Combining modulator outputs - AVS Forum
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post #1 of 54 Old 07-15-2001, 05:37 PM - Thread Starter
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I've seen this discussed briefly but my experiences have been different.

I have two 5000 modulators. I've been trying to combine their outputs so that I do not need to use an antenna switch. I have one unit set for channel 3 and the other for channel 4.

I first tried the cheapo passive signal combiners (basically a splitter in reverse). That did not work. Both signals would disappear the moment they were hooked up.

I then tried an ASKA ASC-4 signal combiner as well as a Quest VCO-1004 signal combiner. Following the instructions produced no signal either. Adjusting the individual pots to limit the bandwidth still produced no signal.

Has anyone successfully combined the channel 3 and 4 output of the 5000 modulators? What did you do and use?

Thanks in advance for any help you can provide.

Regards, Peter
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post #2 of 54 Old 07-15-2001, 07:54 PM
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I read that the ATSC signal is wider than a regular signal so the chan 3 and chan 4 outputs may bleed on top of each other. Unfortunately you may need to use a manual switch.
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post #3 of 54 Old 07-16-2001, 01:02 AM
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Peter -

I've got 3 modulators combined as described in this thread.

Two of these are on adjacent channels (although they are 14 & 15, not 3 & 4). Amplification is definitely key to making this work, as the Dish modulators seem to have inherently low signal levels.

I don't believe the channels are wider, but sometimes harmonics may be a problem. You may need to try a bandpass filter. (The "channel combiners" often add an inexpensive bandpass filter to the "splitter in reverse.") You may want to look for used Cable TV headend filters. These are high quality rack-mount items which sell for $200 - $300 new, but I was able to find them on eBay for a tenth of that.


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post #4 of 54 Old 07-16-2001, 02:32 PM
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I've tried a few different ways to combine my local DTV feeds (all UHF) with my two 5000 modulated channels (VHF 3,4). I have never been able to make it work, but I have always thought if I had the proper RF filters, and added good amplification, it could be done.

A TV engineer I know also said 8VSB has different sideband characteristics and conventional RF filters would probably not work. He suggested I use a good old A/B switch with good isolation and that's what I did.

I would still like to combine them all on one coax, is anyone doing this, with ch 3 & 4?

chap,
Read peterd's thread, he has HBO, Showtime & ExpressVU HD all at one time, for his different PV-HD1000's.

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post #5 of 54 Old 07-16-2001, 09:44 PM
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I'm confused by this whole thread. Why do you need to have 2 different modulators going ot the same device???? Am I missing out on a cool trick? :-).

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post #6 of 54 Old 07-16-2001, 09:47 PM
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The DISH 5000 modulated output has both upper and lower sidebands. Broadcast DTV has only the upper sideband. Thus the lower side-band of channel 4 would overlap channel 3.

You could only combine them if you had a very good filter to remove the lower sideband of channel 4.

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post #7 of 54 Old 07-17-2001, 11:28 AM
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I'm a little confused about this too. Just channel switch the 5000 as balazer said. If the object is to record movies on two or three channels simultaneously (eg., Perfect Storm and Supernova the other night), then you would need 2 complete combos, with one connected to each modulator. It's hard to avoid messing with the 5000 remote because of the need to access the guide.
-Roger

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post #8 of 54 Old 07-17-2001, 11:54 AM - Thread Starter
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Sorry for the slow response. My hard drive crashed and I've been busy rebuilding the system.

PVR, I think that is what I am experiencing. I'm hoping for a more elegant solution.

PeterD, I did read that thread a while ago. I think your situation is a little different in that you have the first modulators that output on 14/15. If you want to swap, I'll be happy to! I tried some of the signal combiners that have filtering capabilities. Using the bandwidth adjustments did not prove successful. I'll keep my eye out for these headend filters you speak of.

Chap, I don't know if it is a cool trick but....I have both DISH and BEV outputting on modulators feeding a Panny combo and AccessDTV. Even if I leave the respective units on the correct channel, I seem to forget to switch the antenna input thereby causing me to record the wrong system. If I can combine the signals onto a single cable all I need to do is tell the systems to record on different channels (3/4).

Balazer, It's is not just different channels, it is different services I am trying to combine (DISH/BEV). AS to the Zenith unit, YIKES!, I was hoping for something cheaper.

Gridleak, Please see above.

Ken H, I've tried every combination of amplifier and bandwidth adjustment via my cheap ($15) adjustable signal combiners and Radio Shack amplifiers. I'll keep using the manual switches while I look for other solutions.

Best regards, Peter
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post #9 of 54 Old 07-17-2001, 12:15 PM
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Hey Ken -- what's an example of a "good A/B switch with good isolation"?

