>>> 2021-02-11 the third and up broadcasts (PDF)
In my RSS reader, I came across the article New Life for the Third Network on the North Korea analysis blog 38 North. I've always been interested in this system because it is an unusual broadcast technology, and it seems it might remain relevant as Kim Jong Un has apparently called for the apparently dilapidated system to be repaired and expanded. What I'm saying is that I don't just write about obsolete technology, but also foreign policy, apparently.
Anyway, onto the obscure technology. If you're not familiar, the "Third Broadcast" is a wired radio system installed in at least some cities in North Korea. Rumors about the system abound, but I will try to stay to facts, which there are very few of. The system is evidently operated in a similar fashion to a radio network with central programming produced in Pyongyang with various slots filled in with programming produced at a regional or local level. The "receiver" consists of a speaker unit with a volume knob that usually seems to be plugged into a socket on the wall, but might sometimes be hardwired (or the plug concealed behind in a recess socket). There is only one channel, so the volume control is all you need.
Technically, one wonders, what exactly is "wired radio?" Well, there are various systems that have been referred to as "wired radio" or "wired broadcast". The term is usually reserved for systems that do not function by transmitting RF into a coaxial cable, because, well, we usually just call that cable. Instead, "wired radio" systems usually don't involve "radio" at all but just directly put the modulated signal onto a wire.
From what little I have been able to figure out, my guess is that North Korea's example is basically a large-scale 70v audio system. High-voltage audio systems are fairly common in commercial overhead paging and some sound reinforcement applications. Basically, the low voltage and high current produced by a conventional audio amplifier requires a very large-gauge wire to span long distances without significant loss, and large-gauge wires are expensive. If you want to install, say, in-ceiling PA speakers throughout a building, you can save money by using a special paging amplifier which produces a high voltage and low current.
Typically in these systems the speakers are just normal four or eight ohm speakers, but each one has a transformer mounted to it that converts the high-voltage audio to low-voltage at higher current for the speaker coil. You can also fit the individual speaker with a volume control using a simple potentiometer, although since paging systems are not usually meant to be adjusted "on the fly" this isn't especially common, with it being more common for there to be multiple taps on the amplifier or an internal adjustment knob so that the installer can do a one-time adjustment for roughly equal loudness throughout a room .
There are a couple of different conventions for these systems, but by far the most common is 70 volt. For a large-scale system like that in North Korea, they may use 110 volt, which is also used in some commercial buildings. I'm not sure how exactly this scales up to a city, do they use yet a higher voltage for longer distances with neighborhood step-down transformers? In any case, high-voltage audio is a well understood, commercially available technology that would be quite consistent with what I've seen of the North Korean system.
That's not to say that I'm sure this is the case. There are other ways of distributing audio over large areas. The Muzak company, for example, got its start by distributing background music to clients using a wired network. As I understand it, the first implementation of Muzak used leased telephone lines (presumably "toll quality" lines for better audio) to get the sound to each building, where there was an amplifier to high-voltage audio for the building's speakers. This was done at a time when recorded music was not common, so the background music in large stores was sometimes actually a live performance--- just performed elsewhere. North Korea might use a similar arrangement to feed neighborhood amplifiers.
My assumption that North Korea uses a high-voltage audio system rather than a low-level signal distribution network is due to the fact that the radios are only connected to a single socket. They don't seem to require power for amplification, so the audio must arrive already amplified. I'm not aware of any good way to do this other than high-voltage audio.
So, is there any other interest I can eek out of high-voltage audio? Of course! Another common application of high-voltage audio technology, which is basically just a special case of paging, is audio masking systems. These are typically used in facilities where sensitive discussions occur in order to prevent eavesdropping, and vary in sophistication. A therapist's office might use a simple white noise machine to prevent casual eavesdropping from the waiting room, while a facility housing corporate or government secrets might use a wide-area sound masking system intended to be resistant against technical surveillance. These larger-scale sound masking systems are often essentially just 70-volt PAs used to broadcast white noise, but the "speaker" designs tend to be a little more eccentric.
