Ancient Cable Modem Reveals Its RF Secrets

Most reverse engineering projects we see around here have some sort of practical endpoint in mind. Usually, but not always. Reverse-engineering a 40-year-old cable modem probably serves no practical end, except for the simple pleasure of understanding how 1980s tech worked.

You’ll be forgiven if the NABU Network, the source of the modem [Jared Boone] tears into, sounds unfamiliar; it only existed from 1982 to 1985 and primarily operated in Ottawa, Canada. It’s pretty interesting though, especially the Z80-based computer that was part of the package. The modem itself is a boxy affair bearing all the hallmarks of 1980s tech. [Jared]’s inspection revealed a power supply with a big transformer, a main logic board, and a mysterious shielded section with all the RF circuits, which is the focus of the video below.

Using a signal generator, a spectrum analyzer, and an oscilloscope, not to mention the PCB silkscreen and component markings, [Jared] built a block diagram of the circuit and determined the important frequencies for things like the local oscillator. He worked through the RF section, discovering what each compartment does, with the most interesting one probably being the quadrature demodulator. But things took a decidedly digital twist in the last compartment, where the modulated RF is turned into digital data with a couple of 7400-series chips, some comparators, and a crystal oscillator.

This tour of 80s tech and the methods [Jared] used to figure out what’s going on in this box were pretty impressive. There’s more to come on this project, including recreating the original signal with SDRs. In the mean time, if this put you in the mood for other videotext systems of the 80s, you might enjoy this Minitel terminal teardown.

27 thoughts on “Ancient Cable Modem Reveals Its RF Secrets

  1. When did a 40 year device (the 80s) become Ancient ;) . There are quite a few of us that are still living you know that lived through the 80s :) . Now back 500 BC …

    Interesting project though.

      1. Most of my cars were around 1995, and I’m not even 35…
        Currently no car (living in Vienna with very good public transport), but I miss the sound of the _5_ cylinder 2.5l Diesel in the good old Audi 100/A6 :-)

      2. At least in the states I’ve lived in, 25 years post manufacturing, a car qualifies as a “Classic”, and no longer has to go through emissions inspections. You can get special plates, too.

        I had a ’97 Audi A4 Quattro until last year (only 38k miles!), but I was too lazy to go through the process, and then I donated it to Habitat for Humanity after inheriting two much newer cars.

        Anyway, in these cases, I think they are simply talking about the legal classification, regardless of whether the car actually subjectively qualifies.

    1. People have a weird idea of time since a while. Stuff from the 90s is now vintage, when that used to refer to things from Victorian era or similar distant times.

      Ancient referred to things like the ancient Greeks. Maybe it’s US American influence and their very short history.

      Older usually refers to people in their 70s+, now it refers to people who aren’t even middle age (and people push back the time when middle age starts). Weird development.

      1. The meanings of terms do change over time, I suppose.
        Especially computer use a different time scale. Especially in the 90s, a PC often was being obsolete within 6 months.

        “Older usually refers to people in their 70s+, now it refers to people who aren’t even middle age (and people push back the time when middle age starts). Weird development.”

        Understandable. We live in fast-paced times in which we are being afraid of reaching an age in which we become helpless. Back then, we had families that cared for us. Now we’re all selfish and don’t want to bother with this anymore. Once you’re nolonger able to work, you’re in trouble. Especially the US job market is somewhat inhumane, I think. On the other hand, even if you can work, it barely is enough for a living. Back in the ~50s, people still had their own house and backyard with an apple tree.

        And a few hundreds years before, people passed away age ~30. No kidding. The lack of hygiene and the hard work on the fields had worn out those people really quickly.

        What’s necessary is more balance in life. People need more time to rest, to recover. A healthy amount of sleep, with good sleep quality. The western model of making career is poisonous. It wears out people, let them age quickly. In mid-late 20th century, the balance wasn’t ideal, but better. People took time to feel their surroundings. Watching the leaves falling down a tree in autumn etc. People who read a book, were willing to increase their knowledge (autodidacts).

        That being said, I suppose you’re from the b generation?
        Because this would explain that “life just starts with 30” attitude.
        It surely was somewhat true in the 1950s-1980s that 30 is no age.
        And it still holds true in some ways, maybe.
        However, you may realize that with 30+ you can’t do a lot of things anymore.
        Do professional sports (soccer or football), a career at military, become an astronaut..
        Your free time is something else entirely. You can still have fun no matter the age. But at 30, you’re nolonger able to be competition.

        Biological, the degeneration process of your body starts with ~25 already.
        Back in the dark ages, girls stopped being working as prostit. when they reached puberty and could get pregnant.
        When they reach puberty, girls have already lost the majority of their egg cells. Long story short, a lot of things aren’t the way we want to see them.

    1. Could have been better even if fibre had gotten the support it deserved.
      In my country, fibre communication lost to cable TV infrastructure.
      No idea why cable gets so much attention. Back in the 90s, in my country, it was all about satellite TV. Gratefully. Cable is evil. Imho.