Is it an electronic switch, or a physical switch?

Can ayone suggest a good electronic A/B RF switch that can handle HD -- possibly even with remote control?

Thanks,

Steve R
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post #10 of 54 Old 07-17-2001, 03:46 PM
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Has anybody pried open a Dish modulator to look for jumpers? There may be a simple way to change the channel (no pun intended).

-Dylan

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post #11 of 54 Old 07-17-2001, 05:31 PM
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Ask and ye shall receive.;) http://home.earthlink.net/~rogercc/_...odulatortm.jpg enlarge
Just happened to look inside yesterday to see. No jumpers, but does appear to be some tunable slugs.

Lousy photo. I'll try to improve it. Wanted to be able to read chip numbers etc.

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Analog -- Always out of adjustment.
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post #12 of 54 Old 07-17-2001, 08:22 PM
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My two 5000's are getting different programming, that's why I want to use both on the same coax, along with my local DTV stations, into one TU-DST51.

As far as a high isolation A/B RF switch, Radio Shack has the 15-1233 for $7.99.

They also have a remote control model, the 15-1957, for $39.99.

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post #13 of 54 Old 07-17-2001, 08:30 PM
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gridleak,

You're a pretty adventuresome guy, when are you going to adjust the slugs, just to see what happens?

Your friend,
Ken

PS. No pressure at all. Hey, I took the 742, didn't I?

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[This message has been edited by Ken H (edited 07-17-2001).]

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post #14 of 54 Old 07-17-2001, 09:27 PM
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Why bother trying to combine modulator outputs?

Use the RF remote to flip between all two HD channels on DISH.

And if you need to schedule it to record something on one channel and then the other later, can't the 5000 do this automatically if you set it up to record?

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post #15 of 54 Old 07-17-2001, 09:30 PM
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Roger,
Nice work! Now we'll just heckle you to play with the slugs. http://www.avsforum.com/ubb/smile.gif Actually, someone should get the datasheet for the Zenith HDTV modulator I.C. That osc-in-a-can sure has a specific frequency...

-Dylan


[This message has been edited by dahester (edited 07-18-2001).]

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post #16 of 54 Old 07-17-2001, 09:33 PM
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You can combine modulator outputs using the Zenith ATSC Translator and a channel translator - ~$7500.

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post #17 of 54 Old 07-18-2001, 08:57 AM
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Quote:
Originally posted by Ken H:


As far as a high isolation A/B RF switch, Radio Shack has the 15-1233 for $7.99.

They also have a remote control model, the 15-1957, for $39.99.
[/b]

I use a remote controlled A/B switch similar to the Radio Shack 15-1957 to switch between my C band LNB and Ku LNB into my Unity Motion HD receiver. There is practically no loss even at these frequencies.


Al Keown
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post #18 of 54 Old 07-18-2001, 10:25 AM
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I looked at the spectrum from a modulator and compared it with the spectra from local DTV channels.

Unlike the analog channels, a DTV channel fills the entire 6MHz spectrum. If you are not suffering multipath, the spectrum should be flat across the entire 6MHz channel (i.e. no dips or notches) and drop off precipitously at the channel edge.

The modulator spectrum is also flat and covers a width of 6MHz, but altough its "skirts" are quite steep, it does not drop off quite as rapidly as the OTA channels. Also, its amplitude is much less than the OTA channels. I can't tell you by how much it is less because my spectrum analyzer is way out of calibration.

By amplifying both the OTA signal and the modulator signal with two separate adjustable gain amplifiers from Radio Shack, then combining using the VHF/UHF combiner from Radio Shack, I was able to adjust the gains of the amplifiers so as to combine the OTA signal with the modulator signal to feed into a Panasonic STB.

If this does not work for two modulators, I suspect that the more gradual "skirts" of the two modulator spectra may be causing interference between Ch 3 and Ch 4. If this is the case, any band pass filter that you would need would have to have a much steeper skirt; I doubt that such a filter is commercially available.
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post #19 of 54 Old 07-18-2001, 01:47 PM
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cymro,

You sum it up quite nicely.


gridleak,

No problem, take your time. I am guessing that because the original dealer versions of the modulator were ch 14/15, there might be a way to 1) adjust the current ones to another channel 2) or replace a part to get a different channel choice. Let us know as things develop.





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post #20 of 54 Old 07-18-2001, 02:57 PM
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Quote:
Originally posted by cymro:
Also, its amplitude is much less than the OTA channels.
Sure could explain why so many here have problems with the modulator/STB connection.

Thanks, cymro, for your comments on this. I've looked at the OTA signals as well and what you say is exactly what I've seen. The NTSC stuff is a mess, and the ATSC signals are neat boxes with steep sides and a little peak on one side. Even two adjacent channels can be distinguished, with a deep dip separating the two.