That's because high-end sound masking systems often use transducers coupled directly to surfaces in order to prevent the capture of audio from the surface by mechanical or optical means. That means, for example, an audio transducer adhered to each windowpane which vibrates the windowpane with white noise in a way that is minimally audible to humans but should be overwhelming to a laser reflection listening technique. Atlas Sound is a major manufacturer of these high-end systems and their products run surprisingly cheap on eBay, I have a TSCM-grade sound masking unit under the bed because I think it helps me sleep, or at least improves the security of my dreams. They also manufacturer sound masking speakers intended for use in air ducts, plenum spaces, transduction to doors and walls, etc. Putting Muzak into such a system might have an amusing result, as all these untuned surfaces would probably rattle and resonate in horrifying ways.
A similar wired radio system to North Korea's existed in the Soviet Union, which might give the impression that wired radio is limited to troubled Communist states. That's not entirely true, but it is certainly more attractive to embattled regimes because it has the property of being extremely difficult to intercept or disrupt (jam) compared to broadcast radio. That said, there have been other wired radio schemes with little to do with state propaganda. Well, depending on what you think of the BBC, I suppose.
Indeed, one of the other cases of a "wired radio" technology I am aware of is the television system in Kingston upon Hull, England. As I understand it, the mid-sized city of Hull is built on geography that made television reception extremely uneven---too many hills and gulches for good coverage. In the 1930s, a company called Broadcast Relay Service operated "piped TV" so that Hull residents could get their television without struggling with reception. This might sound like it's just cable television, and conceptually it's about the same, but the implementation was quite different.
Broadcast Relay, later called Rediffusion, actually used sets of twisted pairs, one to carry each channel. A specially equipped television had a switch box that connected a different pair to the CRT depending on which channel you wanted to watch. This was perhaps technically simpler, but less scalable, than cable television. The scalability problem would not have been evident at the time, because there were very few television channels in the UK, very far into the modern day. My recollection is that a typical Broadcast Relay switch box supported four channels.
The funny thing is, post-war, Broadcast Relay rebranded as Rediffusion and went into the radio business---wired radio. They operated an audio-only service nearly identical to the Third Broadcast, except that homes were connected with multiple pairs for multiple channels. They expanded into various British colonies and other UK cities, and transitioned more into the television and radio rental business... This sounds odd to a US audience, but apparently in the UK it was very common to rent your television and radio.
Of course, by the '70s these wired services had become uncompetitive with the improved broadcast technology and Rediffusion went the way of history, trying out a variety of diversified businesses that never caught on.
I'm not aware of any successful wired radio services in the US for general public consumption, although I would not be at all surprised to learn of one. Similar technology was widely used in the US but generally for background music applications, as in the case of Muzak. Many of these services transitioned over time to other, less-expensive distribution networks like FM subcarrier, mailed-out recorded tapes, and satellite. Of course, today, I assume most background music is done by IP, but I bet there's still at least one chain that uses a hired satellite transponder to distribute music to their stores.
Earlier in the history of telecom, it was typical for major cities to have dedicated wiring run to major businesses for all sorts of specific purposes. Over time, most of these systems moved to leased telephone lines, then to dialed telephone lines, and onto the internet. This could be viewed as the long-term component of the trend of "over the top" or OTT services. Today we talk about telephone and television being OTTd onto internet services, but post World War II they were basically talking about OTTing background music and fire detection onto the telephone system.
And perhaps that leads to another topic: in the not too far future, I will talk about fire telegraphs and their function as perhaps the earliest urban-scale digital communication system. The need to quickly dispatch fire brigades has long driven innovation in telecom, and the fire telegraph is an elegant design which has an ongoing influence today in both fire practice and, well, a few places that just still have fire telegraphs.
 Yes, 38 North calls it "Third Network." I think these are both possible translations of the same Korean term, but "Third Broadcast" is generally more common. This refers, I suppose, to it being third after radio and television.
 Once, in a church, I dealt a bit with an audio reinforcement system that ran at high voltage and had wall panels with volume adjustments for each room. It also used multiple runs to each room to enable a simple matrix arrangement. This was a system installed in, I would guess, the '70s, and I don't think this kind of setup is common today. Last time I had the opportunity to deal with a commercial paging system it was a brand-new one that just used an IP network to link separate paging "nodes" with integrated amplifiers for each zone. It was hilariously complicated and less hilariously expensive.