Posts Tagged ‘X1’

There’s More To The Story (There Always Is!)

A couple weeks ago, I posted a story about a particularly irritating problem I was having with my high-speed Comcast internet service dropping out. After lots of troubleshooting, I thought I had cornered the culprit – a Samsung-manufactured set-top box that Comcast was using for delivery basic Xfinity services (no DVR).

When I connected the cable TV input connection to a spectrum analyzer, I saw some pretty nasty burst of spectral noise that ranged from 11 MHz all the way up to 400 MHz. I figured this might have been the cause of the dropouts and promptly returned it to the local Comcast Store, only to find out that particular model wasn’t in use any more and that I’d have a much smaller, flatter version to take home – one that would link automatically to my main X1 DVR.

Coincidentally, the dropout problem stopped, so I did what any other reasonable person would do: I assumed that was the end of it.

Except it wasn’t. A few days after I replaced the box, the WAN connection started dropping out again. Some days it dropped only a couple of times, but on April 13, it dropped out almost fourteen times in four hours. Out came the test equipment (and plenty of expletives) as I started testing every line in the house, taking more and more things of-line.

At one point, the only thing connected to the Comcast drop was my wireless gateway and my spectrum analyzer, through a brand-new 2-way splitter good to 1.5 GHz. Sure enough, the WAN connection dropped again – but this time, I caught something on the analyzer I hadn’t seen before.

Figure 1. The entire system (two cable boxes and a wireless gateway) are working just fine with these signals.

Figure 1 shows the ‘normal’ levels of QAM carriers coming through the drop. There’s a little up-and-down there, but the entire system – in particular, the downstream QAM carriers above 650 MHz – all measured at least 32 dB above the noise floor (about -87 to -88 dBm). In this condition, the wireless gateway was chugging along just fine and broadband speeds were pretty fast.

Just ten minutes later – while I was watching the analyzer screen – the QAM carriers from 50 MHz through 400 MHz dropped precipitously, as seen in Figure 2. Right on schedule, the WAN connection stopped working! Yet, I hadn’t touched, changed, or re-wired anything. This was starting to look like a classic ghost in the machine, and likely an issue outside my house. (Yes, the Samsung box did need to be replaced in any case – it was quite dirty, RF-wise.)

Figure 2. Ten minutes later, KABOOM – the WAN connection dropped and my analyzer showed some nasty QAM waveforms below 400 MHz. What was causing this?

Well, after escalating this problem to Comcast’s Special Operations unit (yes Virginia, they do have Special Ops guys), I was visited by Jason Litton and Fredrick Finger of Comcast. I asked them to replace the DC block/ground block outside the house and also to sweep the underground cable coming back to the house. I had previously gone out to check the ground block and discovered (a) it was grounded – the wire had come loose) and (b) there was a tiny bit of play in the connections at either end, which I tightened up before they arrived.

Long story short; the block was eventually replaced, re-grounded, and new connectors were installed at either end of the underground drop. During testing, Jason spotted noise coming from a neighbor’s coaxial drop and proceeded to install several more new connectors. I also took the opportunity to have them put in two brand-new splitters in my basement (overkill, but what the heck) and run a new coaxial line to my workbench.

And that finally did the trick. Whatever phantom was haunting my system had finally been exorcised for good. Using Comcast’s brodband speed test to New Castle, Delaware and Secaucus, New Jersey, I saw wired LAN and 5 GHz 802.11ac download speeds hitting 100 MB/s. Using the popular server in Dallas, Texas; I measured download speeds around 30 – 48 Mb/s. Upload speeds to all servers were in the range of 10 – 12 Mb/s.

Figure 3. The final setup with all QAM levels where they should be!


Figure 4. Best of all, there’s no noise below 50 MHz on the upstream channels. FINALLY!

So what was the culprit? Most likely the cheapest thing in the system – the DC block. Noise from the Samsung STB didn’t help, and apparently neither did the noise coming from my neighbor’s cable drop. But the block probably had an intermittent connection and was creating some nasty standing waves, causing tilt on the lower QAM carriers and noise at the uplink frequencies around 30 MHz.

I’ll have more details on this unfortunate series of events during my RF/Wireless class at InfoComm in June. Until then, things are working well (knock on wood, or metal, or coax, or modem…)

Now You See It…Now You Don’t

A few years ago, readers may recall my on-going battles with Comcast to fix a service reliability issue. I had asked from the start that the ‘drop’ – the underground RG-6 coaxial cable running from the sidewalk to my house – be replaced. I suspected the cable’s jacket had been compromised with moisture over time.

Several visits from Comcast later (plus two modem upgrades), they finally did just that. Lo and behold, the underground cable had been spliced at some point with a barrel and simply covered with dirt  – no  insulating tape or waterproofing applied.

