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I recall in the early 90s a type of modem (for analog phone lines) that let you conduct a voice call while maintaining a data connection. It sent bursts of data during the natural silences in conversation. What was this called? I thought it was a "data over voice modem" or "voice over data modem", but Internet searches didn't help me find examples of what I'm thinking of (possibly because the term now means something else).

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The technologies being referred to are Analog Simultaneous Voice and Data (ASVD) and Digital Simultaneous Voice and Data (DSVD), both of which become available in consumer-grade modems sometime around the mid 90s and allowed simultaneous voice transmissions over data connections with the voice transmission generally using 14.4 kbps (later ASVD implementations) or 9.6 kbps (DSVD) of the available bandwidth.

Simultaneous voice-data technologies could not at that time be used for internet connections, but were intended for "point-to-point" connections where (say) one home user could now call up another home user for a quick game of deathmatch Doom and shout "loser" over the phone line whenever they fragged them. Clearly a very important innovation, but work productivity was the main marketing focus of the technologies and provided an early instance of the voice-data sharing technologies such as video conferencing, whiteboarding and collaborative editing that would go on to become important mainstays of modern business.

Although ISDN and to some extent DSL were both in existence at around the same time, very limited availability in many countries plus high price points plus incompatibility issues precluded a wider uptake, whereas almost all home computer users could generally afford a cheap 28.8 kbps or 33.6 kbps modem and had access to a conventional phone line. On the downside, the proliferation of incompatible ASVD/DSVD implementations meant that successfully making a simultaneous voice-data call required the person at the other end to have almost exactly the same model of modem that you had, a situation that early ISDN adopters would also have been familiar with. The eventual widespread adoption of DSL effectively killed off any significant advancements in dial-up modem technology as far as I can tell and ASVD/DSVD became an evolutionary dead-end.

Further Reading/Viewing

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  • The question is asking about something that wasn't "simultaneous", but would instead switch back and forth between voice and data. I can imagine a variety of ways of implementing such a thing that would work reasonably well using a separate earpiece and microphone, using a couple of analog delay chips to avoid cutting off the starts of people's speech or letting the starts of modem tones through the earpiece. Not super cheap, but plausible just using analog circuitry that would have been available even in the 1970s.
    – supercat
    Commented Mar 27 at 15:03
  • Perhaps the OP could confirm what they're looking for? Maybe it just appeared that the data was "bursted" between voice transmissions because the original implementation of the V.61 standard was so slow as to only permit neglible data transfer while a voice signal was being sent. You also would have had to have some way to monitor the data transfer to know what was going on. There are ways to do what you're describing but the timeframe does suggest ASVD/DSVD modems - the relevant point being the OP said it was "a type of modem" rather than "some device" that you could hook up to a modem. Commented Mar 27 at 18:33
  • @supercat What you describe was done in the early 1960's to maximize the capacity of long-distance cables by multiplexing analog voice calls. Time-assignment speech interpolation
    – user71659
    Commented Mar 27 at 21:35
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    OP here. Yes, I am specifically asking about a modem (I think an internal PC card, but it could have been external). It was from a major manufacturer (USR Hayes, Intel, etc).I recall two-page spread ads in magazines that showed a voice waveform or graph, with flatline gaps in conversation, and "data" penciled in with arrows pointing to the gaps.
    – Tankboy
    Commented Mar 29 at 18:13
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The first "high speed" internet service that was available in my neighborhood was ADSL (Asymmetric Digital Subscriber Line), which was provided by the local phone company simultaneous with, and over the same copper pair as our POTS (Plain Old Telephone Service.) The DSL service used a frequency band that was entirely outside of the POTS 300-4000 Hz pass band, and a "splitter" near the point where the phone line entered the house provided low-pass filtering for the telephone sets, and high-pass filtering for the DSL modem.

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    I'm not sure what's funnier - that your answer completely fails to address the question, or that it does so by describing contemporary ubiquitous broadband technology as though it were something that disappeared after the advent of a more recent and vastly superior advancement. Commented Mar 25 at 13:27
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    @Eight-BitGuru, "Completely?" Hows that? ADSL was invented in the 90s (OK, turns out it was the late 90s, and OP asked for "early 90s".) ADSL is a standard for a modem. It works over a residential dial-up phone line, and it lets you use the phone line for voice calls while it also is being used for data at the same time. Even if ADSL is not the specific technology the OP remembers, it sure looks to me as if it ticks off most of the boxes. Commented Mar 25 at 13:42
  • @Eight-BitGuru, Also, So what if ADSL still is in-use today? What if somebody asked a question about some C programming language toolkit from the 1970s or the 1980s, and what if my answer said something about "stack frames." Would that be off-topic just because modern C implementations still use stack frames? Commented Mar 25 at 13:48
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    @SolomonSlow The question asks about the “early 90s” time frame and uses the past tense, giving a strong implication that the technology was common then but is not so now — neither of which applies to ADSL.  (The first ADSL standard was approved in 1998, but wasn't available here until 2000, and wasn't popular for a few years after that.)  Nor does ADSL send “bursts of data [only] during the natural silences in conversation”.  So while ADSL is a partial match for the question, and probably worth mentioning, it's clearly not a full answer.
    – gidds
    Commented Mar 25 at 16:18

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