Show HN: I reverse engineered my cable modem and turned it into an SDR

If that mishap on an SOIC chip occurs, don’t fret, you can usually shave off a bit of the plastic and get enough pin surface to solder a jumper to VCC.

An alternate method is to follow the VCC trace and cut it, then solder your own wire to the unlifted lead and a nearby capacitor with a switch. Beware that you might power more than the chip if the trace continues, but that’s usually fine. Sometimes this is a good thing because you still power pull-up resistors.

Yet another option is to place a glob of solder across 4 pins, heat them all up, and lift half the chip. The other 4 legs will be hot and soft enough to bend easily.

Finally for the non-VCC connections, could follow the traces and find easier places to solder to rather than a bunch of side-by-side leads.

> Enter '1', '2', or 'p' within 2 seconds or take default...

Wait, tens or hundreds of millions of cable modem startups are delayed by 2s on each start just in case somebody has soldered up a serial interface and wants to load a different image?

I bought these boxes which use the Coax at very high frequencies to do Ethernet between sides of my house using MoCa which is used by TiVos and newer set top boxes. I had trouble understanding "how is this secure" when my modem is also connected to the same coax and this is clearly not behind my firewall. Well, of course, you filter out the signal from leaving through the Coax which provides you cable!

I would rather just run ethernet and keep it behind a firewall, but this reduces punching holes in the walls for now.

A buddy an I had this idea ~6-7 years ago, and had a couple of the zoom docsis 3 modems handy. They have something like a 100Mhz fairly high bit depth A/D on them. At the time the price/bandwidth was crazy (and still is if someone managed to pull it off with a modern docsis 3.1 system). I got some boot logs/etc but didn't get very far into reverse engineering the chip. Its too bad that the chip vendors go to such lengths to hide what is basically mostly repackaged 3rd party IP from designware/arm/etc.

Current DOCSIS has ~200Mhz sampling per OFDM channel, given a 3rd party tuner in front of it, the results might be pretty crazy.

You mentioned you were getting distortion/noise when downsampling below 928kHz. Are you lowpass-filtering before you decimate (but after you demodulate)? Otherwise you will get all kinds of aliasing.

Looks like FM radio stations have a bandwidth of 200kHz so you should be able to lowpass with a cutoff of around 50kHz then decimate to 232kHz.

That's some amazing work in my eyes, how long did it take?

Glares at system that ran RS232 levels (-12 & +12) into 5V Atmel MCU 24/7 for a couple years successfully

Sure, the buffers will have some random amount of data in, but by pre-erasing them, it ought to be possible to figure out where it got up to.

Failing that, just keep poking registers - there will certainly be a continuous capture mode, since that's what it will use for real operations.

To further boost performance, it might be worth skipping past interrupts and going straight for DMA, as the CPU probably doesn't need to be doing anything. I imagine figuring out how that works on an undocumented chip would be quite tricky though.

Specifically, I have a basement full of old (some non-working) hardware for various gadgets/computers/electronics and I'd love to upcycle them into ... something else. Instructables and WikiHow are the usual places, but it's usually things that require "the thing you're building"[0] to build or require buying so many new components that by the time your done you've bought a product minus a few motors.

I yearn for the "Make X do Y" with a small number of inexpensive tools (preferably ones I own, but that's a tricky proposition) where "X" is something common, or obsolete and "Y" is something outside of or is an upgrade to it's design[1]. I love doing this sort of thing and I find that about half the time when I'm working on Pi/Arduino projects I have way more parts than I realize -- I can't believe I purchased any loose low power LEDs (IR, colored...) I have a whole box of remotes/miscellania that should really be in the garbage for how useless it is ... until I need a 1.5V LED, have only 5V+, and can't wait a day for shipping. I started salvaging electronics before trashing them. It'd be nice if I could look up the whole obsolete device and see what I can use more of it in... plus, at least for me, I'd throw less away (and store less in boxes).

[0] My go-to is the small number of "Blu-Ray Engraver/Laser Cutters". Every one I could find (A) required a laser cutter, or (B) many of the laser-cut parts required a laser cutter that was bigger than the one being built, so you couldn't even sell the more expensive laser cutter if you wish to be able to produce a second one. It's a small issue in that there are still other parts that need to be purchased, but you have to buy a laser cutter to make the custom, $0.01 in pressed board, set of parts. The others are nearly every upcycled RC-Car.

[1] I had a friend who designed custom sub-woofers for cars when I was younger. He had a Yamaha off-the-shelf sub that he modified to the tune of $250 and some of his own time. It put my $1000 sub to shame. I can't remember all the specifics of the modification, but I know he used the original amp board and some upgraded MOSFETS but didn't touch the 10" speaker or the cabinet (which was what he spent most of his time on, typically).

https://www.rtl-sdr.com/about-rtl-sdr/ https://www.reddit.com/r/RTLSDR/

I tried to search for this forum thread, but my google-fu must be failing me. Does anyone know where it is?

>It can currently stream I/Q data to a peer via TCP

mean? Does it somehow stream data over the modem's ethernet connection?

And a cheap one too!!!

About two years ago I realized I was doing the opposite sort-of without realizing it. I'd want to do something given a set of constraints (say, create some app in some specific programming language or for some platform), google it, find that there was some loose chatter around the idea and I'd dismiss it thinking "well, 'The Internet' hasn't done it/doesn't think it's possible, so I shouldn't bother".

Maybe it's a form of imposter syndrome. For me, I think it's more complicated:

I'd reach a point in a framework/subset of "whatever stack/language I was focused on at that time" where I was running out of obvious "new things to try", so I'd Google and find the same list; 'The Internet' decided for me that there's really not anything else of use to do with these this framework, let's learn a new one.

I need to "do X with Y", I Google it and find "Y" nowhere, but find "Z" everywhere. It's a bad fit but that's how 'The Internet' does it.

The most common, and one I specifically guard against and it happens consistently with endeavors like this: I want to make "X" do "Y" knowing full well that "Y" is not designed for (or more frequently specifically designed to prevent me from) doing "Y". After wading through replies to countless other people daring to ask the question in a forum -- the usual: "You don't know what you're talking about, can't be done, you shouldn't try it", "Why would you even want to do that when you can get 'Y' for $.$$ on eBay?", "Google it" (I did, that's why I'm here after 4 pages of clicking). Lovely how every question has 3-4 unhelpful-non-answers in one of these categories.

I changed my approach more than a decade ago while trying to reverse engineer the obfuscation a large telecom vendor used for their mobile broadband password storage in the Windows Registry[0] and succeeded in writing an "obfuscator" in a day from first introduction to deployed solution. Ever since then, I trust my instinct when I have expertise. When I do not, I read a very positive signal when "a lot of are people asking if something can be done" IIF there isn't a good answer to why it "can't be done" and any reasons why it "shouldn't be done" don't apply to the issue at hand.

[0] I recall, at the time, most of the answers were some form of "IANAL... but..." -- our purpose was to assign a random password to a user's account, then install the client to the user's company-assigned laptop with that password pre-populated (I didn't come up with the idea, I just had to figure out how to do it). The remainder of the answers were variants of "you can't crack AES" and "IM L33t H4x0R - WiLl CrAcK 4U". It was very clearly not AES or anything resembling a hash. It took so little effort to figure out that I would have saved time by "Starting Before Googling". Nobody (searchable) had tried. Probably nobody had a (legitimate) reason to try. It was just surprising how many people had a reason to "not try" while also discouraging others from trying with either wrong information or no information at all.