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Video TRANSMITTER -- RF INTERFERENCE IN AUDIO


wolf

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TRANSMITTER -- RF INTERFERENCE IN AUDIO

1. Audio cables have to be good (best are Canare 4E6S).  Solder joints cannot be cold or partially cold. Shells of XLR connector may not be grounded. The Mixer has to have a good RF resistant input. The cable sends some RF to the following electronics and if they just rectify the RF you will hear it of course. Cooper mixers, with Jensen Trafos are good, so is Soundevices with the Swedish ones they use. Cheap boards are trouble. Some “digital” boards have inputs that go much too high in freq (excessive bandwidth) and will rectify RF.

2. Some older Schoeps CMC electronics are more sensitive than others – test which serial numbers are better.  Use GVC right angle; it reduces noise a lot.  Use Cut 1; it reduces RF noise some. http://www.schoeps.de/E-2004/cmc.html

3. Use newer, past 2007, Schoeps; they are built RF-proof.  Get older ones updated by factory.  Older black Sennheiser 48V are really bad.  They can be updated. http://www.sennheiser.com/sennheiser/home_en.nsf/root/professional

4. Transmitters on lower TV channels cause much more interference than upper channels.

5. CanaTrans at 1 Watt causes much more interference than Modulus.  The new Modulus 3000Y equivalent power on CanaTrans is ¼ Watt. Older Modulus equivalent is ¼ - ½ Watts.

6. Worst-case interference of mic/preamp is 6 feet from transmitter. With GVC or Cut 1, worst case is 3 feet.  Newer Shoeps have no sensitivity above 3” from Tx.

7. Note that some mic positions relative to the antenna are worse than others.  Experiment with positioning both.  Adjust all antennas and mics and experiment. Parallel elements (cable-cable  antenna-mic) are most susceptible of course.

8. Use FM transmitters.  Microwave at 1000 MHz and above has to be within 2” of worst Shoeps to have any negative effect.

9. Coiled microphone cables are bad.  Do not use them.

10. Sankens and other unbalanced mikes are trouble especially with long cables.

11. Try to install the RF filter the Deva makers give away for the XLR at the end of a boom. http://www.zaxcom.com/deva_v.htm

7/30/2008  wolf - comments invited

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TRANSMITTER -- RF INTERFERENCE IN AUDIO

1. Audio cables have to be good (best are Canare 4E6S).  Solder joints cannot be cold or partially cold. Shells of XLR connector may not be grounded. The Mixer has to have a good RF resistant input. The cable sends some RF to the following electronics and if they just rectify the RF you will hear it of course. Cooper mixers, with Jensen Trafos are good, so is Soundevices with the Swedish ones they use. Cheap boards are trouble. Some “digital†boards have inputs that go much too high in freq (excessive bandwidth) and will rectify RF.

2. Some older Schoeps CMC electronics are more sensitive than others – test which serial numbers are better.   Use GVC right angle; it reduces noise a lot.  Use Cut 1; it reduces RF noise some. http://www.schoeps.de/E-2004/cmc.html

3. Use newer, past 2007, Schoeps; they are built RF-proof.  Get older ones updated by factory.  Older black Sennheiser 48V are really bad.  They can be updated. http://www.sennheiser.com/sennheiser/home_en.nsf/root/professional

4. Transmitters on lower TV channels cause much more interference than upper channels.

5. CanaTrans at 1 Watt causes much more interference than Modulus.  The new Modulus 3000Y equivalent power on CanaTrans is ¼ Watt. Older Modulus equivalent is ¼ - ½ Watts.

6. Worst-case interference of mic/preamp is 6 feet from transmitter. With GVC or Cut 1, worst case is 3 feet.  Newer Shoeps have no sensitivity above 3†from Tx.

7. Note that some mic positions relative to the antenna are worse than others.  Experiment with positioning both.  Adjust all antennas and mics and experiment. Parallel elements (cable-cable  antenna-mic) are most susceptible of course.

8. Use FM transmitters.  Microwave at 1000 MHz and above has to be within 2†of worst Shoeps to have any negative effect.

9. Coiled microphone cables are bad.  Do not use them.

10. Sankens and other unbalanced mikes are trouble especially with long cables.

11. Try to install the RF filter the Deva makers give away for the XLR at the end of a boom. http://www.zaxcom.com/deva_v.htm

7/30/2008  wolf - comments invited

All of the video transmitters on broadcast tv channels are illegal in the US.

Jim

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All these are experiences from Northern Mexico ( usually I include this in any post like that) and of course the only legal transmitters on the UHF band in the USA would have a license from the FCC. Somehow our Radio mikes are licensed by the manufacturer  too. They are the caretakers of the spectrum on behalf of the public good. The FCC board is appointed by the president. They are very busy with the changeover to digital TV. There is an interesting thing in that new law that all elected officials get some free airtime on commercial TV. Now you might understand why the giveaway of new frequencies to the huge internationally owned communication  corps. 

and

Law applying to use:  Transmissions emitting from equipment on all radio frequencies are assigned technical parameters and licenses by the US government through its FCC agency.  There are penalties for using specific parts of the electromagnetic spectrum without the proper licenses.  The US government through the FCC sells or licenses the use of the air waves.  Use UHF video modulators  only overseas where it is legal to transmit and approved by the local authorities. The manufacturer and dealer and associated personnel will not assume any responsibility for consequential or other damages resulting from illegal use or sale.    etc...........

w.

