[HN Gopher] More thoughts on the 1670 modem's weird noises
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More thoughts on the 1670 modem's weird noises
Author : kencausey
Score : 51 points
Date : 2025-03-06 16:18 UTC (6 hours ago)
(HTM) web link (rachelbythebay.com)
(TXT) w3m dump (rachelbythebay.com)
| cenobyte wrote:
| Sounds like your Vic 20 and/or power supply isn't shielded very
| well and its making your speakers do weird things.
| kstrauser wrote:
| Every 1.2 seconds, though?
| hermitdev wrote:
| A thought: maybe the pulse could be some sort of link status
| probing? e.g. "is this thing plugged in?"
| NotYourLawyer wrote:
| Sounds like it could be a mechanical relay clicking. Is it
| actually coming from the speaker?
| alnwlsn wrote:
| According to this [0] schematic, there is a relay present.
| Looks like it's job is to be the hook switch for the phone. The
| recording sounds like the characteristic ping of a reed relay
| to me, so it seems like it's hanging up and then going off hook
| again every 1.2 seconds. Not sure why it would be doing this -
| maybe it's trying to get a dial tone? Is it smart enough for
| that?
|
| 0 -
| https://www.zimmers.net/anonftp/pub/cbm/schematics/misc/1670...
| pjdesno wrote:
| Yes, I believe so. You can't expect dialing to work until
| you've received dial tone, and with older equipment [on the
| phone company side] there might be a significant delay.
| pjdesno wrote:
| I'm surprised that you could get a modem pair to connect with a
| plain wire. An old dial telephone (e.g. a 500 set) was an
| entirely passive device, relying on power from the central office
| end of the wire. The carbon mic didn't need an amplifier - it's
| just a resistor, with resistance varying according to sound
| pressure; a transformer subtracted out the mic from the combined
| signal and sent the difference to the speaker.
|
| I did a quick search on 1670 schematics and came up with very
| little. Note however that several sources identify the USR101
| chip as a ROM - that's clearly not the case, as a schematic shows
| no address or data bus, and you can see a line from the coupling
| transformer to an RC network and several pins on the device. It's
| almost certainly the analog front end and the
| modulator/demodulator.
|
| If you could find a datasheet for that device you'd probably be
| able to figure out how the whole thing works, but I'm guessing
| those were never available to the public and have been lost to
| the mists of time.
| pjdesno wrote:
| See https://hackaday.com/2024/11/19/dial-up-internet-using-
| the-v... for a description of a telephone line simulator.
| timthorn wrote:
| It doesn't even need a wire - wet string is enough:
| https://www.revk.uk/2017/12/its-official-adsl-works-over-wet...
| genter wrote:
| ADSL works completely differently than audio as described by
| pjdesno.
| Cthulhu_ wrote:
| Kids these days with their fancy copper wire, back in MY day
| we used CANS and STRING.
| bongodongobob wrote:
| People used to use barbed wire fence for phone lines.
|
| https://www.atlasobscura.com/articles/barbed-wire-telephone-...
| reaperducer wrote:
| As of 2019, there were places in Arizona and New Mexico where
| telephone line was still run along the top of fenceposts,
| alongside the barbed wire.
| pjdesno wrote:
| Here's 1gbit running over 8 strands of barbed wire:
| https://www.sigcon.com/Pubs/edn/SoGoodBarbedWire.htm
| alnwlsn wrote:
| I'm sure I found the same sources you just did. USR101 is
| surely not a ROM by this [0] schematic because the speaker
| audio goes directly to one of the pins on it - that doesn't
| seem right.
|
| I guess enough signal makes it through T1 to let it work on dry
| lines. That sort of thing isn't unheard of [1]. I used to mess
| around with obsolete dial up modems as a kid and I could
| definitely get them to connect with just wires - sometimes. It
| didn't always work, and some modems would not work with other
| ones, etc.
|
| The weird thing here is the 1670's second long pulses. Maybe it
| would do something else if connected to a real phone line? I'd
| surely try it myself but I only have a 1660 modem on hand.
|
| 0 -
| https://www.zimmers.net/anonftp/pub/cbm/schematics/misc/1670...
