RF modulator interference woes - AVS Forum
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post #1 of 17 Old 10-25-2000, 04:15 PM - Thread Starter
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Hello,

I recently (today) bought a Sony MRD-D1 modulator to distribute my Tivo and DVD player through the house. But, I seem to have an interference problem. Here's my setup:

CATV comes in the house, into a Channel Vision 2/4-PIA amp/splitter and out to the TVs. The MRD-D1 output goes into the other input of the amp (which was designed for exactly this purpose).

Now, when I tune cable channels above 72 on my TV, I get pure snow. But, when I try to use any of those channels (up to 86, which is the limit of the MRD-D1), I get a really awful picture. How awful depends on the channel I use, but at its best there is still a _lot_ of interference, I would call it unwatchable. I also tried channel 64 (63-64 are also unused), where the picture is a lot better, it still has some interference there (probably because 65 is used), but it is much better than 73-86.

I know that the problem is CATV interference, because if I unplug the CATV from the amp, the MRD-D1's picture is perfect on every channel.

So, what is up there causing the interference? My guess is digital cable... I searched the Web but I can't find out what freqencies that they typically use (my provider is AT&T), I can't imagine what it would be other than just above the analog channels. So, if that's the case, I guess a low-pass filter won't help me. Or, more accurately, it will help the modulator at the expense of all of the digital channels, right? Maybe if I get a better modulator (that goes up to ch. 135), I can get up above the interference?

Any suggestions would be greatly appreciated... Why isn't this stuff ever as simple as it seems? http://www.avsforum.com/ubb/smile.gif
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post #2 of 17 Old 10-25-2000, 04:50 PM
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This came up in a thread a little while back: http://www.avsforum.com/ubb/Forum22/HTML/000040.html

You need to get a lowpass filter. This should block the interference below a certain channel number (determined by which filter you get).

You might want to check with CHASMM to see if this fixed his problem...

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post #3 of 17 Old 10-25-2000, 06:01 PM
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Hi all...

I ordered a low-pass filter from MT (ChannelPlus ) and it arrived a couple of days ago. I haven't had an opportunity to install it yet.

I did notice however that SatServ.com seems to have lower prices on many of the same items that ChannelPlus carries...you should check there.

As soon I get to install the low-pass filter, I'll pass along my experiences. todd_j_derr, I ran into the exact same issue that you're talking about.

As a side question, where were you able to locate the Sony unit? I'm interested in possibly picking up another one...

Chas.

[This message has been edited by chasmm (edited 10-25-2000).]
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post #4 of 17 Old 10-25-2000, 08:20 PM - Thread Starter
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Thanks for the quick responses - I guess I will try a lowpass filter, although, as I mentioned, I'm a little worried about it filtering out my digital cable channels.

chas: I couldn't find one anywhere either and I didn't want to pay the price of the CP3025, so turned to ebay - first auction that I ever won. If I can't get my issues resolved, I'll be happy to sell it to you.

My second problem is that the amp doesn't seem to be passing the DC return signal for the remote IR. I wonder if I can get around this by doing kind of a 'bypass' using 2 splitter/combiners, i.e.:

Code:
MRD-D1
  |
 ------  < splitter
 |    |
Amp   |
 |    |  
 ------ < splitter
 |
IR sensor/TV
But, I worry that this will also cause a feedback loop - isn't this effectively connecting the amp output back to the input? Maybe if I used another filter (high-pass?) on the 'bypass'? Is there some way to solve this?

Again, I'm open to suggestions, even if it involves changing equipment (which would be unfortunate, but...)

[This message has been edited by todd_j_derr (edited 10-25-2000).]

[This message has been edited by todd_j_derr (edited 10-25-2000).]
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post #5 of 17 Old 10-25-2000, 09:32 PM - Thread Starter
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ok, the more I think (and read) about it, the more sure I am that the MRD-D1 is not going to work for me. I can't think of anywhere else the digital channels could live except in the range I am trying to use. Channel 86 is only approx. 600MHz which is probably in use.

But, there is hope, since other modulators go up to ~860MHz (135), and it seems that most cable systems don't go that high yet. So, i think I'm going to order a different modulator, and if it works, the MRD-D1 will be up for sale. For IR, I will probably use the RCA/RadioShack IR/RF 'pyramids'.

sigh. of course, that was pretty much my original plan until i heard about the mrd-d1.



