Landline text transmission - Page 1
Forum section: electronics / projects

author: Dave101
author_rank: Newbie
timestamp: June 10, 2015, 12:10:39 pm
content: Good day everyone:I am considering to take on a new and fun project. I would like to make use of the free landline services provided by our service provider. Phone calls are free after 8:00 PM, but nobody uses it, so for a start, I would like to use the line to transmit text data to my friends, this could save some internet cap, and serve as a backup for sending messages or email, this is in my opinion also more private when properly encrypted. (This is a long shot; but maybe one day, the system can be so refined that I could save more significant amounts of data.) But I'm more concerned about proving the concept of text transmission, and building a working model rather than saving money. So, I want to gain experience, and learn about all the small stuff that makes the real thing different from the basic concept on paper, like noise cancellation, data reliability, signal timing, incoming signal amplification if necessary etc.I want to know what problems I might expect, and how I should overcome them.Here's the idea:For the example, assume we're only interested in the characters of the alphabet, the real design will have the other characters like punctuation etc. (I saw that there's command characters too like the ones that signals the start and end of transmission. This gives me the idea of sending the text to a basic HTML interpreter and display simple webpages. I am also interested in sending images later. But this is beyond the scope for now.) I want to transmit the characters by combination of different frequencies. The alphabet has 26 characters, and the smallest power of 2 that is greater than 26 is 32, which means we need 5 different frequencies.(I want to know more or less which frequencies will have the best quality over the phone line and how far apart they typically should be.)What should the signal duration be for one character?The characters will be translated to binary as follows:A = 00001B = 00010C = 00011D = 00100:etc.Example frequencies:1 1 1 1 10 0 0 0 0(100Hz) (200Hz) (300Hz) (400Hz) (500Hz)Each bit will have a different frequency, and the characters are made by adding the frequency signals as in above, to create a complex signal. I want to use the audio output of a PC, or an Arduino analog output and then transmit the signal through the microphone wires of an old telephone plugged into the line. At my friend's home, the signal will be filtered into its component frequencies (Writing a program would be the easiest way to filter the frequencies in my opinion, like using the fast fourier transform,maybe someone knows an easier algorithm). The component frequencies will then be either on or off, so the binary signal can be translated back to text.So I want to know what are the biggest problems I might face, and how I can overcome them.What books do you recommend to help me on this topic?

author: c4757p
author_rank: Super Contributor
timestamp: June 10, 2015, 12:28:04 pm
content: Why invent your own scheme? This is how most people connected to the Internet for a while. The technology already exists, do some research. Or hell, buy some old modems.

author: Dave101
author_rank: Newbie
timestamp: June 10, 2015, 12:32:23 pm
content: I know, but I want to learn how to build it myself. I can learn a lot of useful lessons that I can apply in future projects.

author: jwm_
author_rank: Frequent Contributor
timestamp: June 10, 2015, 12:37:08 pm
content: I am assuming you don't want to use a modem (which can be found in any trash heap nowadays..) in favor of a DIY solution for fun.Best thing would do tones that we already know work well over the phone. DTMF dial tones. You can get single chips in nice breadboardable dip packages that will decode DTMF and spit it out as binary, the nice advantage of this is that even if someone doesn't have an encoder on their side, they can manually press keys on the phone to make stuff happen.If you just search for DTMF decoder on ebay, you will find tons of chips (vintage and modern) to do it as well as random modules. This one seems like it will do the deal (have not used this particular one) and costs 40 cents or so on ebay when buying ten or a buck or so from other suppliers.http://www.microsemi.com/document-portal/doc_download/127041-mt8870d-datasheet-oct2006John

author: Kalvin
author_rank: Super Contributor
timestamp: June 10, 2015, 12:53:57 pm
content: Simples solution is to use FSK-modulation with sufficient error correction. In the transmitting end you just need to generate two different frequencies depending of whether you are transmitting 1 or 0. The receiver will contain two bandpass filters (implemented either analog or digital). Just compare the bandpass signals which has higher enegry, thus you have received 1 to 0. Very simple, proven technology.

author: Dave101
author_rank: Newbie
timestamp: June 10, 2015, 01:30:25 pm
content: Thanks, the datasheet answers the question about the signal duration for the DTMF chip, now FSK sounds like a very good idea. So, I'm going to use FSK.

author: Kalvin
author_rank: Super Contributor
timestamp: June 10, 2015, 01:50:13 pm
content: With FSK the attainable data rate will be 300 bit/s up to 1200 bit/s. Higher data rates require more advanced and more complicated modulation techniques.

author: mark03
author_rank: Frequent Contributor
timestamp: June 10, 2015, 04:26:14 pm
content: If you could find some old acoustic coupler modems (designed to fit a telephone handset), you could use that as a fun chassis even if you scrap the old circuit and build your own for educational purposes. Go for the retro look! O0 Although, you would also need to find old-fashioned telephones to distribute with the acoustic couplers

author: JackOfVA
author_rank: Unknown
timestamp: June 10, 2015, 04:41:46 pm
content: The modulation methodology suggested in the original post is known as MFSK these days, or multiple tone frequency shift keying. Lots of references on it via the 'net.Selecting bit tones that are harmonically related to each other is a bad idea since it puts a premium on distortionless tone generation and transmission. (How do you know that a detected tone at say 400 Hz is not the 2nd harmonic of a transmitted 200 Hz tone or the 3rd order intermodulation product of, for example, 1000 Hz and 1600 Hz or the 2nd order IMD product of 1000 Hz and 600 Hz. Hence one tries to select a set of tone frequencies that are not harmonically related or that are not generated via low order intermodulation products.)MFSK is fine for tinkering and experimenting and has advantages when sent over channels with selective fading, frequency dispersion and impulse noise (as is the usual case with radio data transmission), particularly when low or medium data rates are all that is necessary.For serious speed data transmission over the twisted pair telephone network, there are far better, more modern approaches.

author: m100
author_rank: Regular Contributor
timestamp: June 10, 2015, 05:14:22 pm
content: I'd hazard a guess that most of the DTMF chips that were commonplace in the 80's are long obsolete, nothing that an arduino couldn't do of course but even as a fun project the question has to be why now, just for the retro element? I can't even recall the last time I used a dial up modem either point to point or to an ISP's point of presence, maybe late 1990's. Interfacing to the phone line in a 'legal' way is also a PITA, back in the day of dial up many manufacturers of systems that needed to communicate across the public network bought off the shelf modules with RS232 connections rather than roll their own and get them appoved .If I wanted a low rate 'local area' data channel then I'd not be thinking of the phone line I'd be using radio, maybe Lorahttp://www.semtech.com/wireless-rf/lora.htmlIf higher speeds then maybe Zigbeehttp://en.wikipedia.org/wiki/ZigBeeOr even wifi with a high gain antenna. Could be up and running in a few days with can antenna leaving more time to do something else.

author: mark03
author_rank: Frequent Contributor
timestamp: June 10, 2015, 07:44:44 pm
content: There is also a theoretical basis for the tone spacing in FSK. For noncoherent detection (receiver doesn't know the phase of the transmitted tone, only the frequency), the minimum spacing should be 1/T, e.g. 300 Hz for 300 baud.Just now I looked back at the old Bell 103 and 202 standards, and noticed they do not satisfy this condition. Anyone know why?

author: m100
author_rank: Regular Contributor
timestamp: June 10, 2015, 08:59:30 pm
content: The ITU standard V21 for 300bps, while having different base frequencies, also has a shift of 200Hz the same as Bell 103https://www.itu.int/rec/dologin_pub.asp?lang=e&id=T-REC-V.21-198811-I!!PDF-E&type=items