Old dial up modem noise explanation

Question

I am looking for a technical documentation on the connection procedure used on the old Dial Up modem that generate this sound: http://www.youtube.com/watch?v=iHW1ho8L7V8

I can't find anything on Google about this, do you know where can I find documentation about this ?

Sure it's a little bit useless today, but I often think of "What is it doing with this noise ? What is transmitted ?"

Thank you

What's with the "old"? It's not that long ago that they were the most common way to connect to a remote network. Now, acoustic couplers might be getting to the point where some might consider them to be a little oldish. — John Gardeniers, Jul 21 '10 at 03:47
Just a note that while this knowledge may at first appear a little useless these days it must be remembered that the same technology still exists in FAX machines (only the handshake is a little different). If you are familiar with the sounds it can be helpful when diagnosing connection issues. — John Gardeniers, Jul 21 '10 at 05:42
It's actually not all that useless - many places still have an analog modem as an emergency terminal access. — voretaq7, Jan 31 '13 at 18:59

score 11 · Answer 1 · edited Jan 31 '13 at 18:57

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What you're listening to is the analog negotiation of various features between two modems. Those of us of a certain age can identify baud rate from just the noise, even 15 years after we stopped regular usage of the things. Ahem. What I can't tell you off the top of my head are the protocol versions.

Wikipedia has a nice list of these, as they evolved a LOT over the years. 100-baud is a very different critter than the 56K we had when dialup died.

http://en.wikipedia.org/wiki/ITU-T_V-Series_Recommendations

And if you really do want the technical definitions for all of this, the ITU still has these published:

http://www.itu.int/rec/T-REC-V/en

There were a few other protocols in use in line negotiations, but I've lost my modem manuals a move or two ago and can't look them up. Digging around I find a familiar friend, MNP as well as a host of vendor-specific protocols that never took off very far.

edited Jan 31 '13 at 18:57

voretaq7

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answered Jul 21 '10 at 02:13

sysadmin1138

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Oh, you wanted to know the higher-level function of what it was doing... my mistake. I'll leave my answer for the curious though. – Mark Henderson Jul 21 '10 at 02:57
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I remember being able to hit "retry" before my computer told me the connection speed just from knowing the *sound* of the failed "high speed" 56K handshake going down to 28.8K – Chris Nava Jul 21 '10 at 03:17
Next you'll claim there was once such a thing as "acoustic couplers". Now get off my lawn! – Dennis Williamson Jul 21 '10 at 03:46
Aahh, good old Hackers, circa 1994, where all mad hacks were done from a public phonebooth, a laptop covered in stickers and an acoustic coupler. And not a single person looks at you strangely. – Mark Henderson Jul 21 '10 at 04:12
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@Chris Nava FOR THAT REASON I never understood people who turned off their modem speaker. – sysadmin1138 Jul 21 '10 at 04:20
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Someone even [took the time to run a dial up sequence through a frequency spectrum analyzer](http://i.stack.imgur.com/OwPye.jpg) -- It's really cool to look at, and has an overview of what's going on.. – voretaq7 Jan 31 '13 at 18:57

score 8 · Answer 2 · answered Jul 21 '10 at 02:22

What you're asking is a simple question with a very complex answer.

Each tone you're hearing when an oldschool modem dials up is indicative of a set of data that is being transmitted. Sound pitch is measured in Hz. Your average adult can hear from around 20Hz to 20,000Hz (but we can't always distinguish the difference between 20Hz and 21Hz).

So say for example, a 20Hz pitch means 0 and a 21Hz pitch means 1. So to transmit

00000110

You would transmit 20Hz 20Hz 20Hz 20Hz 20Hz 21Hz 21Hz 20Hz. At something rediculously slow like 1-baud, that would take 8 seconds to transmit that data.

But why bother, when you can say that:

30Hz = 00000000
31Hz = 00000001
32Hz = 00000010
33Hz = 00000011
34Hz = 00000100
35Hz = 00000101
36Hz = 00000110
37Hz = 00000111

etc so on and so forth. So the same dataset (00000110) can be represented as 36Hz and you've transmitted 8 bits in 1 second, rather than 8. Congratulations, you've sent 8 bits of information in a single cycle.

Now a MODEM over a crappy telephone line, can only distinguish so many different freqencies, and there needs to be error checking etc in place, but to transmit:

000000110000011100000100

You're going to need a lot of different audio pitches played after eachother super fast (56,000 of them per second in the case of a 56K modem). When you play 56,000 different tones within the space of a second, that dialup sound is what you get.

@John, Then I'll have to explain Phase-Shift keying and I just can't be bothered right now. That's assuming that people understand what moudulation is in the first place. Even though modulation is mentioned in the term MODEM twice. I think most pople assume MODEM is a noun, not an acronym. — Mark Henderson, Jul 21 '10 at 04:09
My mistake, not phase-shift keying (That's for digital modulation), just phase modulation (thanks Wikipedia) — Mark Henderson, Jul 21 '10 at 04:10
Most people can hear to around 20kHz, but the traditional/analog telephone system used at the time of the modems in question was specced only for 4kHz, and since what telco called 'outside plant' was built to last decades in often harsh conditions you often got only 3500Hz or less. FSK with 1 bit per baud = 2 frequencies only went up to 1200bps or maybe 2400; everything higher used more complicated modulations (some of them with error correction). — dave_thompson_085, May 06 '21 at 03:52

score 3 · Accepted Answer · edited May 05 '21 at 18:23

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I just found a graph explaining everything: https://upload.wikimedia.org/wikipedia/commons/a/ad/Dial_up_modem_noises_explained_final.png

edited May 05 '21 at 18:23

Matteo Italia

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answered Jan 31 '13 at 11:05

Kedare

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Old dial up modem noise explanation

3 Answers3