|
| 1 - https://en.wikipedia.org/wiki/Sound-powered_telephone
| 3x35r22m4u wrote:
| I used to work at an ISO in 1997 and we tested Mototola Sufr
| 28.8 and USRobotics Sportster 33.6 modems this way, just a
| short pair between them back-to-back. ATX1D on one side, ATA on
| the other, connection established.
|
| However, recently a friend of mine tried to use the same trick
| with two USB modems from Trendnet and it failed. It seems they
| either don't have oomph enough or they rely on 9vdc off-hook
| carrier voltage to work.
| pjdesno wrote:
| Cool! I was an intern with the modem folks at Motorola Codex
| back in the 80s; I probably worked with some of the folks who
| did the data path on that one.
|
| (hmm, actually by that time it's likely that all the Codex
| tech had been killed, and your modem came from the Motorola
| UDS folks in Huntsville. Makes sense - Motorola Codex
| couldn't build a $500 modem to save their lives, never mind a
| $100 one, and a lot of the folks interested in building
| consumer modems left to found Zoom Telephonics)
|
| > 9vdc off-hook carrier voltage to work
|
| In the US it's nominally 48v, although it drops to a small
| fraction of that with any significant (or even fairly
| insignificant) load.
| jcrawfordor wrote:
| Battery power on common battery phones (in a common battery
| phone there isn't actually a battery, the terms here are mostly
| historic) is required for a carbon transmitter to work and, for
| DTMF dialing, it's used by some DTMF keypads. But battery isn't
| actually that important to most modern phones, which use
| ADC/DAC instead of an analog audio setup. Modems are similar,
| they have their own audio electronics that rely on their
| external power source. That's a good practice in general
| because battery power on phone lines is pretty variable and not
| always very clean, so you don't want to run ICs off of it if
| you can avoid it. The current is also too limited to use it for
| much, even indicator lights will run into certification
| problems.
|
| The issue with the USB modems might actually be a bit
| different... more frequent use of single-pair digital business
| telephone systems in the '90s lead to problems with people
| connecting them to conventional modems. Some of these digital
| systems used unusually high battery voltages (sort of a PoE-
| esque method of providing meaningful current to the desk phones
| for their lights and displays), and there was a concern that
| they could damage the ADC on modems. So a lot of newer modems
| actually check the voltage on the line before they connect the
| audio electronics to it, as a safety precaution. There's an AT
| response that indicates that the modem is unhappy with the line
| voltage and I tenuously remember that at least some modems have
| an AT command to defeat this protection and make them connect
| anyway. Digital Line Detect is what the feature is usually
| called, but you might more generally call it a line voltage
| check.
| toast0 wrote:
| Some modems want to detect the line before they will work
| (voltage and/or dialtone), but if you can get past that with
| settings or a battery/etc, I'd think you can just listen and
| send audio on a plain wire and it should work.
| hinkley wrote:
| Sounds a bit like constructive interference between two sound
| sources that are almost the same frequency but not.
|
| Or RC "ringing". That click stopping the sound makes me curious
| to see a plot of bytes per second sent and received. Is the modem
| resetting itself every time it clicks and introducing a small
| pause or data loss? Or is it completely divorced from the sound.
|
| These modems don't have fans built in right? If you isolate the
| speaker does it still make the noise?
| einr wrote:
| _That click stopping the sound makes me curious to see a plot
| of bytes per second sent and received. Is the modem resetting
| itself every time it clicks and introducing a small pause or
| data loss?_
|
| There is no data being sent or received yet when it makes this
| sound; as soon as it dials and starts handshaking it goes away.
| You can hear that here:
| https://www.youtube.com/watch?v=S-ChICic2TA
|
| EDIT: actually that's only partially true -- in the video above
| it's still making the noise when the dialling is happening. It
| disappears as soon as it detects a carrier from the other
| modem. This is weird because the click really sounds like a
| relay and the function of the relay should be to hang up the
| connection. Obviously, hanging up while in the middle of
| dialling is not a good idea.
| pjdesno wrote:
| It might be trying to detect dial tone, then hanging up and
| trying again after a second or so - the click would be the
| mechanical relay you see in the schematic. (note that standard
| phone lines don't send dial tone until they detect an off-hook
| condition, i.e. current flowing down the line. Even then, older
| exchanges had a limited number of "dialers", and wouldn't send
| dial tone until one had been connected to the line)
| cluckindan wrote:
| Testing for echo? Impedance calibration?
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