How's it going? I hope everyone had a good weekend. I recently picked up an interesting Philco test set, a 322a N-Carrier Generator. Can't really find any documentation on the internet about it, much less another unit, so I'm guessing it's quite rare. The seller said it powers on, but had no idea how to test it, so it was sold as Parts/ Not working, so I got a great deal on it (assuming it does work :) ).
Does anyone here know how this device would be used? There's about 12 toggle switches/keys and knobs, one for each carrier frequency, an AC input, a battery supply in the front cover of the case that takes a bunch of D cells, a carrier selection knob, output slope dB knob, group output & impedance knob, group select knob (for West-East groups & vice versa )along with 2 line out jacks, DC feed jack, and a terminal strip thing with 10 contacts.
It looks to be in excellent condition, as far as I can tell without knowing a way to test it or how it was used. Any input or info would be greatly appreciated, thanks in advance! ;D
In the U.S. telephone network, the 12-channel carrier system was an early frequency-division multiplexing system standard, used to carry multiple telephone calls on a single twisted pair of wires, mostly for short to medium distances. In this system twelve voice channels are multiplexed in a high frequency carrier and passed through a balanced pair trunk line similar to those used for individual voice frequency connections. The original system is obsolete today, but the multiplexing of voice channels in units of 12 or 24 channels in modern digital trunk lines such as T-1 is a legacy of the system.
The twelve channel scheme was first devised in the early 1930s to provide a line spectrum covering 60 to 108 kHz for the Type J Carrier Telephone System, an equivalent four wire (on two wire facilities) open wire carrier that was used almost exclusively for interstate long haul toll telephony. This became the basic building block, the "channel group", for all succeeding long haul systems, such as Type K and all the Type L systems into the late 1970s. All long haul "channel groups" used the single-sideband/suppressed carrier heterodyne scheme that was produced by a Western Electric Type A-1 through A-6 channel bank.
The twelve channel scheme, in order to maintain some bandwidth and routing compatibility, was carried through to the short haul carriers, as well, as they started developing to eliminate voice band open wire trunk lines in the 1950s. The Bell System vacuum-tube driven N-1 Carrier of the early 1950s was the most used twelve channel carrier system, using double sideband/unsuppressed carrier operation which didn't need network timebase synchronization to maintain frequency accuracy. N-2 was similar in heterodyning scheme, but in discrete transistorized "plug-in unit" architecture, while N3 used the same frequency plan but a scheme of using single sideband with a different voice channel on each side of the carrier, a technique first seen on the 16 channel Type "O" open wire short haul carrier of the 1950s. This doubled the capacity to 24 channels, the same as a basic digital Type T PCM carrier introduced in the late 1950s, which became the now-ubiquitous "T-1" of the digital world.
Repeaters were spaced approximately 6 miles (10 km) apart, depending on wire gauge. With few exceptions, N-carriers used 19 gage unloaded toll pairs in two-wire operation. Each repeater either received from both directions at a low frequency band and sent in both directions at a higher band, or vice versa. This frequency frogging allowed equivalent four-wire operation on a single cable pair in two-wire operation.
During the period when Type N-1 was in widespread use, Lenkurt Corporation, owned and controlled by General Telephone, fielded a variant competitor, the Type BN. BN used the same pairs and repeaters as did the Bell N-CXR, but used four channel "groups," lower single-sideband heterodyning, and 24 channels per carrier, as later seen on Western Electric's Type N-3. Type BN was used at times by Bell Operating Companies after the 1956 Hush-a-phone Decision of the US Supreme Court, a landmark case which challenged AT&T's "benign monopoly" of US telephone equipment industry. Part of this settlement was for AT&T's Bell Operating Companies to buy and use small amounts of Lenkurt and Collins microwave and carrier systems. In California, Type BN was used almost exclusively to provide trunk and private line connections between Pacific Tel. & Tel. toll offices and local General Telephone end offices.
The N1 Carrier System is a 12‐channel, double‐sideband system for single cable application. It provides low loss, stable, high velocity service for toll and exchange circuits in the range from 15 or 20 miles to 200 miles. Units and sub‐assemblies are miniaturized and arranged on a plug‐in basis. Emphasis has been placed on reduction in cost of components, as well as simplification of manufacturing methods, engineering, installation and maintenance. Economy is achieved by many novel features, principal among which is a built‐in low cost compandor. By compressing and expanding the volume range of speech, the compandor permits much higher tolerance of noise and crosstalk, thereby substantially lowering the cost of both line and terminal facilities. Other important features are self‐contained dialing and supervisory signaling, an individual channel regulator, and automatic equalization through the use of "frequency frogging," or interchange of high‐ and low‐frequency groups at each repeater.
Thanks, RB, lots of good info about N-carrier in one place on your post. Sorry for the late reply
I've been wondering if it'd be possible to make a little FDM based switch using this thing, modulating the carriers by voiceband signals to multiplex, putting them through a couple of Extron BNC/ Coax crosspoint switches (like you'd find at a sports bar for routing multiple CATV signals to TVs), then, using SDR radios tuned to the carrier frequencies to demux.
I was looking at kHz bandpass filters for that aspect, but I saw very few that were the right frequencies, so I figured SDRs would be a cheaper alternative, considering they can tune to whichever carrier freq is needed, and filtering could be done in SDR software as well.
I'm sure this idea would likely be much more trouble than it's worth.
Ideally, the jist of it is that each input of the crosspoint switch can carry a bandwidth of about 150 MHz, which is more than enough to carry the 12 channels for N-carrier, after all, it's made for cable TV signals. The switch itself can be controlled via Ethernet using simple commands to open and close crosspoints, and the inputs can connect to multiple outputs, which would be useful for one way trunks for ringing, busy, reorder, intercept recordings, etc, also possibly for dynamic trunking based on routing.
In my original concept, I was thinking of using one 12 i x 8 o switch to handle incoming calls, another for outgoing and originating calls, both connected to some kind of Asterisk switch or an ATA, but I figured if it'd be possible to use FDM on the "video" switches, it could essentially utilize bandwidth that'd otherwise be wasted, increasing the amount of traffic it could carry (assuming I'd be able to get the multiplexing right), and make for another possible layer of switching. Each 12 x 8 switch would become a 144 x 96 one way switch, and the inputs and outputs would be used as trunks. The majority of the actual switching would probably have to be done by Asterisk; the physical crosspoints would be connecting trunks to trunks, so using them may be useful for tandem functions.
I really have no idea if something like this would even work though, and it'd no doubt involve coming up with a way to interface Asterisk boxes with both the crosspoints, and the SDRs to split the FDM trunks into audio channels that Asterisk can use. Supervision would probably have to be handled in Asterisk as well.
At any rate, I appreciate the response and wish you well, especially with all this crazy stuff going on in the world. God bless