[This message has been edited by todd_j_derr (edited 10-26-2000).]
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post #6 of 17 Old 10-26-2000, 08:19 AM
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A couple of comments:
  • I believe that you can also get filters that take out interference for a specific channel. These are designed for cable TV operators to be able to block a specific channel before it enters the house (an old design before decoder boxes). This might work better for you than the lowpass filter, which blocks out all channels above a specific level.
  • The IR function requires a straight run of cable between the MRD-D1 and the target. You can't put splitters or an amplifier in between or you will lose IR capability. The exception is special splitters offered ChannelPlus that are designed to maintain IR signal.
By the way, I'd be willing to sell my MRD-D1 to chas. or whoever else might be interested. I'm in the process of migrating to an 8 input / 12 output configuration.

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post #7 of 17 Old 10-26-2000, 08:31 AM - Thread Starter
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Hello,

Channel Vision (or Channel Plus, probably either) will make a custom filter for you... but I probably don't want to know how much they cost... I haven't really seen anything generally available except the low-pass filters.

Also, all my splitters are the Channel Vision DC-pass type. I verified that the signal path (only through 1 splitter) is OK if I bypass the amp.

this whole thing is starting to make my head hurt...
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post #8 of 17 Old 10-26-2000, 10:15 AM - Thread Starter
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yeah, i was worried about overdriving the amp as well, but as I mentioned, if I disconnect the CATV feed, then the picture looks perfect on all of the TVs. I have tried both the 'local' and 'distant' outputs, the results were about the same.

Using the MRD-D1 as a distribution amp would be 'simpler', except that the sources that I am modulating are not at the headend, and I don't want to move them. I also don't think that it would solve the problem (well, it would solve the IR problem, but not the interference problem), or I'd give it a shot.
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post #9 of 17 Old 10-26-2000, 09:46 PM
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In the set up that you're describing, I'd also be concerned about overdriving the amplifier, as it's being fed an amplified signal by the MRD-D1. I had a similar issue in feeding the output of my 4-way modulator into the MRD-D1 & ended up needing to put an attenuator in between them.

Have you tried putting your TV feeds directly into the MRD-D1 (which has 5)rather than through the Channel Vision amplifier? This would simplify things greatly & possibly give you better performance.

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post #10 of 17 Old 10-28-2000, 04:01 PM
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Okay...I installed my ChannelPlus low-pass filter (filters everything about channel 55) and two Sony MRDD1s last night. Everything is working well! I've got a bit of interference in the regular cable channels that seem to be caused by the MRDD1s but I'm troubleshooting it.

I've got one of my VCRs on channel 69, DSS 1 on channel 71, DSS 2 on channel 73, and DVD on channel 75. At this point, the kids and wife are happy to not have to worry about switch boxes.

I'm still in the market for another modulator, but I'm looking at an Aska Triple Channel Digital Modulator. Has anyone has any experience with them. The triple channel version is about $150 and has the advantage of being able to set the channel digitally. I could then feed the output of this into one of the Sony MRDD1s (used in combination I could modulate 5 sources).

Any experience with an ASKA unit?

Chas.

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post #11 of 17 Old 10-29-2000, 03:30 PM
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The reason you cann't get the ir signal to pass is that you need a power passive splitter such as the ChannelPlus 2512. If you use a conventional splitter, it blocks DC voltage.

Greg C.

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post #12 of 17 Old 10-29-2000, 03:58 PM
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I just got an MRD-D1 on Friday. I went to install it on Saturday morning, thinking I'd have everything working in an hour. It was a much bigger headache than I anticipated. I have basic cable (locals, superstations, etc.). One thing I discovered is that I can tune in some of the "Extended Basic" channels. Perhaps there is a special free weekend/month going on that I don't know about, but I tend to think that it's a mistake on their end, a mistake the installer made, or just a "loophole". While this may seem like a nice extra, it actually causes me trouble because I can't find any good unused stations. My problem is compounded by the fact that my bedroom TV (an old but excellent NEC 20" set) won't let me select channels above 65. Bottom line is that I'll probably be getting a low-pass filter to block out everything above channel 50. I tried a Radio Shack variable attenuator (0 to 20db) but even at the highest level, I still get an unsatisfactory amount of herringbone. And at that level, the cable signal gets very snowy. The worst of both worlds.

