
tuner? All the online articles I see (mainly googling/wikipedia) are vague,
or I'm missing, the relationship between composite and broadcast RF
signals.  I have seen some people make "DIY video modulators"  and one took
a transmitter, lopped off half the PCB and fed the composite in and changed
a few components.  He didn't have a schematic, but It would seem to me, the
latter half of a transmitter would be upmixing, filtering and amplifying.

Thanks,
Jonathan Hallameyer

2010\04\14@145953 by peter green

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Jonathan Hallameyer wrote:
>  Hello all,
> I have a question about composite (CVBS) video and analog broadcast TV in
> NTSC land.  Going from a composite video source to an RF input on a TV
> usualy would be done with a "modulator'  but the real question is,  does the
> 'modulator' actually modulate anything, given that for NTSC you are going to
> have to Amplitude modulate the luma,  frequency modulate the audio, and use
> QAM for the chroma signals
My understanding (this is from PAL land but afaict NTSC is much the
same) is that composite video already has the luma and chroma
information merged together. So the final modulator just has to
amplitude modulate the video and frequency modulate the audio.

Afaict the modulators used in home AV gear are pretty shittilly made
devices, indeed i've heard reports that the tuners in modern HDTVs often
have problems receiving them due to their frequency drift.

2010\04\14@151700 by Peter

picon face
> I have a question about composite (CVBS) video and analog broadcast TV in
> NTSC land.  Going from a composite video source to an RF input on a TV
> usualy would be done with a "modulator'  but the real question is,  does the

The CVBS signal is 'air ready'. Simple modulators indeed take the CVBS input and
mix it in a RF mixer with a VHF or UHF carrier to yield a double side band
signal which is interpreted by any TV receiver as 'valid'. Sound modulation is
usually a separate issue, and a second FM carrier is generated for this. A
simple video only modulator consists of a single transistor oscillator and a
diode mixer. One with sound adds one more transistor (i.e. 2 total) and a few
more diodes. The signals produced by such modulators are not OK for transmission
over the air or in large cable systems. Modulators which generate the accurate
signals needed for professional use are significantly more complex (100 times
and more).

Component signal modulators (non-CVBS i.e. separate luma chroma and sync) are
much more complex.

Here is a very simple video only modulator (color but no sound) which uses a
special harmonics generator technique to cover all bands simultaneously:

http://petlibrary.tripod.com/rfmod.htm

-- Peter


2010\04\14@164705 by Olin Lathrop

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Jonathan Hallameyer wrote:
> but the real
> question is,  does the 'modulator' actually modulate anything,

Yes.  For one thing, it puts all this onto a carrier of the simulated
station's frequency.  Most modulators can be set to either channel 2 or 3,
and I vaguely remember the FCC guarantees one of those will always be open
in any one area.

Then there is other modulation, like the hue encoded as a phase offset from
the color carrier, which itself is amplitude modulated onto the RF carrier.
The audio is also modulated onto the RF carrier.


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2010\04\14@171431 by Carl Denk

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In the USA, nearly all built-in modulators (VCR, DVD players, etc.) are
switch settable to either channel 3 or 4. Stand alone modulators with
S-video or composite (yellow RCA) input, and are readily available with
3 or 4 (cheap), or all off the air and cable channels. I have numerous,
output, 59, 63,65,67, and 69. They are digital setting, just push the
little buttons. The input to these include DVD, VCR players, 2 Directv  
receivers, computer.

On 4/14/2010 4:46 PM, Olin Lathrop wrote:
{Quote hidden}

2010\04\15@072456 by Dave Tweed

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Jonathan Hallameyer wrote:
> I have a question about composite (CVBS) video and analog broadcast TV in
> NTSC land. Going from a composite video source to an RF input on a TV
> usualy would be done with a "modulator' but the real question is, does the
> 'modulator' actually modulate anything, given that for NTSC you are going to
> have to Amplitude modulate the luma, frequency modulate the audio, and use
> QAM for the chroma signals, I would expect that the commercial 'modulators'
> wouldn't be doing this, for how cheap they are. Is the composite signal in
> actuality just upmixed with a local oscillator, filtered to the standard
> bandwidths and sent down the coax, for the reverse to be done in the TV
> tuner?

Amplitude modulation and upmixing from baseband are essentially the same
thing.

The thing that you and most of the other responders seem to be missing is
that the CVBS signal can include both the color subcarrier (3.579545 MHz,
QAM) and the audio subcarrier (4.5 MHz, FM). When applied to an AM modulator
along with the luminance signal, these signals end up in exactly the right
place in the resulting RF.

What the cheapest modulators don't do is filter the resulting RF spectrum
to remove most of the lower sideband in order to create a proper VSB
(vestigial sideband) signal. But this isn't very important unless you want
to broadcast the signal over the air.

Of course, the point is mostly moot now that commercial NTSC broadcast has
been switched off in the US.

-- Dave Tweed

2010\04\15@092253 by Carl Denk

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Not really, trying to distribute numerous HDTV sources to numerous HDTV
receivers (4 each in my house) is very expensive, plus having to run
many cables everywhere. Our largest receiver is 39" which does have HD
off the air (antenna) and Directv. The other sets are 26" and HD isn't
that much better on those small screens. An RG-6 network with a 8 >1
combiner and a 1 > 8 splitter and an inexpensive modulator on each
source works just great. I can play from 2 Directv DVR's, DVD player and
VCR. All this distribution is NTSC. The neat thing is when going from
one room to another, don't have to remember it's Directv channel 345,
just that it's the living room receiver with modulator on channel 63.

