With this posting of the Bell System Technical Journal from volume 1 issue 1 in July 1922 to the final issue published in December 1983, we are pleased to be able to open the vault of this knowledge to our global technical colleagues.
***** Moderator's Note *****
Yes, we've seen this before, but I couldn't resist the chance to play "Ultimate Telecom Trivia"!
Here's the question: *why* does a T1 line have 24 channels?
- Harry Nyquist tells us that the sampling rate must be twice the highest frequency within that bandwidth, or 8000 Hz.
- The circuit uses Pulse Code Modulation with a sampling rate of 128 steps, so 7 bits (plus one check bit) must be transmitted for each sample.
- Manholes in urban areas are about 6000 feet apart, which is the distance between loading coils in a voice loop.
- Data repeaters have to be accessible for service (i.e., in manholes), so they must be 6000 feet apart.
- The circuit must operate over No. 22 twisted pair.
- Research at Bell Labs in the 1960s showed that the maximum data rate that could be reliably transmitted over a distance of 6000 feet over No.
22 conductors is about l.6 Mb/sec. Presumably, this research included allowances for overspaced manholes, wet cable, corroded splices, ambient temperature variations, and anything else that Murphy could dream up.
The standard multiplexing scheme in use when T1 carrier was introduced was based on a 12 channel group. The digital line was known as a "digroup", or two groups. The choice was made to facilitate connection to the existing long haul analog network (a single DS- 1 bank to two group banks). European providers use a 30 channel system (plus a framing and a signaling channel for a total of 32 timeslots) instead of our 24 channel arrangement.
First, they used a multiple of 12 channels, because it had to interface with analog channel banks and the analog equipment used multiples of 12 channels, called a "channel group."
Second, they could reliably transmit and decode 24 channels between manholes, but not 36 (too many errors due to transmission distortion). Another point of trivia: At one time, they had digital switching of analog carrier channels: Transmission between cities was analog, but the switches were digital.
***** Moderator's Note *****
Although T-Carrier banks had to interface with Analog banks, the connections were always at "baseband", i.e., 300-4,000Hz connections on pair wire. The Analog banks could never interoperate with T-Carrier banks on the "high speed" side of the banks: a "T-1" 1.544 Mbps circuit can't feed an analog carrier system. Apples and Oranges, really.
The first cable TV systems *were* community antenna tv systems. For the first 25 years of the industry's existence, broadcast retransmission (especially distant stations) was about the only thing they had to sell. Many systems originated local programming ("local origination") and carried PEG (public, educational, and government) access programming, but these services didn't sell many subscriptions.
The term CATV originally stood for community antenna TV. Somewhere around the mid-70s, when CATV companies began carrying satellite- delivered non-broadcast programming, the term CATV morphed into cable TV.
So the question refers to the first CATV system.
As for your second question, I was thinking of the US, but the question could certainly apply worldwide.
Generally true, but #4ESS had a frame called the "L to T connector" that actually took analog group signals and interfaced them directly to the digital switch fabric without intevening voice band connections. The interesting technical part of it was the use of digital signal processors (DSP1, actually, manufactured by Western Electric in Allentown PA) instead of analog signal processing. This was the first use of DSPs in the Bell System's network.
BTW, are you looking for the N carrier connection?? N carrier was the dominant form of short haul carrier and the version in use in the early '60s put 24 voice channels on a four wire cable circuit. T carrier was intended for the same short haul market of interexchange and toll connecting trunks.
As I recall, long haul trunks were administered at the group level, so a multiple of 12 was desirable.
When I was a child we lived in Ashland, Kentucky for two years. We had cable television service. Ashland itself had only a PBS station. It was difficult to pull in stations from West Virginia. We had ABC, CBS, and NBC from Charleston, WV, the local PBS station, and an indepdendent station from Cincinnati known then as "Metromedia 19".
We had no cable box. The stations were mapped to the VHF dial. Even though I was only in third grade I noticed a flicker as I turned the knob between channels 6 and 7. I discovered if I turned the knob just right so it was between those two channels we could get HBO.
I seem to recall also some sort of information type channel. So of the 12 VHF channels (2 through 13) we had only 6: ABC, CBS, NBC, PBS, independent, and the information channel.
The thing I remember most about early cable TV was a surprise I got when I was visiting some friends in Allentown in May of
1970. I was surprised to learn that they had not one but TWO local cable providers from which to choose, and that they had recently signed up with one.
This was in the days when Sterling-Manhattan was struggling to get their initial NYC cable system launched in spite of all kinds of political and technical issues. The idea of competing cable systems in the NYC market was something not even dreamed of. ;-)
My impression was that the Allentown system was more than just 'community antenna' TV, although I don't remember anything more than local, NYC, and Philly broadcast stations being on it.
It did use a decoder box, but I don't remember much about it other than a rotary channel selector knob.
