Modems and Baud and Bits, Oh My...

Several people have been talking about modems and the like, and even acoustic couplers. Here are some points: The 103 Modem frequencies are: 1070/1270 and 2025/2225 ('mark' is the higher frequency). One group is used in each direction. As I remember it the high group was used as transmit on the answer side, as the "answer tone" (one of the high frequencies) was one that tripped the echo suppressors to "off" so they wouldn't interfere with the transmission of the data. The 103 modem has a 200 Hz shift (difference between mark and space) and doesn't like much above twice the shift frequency in data rate (that is why things changed after 300 bps). Some people were lucky (real lucky if you ask me) to get 450 bps out of the thing. Acoustic couplers go WAY back. In the 60's (some might think this is ancient history) it wasn't good to attach things directly to the phone line (people did it any way, but I digress). So, many schemes were used to couple things to the phone line. One of the most successful vendors was Anderson Jacobson. Tymshare (yeah, the guys with the SDS 940's) also developed a coupler. Their original one used magnetic coupling for the receive side as is was less prone to noise. Since you had the coupler near the ASR-33 you can imagine the noise. Magnetic coupling is still used in "hearing aid compatible" telephones (you will see it on pay phones with blue strain reliefs on the handset cord). At higher speeds several standards abounded. Bell "202" modems used a single FSK carrier with a wide shift to accommodate 1200 bps in ONE direction. The standard (Bell Standard) provided for a 5 bps back channel (so you could tell the other guy to "turn the line around"). Others expanded on the back channel to make it an FSK pair (around 300 Hz as I remember) that would about 150 bps. If you were using a CRT terminal, this was OK since you could hardly type that fast, and having the (big computer like pdp-10, or others) respond back at 1200 bps seemed like lightening fast. The next standard (The phone company was allowing direct connections by then) was 1200 full duplex. The first one was Vadic 3400, followed by Bell 212. While similar, they had opposite answer/originate pairs and were not compatible. These modems took in the 1200 async data and converted it to a synchronous stream that was sent over the carrier. What was nice is that if you used the modem and connected to a "low speed" (Bell 103 type) modem, it used those frequencies (like a second modem). Later developments raised the speed up to 33k bps full duplex using lots of computing power in the modem (which got cheaper to do). with various "V.xxx" standards. All of this finished up with 56k modems which was the limit for a DS0 channel (8 bits, less one for signalling and 8k samples per second). Just remember, programming on an ASR33 is a VERY humbling experience. Everyone should try it for one project just to get it. Unfortunately, comments get sacrificed but you do make up for it in the scribbles on the real paper. -- Tom Watson tsw at johana.com ____________________________________________________________________________________ Get your own web address. Have a HUGE year through Yahoo! Small Business. http://smallbusiness.yahoo.com/domains/?p=BESTDEAL