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Calling the heart line
October 23, 2009 8:02 PM   Subscribe

Acoustic modem + cell phone. Can it be done? More inside...

We just left the NICU with our new daughter (YAY!) but they've given us a heart monitor we might have to use that communicates with the hospital via an old school acoustic modem not wholly unlike this one (erm...wot?). We have a house full of iPhones, but no land line. Will we need to get one for this gadget (which I will gleefully do without a moment's hesitation) or is there a way to make it work with our iPhones? Thanks!
posted by littlerobothead to Computers & Internet (10 answers total)
No, that won't work with your cell phone. Among other things, cell phones use different and much more restrictive audio compression schemes than landlines, and modems of that type are designed to deal with the characteristics of landline audio transmission.
posted by Juffo-Wup at 8:11 PM on October 23, 2009

Possibly, but the connection would be really slow and flaky. I seem to recall reading that it was at least possible, but it might depend on how forgiving the monitor's modem is.
posted by meta_eli at 8:17 PM on October 23, 2009

This phreaker's guide seems to indicate you could get online at up to 1200 baud.
posted by meta_eli at 8:19 PM on October 23, 2009

It might have worked at low connection rates on analog cell phones but not with digital, and everything is digital now.
posted by Juffo-Wup at 8:31 PM on October 23, 2009

No, it's not possible. The bandpass and voice compression used to reduce the signal's bandwidth on a cellphone just murders acoustic-coupled modems.

I know this from personal experience, as I have tried it.
posted by Netzapper at 8:48 PM on October 23, 2009

Congratulations and welcome to the parent club. I am a proud parent of 2 daughters, an iPhone, and lots of experience with modems. My advice is not to count on it at all. Rather make arrangements with a friendly neighbor, or order a regular phone for the interim.

posted by bright77blue at 8:55 PM on October 23, 2009

There's no reason why an analog modem wouldn't work over a digital connection. It might not work at the full bandwidth, but it should be able to communicate at some speed. Digital compression artifacts are no different then analog noise. Something like skype might work better though.
posted by delmoi at 9:08 PM on October 23, 2009

Ask at before saying no to this option. If anyone would know, it would be them.
posted by fantasticninety at 9:48 PM on October 23, 2009

There's no reason why an analog modem wouldn't work over a digital connection. It might not work at the full bandwidth, but it should be able to communicate at some speed. Digital compression artifacts are no different then analog noise.

There is a reason. The codecs used in GSM cell phones are specifically designed to encode speech while ignoring non-speech audio. In fact, they're designed so that the coding artifacts are specifically pushed into the non-speech portion of the signal. Whereas conventional acoustic modem modulation schemes are designed to work over the entire bandwidth of a POTS line.

This is not like MP3 or OGG, where audio redundancy is reduced to achieve compression. Rather the codec is essentially incapable of producing many non-speech sound. This is why, for instance, hold music sounds bloody terrible over a cellphone, even while it sounds okay over a landline phone--which just uses a bandpass to reduce the bandwidth. The entire algorithm is designed with the human ear and speech perception in mind. It is not a general purpose audio codec. And all of this is ignoring the other speech-oriented processing that goes on in a cellphone, like echo-cancellation.

Even assuming the codec was capable of handling non-speech sound viably, the maximum, perfect-link bandwidth of the GSM voice channel is 13 kbits/s. That's your Shannon limit for the channel. The sample rate for EFR GSM coding is 8000 samples/s. Your Nyquist frequency is then 4000 samples/s. So at best, you can do 4000 modulations per second.

Now, that doesn't sound too bad--you could definitely do 4000 bits/s with FSK, or significantly more with PSK... except that the signal is being compressed. The actual number of successfully-demodulated signal frequency changes is going to be a function of the interaction between your modem and your compression scheme. If the compression scheme picks a bad codeword, then you'll have to detect the error and retransmit. Since this is not random, but rather deterministic, the chance that your retransmission will get the right codeword is actually pretty low.

All of this boils down to the following: if the error and transmission rate of the modem is determining the survival of an infant, don't fucking fuck with it. If we were talking in the abstract, I'd be inclined to agree with you, delmoi, that some modulation scheme could be found that would transmit at at least 4 kbits/s. But given that the OP has no control over the modulation scheme, and his child's life is at stake, the claim that "it should [work] at some speed" is downright irresponsible.
posted by Netzapper at 10:29 PM on October 23, 2009 [6 favorites]

Back in the day, many cell phones had an option to force them to use the analog cell network in order to make modem and fax use possible. (The AMPS network used narrowband FM for the voice channel, no funky compression.) This pretty much went away when newer digital networks (GSM, etc) showed up with support for actual data transmission.

I don't think a modern modem has any protocols which use a symbol rate slower than 600 baud. GSM transmits frames at 50 frames/sec (or 200 subframes/sec), so anything that tries to get data through that codec at faster than 50/200 baud is going to have to be very much adapted to the GSM codec. Which a normal modem won't be. If you can crank the speed down to 150 bps or 75 bps, there's a possibility it'd work, I guess.

I think the right answer is just to get a landline. They're probably still more reliable than cell phones anyway.
posted by hattifattener at 11:04 PM on October 23, 2009

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