Talk:Carrier telephony

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Article scope and structure

This article covers the development of analog carrier telephony systems using frequency-division multiplexing (FDM). It covers the period from early experimental work (e.g., Squier, 1910) through large-scale coaxial carrier systems in the mid-20th century. Largely US based, need expansion to other develpers.

The focus is on:

  • historical development and system evolution
  • technical approaches to multiplexing, frequency planning and transmission
  • major Bell system families (e.g., Types A–D, J, K, L, N)
  • coverage of open wire, cable, and coax
  • some competative systems

Digital carrier systems (e.g., T-carrier) are mentioned for context but are not covered in detail.

Sources

Relies largely on contemporaneous technical literature, including American Institute of Electrical Engineers (AIEE)/ IEEE and the Bell System Technical Journal (BSTJ).

To do

  • Expand lead section with concise summary of later developments
  • Review and normalize citation formatting
  • Consider adding diagrams of frequency plans or system architecture
  • Check for overlap with related articles (e.g., Frequency-division multiplexing, 12-channel carrier system)
  • Add non-US view

As always, comments, ideas sources welcome, especially non-US sources

I'm spliting the history from the technology, and splitting the technology into separate areas. I have a first cut on the filter part. Mixers and amplifiers next. I think we should merge in 12-channel carrier system thoughts?

added section on amplifiers

Copy from my sandbox. Try to detail the invention of negative feedback amplification, the problem of long haul signal transmision WhaleFarm (talk) 01:52, 26 May 2026 (UTC)

Merge from 12-channel

Merging content from 12-channel carrier system; see its history for attribution. WhaleFarm (talk) 22:01, 1 June 2026 (UTC)

GA review

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Reviewing
This review is transcluded from Talk:Carrier telephony/GA1. The edit link for this section can be used to add comments to the review.

Nominator: WhaleFarm (talk · contribs) 14:35, 5 June 2026 (UTC)

Reviewer: Dreamyshade (talk · contribs) 21:32, 24 June 2026 (UTC)


This is an interesting topic, and the article reflects a lot of thought and research. Just starting to draft a review! I'll keep working on it over the next few days. Comments are below.

GA review – see WP:WIAGA for criteria

  1. Is it well written?
    A. The prose is clear, concise and understandable to an appropriately broad audience, and the spelling and grammar are correct:
    B. It complies with the manual of style guidelines for lead sections, layout, words to watch, fiction, and list incorporation:
  2. Is it verifiable with no original research, as shown by a source spot-check?
    A. It contains a list of all references (sources of information), presented in accordance with the layout style guideline:
    B. Reliable sources are cited inline. All content that could reasonably be challenged, except for plot summaries and that which summarizes cited content elsewhere in the article, must be cited no later than the end of the paragraph (or line if the content is not in prose):
    C. It contains no original research:
    D. It contains no copyright violations nor plagiarism:
    No copyvio detected with automated tool
  3. Is it broad in its coverage?
    A. It addresses the main aspects of the topic:
    B. It stays focused on the topic without going into unnecessary detail (see summary style):
  4. Is it neutral?
    It represents viewpoints fairly and without editorial bias, giving due weight to each:
  5. Is it stable?
    It does not change significantly from day to day because of an ongoing edit war or content dispute:
  6. Is it illustrated, if possible, by images?
    A. Images are tagged with their copyright status, and valid non-free use rationales are provided for non-free content:
    B. Images are relevant to the topic, and have suitable captions:
  7. Overall:
    Pass or Fail:


Criteria 1 (well written)

