Comments on: Review of Antec skeleton case neglects to mention RFI issues

By: Ed

Ed — Thu, 06 Nov 2008 19:35:20 +0000

eheheh… I have to take one of those jacobs ladders to work one day… Maybe I can give it as a present to my boss.
More seriously, very interesting stuff guys, since neither me nor my friend live in the u.s. I had no idea your cable tv was like that. Here in the u.k. my cable operator installed top quality cabling at my home, hence my comments. TV here is basically all digital, there are only 4 analog signals. I never experienced any squares or blocks. Fiber optics bring the signal to the kerb, only the last few hundred meters are coaxial.

By: Greg Maxwell

Greg Maxwell — Sun, 26 Oct 2008 20:41:22 +0000

In a prior professional role I had the interesting job of acting as a government regulator over a local cable company. In short: Preventing leaks is hard. Fixing leaks is expensive. The cable company was not evil, but the strategy that would maximize their profit was to perform the minimum they could get away with. The worst signal quality/interference issues tended to show up on long runs in rural areas which were generally unprofitable for the cable company (but which they were required to serve in exchange for their monopoly), so they were especially disinclined to deal with them. The FCC had nothing really motivating enforcement (FCC enforces and they catch hell from industry lobbyists), local issues simply do not get traction with national regulators. At least the people in my community had something of a voice, since as a local agency we were forced to be responsive to the community, though our power was not the same as the FCC’s (in some ways we had more regulatory power, in some ways we had less).

Of course, interference goes both directions.

Modern PCs are pretty noisy. To get transistors to switch faster you push harder clock driving levels, to keep chips synchronous you use big clock distribution trees and delay lines which create lots of potentially radiating surfaces. Reduce lead solders are more likely to gain semiconducting contaminants over time and produce spurious harmonics. Most modern systems contain a feature called “Clock spread spectrum” which basically adds a ton of phase noise to the system clocks, it doesn’t reduce the noise, in fact it often increases it (by worsening various impedance matches), but it gets the system manufacturers around various regulatory requirements which specify flawed measurement techniques.

My own computers, even in properly shielded cases, produces interesting birdies all over the electromagnetic spectrum. Equipment in the same room typically produces *much* stronger than the intentional radiators which are far away. The laptops are especially bad. It can be quite challenging to hunt down and correct all the sources of spurious RF. Many years ago I had a cute computer housed in a custom made cardboard case. Operating my jacobs ladder would cause it to reboot or freeze, though I don’t know how much of the RF was conducted via the power lines vs through the air.

So I think the issue is real, although perhaps not material if you’re neither not conducting radio science (or a HAM), and if you’re running Windows (in which case an occasional extra lockup won’t be noticed over the background noise.

By: Cyde Weys

Cyde Weys — Sun, 26 Oct 2008 16:15:32 +0000

Yeah, I have to back up Kelly here. I pick up all sorts of interference from cable TV on my ham radio transceiver. I know it’s cable TV because cable TV audio is FM, meaning I can actually listen to it on transceiver. I’ve used frequency charts showing where the cable channels and local listings and, sure enough, I can reliably tune into just about every cable program being broadcast in the area at 6 MHz increments.

Even worse, some of it interferes on the ham radio bands. Grrrr.

By: Kelly Martin

Kelly Martin — Sun, 26 Oct 2008 14:54:21 +0000

Ed:

Properly-shielded cable TV cables? Bwa hah ha. There isn’t a cable company in the country that has its cabling properly-shielded end to end. Why do you think that most cable companies put an unimportant channel on Channel 18, or have that channel unassigned entirely? Why do you think that W9DUP had to move from 145.25 to 145.43? Cable companies leak massively, and rarely bother to repair the leaks because they have lobbyists who will just lean on the FCC should anyone complain. FCC regs on EMI are widely ignored, and the FCC doesn’t care unless it interferes with a licensed service (and even then only if the licensed service is operated by a company with lobbyists). See also this article from my blog, on the vagaries of FCC enforcement.

Digital signalling is not less prone to interference, it’s just that the interference is less obvious. Instead of getting loud, distorted speech you just get dropouts. We see this all the time on our digital cable channels; black squares of doom when the mpeg stream loses too many segments, or no audio for a few seconds. Digital TV (and digital radio) actually require higher signal-to-noise ratios in order to get a usable picture because of the deep compression the broadcasters use to squeak every last dreg out of the bandwidth. This makes them more prone to interference, not less, because any interference cuts into the available SNR. Also, digital signals generally do not gracefully degrade the way analog does; with digital it’s generally all or nothing.

About the only thing that avoids these are solutions that are optical fiber end-to-end, and last-mile fiber is, as of yet, not widely deployed.

By: Ed

Ed — Thu, 23 Oct 2008 19:50:57 +0000

A friend’s computer always has all the panels removed, basically working like a frame that supports the rest. He noted that he hardly notices any interference nowadays, having had a number of different motherboards in it throughout the years. I’m guessing newer motherboards cannot afford to dissipate power in the form of radio energy. An extra guess is that interference created on one side of a board would cause parasitic signals to show up on another side, so it’s better do do your homework right and ensure all the lanes and connections have a properly adapted impedance.
Another point to note is that almost all equipment is now digital. Before, we had analog radio and analog tv, now radio is mostly gone and tv comes through a properly shielded cable into our homes. All this new digital equipment is less prone to letting itself get interfered. Surely, I agree that getting rid of all these guesses is the way to go. RFI should be properly measured. In order to comply with the FCC rules, I’m sure most manufacturers do it when they release a new product. (ehehehe, I bet even Antec did some of this, but just kept quiet.)