      1. Cable got upgrades too. When I moved to my current address 15 years ago I got around 12 mbps on cable, but I jumped to fibre as soon as it was available. Just recently I was helping a neighbour with their internet access and expected that their ‘old’ cable connection bandwidth was the issue (poor streaming/VOIP performance). A quick speedtest revealed the cable connection was over 250 mbps i.e. not the issue

        1. I understand, what I meant was that satellite TV meant freedom back in the day. I was thinking of Astra satellites in particular. The programme could be received everywhere in sight of a given satellite. Censorship and national borders didn’t exist like it’s technically possible with cable. So you could watch news programs and sports programs from other countries, too. Often, multiple languages could be selected, too, by rotating a knob on the satellite receiver. Satellite radio also was a thing. It was digital, sometimes, even (PCM?). Last but not least, many cable stations did use TV satellites as their source. Local stations, excluded, of course. Some smaller TV stations didn’t have satellite broadcast.

          That being said, in our recent times the cable infrastructure has a good side, as well. It’s like a huge 10Base2 Ethernet network. The thick coaxial cable has excellent shielding and can provide hundreds of MHz of bandwidth (with loss increasing at higher frequencies). Only downside is that it’s a shared medium, maybe.

        2. Well done! 😎👍 Tip: You can attach snap ferrites to power cords of modems and coaxial or DSL cables (but don’t use ferrites that are too tight, power cords shouldn’t be squeezed). This will reduce noise both ways. It will reduce RFI in your house and reduce sheath currents. The only “harm” it could possibly do is dampening a signal too much (if too many are installed). The modem’s GUI has spectrum information, which help to check the situation. 🙂

  2. Interesting! I never knew cable modems were around in 1983. It’s really need to see the design, it’s a cable modem with no upstream capabilities. It just receives a data stream non stop and you pick out what you want to save / capture on the computer with software. It’s truly a broadcast (haha!) network topology……

    1. Intellivision PlayCable.

      I could have swore there was a similar service for the Atari 2600, but all I am finding is GameLine which was a POTS-based dialup service.

  3. This reminds me of the old analog modems. They started as relatively simple devices, Then became ever more complex and contained DSP’s for the (de) modulation. And then at some point (I guess when 80486 was common and 56k6 modems) PC’s became powerful enough to take over all the calculations and the cost of such modems dropped sharply, because all they contained was some simple circuits. Some audio I/O, a way to recognize and generate ringtones, and galvanic isolation to the POTS line.

    1. There also were RTTY and Packet-Radio.
      The modems were using AFSK usually at 45,45 to 1200 Baud. Popular tone pairs were 1200 and 2200 Hz.
      ICs like X2211 were popular decoder ICs.

      In the 90s,so called hamcomm universal modems were a thing among amateurs, too.
      They consisted of an 741 op-amp as a comparator.
      Basic devices could be built for a few cents.
      They were data-slicers, really. Converting AF into square wave pulses of varying length. The software was were the magic happened.

      That was in middle of the 486 era, really. When power users had VLB VGA graphics cards, data/fax modems and a 486DX2-66 with 8 to 16 MB of RAM. And an stereotypical 14″ VGA monitor (flight sim surely fans had bigger ones). 😃

    2. [Sorry for the duplicate message, this comment first appeared under an unrelated comment, despite my best effort. Hopefully this one will show up in the right place.]

      If only there was a guide about how to do that. Oh wait, there is! :-)

      I’m pretty proficient with KiCAD, so I’ll try what you suggest. I fear it might be pretty slow-going for me, compared to just drawing the schematic directly and marking off the completed traces on my tablet. But there would be a big benefit to being able to check against the netlist at the end, after I’ve started understanding the circuit and done a bunch of cleanup on the schematic.

  4. One thing I do not understand is at 18:00 where he reverse engineered the circuit and used a generic paint program. You can directly load the pictures into KiCad with: PCB Editor / Place / Add Reference Image. An external graphics program is still useful to do some basic image manipulation such as mirroring the image from the bottom layer, maybe reversing lens distortion or do contrast enhancement. The quality of the pictures is quite important, but the video quality in the youtube video is already quite sufficient.

    1. For RTTY or the HamCom software, surely. If you really want, you can torture those old metal bricks with STS Orbit Plus, as well. An Amstrad PC1640 with EGA technically can run that. Programs like JV-Fax 7 or DL4SAW’s GSHPC are happy about an 80286+ with Super VGA or VBE graphics card, though. 😁

    1. I think it is a coincidence. They used an 8 MHz crystal, and divided it down by a nice round binary number, 512. Then they multiplied it back up via the divide-by-64 prescaler and the frequency synthesizer N divider. 8 MHz / 512 * 64 is 1 MHz. So the N divider value is the resulting frequency in MHz of the local oscillator. Very elegant! The microcontroller only has about 1K of program memory and 64 bytes or so of RAM. So it doesn’t have many resources to do complex math. Being able to tune in increments of 1 MHz when cable TV channels are spaced 6 MHz apart would keep the micocontroller code very simple.

    2. If only there was a guide about how to do that. Oh wait, there is! :-)

      I’m pretty proficient with KiCAD, so I’ll try what you suggest. I fear it might be pretty slow-going for me, compared to just drawing the schematic directly and marking off the completed traces on my tablet. But there would be a big benefit to being able to check against the netlist at the end, after I’ve started understanding the circuit and done a bunch of cleanup on the schematic.

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