Done
  1. Wikipedia:Make technical articles understandable: I suggest adding a bit more context to the lead section to help people new to the topic. For example, consider putting the purpose before the technical terminology in the lead sentence, something like this: "Carrier telephony is a long-distance telephone transmission method that carries multiple simultaneous long-distance calls over the same telephone lines by using frequency-division multiplexing (FDM)." Done
  2. Since Bell has a significant role throughout the article, this should be noted in the lead. Link to Bell System and Bell Telephone Laboratories upon first mentions. Done
  3. Some of the paragraphs are short and could be consolidated. As MOS:PARA says, "Single-sentence paragraphs can inhibit the flow of the text; by the same token, long paragraphs become hard to read." (WP:PARAGRAPH is also helpful.) Done
  4. The end of the lead doesn't seem to be fleshed out in the body: "Analog carrier systems were gradually replaced by digital transmission systems using pulse-code modulation (PCM), including the T1 system introduced in 1962, although many of their multiplexing and network-management methods continued in digital telephone systems." You could add a short section after "Technology" to cover this part, even just a couple sentences and a citation. Done
  5. Add wikilinks for the first mentions of technical terms in general, so that a reader can easily click for background information if needed. (I put in a few as an example; please feel free to check them for accuracy.) For unusual terms, especially terms that don't have articles, adding a tiny bit of inline context is helpful too. For example, I've visited multiple telecommunications museums just for fun, but I don't have any engineering education, and I'm not already familiar with terms including 12-channel group, compand, pilot tones, lattice filters, and thermistors. Done
    1. (I made a pass through, added explainers)
  6. The sentence "In 1929, Bell applied for a patent to multiplex telephony signals onto a coaxial cable. It described telephone with video capabilities, and up to hundreds of voice channels." seems a bit out of context. I think you just need to add something to connect it to the rest of the section, like "This was the basis for the development of high-capacity coaxial systems in the 1950s." Done
    1. (good catch. I re-read this, and decided it was drivel. I re-wrote this, added a bit more context, and blended int L3 better)
  7. Additional terms unfamiliar to me that I couldn't easily find a wikilink for: "quartz blank", "double-balanced modulators". Done
    1. (quartz blank. It's the term for the piece of quarts that a crystal is build from, the advance here was using that blank to make a more compex structure that just a resonantor. It's sort of covered in crystal filter, but that's a real mess, technically wrong, etc. I linked to it, will try to fix later. Double balanced is linked, inside frequency mixer.)
  8. "Affel, Demarest, and Green" are mentioned without context. Done
    1. (they are the source of the following quote, referenced.)
      1. I see that Affel was mentioned earlier, since the reader doesn't need to track who Demarest and Green are, I don't think we need to refer to them by name; I edited this.
  9. Is "Thin-film hybrid IC" supposed to link to Thick-film technology? Should that link to hybrid integrated circuit in addition, or instead? Done
    1. (there isn't really a good thin-film link, think is similar, but I like hybrid, the thick/thin is not important. I used hybrid)
  10. Very minor: not familiar with the term "receive filter" and couldn't find a link for it. Done
  1. Looking at History of AT&T, I realized that I was probably wrong when I corrected AT&T to Bell System a couple times. Ideally, to reduce confusion, the lead or background section should briefly mention the relationship between the companies, and the article probably needs checking to make sure the right company is mentioned in the right places overall.

Criteria 2 (verifiable)

Done
  1. Agree with User:RoySmith's recent addition of 11 citation needed tags. However, those statements seem factual, and I imagine you could find citations for them without much trouble, so it seems reasonable to start this review while that work is pending. Done
  2. The tables that start with "Forward carrier (kHz)" and "Repeater spacing" doesn't seem to have citations. They would also benefit from a high-level caption like the "A-type channel bank development" table, and that caption is a reasonable place to put the citation. Done

This table lists 22 random passages from throughout the article (14.9% of 148 total passages). These passages contain 25 inline citations (14.5% of 172 in the article). Generated with the Veracity user script.