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post #21 of 54 Old 07-18-2001, 09:23 PM
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Ken,

Not as adventuresome as I used to be. When I was 8 yrs old I offered to fix my grammar school Principal's Atwater-Kent radio. Big old wood case, dual tuning dials, tubes with numbers like "27". The Principal was a tough old broad with a big paddle in her office, so it's a good thing I didn't screw that job up. She gave me a shiney new quarter.

I have to admit taking 742 was gutsy, but 5000's are dime-a-dozen compared to modulators.http://www.avsforum.com/ubb/smile.gif I wouldn't want to try messing with one blind so I'll have to drag a spectrum analyzer down to the house. I was going to do it anyway, because I'm curious about the double sideband stuff. Might be a few days.

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post #22 of 54 Old 07-18-2001, 09:59 PM - Thread Starter
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Here's a link to what I tried. Evidently, this unit was not able to handle the "skirts" mentioned above.
http://www.askacom.com/pdf/sign_combiners.pdf

Cymo, I will try the mod > Amp > regular combiner, and play with the gain to see if this will work in my setup.

Thanks and regards, Peter
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post #23 of 54 Old 07-18-2001, 10:59 PM
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Ken -

A reasonably logical supposition about 14/15, but I don't think it pans out.

The original pre-production modulators were assembled using 2 circuit boards in a huge chassis (sits under the 5000, same footprint & 1" high). One card is connected via a riser to the card-edge connector in the 5000 expansion slot. The other card is the actual RF modulator.

Here's where it gets really scary... That second card is actually an ISA bus card. The ISA connector is not being used, however. A ribbon cable connects the two cards (attached to a header at the top edge of the modulator). So, it appears that they found a source for a modulator which had been originally designed for a PC, then adapted it for their prototype. Therefore, I think the fact that it used 14 or 15 is not relevant to the capabilities of the production units. (This may also explain why they are happy in an adjacent channel setup.)

When I first got them, I disassembled one to retrieve a loose screw. Unfortunately I didn't think to grab the digital camera (and this was before Roger's many illustrated posts http://www.avsforum.com/ubb/wink.gif ). If I have one open again, I'll be sure to post photos.


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post #24 of 54 Old 07-19-2001, 07:04 AM
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Yikes!

As I wrote that post, I knew I was too optimistic.

Thanks for the info, Peter. It looks like I'm stuck with the a/b switch, until someone smarter than me figures this out.

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post #25 of 54 Old 07-20-2001, 12:08 PM
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Maybe it's a Sencore board. Their HDTV signal sources output on 14/15.

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post #26 of 54 Old 07-20-2001, 06:07 PM
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As I sit here thinking about this, it gets worse. I may live in the only (or one of the very few) DTV market in the country with a DTV station on ch 14. So even if the standard HD Modulator output could be changed to 14/15, I'm still screwed.

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post #27 of 54 Old 07-20-2001, 08:46 PM
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Ken -

The Zenith solution mentioned earlier is designed for OTA retransmission (including the ability to change the PSIP info) and therefore is expensive. But there might be a cheaper solution thanks to eBay...

Have you thought about a CATV channel processor? These are the units used in headends to move OTA channels around. It appears that, depending on the sophistication, some deal only in RF while others may perform demodulation (to audio + video or just to IF) & then remodulate the signal. If you found one which operated only with the RF signal, it might allow you to move the Dish modulator output to another channel.

Try searching eBay for "Blonder Tongue", "Pico Macom", "Jerrold", etc.


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post #28 of 54 Old 07-21-2001, 06:02 PM
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I had hoped that it might be possible to breadboard a homebrew copy of the DISH HD RF modulator until I looked at the board shot posted above. Looks like too much surface-mount to fool with. I wonder if there is anything made for some type of pro use that could be kludged together to work with the DISH/Panny combo. Perhaps even a commercial broadcasting unit that takes the HD signal off a satellite feed and converts to a terrestrial antenna broadcast freq to be sent to the RF preamp for broadcasting.

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post #29 of 54 Old 07-22-2001, 12:42 AM
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So, the 169 Time approach wasn't sound. But, as the thread title says, "Dish 5000 modulator worth its weight in gold." Anyone want to try producing clones of the modulator? http://www.avsforum.com/ubb/wink.gif

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post #30 of 54 Old 07-22-2001, 08:51 AM
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It's likely that the Dish5000 HDTV modulator is firmware controlled. If so, and Dish has copyrighted the firmware, you'd have to reverse engineer the firmware (without actually copying it) to avoid copyright infringement.

LOL

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