More thoughts on the 1670 modem's weird noises
I recently wrote about getting a pair of VIC-20s and attached 1670 modems to connect over a length of line cord with nothing else in the middle. At the end, I tossed out a side note about the odd noises these things make in dial mode.
Some readers have been asking if it could be a relay, like for pulse dialing, and so I figured I should try to clarify as much as I can.
First, here's a recording from the other day's shenanigans, complete with me bopping the keys on the less-reliable computer to make it Do Stuff.
For those who can't hear it for whatever reason: there's some background humming in the whole recording from other devices in the room. But, then there's a source of white noise that's much closer and louder - that's the modem joining in. It "dips" every now and then and is replaced with the "pa-tink" noise I mentioned and then it comes back up to the previous volume level. This happens a few times, and I smack a key or two, and then the recording ends.
Looking in Audacity, it hits at about 1.2s, 2.4s, 3.6s, and 4.8s. That's curious, right? About 1.2 seconds each time?
To be crystal clear: this is not pulse dialing. That sounds completely different, and besides, I didn't tell the modem to dial anything. I sent it "ATD" which basically means "go into originate mode without actually trying to generate any pulse or tone signals - just grab the line and start listening".
Another rejected hypothesis: this is not just something broken in my originating test unit here. Both of them do it when they're in that role. It's also not a case of "they're almost 40 years old and they both degraded into this state", because the original I had way back in 1988 definitely did this, too. That's how it got stuck in my head in the first place!
One thing I don't know is whether this is solely a function of the local speaker, or if it's actually putting that clicking onto the line. I'm guessing it's just the speaker, but I'm also not feeling like setting up the test rig and somehow finding a way to listen to one side and not the other.
For whatever reason, the 1670 makes those noises when it's sitting there waiting for an answer tone from the other side, and I've never understood why. Obviously, at this point, it doesn't matter, but it's just one of life's little nerd mysteries.