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nycxzero
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Last edited by nycxzero on Sun Aug 09, 2009 7:48 pm; edited 1 time in total
Sat Feb 22, 2003 5:22 am View user's profile Send private message
znight
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Yet another challenge for you marko.

The TV MAX.
Sat Feb 22, 2003 6:19 am View user's profile Send private message AIM Address
Drummer
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it would be a waste through the airwaves I think. Here in the US, everyones got cable, and I think thoes TVs dont recieve airwaves. I could be wrong

Dave
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nycxzero
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Sat Feb 22, 2003 5:25 pm View user's profile Send private message
goaway
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Get an RF Modulator and hook it to a linear amplifier, then an antenna.

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Sun Feb 23, 2003 7:16 pm View user's profile Send private message AIM Address
nycxzero
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that does?
Sun Feb 23, 2003 10:03 pm View user's profile Send private message
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sends tv signals, what else?

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Mon Feb 24, 2003 10:26 pm View user's profile Send private message AIM Address
Drummer
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correct me if im wrong but isnt TV signals the ones with the repeater stations/towers on the mountains? (well, for commerical uses, you dont see too many FM commercial repeaters)

Dave
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Mike
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One of the problems with using a RF modulator for broadcasting is most of the cheap ones have no vestigal (lower) sideband filter which means your signal takes up two tv channels instead of one so If you broadcanst on Channel 25 youre putting out a signal on 24 as well

The reason there are so few good TV transmitters available at a reasonable price is that it is uneconomic to do Transmitters for the various TV standards (4.5 5.5 6.0 6.5 AM and 6.5 FM sound positive and negative video modulation and different bandwidths) also in some countries VHF is the main band while in others its UHF
Sat May 03, 2003 12:02 am View user's profile Send private message
StormiNorm
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Mike wrote:
One of the problems with using a RF modulator for broadcasting is most of the cheap ones have no vestigal (lower) sideband filter which means your signal takes up two tv channels instead of one so If you broadcanst on Channel 25 youre putting out a signal on 24 as well

The reason there are so few good TV transmitters available at a reasonable price is that it is uneconomic to do Transmitters for the various TV standards (4.5 5.5 6.0 6.5 AM and 6.5 FM sound positive and negative video modulation and different bandwidths) also in some countries VHF is the main band while in others its UHF



==================================
Greetings(Greetz)

Here are a few more thoughts to augment the above rhetoric by Mike.

There are a few Commercially available Cable modulators that incorporate SAW filtering and other selectable parameters that would enable one to utilize them as the first Low power/exciter/predriver stage of a TV TX.

And even then ,if one is fortunate enough to procure such a unit(Commercial CATV MODULATOR)with an adequate amount of vestigial filtering,There will still be a fair amount of residual intermod leak into the adjacent chanell. etc.etc.

One of the biggest problems even with a "high" end CATV modulator used in such a manner,is the production of a 920 kHz "beat" interference frequency between audio and colour subcarriers,which produces additional undesired IMD products in the following power amplifier stages.

Also there is a loss of RF power in the lower sideband audio subcarrier due to the the 4.5 mHz FM carrier being mixed wih baseband video in the video modulating cct. of the CATV modualator.

One low cost and very stable and clean method of building a TV station that would rival that of any Commercial Station is to use two individual TX transmitters and configure (modify) them to achieve the following.

Lets explore a hypothetical idea of say Setting up a TV Station on Channel 6 NTSC format.

The Video freq. for CH6 is at 83.250 mHz
The Audio freq. for CH6 is at 87.750 mHz

One can procure Two individual Transmitters,to develop the video and audio subcarriers.

One TX would be operated at 83.250 mHz.AMPLITUDE MODUALTED with the video content....and another TX on a freq. of 87.750 mHz FREQUENCY MODULATED with the aural content to serve as the aural subcarrier.

