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Sound Devices 788T can cause CH38 Radio Mic interference


Simon Koelmeyer
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Hello All

This primarily concerns UK based recordists (or others on channel 38, 606.5-613.500MHz)

just had a fascinating chat with Kish Patel, MD of Audio Ltd, who's radio mics many (most?) of us use.

He tells me that Audio Ltd found that the Sound Devices 788T emits RF, via input and output cables, which cause interference with (and decreased range to) our 2040 radio kit, and for that matter any other radio mics tuned to certain frequencies.

The precise physics I'll leave to him to explain, but suffice it to say, they have produced the following list of frequencies which the SD788T emits in CH38/39/40. Simply put, our audio is at higher risk to 'hits' on channel 9 and increased (but lower) risk on channels 3 and 5 of our packs as they are too close to interference. Channels 14 and 26 are also affected, but these are in the CH39/40 range.

Emissions from 788T in the CH38 region.

L 611.0

H 614.4

L 608.3

L 604.5

H 602.1

H 617.4

H 626.7

(L = low level, H = High level)

Not sure if everyone else was aware of this or whether or not it was already common knowledge - but it certainly is news to me and a few of my colleagues. It would appear there is now even less choice in already congested airwaves in UK.

Anyone from Sound Devices care to comment?

Hope all healthy and busy Stateside - good luck at the polls tomorrow (whichever camp you're in!)

S

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This has been covered here and on the Sound Devices forum, SD are aware of it and I believe their RF people are in touch with Audio Ltd. I'm curious to know if all 788's output rf on exactly the same freqs or if it differs from unit to unit, also, did they test a SSD and was there any difference.

http://forums.sounddevices.com/showthread.php?3610-RF-Splash-from-788

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I use a 788t and 2 lectro SR out of the bag, have not have any problems. Will do a comparison frequency scan with the 788 off and with it on and will put on results.

Just remember if you leave a Lectro scanning it will add new RF, but not remove RF that went away. Meaning, if you scan with your 788T on, see some RF, then turn off the 788T while the Lectros are still scanning, they won't clear the RF that is no longer there.

You either have to go in and out of scan, or actually power cycle the Lectro (I usually power cycle to be sure). Sorry if you already know that, but a LOT of people don't realize it and say "the RF didn't go away after I turned off ******, so it must be something else".

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From page 20 of the SRa manual (typical):

"Data gathered during a scan is stored until it is intentionally

erased or the power is turned off. Previous

data will remain and subsequent scans can be made

to search for additional signals or to accumulate higher

peaks."

Best,

Larry F

Lectro

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I own the 788t with a SSD and have noticed the RF issue. I have 2 blk 19 SRa's and when using them in the bag it will show RF when the 788t is on. It goes away when the the 788t is turned off. When I outboard my SRa's to pouches on my petrol bag it also goes away. My question is to anyone who has a Quadpack from lectro. Will the quadpack help reduce that RF and improve the SRa's performance? Thanks for the info

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I understand some people have issues with this. I use a 788T SSD, and have Lectros in blocks 21, 22, and 25. My scans show no RF spray at all. Maybe Im just lucky, but I would suggest that people run tests before jumping to conclusions or getting overly worried about it.

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With 788T on, shows some spray on Lectro BL19 and BL22 here. Much less than the 744T but it's there.

Yes, that's my experience as well with BLK19 -- barely one notch of RF on the Lectro scale, depending on which channel. I can minimize it just by putting about two inches between the 788 and the 411 receivers in the bag. I've never had it really be a problem (knock on wood). Certain frequencies have zero interference, I think at the high end and low end of BLK19.

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The workaround for me has meant putting the CL8 on top (prefer it there anyway) and the Micron's nice and high in the bag (antenna bottoms at CL8 top). It's not perfect, but it's OK.

It's got the SSD in there. Don't know how much difference it makes compared to the HDD version.

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I understand some people have issues with this. I use a 788T SSD, and have Lectros in blocks 21, 22, and 25. My scans show no RF spray at all. Maybe Im just lucky, but I would suggest that people run tests before jumping to conclusions or getting overly worried about it.

It depends (and I'm not being a smart Equus asinus).

Best,

Larry F

Lectro

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  • 2 weeks later...

I am interested by the article that mentions tying the ground to the shell.

http://www.sounddevices.com/notes/recorders/788/788t-w-wireless-in-38-40/#more-2564

I notice pretty bad spray on block 21. I am going to tie the ground to shield. Also I met one mixer who liked to use Neutrik EMC connectors do you guys think this would further eliminate the RF Spray? Finally the article that mentions tying the ground to the shell made no difference on connectors 5-8 I would be interested to know which connectors they are using.

If I build the EMC cables ill do side by sides to let you know if it makes a difference.

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