> On Wed, 27 Apr 2005 11:12:20 -0400, Jerry Avins <jya@ieee.org> wrote:
>
>
>>The sound carrier is usually part of the video signal that is modulated.
>>Sound is usually recovered after the video detector with an FM detector
>>set at 4.5 MHz.
>
>
> Perhaps I'm mis-communicating. Broadcast transmitters have always (?)
> used separate pix and sound RF amplifiers. Perhaps you mean the chroma
> subcarrier ?
>
> Anyway, all I'd meant to suggest was the suppressed-carrier DSB with
> following filter model for the OP's purpose. Dunno, might be useful.
I'm the miscommunicator. Sound and video carriers are amplified
separately to relax the video final's bandwidth requirements. The
composite signal is treated as a whole by the receiver, and instead of
pulling the audio carrier separately from the IF, it is taken from the
output of the video detector. this "intercarrier sound" method avoids
problems that might arise from mistuning the IF.
Jerry
--
Engineering is the art of making what you want from things you can get.
�����������������������������������������������������������������������
Reply by Chris Hornbeck●April 28, 20052005-04-28
On Wed, 27 Apr 2005 11:12:20 -0400, Jerry Avins <jya@ieee.org> wrote:
>The sound carrier is usually part of the video signal that is modulated.
>Sound is usually recovered after the video detector with an FM detector
>set at 4.5 MHz.
Perhaps I'm mis-communicating. Broadcast transmitters have always (?)
used separate pix and sound RF amplifiers. Perhaps you mean the chroma
subcarrier ?
Anyway, all I'd meant to suggest was the suppressed-carrier DSB with
following filter model for the OP's purpose. Dunno, might be useful.
Chris Hornbeck
"Don't panic."
Reply by Mark●April 27, 20052005-04-27
this is true for most small consumer grade modulators
CATV often uses separate modulators but shared amplifier stages
broadcast TV typically uses separate transmitters and the RFs are
diplexed to the antenna feed with lots of big plumbing
Reply by Jerry Avins●April 27, 20052005-04-27
Chris Hornbeck wrote:
> On Mon, 25 Apr 2005 23:45:15 -0400, Jerry Avins <jya@ieee.org> wrote:
>
>
>>Is it mitigating that I last dealt with this in 1954?
>
>
> When, as a three year old, I was playing on the work bench
> my father built me for my third birthday. Gotta love the depth in
> this newsgroup.
>
> To the OP: aren't conventional modern TV transmitters built
> as a DSB modulator passed through a (high-pass) filter added
> to a separate carrier (and to a separate sound carrier)?
>
> Would this architecture be useful to you?
>
> Good fortune,
>
> Chris Hornbeck
> "This has been an account for those who don't keep them"
> J-LG, _Tout Va Bien_ 1972
The sound carrier is usually part of the video signal that is modulated.
Sound is usually recovered after the video detector with an FM detector
set at 4.5 MHz.
Jerry
--
Engineering is the art of making what you want from things you can get.
�����������������������������������������������������������������������
Reply by Ajay●April 27, 20052005-04-27
"What is your application? Is it a low cost consumer device for
connection to a single TV or a high quality CATV or broadcast device?"
-- high quality CATV or broadcast device
Reply by Steve Underwood●April 27, 20052005-04-27
Mark wrote:
>I don't understand your question. Baseband video is a real signal from
>DC to about 4.2 MHz. When applied to a DSB AM modulator, it creates
>symmetrical sidebands. Filtering removes part of the lower sideband
>(at RF, at IF the upper sideband is removed). Alternatively the
>phasing method can be used to remove part of the unwanted (vestigial)
>sideband.
>
>
>Its easy for a SAW designer to design the required SAW filter difficult
>for me :-) )
>I think the process is similar to designing an FIR filter.
>
>
The first part is the same. When you get to the "allowing for
quantisation effects" stage with an FIR, you replace that with an
"allowing for diffraction and other nasties at the ends of the fingers"
stage.
>There is a large selection of IF and also Ch 3 and Ch 4 SAWs
>commercially available for low cost modulators. There are also high
>quality expensive ones.
