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Sound Reinforcement - Forums for Live Sound Professionals - Your Displayed Name Must Be Your Real Full Name To Post In The Live Sound Forums => Wireless and Communications => Topic started by: Mark Hannah on March 09, 2018, 09:36:34 AM
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Hello all,
Multicouper usage with digitally modulated IEMs?
We are considering Lectrosonics' Duet system to replace our 600Mhz IEMs. Lectrosonics is recommending Shure PA421/821B combiners. We already own PWS GX-8s.
What are your thoughts about using digitally modulated gear with the currently available combiners? How about using the combiner with both FM and digital at the same time?
I would appreciate a simple and something that gets into more detail (to help expand my knowledge).
Thank you.
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We are considering Lectrosonics' Duet system to replace our 600Mhz IEMs. Lectrosonics is recommending Shure PA421/821B combiners. We already own PWS GX-8s.
I seem to recall reading something about the Automatic Gain Compensation in the GX-8 not playing nicely with digital carriers, but my google skills are coming up short. If thats the case the PA821 or RAD TX-8 shouldn't be affected as they are fixed or switchable gain.
I'm sure Henry or one of the PWS guys will be along to point us in the right direction shortly :)
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Multicouper usage with digitally modulated IEMs?
We are considering Lectrosonics' Duet system to replace our 600Mhz IEMs. Lectrosonics is recommending Shure PA421/821B combiners. We already own PWS GX-8s.
What are your thoughts about using digitally modulated gear with the currently available combiners? How about using the combiner with both FM and digital at the same time?
I would appreciate a simple and something that gets into more detail (to help expand my knowledge).
First, on a pedantic note, in the real world of RF, "multicoupler" refers to a receive path device that takes a single input from an antenna and splits to several outputs. "Combiner" is the correct term for combining multiple transmitters into one antenna.
On to the topic at hand . . . The GX8 will not work under any circumstances with digital transmission schemes. As Neil alluded to, the AGC circuit simply can not lock onto the wideband carrier and thus sends erroneous voltage levels to the control amp. It's also not nearly linear enough.
The Shure units will definitely work. Lectro is also finishing up testing on the RAD TX-8U; Maybe Karl can chime in on their findings but it worked for us. Do note however, it must be the latest hardware version with the "U" in the model name; earlier units will not work with the digital modulation.
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First, on a pedantic note, in the real world of RF, "multicoupler" refers to a receive path device that takes a single input from an antenna and splits to several outputs. "Combiner" is the correct term for combining multiple transmitters into one antenna.
On to the topic at hand . . . The GX8 will not work under any circumstances with digital transmission schemes. As Neil alluded to, the AGC circuit simply can not lock onto the wideband carrier and thus sends erroneous voltage levels to the control amp. It's also not nearly linear enough.
The Shure units will definitely work. Lectro is also finishing up testing on the RAD TX-8U; Maybe Karl can chime in on their findings but it worked for us. Do note however, it must be the latest hardware version with the "U" in the model name; earlier units will not work with the digital modulation.
We haven't had a chance to thoroughly test the latest RAD combiner, but we're hopeful it looks good because it has a nice set of features.
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Do note however, it must be the latest hardware version with the "U" in the model name; earlier units will not work with the digital modulation.
Thank you for the info, Henry. I was unaware of this HW update.
Sent from my Nexus 5 using Tapatalk
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First, on a pedantic note, in the real world of RF, "multicoupler" refers to a receive path device that takes a single input from an antenna and splits to several outputs. "Combiner" is the correct term for combining multiple transmitters into one antenna.
Henry,
Ah yes. Quite confusing as I used both combiner and multicoupler in the same post... Updated the Subject.
What about combining analog and digital in the same combiner?
We haven't had a chance to thoroughly test the latest RAD combiner, but we're hopeful it looks good because it has a nice set of features.
Karl,
Would you be willing to follow up on this post when Lectrosonics completes testing on the RAD combiner?
Thank you in advance.
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What about combining analog and digital in the same combiner?
Should not be a problem. Just adhere to proper frequency spacing requirements as per device.
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Henry,
Ah yes. Quite confusing as I used both combiner and multicoupler in the same post... Updated the Subject.
What about combining analog and digital in the same combiner?
Karl,
Would you be willing to follow up on this post when Lectrosonics completes testing on the RAD combiner?
Thank you in advance.
Yes - I'll be happy to do so.
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Karl,
Would you be willing to follow up on this post when Lectrosonics completes testing on the RAD combiner?