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DIGITAL TV is here now on the UHF and VHF band.  How to deal with it in Northern Mexico.

How to optimize TV reception on the movie set.  This is all for non-USA use.

Full power Digital TV is on the air and covers most of the UHF band in the larger cities and it spills over into Northern Mexico of course..

Analog tuners do not show us if and where there is a digital signal.  If you tune an analog tuner (say a Sony 1041) to any UHF station, it will show “snow,” (noise) if there is a digital station or nothing.

How to tell if there is a digital station on a particular UHF channel?

The only way I found is to look it up on one of the databases by frequency.  For example:  KABC digital in Los Angeles is at 705.25 MHz – that is UHF Ch 53.  Tune your Sony 1041 to UHF 53 and you see snow.  Tune your digital TV to Ch 7.1 and you see ABC digital.  Of course, the TV displays #7.1  for marketing reasons only; you are really watching UHF TV 53 or 705.25 MHz demodulated digitally.

What this means is simple: Do not use UHF Ch. 53.  If you are 30 miles away from a digital transmitter, and use a modulator on this channel you will notice it works but its range is cut in half. Ss northern Mexico is pretty safe.

Another example:  KCET is on UHF Ch 28 analog.  You know your modulator in Northern Mexico does not work on this channel.  But, KCET digital transmits on 741.25 Mhz which is UHF Ch 59.  So that channel will give your modulator only half the normal range.  This applies to digital transmitters that are at least 30 miles away from your receiver.  If you are closer to the mountaintop where Ch 59 digital transmits, the analog tuner will work less well.

Weak analog channel:  the only channel you find with no digital TV on it is one with a weak analog signal.  You put your modulator on that channel and again you will notice that the range is half of what it should be.

The whole UHF spectrum today is taken up by digital TV and analog TV.  So, what to do?

In Mexico, there is UHF Ch 37 open.  In the USA, it’s reserved for astronomy work.  Also, CH 64 seems to be used mainly for commercial (plumbers and taxis) communication not TV and does not interfere much.

In the Northern part of Mexico, using Ch 37 or Ch 64 seems to double the range of modulators.  Also, in Northern Mexico, the channels under 21 seem to be little used.  In L.A., Ch 14-16 is used by the sheriffs for their digital communication to their car computers.  Chances that an analog modulator would interfere are minimal, but of course, you would not be using a modulator on the air in the USA anyway.

Tuner quality:  are tuners in LCD monitors as good as external expensive 1041s?  The Aquos 20”, 21” and 26” LCD tuners are very slightly less sensitive than the 1041.  This translates into a range difference of 2%, at worst.

For 2009:  Everything in VHF will change with the “turn off” of analog TV.  UHF will stay the same except maybe there will be holes or “white space” where the analog stations were.  Maybe Bill Gates will get that space to rent for data/internet use.

2008 News:  IDX CW-5HD has a license-free digital transmitter/receiver in the 5.2 band with no delay and 50-yard range for $6,000 – it’s SDI only, not 23.9 yet. 2 can be used in proximity. One alone is encoded. Excellent picture with no dropouts!!  http://www.idxtek.com/pdf/cw5hd_flyer.pdf

05/20/08

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  • 11 months later...

Legality- We just had an interesting lecture by Tim Holly at Wexler and there is new law and regulation that allows video assist (with restrictions) and Radio mike transmitters and a licensing procedure finally. He will post the info on 695.com. FINALLY  wolf

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Legality- We just had an interesting lecture by Tim Holly at Wexler and there is new law and regulation that allows video assist (with restrictions) and Radio mike transmitters and a licensing procedure finally. He will post the info on 695.com. FINALLY  wolf

Correction: the event was held at Bexel, not "Wexler" (Wexler Video, no relation).

-  Jeff Wexler

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  • 2 weeks later...

this is what some manufacturers are including in their sales pakage:

Legal  Newsflash:

There is provision in the federal regulations now to use Video Assist transmitters with some restrictions with permission in the USA.

Look at:  Federal Communications Commission §74.870  47 CFR Ch. I (10-1-04 Edition) They ask the manufacturer to:  include with a wireless assist video device information regarding the requirement for users to obtain an FCC license, the requirement that stations must locate at least 129 kilometers away from a co-channel TV station, the limited class of users that may operate these devices, the authorized uses, the need for users to obtain a license, and the requirement that a local coordinator (or adjacent channel TV stations, if there is no local coordinator) must be notified prior to operation.

[68 FR 12772, Mar. 17, 2003, as amended at 68 FR 69331, Dec, 12, 2003]

You may read the details regarding above at:

http://edocket.access.gpo.gov/cfr_2004/octqtr/pdf/47cfr74.870.pdf

http://edocket.access.gpo.gov/cfr_2008/octqtr/47cfr74.870.htm

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