pretty much OT: acoustic coupling

On 24 Mar 2007 at 17:57, Randy Dawson wrote: The old ASR 32/33 modems were strictly analog devices. I doubt that they could have responded at a 1200 baud rate. I thought that Bell 103 frequencies were used, but I'm not certain. The original acoustic couplers were designed for TDD use and ran at about 50 baud. I can tell you that 'way back when, a couple of us tried to see how fast a pair of old Anderson Jacobsen modems would run and we didn't get past 450 baud--and much less than that when calling Los Gatos (Bay area old-timers may remember this). Again, other than for the level conversion to RS-232C levels at the output, these were strictly analog devices. "Modern" 56K modems rely on the the digital 64Kbit nature of phone lines and require a digital interface to the line at the far end. The 56K arises out of the simple fact that, while the telco uses a 64K bit rate, some of the bits are reserved by the telco for signalling and control. You can't do 56K over a strictly analog phone line--if digital capabilities aren't sensed, the protocol degrades to V.32/34/34 bis. I believe that 8 carriers are used with 128 levels per carrier, although it's been awhile since I read the standard. Cheers, Chuck