w8sdz@brl.arpa (03/31/83)
From: Keith Petersen <w8sdz@brl.arpa> The forthcoming 300/1200 baud PMMI S-100 modem will NOT have the Vadic 3400 protocol included. Several people asked about that. Vadic isn't including it in most of their new OEM units. There has been some discussion about whether 3400 is better than 212A, and one comment I read was that it really wasn't a better protocol. The only reason it SEEMED better, the sender said, was that the 212A part of the "Triple Modem" wasn't as well designed as it could have been. I assume that if this is true, it could be the reason that Racal hasn't included the 3400 protocol in the new units. They may have improved the 212A part instead, in the interests of gaining a wider market. This is all conjecture, however. For those interested in the new PMMI modem (when it's available in June), here's the address and phone number: PMMI Communications 5201 Leesburg Pike Suite 604 Falls Church, Virginia 22041 Phone (703) 379-9660 --Keith
dan (04/05/83)
If Vadic is losing interest in the vadic 1200 baud protocol, it must be because Vadic customers are losing interest in the Vadic protocol (I don't mean to imply that this is really happening, though it seems reasonable). The Vadic protocol can be made to work with accoustic couplers while the Bell protocol cannot. This was a good advertising point for Vadic back in the bad old days when Ma Bell was allowed to make direct connection to the phone lines artificially expensive. (It has been suggested that the Bell protocol was deliberately designed to make accoustic couplers and therefore nonbell modems impractical. The problem with couplers is that they distort signals, introducing harmonics. The Bell protocol has the originating end transmission carrier at half the frequency of the reception carrier. The Vadic protocol does it the other way. Draw your own conclusions.) Now that direct connection of modems is the rule rather than the exception, the Bell protocol works as well as the Vadic. In the absence of any compelling reason to use the nonbell protocol, the tendency must be to go with the protocol belonging to the bigger company. Since triple modems are more expensive, I guess people are mainly buying just the bell 212a protocol these days. Dan Strick idis!dan
jim (04/05/83)
The argument I have heard for Vadic 3400 being "better" than Bell 212 is that Bell's transmit and receive frequencies are very close to being a 2:1 ratio, whereas Vadic's are farther apart. For direct connect this makes little difference, since harmonic distortion in electronic circuits tends to be odd orders (3, 5, etc.). But if you try to make an acoustic coupler, you find that carbon mikes have a lot of 2d harmonic distortion, so the transmit carrier gets mixed up with the receive. This argument makes sense to me, and in fact I have not seen any 212 compatible acoustic couplers, but the acoustic Vadic modems are common. The modulation schemes are identical other than the frequencies, according to Vadic's product info. They are both dibit phase encoded. The Vadic method has been around longer, but Bell is a bigger company, so I expect Vadic 3400 to fade away before too long. Does anyone know anything about the 4800 baud modem from Microperipheral?
smb (04/06/83)
There's another non-technical reason why 212-type modems are eclipsing the Vadic protocol -- competition. Look how many folks make 212s. Then look for who else makes Vadic-compatible modems.