Why must my electronic device accept any interference it receives?

I recently purchased a new Bluetooth speaker and, as do many devices, it sports a quotation from Part 15 of FCC rules. It says that the device must not cause harmful interference. Fair enough. I shouldn’t be allowed to operate a device that causes problems for other people. However, the second part of Part 15 says that the device must accept any interference it receives, even if it causes undesired operation. Why? Why can’t the manufacturer of my little speaker design it in such a way as to reject any interference it may receive that would interfere with my listening pleasure? I get the first requirement, but the second doesn’t make sense to me.

The quick and dirty answer is that it’s operating in an unlicensed portion of the RF spectrum. If you had official government authorization to use that portion of the spectrum, you would have the government ready to step in to help you. But you don’t, so you don’t.

A more complete answer will have to wait for tomorrow when I’m sober and not busy.

You are mis-interpreting the statement.
It’s not that the device MUST be designed so that interference causes problems, but rather, if after all the effort the manufacturer’s efforts at shielding it, it still exhibits undesired operation, you can’t complain to the government.

This is an assumption on the FCC’s part that all devices in operation have passed Part 15, and so are operating within the emission boundaries set by the FCC. So, if another (authorized) device causes your speakers to malfunction, that’s the manufacturer’s (and your) problem, not the FCC’s.

Note that this is different in Europe, where devices undergo CE testing and are supposed to pass an “immunity” test, where they are subject to bombardment by RF energy. If they fail the test, they are not allowed to be sold. The FCC has no similar requirement for consumer electronics.

FWIW, one of the devices I designed failed CE immunity testing, and had to have some filtering added to it to pass.

Unlike analog radios, Bluetooth and WiFi are digitally modulated. It’s not like “channelized” licensed radio spectrum. They are intended to overlap within the same 2.4Ghz spectrum, along with everything else using that.

There are multiple versions of each BT and WiFi standard, often using different methods to avoid or mitigate mutual interference.

BT uses frequency hopping spread spectrum, and newer versions I think may use a “hear and avoid” algorithm to set up a hopping pattern to reduce interference. WiFi originally used Direct Spread Spectrum but newer versions use ODFM (Orthogonal Frequency Division Multiplexing).

Even within one manufacturer (say Apple) the various BT devices may use different BT versions. E.g, Magic Mouse vs Magic Mouse 2 vs Magic Trackpad vs AirPods, etc.

So it’s not necessarily like the analog case where the receiving radio needs better filtering. It could be an inadequate “sense and avoid” algorithm caused by digital RF devices or old drivers. Even if the consumer device is new, it could be using an old BT chipset. Figuring out what version it’s using is often difficult.

Some manufacturers state the BT version. E.g, the JBL Charge 3 is version 3.0. The Anker SoundCore Flare is version 4.2: https://www.techradar.com/news/audio/portable-audio/10-best-portable-speakers-1069079

That said, BT and WiFi can be sometimes blanketed by RF interference which they can’t hop around or use spread spectrum techniques to avoid. There are lots of old cordless phones, garage door openers, baby monitors, security monitors, etc. which might be inefficiently contending for that spectral space somehow misbehaving.

Whether modulation is analog or digital, if a competing RF signal stays in your “receive passband” this often cannot be filtered out. If it’s any consolation, the RF noise problem is less on microwave frequencies like 2.4Ghz and above. At lower frequencies it can be really bad: https://spectrum.ieee.org/telecom/wireless/electronic-noise-is-drowning-out-the-internet-of-things

There are microwave RF spectrum analyzers available but they are expensive and require lots of expertise to interpret. For WiFi problems there are some good apps like WiFi Explorer (for Mac): https://itunes.apple.com/us/app/wifi-explorer/id494803304?mt=12

For Macs there is a similar Bluetooth Explorer app but it’s part of the XCode development environment: https://robservatory.com/debugging-bluetooth-issues-in-macos-sierra/

For regular consumers the best approach might be trial and error product evaluation. If your BT speaker has interference, evaluate all the other BT devices and try turning them off. Or get a different BT speaker while the 1st can still be returned and if it doesn’t have the problem keep that one.

I had an old landline phone that picked up radio station interference when we moved. That was the fault of the cheap unshielded phone, and I threw it out.

I always assumed it meant that you can’t have your device somehow jam competing signals, and if they make it through despite shielding and other techniques, you’re just going to have to live with it since you’re in a public band.

Drum God:

I think maybe you misunderstood a little, due to ambiguous wording. Your device is not required to accept interference as input - it’s required to accept (and deal with) the fact that interference will happen.

In the early days of personal computers this was a major issue. The clock speed of even those computers was in the low megahertz (at best)- just above the AM band and right in the range of a lot of other useful bands.

Even worse was the monitor connection. People used TV-like monitors (or even TVs). The frequencies used in the video signal was also problematic and all the electronic noise would easily escape the box thru the video output.

Making a properly shielded virtually home-brew PC and paying to get it certified was a non-starter.

Someone noticed an exception in the FCC rules. Slap the right words on the box and you’re good to go.

Note that microwave ovens, vacuum cleaners, etc. also dump a lot of noise out. Sometimes thru the house AC wiring. Some people have all sorts of issues when those appliances run.

I recall pacemaker jokes back when microwave ovens were just becoming ubiquitous.

The Robustness Principle: Be conservative in what you do, be liberal in what you accept from others.

The wording isn’t just ambiguous, it’s self-contradictory. If consumer devices aren’t allowed to cause harmful interference, there will be no interference for them to accept, because any devices I am likely to be near will also be subject to the no harmful interference rule.

FCC Part 15 arose from the Ham Radio era, where licensed operators were legally generating large signals. If they were operating legally, an unlicensed device (operating under part 15) had to accept their emissions without complaints. Most hams would try to address interference, but they were not necessarily compelled to.

So what is Part 15 telling me, or my device’s manufacturer, to actually do? Why must these words be printed on the actual device itself?

It’s tell you that the device is certified to operate without a license from the FCC (unlike, say, a ham radio transmitter). Part of that privilege is the burden that you must bear regarding undesired operation due to interference caused by other licensed or unlicensed devices.

It’s telling you that, in the great hierarchy of electronic devices, yours is at the bottom, and informing you that’s all you get.

Oh yeah. I remember the early days of TV (1960s or before). The electric noise from vacuum cleaners and the mixer in the kitchen made the TV unwatchable. Well, if it was mom running the appliance, you put up with it, but anyone else you could complain to.

Somewhere around 1970, they started to put filters in TVs to eliminate the interference.