As for the MRD-D1 itself, I've got a couple of gripes. I'm not sure if they're all built this way or it's just my unit, but tuning knobs don't seem to be aligned accurately or consistently. I may think I'm tuning it to channel 83 but it's really being set to 73. All of the knobs are slightly different in this regard. Once you figure it out, it's not a big deal. It would also be nice if the "first-digit" knob "clicked" at each number. I can see why the second digit would be loose, allowing for finer fine-tuning, but having a loose "first-digit" knob combined with it not aligned properly, makes it more difficult.

My biggest problem, however, is that my Video 1 signal seems to be creeping into my Video 2 signal (I guess I'd call it ghosting that rolls from left to right). It's very mild, but I can notice it (my wife might not). I tried setting the channel distances way apart, using different cables (trying both "cheap" cables and monster cables), switching which source (DVD, TiVO) was Video 1 and which was Video 2, and plugging the MRD-D1 into my surge protector/line conditioner, but nothing worked. It's always Video 1 creeping into Video 2 (the Video 1 signal seems clean). I'm going to try and find another unit to see if mine is just defective. I could probably live with it as it's not THAT bad, but, knowing me, I'd probably be bothered by it enough to end up just using the Video 1 input and switching it via my A/V receiver (or my $50 Sony manual video switcher). Any other ideas as to what the problem could be?

Scott R

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post #13 of 17 Old 10-30-2000, 04:01 AM
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Many of the lower price modulators don't perform well with adjacent channels. For example, if 55 is in use (OTA, Cable or in-house channel), channels 54 and 56 cannot also be in use. Most of the more expensive (analog or digital) modulators have better adjacent channel rejection characteristics.

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post #14 of 17 Old 10-30-2000, 08:27 AM - Thread Starter
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peeps: I don't know what is causing the interference, digital cable was just a guess. As I said, if I tune ch. 73 to 86 using my TV tuner, I get snow. If I try to modulate on those channels, I get a lot of interference. If I disconnect the CATV feed, the modulated picture is perfect. So, something on the CATV is causing a problem.
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post #15 of 17 Old 10-30-2000, 10:51 AM - Thread Starter
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bb: thanks for confirming that - he didn't happen to know what the common highest frequency used by the cable co. was, did he?

As far as the filters, if you go to channel vision's site and look at the instructions for their modulators, i.e.:

http://www.channelvision.com/resourc...ts/mod-ins.pdf

At the bottom of page 4, they say 'Call for pricing on a custom filter to block out a specific channel or multiple channels.' Unfortunately, I can't imagine that would be very economical, not to mention I still don't know what channels are available for me in the first place!

I think I might just buy a channel vision stereo modulator (if I can find somewhere that will let me return it without a big restocking fee if it doesn't work out) and see if I can find an unused frequency. If not, well, I've got a 1000 ft. spool of coax in my garage, and some A/B switches - pain in the butt, but better than nothing at all...
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post #16 of 17 Old 10-30-2000, 06:12 PM
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I should probably start a new thread so that I don't confuse these two separate issues.

The bleed-through is definitely coming from the Video 1 signal (I can make out the text of the TiVO menu screens rolling by if I look close enough). Again, the Video 1 channel ends up looking fine. I ordered another one through the store. They didn't have any more at my local store so they're having one of their other shops ship it to them. BTW, the store I got it from is Bernies (CT and MA based). I got it from them for $60! I'm quite pleased with this deal, though I'd be even happier if I didn't have the bleed-through. If the replacement has the same problem, I'll give ChannelPlus a call and see what they think. If the replacement works OK, I'll return the one I've got and order a second one (at this price, it's worth it just for the IR - I could always sell the second main unit via ebay). Tonight I need to order a low-pass filter. Thanks to my old 20" NEC's upper channel limit of 65, I guess I'll need to go with the 50+ blocker.

Scott

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post #17 of 17 Old 10-30-2000, 09:21 PM
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I spoke with a friend who know a lot about cable systems about this last night. He confirmed that digital cable, as well as cable modems, will interfere with the upper end of the cable spectrum. Using a lowpass filter may fix the modulator problems, but would cause problems for digital cable and/or cable modems.

The best solution to these problems would be filters that block out specific channels, so that you could replace specific channel(s) with your modulated channel(s). I don't know of a source for filters like this, but if anyone does, please post it.

Scott: I feel your pain re: channel selection on the MRD-D1. The higher-end models have push button channel selection that is easier to use. The bleed-through between video sources is unusual. If you disconnect one video source from the MRD-D1 & the bleed-through disappears, then (I think) you can isolate the problem to be a defect in the MRD-D1. You could talk to ChannelPlus tech support -- I've found them to be very helpful.

Regards
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