For those who don't know the geography, I might describe the location of Allentown as being in the fair-to-almost-good range for Philadelphia broadcast stations and the crummy-to-fair rance of the NYC stations. Prior to cable, my friends had a fairly large rooftop antenna with a motor-driven rotor to swing it in the direction of either NYC or Philly.
In the book "Transmission Systems For Communications", Bell Telephone Laboratories, Inc., Fifth Edition, 1982, Page 596:
"FDM·to·TDM Connector. Long-haul transmission tends to be dominated by analog FDM (frequency-division multiplex) systems (primarily line-of-sight radio relay). On the other hand, digital switching of longdistance telephone traffic is rapidly being introduced. Thus, it is necessary to have an economical interface between FDM transmission and digital switching. The most convenient place in the hierarchy to perform this function is at the group level. In its simplest conceptual form (LT-1), this FDM-to-TDM connector consists functionally of two A-type channel banks (see Chap. 15) which demodulate two FDM groups into 24 individual voice channels and a digital channel bank which encodes these channels into DS1 format. As A/D conversion and digital processing become cheaper, however, it will become economical to implement the FDM group to TDM converter by performing an A/D conversion directly on the group signals and then translating the encoded FDM signal directly into DS1 format (see Fig. 27-2) by digital processing techniques. This eliminates many of the more expensive analog filters and modulation equipment (see Chap. 35)."
That's the one I was thinking of. I was an application engineer for the components division of WECo at the time. As I recall the LT-1 digitized the group signal and used the DSP-1 digital signal processor to extract and digitize the voice signals, then multiplex them together. Ditto in the reverse direction. The frame replaced the Voiceband Interface Frame (VIF) of the #4ESS which interfaced the A channel banks at analog voice band level. At about the same time, the Digroup Terminal Frame was replaced by the Digital Interface Frame for another big cost reduction. This was a time when the scale of integration was driving down the cost of components very rapidly and all the equipment (my customer was Long Lines applications) was shrinking and dropping in price dramatically.
An interesting sidelight is that one of driving forces for the DSP-1 was Carl Kurth at Merrimac Valley who also was a big player in the thick film analog filter work up there. The digital world would obsolete the thick film circuits (which combined active components and passive ones on the same ceramic substrate) in short order.
Lots of really good stuff was going on in those days (including a crash echo canceler program)...
Back in the mid-1960's, I visited Williamsport, PA. I saw _three_ CATV cables on the power poles. Talking with a resident, I learned that one company started a CATV system; then after signing up many people they raised the rates. A second company saw a business opportunity and started a second CATV system; they lured customers away from the first one and then they raised rates. At which point, yet a third system started up. Interestingly, all three systems got their TV feed from a single site located on a local mountain top.
Maybe Pennsylvania had no laws granting exclusive franchises for CATV.
I used to live in Salem, NH. In the mid-1980's, the franchised cable TV company was planning on a major upgrade, but had only two years left on its franchise agreement. They didn't want to do the expansion if they might lose the franchise in two years. They approached the town government about extending the franchise term. The town gave the following conditions: give us several government and public access channels, and wire the _entire_ town including very sparsely populated areas. The company agree. Under the company's old rules, they would not serve my residential street with one-acre lots and 125-foot frontages because it was not dense enough. Also there were some working dairy farms in the outskirts of town on 50 or more acres, with up to a quarter mile between houses. The cable company agreed to the conditions and wired the whole town, probably to salvage their current investment in outside plant.
That was typical before 1975. The first CATV system I worked for (Madison WI) was equipped to distribute 12 channels, but we only carried video on seven: four locals (ABC, CBS, NBC, PBS), Milwaukee PBS, Milwaukee independent, and Chicago WGN-TV.
We also carried the usual three PEG channels and one channel that we originated ourselves. These channels were mostly filled with character-generated "bulletin boards."
We did our best find video programming to fill the unused channels. We offered a strange concoction of church services, government meetings, a locally-produced quiz program sponsored by a pizza shop (free pizza for the correct answer), a movie service called "Home Cinema" (on U-Matic cassettes) and a sports service (also on U-Matics) that featured such ever-popular attractions as South American "futball" games, mountain climbing, and bowling.
I remember one memorable day when we actually managed to provide full-motion video on *all twelve* channels! When I got home and told my family about it, the kids duly verified it and spent the next hour channel surfing.
In 1975, when HBO began satellite delivery, we added HBO on Channel 17, in the midband between 6 and 7. As you note, it was sometimes possible to get HBO by fiddling with the tuner.
Even more common were modified TV sets. Those were the days of "rotary" VHF tuners that used 12 circuit boards, one for each channel, mounted on a rotating mechanism. Rotating the front panel knob changed channels by switching circuit boards. Local TV repair shops soon discovered a sideline business: tuning an unused circuit board to Channel 17.