Reference #LetterSourceArchiveStatusNotes
Open-wire construction made long-distance service possible but required large amounts of copper. In 1911, Bell System service reached from the East Coast to Denver using voice-frequency open-wire circuits. In 1915, vacuum-tube repeaters made transcontinental service possible between New York and San Francisco.
2aA History of Engineering and Science in the Bell System: Transmission technology (1925-1975). [Shor…archive.org DoneThe bit about "large amounts of copper" wasn't well-supported by those pages, but I used the next few pages to adjust the statement.
Ralph Hartley later analyzed the relation between carrier and sideband components, showing that transmitting only one sideband saved power and bandwidth, allowing more signals to share the line. He also described a balanced modulator circuit for suppressing the carrier component.
10aHartley, R. V. L. (1923). "Relations of Carrier and Side-Bands in Radio Transmission". Bell System … Good
Early systems also added multiple telegraph signals to lines used for telephony.
11ieeexplore.ieee.org Good
The Type D system was developed for shorter toll circuits of about 50 to 200 miles. It added one carrier circuit to a plain voice-frequency circuit, doubling the capacity of a pair at lower cost than the long-distance Type C equipment. For circuits longer than about 125 miles, an added amplifier produced the D-A version.
13aieeexplore.ieee.org ⏳
By the late 1930s, Bell System carrier telephony had standardized on the 12-channel group as a basic multiplexing unit. The group combined twelve voice channels into a band 48 kHz wide, using 4 kHz channel spacing and single-sideband transmission. Standardizing the group allowed much of the same translating equipment to be used on open-wire lines, paired cable, coaxial cable, and later microwave radio-relay systems.
2gA History of Engineering and Science in the Bell System: Transmission technology (1925-1975). [Shor…archive.org ⏳
The system initially used 5 kHz channel spacing, with provision for later conversion to 4 kHz spacing. Eight-channel groups occupied the band from 60 to 100 kHz, with provision for expansion to ten channels within the same frequency range. Crystal filters were used in the channel-group translating equipment. Five groups formed a 200 kHz supergroup, and eight supergroups occupied frequencies from approximately 500 kHz to 2.1 MHz, allowing up to 320 telephone circuits on a single coaxial system.
23bMumford, A. H. (October 1937). "The London-Birmingham Coaxial Cable System. Part I: Description of …archive.org ⏳
In the Type ON2 system, 24 voice signals were transmitted using the same frequency plan as the N-1 system, doubling the number of signals by using single side band. The system utilized components from the N-1 and the O systems.
32adoi.org ⏳
In 1963, General Dynamics described a solid-state 12-channel carrier system for open-wire and cable applications. Its architecture and frequency plan maintained line compatibility with Type J and Type O systems. The system used amplitude modulation up to 350 kHz, with optional companding. Carrier frequencies were staggered with 8, 12, and 16 kHz spacing to simplify filter design. Modulation mixers were simple four-diode single-balanced designs. Demodulation used a single-diode AM detector, with carrier level feedback to set gain using a diode attenuator.
38ieeexplore.ieee.org ⏳
Placed in service on January 3, 1974, the L5 system supported 10,800 telephone circuits on a coaxial pair. A cable with 10 coaxial pairs could support 108,000 simultaneous telephone conversations.
20bdoi.org ⏳
Harry Nyquist's stability criterion, and later equalizer and network-design methods by Hendrik Wade Bode and Sidney Darlington.
49adoi.org ⏳
The Type C system added three carrier channels in each direction in addition to the baseband voice circuit, increasing the capacity of a line to four simultaneous bidirectional telephone conversations. The system was designed for open-wire lines, where crosstalk between adjacent pairs was significant. Because LC filters provided poorer selectivity at higher frequencies, wider spacing was used for the upper channels.
12cieeexplore.ieee.org ⏳
The Type K system was designed for cable transmission systems, where crosstalk was lower than on open-wire lines but attenuation was substantially higher because of cable capacitance. Separate cable pairs were normally used for each transmission direction. The carrier allocations occupied the 12 to 60 kHz range and were derived from the standard 60 to 108 kHz groups by frequency translation.
16cieeexplore.ieee.org ⏳
Early carrier systems depended on the accuracy of their local carrier oscillators. In a single-sideband system, a difference between the transmitting and receiving carrier frequencies shifts the recovered speech frequencies. The Type A and Type B systems used LC oscillators.
7iieeexplore.ieee.org ⏳
The initial signal modulation to form the groups was identical in the J, K and L systems, and was based on double-balanced mixers made with copper oxide rectifiers
61doi.org ⏳
The Type C system used the same amplifier design in both terminal equipment and line repeaters. Low second-harmonic distortion was important because harmonics generated in the lower-frequency bands could fall inside higher-frequency carrier channels. A balanced push-pull circuit reduced second-harmonic distortion by approximately 15 to 20 dB.
12gieeexplore.ieee.org ⏳
The Type J open-wire system used similar pilot-tone regulation techniques, although the pilot frequency was transmitted on the same wire pair as the carrier channels, because a nextra open-wire pair for the pilot was impractical.
17eieeexplore.ieee.org ⏳
Negative-feedback networks were used both to shape the gain and to match the input and output impedances of the cable. A regulating stage between the two amplifier stages used a thermistor/heater combination to track changes in cable loss with temperature. The equalization and feedback design drew on network theory developed by H. W. Bode
67patents.google.com ⏳
49cdoi.org ⏳
68Bode, Hendrik W. (1945). Network Analysis and Feedback Amplifier Design. New York: D. Van Nostrand …archive.org ⏳
Building on this work, W. P. Mason developed crystal lattice filter networks for Bell System carrier telephony applications.
79Mason, W. P. (July 1934). "Electrical Wave Filters Employing Quartz Crystals as Elements" (PDF). Be…dn710306.ca.archive.org ⏳
In the late 1960s and early 1970s, Bell Laboratories developed monolithic crystal filters for carrier telephony channel banks, including the A6 channel bank system.
85aieeexplore.ieee.org ⏳
82bieeexplore.ieee.org ⏳
Other manufacturers developed related technologies. Lenkurt's 60-channel carrier systems used polylithic crystal filters for compact multiplex equipment.
86ieeexplore.ieee.org ⏳
The A5 channel bank, introduced in the early 1960s, was a transistorized replacement for earlier vacuum-tube channel banks. Bell System authors described it as a "radically new version" of the A-type bank.
92bieeexplore.ieee.org ⏳
In the Bell System L600 multiplex equipment, five 12-channel groups were first combined into a 60-channel supergroup occupying 312 to 552 kHz, a bandwidth of 240 kHz. Ten supergroups were then combined to form a 600-channel mastergroup. The mastergroup occupied the band from 60 to 2788 kHz; the difference between the ten 240 kHz supergroups and the full mastergroup band provided guard spaces. The resulting 600-channel structure was compatible with the CCITT multiplex hierarchy.
58bdoi.org ⏳