Thank you in advance.
Hi Mark,
OK, we have our report back from engineering on the TX-8U combiner from RAD:
"This unit seems to be satisfactory for use with our digital and analog systems. Testing was performed using both our 50mW digital systems (Dba/DR - similar to M2 in terms of modulation) and our 250mW IFB system (IFBT4/IFBR1A). IMD performance was good in the TV broadcast band. Here we see 4 carriers applied to inputs 1-4. No degradation of the input signals was noticed, and the walk testing revealed no impairment of system performance."
-Karl
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OK, we have our report back from engineering on the TX-8U combiner from RAD:
"This unit seems to be satisfactory for use with our digital and analog systems. Testing was performed using both our 50mW digital systems (Dba/DR - similar to M2 in terms of modulation) and our 250mW IFB system (IFBT4/IFBR1A). IMD performance was good in the TV broadcast band. Here we see 4 carriers applied to inputs 1-4. No degradation of the input signals was noticed, and the walk testing revealed no impairment of system performance."
-Karl
[RAD hat on]
Thank you Karl. I would like to clarify some suggested settings for users:
- 50-60mW CW is the recommended input level into the TX-8U. This means using a 6dB attenuator on the output of the IFBT-4 (or T-1). It also means you may have to set the Duet power output to 25mW as its broader band spectral mask is more composite power than that of an analog CW carrier.
- The output of the TX-8 can be set to 250mW for an IFBT-x but probably should not be set higher than 100mW for the Duet.
- At minimum, follow standard practice for channel spacing, but adding another 50% or so (spectrum permitting) is advisable for the higher output TX-8U power settings.
[/RAD hat off]
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- 50-60mW CW is the recommended input level into the TX-8U. This means using a 6dB attenuator on the output of the IFBT-4 (or T-1). It also means you may have to set the Duet power output to 25mW as its broader band spectral mask is more composite power than that of an analog CW carrier.
Hi Henry,
So in this context, CW is continuous wave? as in traditional analog FM carrier?
Looking at some of the typical IEM /IFB systems, it looks like the typical bandwidth is somewhere between +/- 25kHz and +/- 75kHz from centre frequency.
Looking up the emissions designator for the Duet it looks like it has 200kHz Bandwidth, and thus the channel power needs to be lower because it takes up more spectrum?
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So in this context, CW is continuous wave? as in traditional analog FM carrier?
Yes. FM or AM.
Looking at some of the typical IEM /IFB systems, it looks like the typical bandwidth is somewhere between +/- 25kHz and +/- 75kHz from centre frequency.
Looking up the emissions designator for the Duet it looks like it has 200kHz Bandwidth, and thus the channel power needs to be lower because it takes up more spectrum?
No. Don't confuse the occupied channel bandwidth (the actual amount of spectrum the carrier occupies) with maximum deviation permitted by FCC technical rules, which is generally, but not always, indicated in the emissions designator ("200K . . ."). The issue is that of composite power over any given amount of spectrum, whether it be 40kHz, 150kHz, 200kHz or 10MHz. With an analog carrier at full practical/usable deviation, only about 50%-80% of the allotted channel bandwidth (depending on the type of signal, e.g. two-way radio with limited audio frequency range and dynamics versus wireless microphone with very wide audio frequency range and dynamics), as indicated by the emissions designator, has energy within the top 6dB of the peak. Conversely, a digital spectral mask will occupy nearly 90% of the allotted channel bandwidth with the carrier amplitude varying only 10ths (if not 100ths) of a dB. Thus, within the same 150kHz or 200kHz, a digital carrier will have a composite power far greater than that of an analog carrier with the same rated RF power output.
This can easily be seen with a spectrum analyzer looking at analog and digital TV signals, analog and digital wireless microphone or IEM signals.
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[RAD hat on]
- 50-60mW CW is the recommended input level into the TX-8U. This means using a 6dB attenuator on the output of the IFBT-4 (or T-1). It also means you may have to set the Duet power output to 25mW as its broader band spectral mask is more composite power than that of an analog CW carrier.
WRT the 6db attenuator, IF you are permanently installing IFBT4's in a rack with a combiner, there is an internal option (in the IFBT4) to select either 100mw or 50mw output power. It's a case of joining different solder pads, which are very close together, so best left to a qualified technician. However, I recommend having them set to 50mw so that someone can't come along later and remove the attenuator because "more is better" ::). You may be able to order them from Lectrosonics already set to 50mw...I'm sure Karl can answer that one.