The RF output from each TX/EXCITER is then combined in a Diplexer..which is nothing more than a ordinary Wilkinson Power Combiner//Divider,which is easily constructed from two quarter wavelength sections of 75 ohm coaxial transmission line.The combined RF output consists of a fairly high degree of RF isolation between each exciter,and the resulting RF ouput can then be further "kicked up a couple nothches"(Amplified) to whatever level of RF Power that one is desirous of Radiateing.

Now from a real world perspective..I have quite a few systems in South America(Bolivia) whereby i implemented this approach useing the stripped out Pre driver stages of Some Motorola VHF/UHF units and also the 800-900mHz series.Individual Video and audio Modulation ccts. and modifications were added and WHAM cranked it up to around 300W and on up the power scale.

This is where the AR 313 (Communication Concepts) MRF141G(28VDC) AND MRF151G(50VDC) excells in provideing Linear Amplification for such use.

As far as i am concerned those Designs are more favourable to being used with a greater degree of Spectral purity on the Lower Freqs,as opposed to being used for the 88-108 mHz spread.If they are used for that spread of spectrum then certian precautions and at least two minor modifications have to be made to ensure spectral integrity..so far i have not seen NO information relateing to those precautions Posted or documented anywhere...Dont even know if the end users of such Designs are aware of the Problems that can arise by pressing such designs into service.The concept of provideng multi octave no tune amplification(10-175mHz) is a novelty,and a dream come true for those with a plug and play mindset...ain't no plug and play with those designs.One has got to have a fair degree of RF sleuth knowledge.

'cos those amps are capable of "Hogging" the spectrum.even more so than the manner in which a Harley Davidson Hogs interstate 75...especially when a Lumina is pegged at 85MPH on the speedometer,but the wheels are turning and burning at 100mph...the midnight Train to Georgia...Oh S***..Blue lights..Red Lights....."Hey Boy" where is de fire at Boy"?...."Sir i have ten pounds of swinging **** and a buckett full of b***s and u callin me Boy?...you gotta be outta u cotttin pickin Redneck Mind Sir"....."G****n Boy..u funny as hell Boy"!......."Sir Please dont call me Boy"!!!...."Boy we calls everyone around these here parts Boy"...................Reached out to hand him the obligatory Registration and DL.......He apparently was unable for a short duration of time,to even lift his hands from his hips to retrieve the paperwork......so i looked him striaght in the face and said...."Have a nice Day Boy"...Got back in the Lumina..cranked up the Bass..and continued Kicking ass and tearing up the mileage between Miama and Ohio..and waved at every other 'Goergia' Highway Patrol that i sped past on my way back to Miama.....Hey right Boy my A**!!!

Anyway got a bit side tracked there...Love it...A whole lotta Mountian Dew was consumed on the 'Goergia' trail.

I have experiemented with and modified two MAX1 PRO'S to verify that the dual TX. idea was feasible useing that particular PLL...and i achieved some worthwhile results useing the method that i described above..(though not perfect)Amplitude modulated the final stage of one unit and added a external video modulation cct..that was the most time consumeing part of the exercise,the results obtained proved that indeed (after forceing the unit to tune down to 83.250mHz) the Max1 can be pressed into service as a LPTV EXCITER WITH THE PROPER MODIFICATIONS.

The video and Audio modulateing and conditioning ccts. i used were from a commercial Canadian design,that has a host of jumpered configurations,which makes it quite easily to adapt to a variety of external options.etc.etc..very simple and uncomplicated design.

The method of useing two seperate exciters with off board VIDEO AND AUDIO Modualtion ccts. can allow one to accomplish a LPTV setup that allows one to see on the Recieveing end a picture that is essentially a very accurate colour reproduction...The phase response from the chrominance sub-carrier (3.58 mHz.) falls within the +/- 10 degree specification used by commercial Broadcasters..

Respects...StormiNorm
Sun Jun 15, 2003 7:17 pm
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