Criteria 3 (broad)

Done
  1. It makes sense to focus the article on the technology, but I believe readers would benefit from a tiny bit more of the business context. For example, the following statements are in passive voice, which makes me curious: who did these things and why? (Presumably the "who" is mostly Bell, and the "why" is mostly "make more money", but in accordance with WP:OBVIOUS, it's helpful to state that for the reader.) Done
    1. "Commercial carrier systems were introduced after World War I." Done
    2. "Carrier telephony became one of the main technologies of 20th-century telephone communication." Done
    3. "By the late 1940s, coaxial carrier systems had expanded rapidly, with thousands of miles of L1 cable in service in the United States." Done
    4. "As carrier systems increased in capacity during the 1930s, more standardized frequency plans were adopted." Done
  2. By the "Postwar refinement and solid-state transition (1950s–1960s)" and "High-capacity coaxial systems (1950s–1970s)" sub-sections, I start losing the narrative and feel like I'm looking at a lot of specific details. Consider shaping the headings around the high-level themes instead of the system identifiers? Check for less important details that could be trimmed? [I read this part again, more slowly, and I see why you have it set it up like this. It's just for the more advanced reader, which is ok for later sections of the article. Still worth considering any minor changes to improve readability.] Done
    1. (I've added a mini-lead to try to tie this stuff together with prose. It talks to growth, economics, competition, tech advances)
      1. Very helpful, thank you!
    1. I've combined the O and N families into one section, also cuts the short para problem down. Hopefully makes the evolution rational a little clearer.
      1. Thanks! Is "to make carrier economical" missing a word, or using "carrier" as a noun?
        1. bstj uses carrier as a noun , but maybe carrier transmission is cleaner? What do you think

Criteria 6 (illustrated)

Done
  1. MOS:CAPCONTEXT: Suggest working a bit of context into the caption for the first image to explain its significance. For example: "Carrier telephony circuits in the 1910s (inset) and 1940s, showing expansion of capacity for long-distance calls". Done
  2. Checking whether media is properly tagged with copyright statuses: Done
    1. File:Bell telephone magazine (1922) (14569514478).jpg - "Please add additional copyright tags to this image if more specific information about copyright status can be determined."
    2. File:Open wire loading 1926.png - Copyright tag looks ok, but in the "Source" field, along with a raw link to the source, it's a good practice to provide the specific details of where the image came from, such as: name of the publication, issue number, date, page number.
    3. File:Carrier growth 1928.png - Same as #2.
    4. File:Typec C carrier terminal.png - Same as #2.
    5. File:Four crossarms.png - Same as #2.
    6. File:Carrier type D frequency allocation.png - Same as #2.
    7. File:Carrier frequency translation.png - Same as #2.
    8. File:BSTJ mixer 1921.png - Best to include a link and specific details.
    9. File:Bell type C carrier amplifier.png - Same as #2.
    10. File:Bandpass filter.png - Same as #2.
  1. This is not required for GA, but I encourage going through the images you uploaded to Commons and adding categories: Commons:Categories. I find the easiest way to identify relevant categories is to search for similar images and see how they're categorized. You can also check out the options on high-level categories like Commons:Category:Signal processing, Category:Telecommunications in the United States, Commons:Category:The Bell System Technical Journal.