>What is your application? Is it a low cost consumer device for
>connection to a single TV or a high quality CATV or broadcast device?
>
>
Regards,
Steve
Reply by Chris Hornbeck●April 26, 20052005-04-26
On Mon, 25 Apr 2005 23:45:15 -0400, Jerry Avins <jya@ieee.org> wrote:
> Is it mitigating that I last dealt with this in 1954?
When, as a three year old, I was playing on the work bench
my father built me for my third birthday. Gotta love the depth in
this newsgroup.
To the OP: aren't conventional modern TV transmitters built
as a DSB modulator passed through a (high-pass) filter added
to a separate carrier (and to a separate sound carrier)?
Would this architecture be useful to you?
Good fortune,
Chris Hornbeck
"This has been an account for those who don't keep them"
J-LG, _Tout Va Bien_ 1972
Reply by Jerry Avins●April 26, 20052005-04-26
Ajay wrote:
> In my application, passband has to be from -5Mhz to 750KHz. As it is
> easier to get better performance from filters at low frequency, I am
> willing to stick with Hilbert option. however, normally hilbert
> transfrom does 90 phase shift for all postive frequencies and -90 for
> negative ones. I want it to do 90 phase shift for frequencies above
> 750Khz - 5MHz and -90 degree phase shift for -750KHz to -5MHz. This
> will create two components which on combining would elide upper
> sideband starting from 750KHz.
>
> I want your help in desiging such a Hilbert Transformer. Please let me
> know if something is still not clear.
>
> -Ajay
HTs can also be designed by filter-design programs. I can show you how
to do it by hand, but you'd be better off with a book. Unlike a sync,
the function is antisymmetric but monotonic on each side of center.
Odd-length filters are a bit easier to explain. Number the center
coefficient h[0], and set it and all even numbered coefficient equal to
zero. For every odd coefficient n, make its value 1/n, including sign.
The Gibbs phenomenon will cause heavy ripple, extending well toward the
center. A Blackman, Nuttall, or other "strong" window will flatten the
response at the expense of bandwidth. Add more terms to restore the
bandwidth. For unity gain, you will need a factor of 2/pi. The signal at
tap zero is a replica of the input delayed as much as the quadrature
that results from the convolution.
Take all this with a grain of salt. It's all from memory and it's been
years since I did an HT the hard way.
Jerry
--
Engineering is the art of making what you want from things you can get.
�����������������������������������������������������������������������
Reply by Mark●April 26, 20052005-04-26
I don't understand your question. Baseband video is a real signal from
DC to about 4.2 MHz. When applied to a DSB AM modulator, it creates
symmetrical sidebands. Filtering removes part of the lower sideband
(at RF, at IF the upper sideband is removed). Alternatively the
phasing method can be used to remove part of the unwanted (vestigial)
sideband.
Its easy for a SAW designer to design the required SAW filter difficult
for me :-) )
I think the process is similar to designing an FIR filter.
There is a large selection of IF and also Ch 3 and Ch 4 SAWs
commercially available for low cost modulators. There are also high
quality expensive ones.
What is your application? Is it a low cost consumer device for
connection to a single TV or a high quality CATV or broadcast device?
Mark
Reply by Ajay●April 26, 20052005-04-26
Mark wrote:
> Jerry,
>
> I think we are in agreement, what you described above is the spec for
> the receiver (in North America) (if you change 1 MHz to 1.25 MHz).
>
> The (professional) TV transmitter however is flat (i.e. full DSB AM)
> down to -1.25 MHz then rolls off sharply, so it is DSB for modulating
> frequences <1.25MHz and SSB (with carrier) for modulating frequencies
>
> 1.25 MHz.
I wonder how can it happen. If we have got baseband video signal whose
spectrum ranges from -5Mhz to 1.25Mhz, for all modulating frequencies,
it will remain unsymmetric spectrum centred at the modulating
frequencies. Correct if I am wrong. So it should remain DSB you prefer
to call it instead of VSB.
> The cheap game type modualtors are sometimes simple full DSB AM. The
> avaialbility of cheap SAW filters has changed that somewhat.
how difficult is to design SAW filter for bandpass frequencies like
61.25Mhz??
-Ajay