On 4/15/2010 7:24 AM, Dave wrote:
> Of course, the point is mostly moot now that commercial NTSC broadcast has
> been switched off in the US.
>
> -- Dave Tweed
>    

2010\04\15@141639 by Herbert Graf

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On Thu, 2010-04-15 at 07:24 -0400, Dave Tweed wrote:
> What the cheapest modulators don't do is filter the resulting RF spectrum
> to remove most of the lower sideband in order to create a proper VSB
> (vestigial sideband) signal. But this isn't very important unless you want
> to broadcast the signal over the air.

Or you want to use two of those things (each set to a different channel)
on the same cable, very frustrating...

> Of course, the point is mostly moot now that commercial NTSC broadcast has
> been switched off in the US.

True for now, but I'm sure ATSC modulators will eventually come to
market. TTYL

2010\04\15@150602 by Carl Denk

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I have been looking for more than a year for ATSC modulators, that I
could plug some sort of HDTV signal (component, DVI, etc.). I keep
getting the answers back: 1: They are available at high cost ($2000+)
for broadcast quality, 2: There are some copy protection issues that
prevent low cost ones. If someone finds a source, please let me know. It
would be nice to put those Directv receiver's HD signal on the RG-6.
Then again Directv is going to have the capabilities of ATT with multi
rooms in the fall. A HDTV tuner card for the computer is around $100,
would think a modulator would be available for less than that.


> On Thu, 2010-04-15 at 07:24 -0400, Dave Tweed wrote:
>    
>
> True for now, but I'm sure ATSC modulators will eventually come to
> market. TTYL
>
>    

2010\04\15@153417 by peter green

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Carl Denk wrote:
> I have been looking for more than a year for ATSC modulators, that I
> could plug some sort of HDTV signal (component, DVI, etc.). I keep
> getting the answers back: 1: They are available at high cost ($2000+)
> for broadcast quality, 2: There are some copy protection issues that
> prevent low cost ones.
As well as the copy protection issues with HDMI (I don't see how such a
modulator could possiblly be HDCP compliant) there is also the
fundamental issue that ATSC and it's european eqivilent DVB are far from
trivial to encode. Lets assume you have an unencrypted DVI/HDMI signal
you have to

Read the bitstream in real time to a series of framebuffers (you need
multiple frames for the next step)
Encode it in real time with an appropriate codec to produce a compressed
bitstream
Modulate it using a complex digital modulation scheme for transmission.

To get that kind of hardware down to a price consumers will pay (e.g. a
price where it's more attractive than just running some propietry system
and/or getting more boxes from the cableco) requires production to be
performed on a massive scale. Even then it will probablly never be
anywhere near as cheap as the simple modulators that can be used for
analog TV.

2010\04\15@163103 by Michael Watterson

face picon face
peter green wrote:
>
>
> To get that kind of hardware down to a price consumers will pay (e.g. a
> price where it's more attractive than just running some propietry system
> and/or getting more boxes from the cableco) requires production to be
> performed on a massive scale. Even then it will probablly never be
> anywhere near as cheap as the simple modulators that can be used for
> analog TV.
>  
I've used PCI cards from Dektec. Cheap at 1,200 euro. See how much
Tandberg charge
COFDM/DTT and QAM/DVB-C capable.  Those are  "just"  modulators.  You  
need a suitable MPEG2 Transport stream with MPEG2 or MPEG4 video in it.
OTH I was able to offline encode with PCs, create PS files, multiplex
into a single file representing a TS and then have on one coax a mix of
8 1920x1080i and 720x576i 50  TV "modulated" by the single "modulator"  
card.  I think  one of the cards had  the ATSC US option also.

Encoding at real DVD / decent BBC broadcast quality  even SD video ( 720
x576 x 50) wasn't possible in real time on a 2.4GHz AMD64.  However  I  
was only using  MPEG4 AVC  to conserve bandwidth and trying to have peak
of 5Mbps but average of 1.5Mbps per channel, yet quality same as 4Mbps  
MPEG2, with  better  handling  of sudden  fast pans etc..*

An analog(ue) modulator essentially has an FM modulator for sound
(4.5MHz to 6.5MHz depending on country) and then a VHF or UHF mixer +
Osc fed with composite video + FM subcarrier. None filter the RF
properly and none do Nicam stereo or Zweikannelton. They are very cheap.

For digital you could use the cheap fairly rubbish MPEG2 encoders in
standalone DVD recorders and cheap cameras  and then an ASIC for the
modulator (to IF), then a up converter to VHF/UHF. Even in volume it's
likely to be $40 to $50 and poorer than a decent analogue modulator. The
studios would never agree to it decyrpting HDCP, so most HDMI of any
value won't work on it, as it would be a low cost HDMI to unencrypted
recording bridge.

With a high end graphics card and PC you CAN actually encode to MPEG2
and use the card as DVB-c, DVB-t or ATSC modulator of a poor nature by
utilising one channel of the RGB DAC as the RF on a VGA output. The
quality would be poor, but it's been demoed. Forget low end integrated  
graphics and Atom for experiments like that.


(* my plan was get round limitation of  not enough bandwidth for Video
on a Microwave Point to Multipoint Broadband distribution system by
simply having a non-standard 4MHz wide DVB-c mux beside the IP downlink
channel and essentially use the same domestic end-users outdoor receiver
that feeds a cable modem, feed a Motorola hybrid cable TV/VOD set box
too. The Microwave broadband delivers about 8Mbps down, 1Mbps up for
about 200 Euro per outdoor radio at up to 12km
http://www.ogierelectronics.com/broadband_solutions.php  It works well
by having a 30Gbyte 30day  rolling cap to limit contention)



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