In the middle of all this fun and games, I received an offer from Comcast to upgrade to their X1 platform. That meant two new set-top boxes – one with a DVR function (not quite an accurate description, as the “DVR” is actually a cloud server), and one ‘basic’ box without DVR access.

So I went ahead and replaced everything. My network speeds had also gone up considerably along the way, and I also had a new 802.11ac channel bonding modem with 2.4 and 5 GHz WiFi connections. Life was good, right?

Well, it was – until last fall, when I started experiencing intermittent dropouts of my Internet service. They came and went randomly and could last as long as 15 minutes before service was restored.

“Here we go again!” I thought to myself as several rounds of diagnostics ensued. Was it the drop to the street? Nope, my spectrum analyzer showed a full spectrum of strong, level QAM carriers all the way up to 700 MHz. Was it a modem problem? I switched out the 802.11ac Arris modem I originally received as part of my upgraded street drop for a Technicolor model.

That didn’t solve anything, either. I looked at all of the coaxial lines running through my house and also checked out the Cat 6 cables I had run after we installed hardwood floors. Was it possible that energy was coupling from the bundle of category cables into the modem? Not likely, as that arrangement had been working well for over a year.

Hard at work, running new network and coaxial cables. (Did you know you can shoot vertical panoramas with a smartphone? I didn’t…)

Finally, I got a Comcast technician to stop by the house and we started sweeping all of the coaxial lines for noise. Lo and behold, the line that feeds my family room and master bedroom boxes had noise around 26-29 MHz, which could have affected the upstream signal from the modem.

Temporarily running an RG-6 cable directly from my X1 DVR to the noise meter showed a clean spectrum, so I got to work running a new cable connection back to the two-way splitter in my basement. After dressing the wires and reconnecting everything, I figured I was out of the woods.

Except I wasn’t. Not long after, the intermittent drop-out started again. In the middle of critical projects, I took to using my Samsung Galaxy as a temporary WiFi hot spot to wait out things out. I rebooted the modem numerous times and was at wit’s end.

Calls to Comcast revealed there weren’t any service outages. Was it RFI from the category cables, by some crazy chance? I replaced the bundle with a single piece of Cat 6 STP (shielded) wire – drains soldered at both ends – and installed an 8-port 1 Gb switch to feed all of my networked devices behind my family room TV.

That didn’t fix the problem.  ARRGGGH! What could it be? The noise HAD to be coming through the coaxial cable – but from where?

After pondering my next move, I decided to connect my spectrum analyzer to each coaxial cable run and look very carefully for noise and unwanted energy around 25 – 30 MHz. Connected to my X1 DVR, I saw nothing. It was as clean as a whistle.

The guilty party under test. A short piece of coax runs from the CABLE IN connector directly to my spectrum analyzer.

However, connected to my ‘basic’ X1 box in the bedroom, I saw the noise floor briefly jump from about -85 dBm to -80 dBm – and this was happening about every two seconds, as regular as a clock. Well, this looks promising! So I switched on “persistence” mode on the analyzer, which would allow whatever momentary spikes of energy to remain visible on screen.

What I saw next was jaw-dropping. High-energy spurious RF carriers dotted the spectrum, starting as low as 6 MHz and running all the way up past 50 MHz. Expanding my scan width to 100 MHz, I found even more of these little buggers – some of them as strong as -64 dBm. (For reference, the QAM carriers in my system are around -40 dBm.)

I expanded the upper limit of my spectral scan to 200, 300, and 400 MHz, only to find RF spikes everywhere. Yikes! I ran upstairs, unplugged the X1 ‘basic’ box, brought it down to my lab, and connected the CABLE IN port directly to the input of my spectrum analyzer.

These RF spikes were enough cause for alarm, so I expanded my spectrum scan.


Things were looking even worse at 100 MHz!


What a mess! And some of the spikes were only 20 dB down from my QAM carriers, like the one at about 300 MHz.

And there was the answer. The X1 box (manufactured by Samsung) had apparently been generating broadband RF interference every 2 seconds for several months, and it was coupling back through the RF input into my system. When my modem sent an upstream request to the Comcast server that happened to coincide with one of these “bursts” of RFI, the connection to the server was lost…and our Internet was out. DOH!

A trip to the Comcast “store” (that’s a generous description as it’s more like a crowded reception room) to return the offending box revealed that design isn’t used any more. Instead, a small (about 6” x 6”) flat terminal was exchanged for my RFI generator. This new STB had just four ports – power, HDMI, RF input, and USB – and now syncs as a slave to my X1 DVR, emulating all of its functions.

In the meantime, our Internet service is now as clean as a whistle and drop-out free. Once again, some thorough detective work and analytical thinking solved what had become a frustrating problem with no apparent resolution.

I’ll discuss this fun episode from my life in more detail during my RF and wireless class at InfoComm this coming June. Come on by, and learn more about troubleshooting RF interference problems in the real world! (And keep an eye on your cable box while you’re at it…)