Other suggestions

Done
  1. MOS:NOTSEEALSO: "As a general rule, the "See also" section should not repeat links that appear in the article's body." Done

Discussion

I have revised the lead to put the purpose of carrier telephony before the technical description, see if that is what you had in mind. I added the Bell System and Bell Telephone Laboratories upfront, given their contribution, that makes sense. I have also started consolidating the short paragraphs. I am working through the citation-needed tags, they should be covered now. seealso should be OK, addressed legend issue, address passive voice issue.— Preceding unsigned comment added by WhaleFarm (talk • contribs) 02:07, 25 June 2026 (UTC)

  • @WhaleFarm: Thanks for hopping in and making some good changes already! Adding more comments, and I'll check off the completed ones as I go along. Dreamyshade (talk) 05:07, 25 June 2026 (UTC)
    I couldn't find the source for image 1, so I've replaced. In the process, I found some good buisness discussions from Bell, I've added a few short paras. WhaleFarm (talk) 18:43, 25 June 2026 (UTC)
    I put full source info on each image. WhaleFarm (talk) 19:43, 25 June 2026 (UTC)
    You're going to need to give me some guidance on catagories. Nothing seems even close, should I invent a whold to set of catagory trees? WhaleFarm (talk) 19:52, 25 June 2026 (UTC)
    Great! For example, I added the following images to the following categories. Doesn't have to be a perfectly precise category; even just adding an image to Commons:Category:Telephony is better than no categories.
    Dreamyshade (talk) 23:01, 25 June 2026 (UTC)
    I want to thank you for jumping in and making improvements. Editing WP is far more rewarding when it's colaborative. I'm well versed in the tech, having heard my father prepare lectures on it in the 1950's, but I don't know the WP ecosystem, and probably will never learn. Oh, and I can't spell to save my life. WhaleFarm (talk) 14:51, 26 June 2026 (UTC)
    You're doing great at working with the Wikipedia ecosystem! This was a strong article even before review. I've been editing for many years and still learn new things from other editors all the time. It's fun to work on this topic because my grandfather was a technical editor at Bell Labs for decades. He didn't talk about it much, but I inherited his folder of papers related to his work, and I've enjoyed learning about his projects. Dreamyshade (talk) 16:25, 26 June 2026 (UTC)
    large amounts of copper - page 8, section 4.3. "required over 1200 tons of copper". At current prices, $16M per phone circuit. WhaleFarm (talk) 03:49, 27 June 2026 (UTC)
    Based on pages 7-9 (and reading "Recent Developments in Toll Telephone Service" for additional context), it reads to me like the amount of copper was only part of the challenge that led them to eventually implement multiplexing, including because the engineers kept figuring out how to carry more signal on thinner wires and with coaxial cable? Dreamyshade (talk) 07:01, 27 June 2026 (UTC)
    To take advantage of coax, you have to go to carrier. I think the 29 paper has some images of early coax, if not, i'll find you some. For thinner wires you have to space the repeaters much closer. With 1200 tons of wire, the origonal system was a few repeaters, i think 3 or 6. With thin wires, it eventually got down to 1000 repeaters. The economies of doing that only make sense when carrier allows you to share that expense over 12-1000 channels.
    even going to 19 ga twisted pair only cuts the copper by 1/4 per pair compared to 13. That's still $4M of copper.
    Copper was really the only choice WhaleFarm (talk) 13:22, 27 June 2026 (UTC)
    Thank you! Since page 8 says, in addition to the copper requirements, that "the cost of loading itself was not trivial", and that loading coils were iron-core and then a nickel-iron alloy, it seems reasonable to say that both the amount of copper needed for the transmission lines and the amount of associated equipment (loading coils) contributed to pressure to find efficiency? I could easily be confused about something here though, and I appreciate your deep understanding of the subject.
    I'll get back to the source review very soon, just got involved in a couple other projects as well! Dreamyshade (talk) 07:34, 29 June 2026 (UTC)
    You're right. It's actually a good point, and an interesting complication. I pretty much avoided getting into the loading issue in the carrier tech section. Loading is a big subject, and there were a lot of discoveries, innovation, much to much to add to this article. It's a good subject to target a new article to, the story is pretty much lost in modern literature, but shows a lot of 1900s thinking. I'm trying to make something for crystals right now that's clear. I have a special interest in educating engineers using examples of some of the earlier designs, the creativity shown under the constraits highlight what we could do with our capabilities today.
    Don't sweat the schedule, I'm traveling in the Canada Rockies right now. WhaleFarm (talk) 15:07, 29 June 2026 (UTC)
    I've treated the table labels, with references WhaleFarm (talk) 22:32, 25 June 2026 (UTC)
    You realize that linking to carrier system is going to force me to do a more serious clean up there........ WhaleFarm (talk) 14:44, 26 June 2026 (UTC)
    Excellent! Yes, I also noticed several of the linked articles would benefit from some improvement, like long-distance calling. Dreamyshade (talk) 16:59, 26 June 2026 (UTC)
I treated the transition to digital, updated the end of the lead and added section near the end. I'l do a search for links, and short explainers WhaleFarm (talk) 20:49, 25 June 2026 (UTC)