Has Analog TDMA ever been a thing?

[fog]

Do you ever get a nutty idea in your head and you can't quite shake it?

Back when I was first getting into DMR, I got to reading up on TDMA. One oversimplified explanation of DMR in particular was that it took 60ms of audio and "compressed" it so it could be transmitted in 30ms. And it left me wondering if TDMA really had to be digital.

Has analog TMDA ever been attempted (or deployed)? It seems that, in theory, you could buffer 60ms (or whatever) of audio, speed it up 2x, and transmit it in a 30ms block, with the receiver then listening to one "slot" or the other and playing it back at half the transmitted speed. I suspect the technology to do this reliably probably emerged at about the same time as usable digital voice codecs, since I can't imagine that trying to buffer 60ms of audio and then speed it up 2x near-instantaneously would be remotely practical with tape. But I've had a hard time trying to Google "analog tdma," since the matches are all comparing the two things. So I'm curious if there were some attempts to do TDMA with two analog voice signals.

To be clear, I'm not remotely trying to argue that there should be a two-slot analog TDMA system, or that it would confer any advantage. I just wonder if it's a thing that has been tried before.

 

[budevans]

Do you ever get a nutty idea in your head and you can't quite shake it?

Back when I was first getting into DMR, I got to reading up on TDMA. One oversimplified explanation of DMR in particular was that it took 60ms of audio and "compressed" it so it could be transmitted in 30ms. And it left me wondering if TDMA really had to be digital.

Has analog TMDA ever been attempted (or deployed)? It seems that, in theory, you could buffer 60ms (or whatever) of audio, speed it up 2x, and transmit it in a 30ms block, with the receiver then listening to one "slot" or the other and playing it back at half the transmitted speed. I suspect the technology to do this reliably probably emerged at about the same time as usable digital voice codecs, since I can't imagine that trying to buffer 60ms of audio and then speed it up 2x near-instantaneously would be remotely practical with tape. But I've had a hard time trying to Google "analog tdma," since the matches are all comparing the two things. So I'm curious if there were some attempts to do TDMA with two analog voice signals.

To be clear, I'm not remotely trying to argue that there should be a two-slot analog TDMA system, or that it would confer any advantage. I just wonder if it's a thing that has been tried before.

TDMA is by design digital only. So, no it can't do analog.
FDMA can be analog, but it uses the full bandwidth of the frequency. So you can't have slots.

 

[fwradio]

There was a multi carrier technology that started out analog and then got scrapped for digital. The company that invented it was called Comspace and the technology was DCMA— Division Coded Multiple Access. It actually had multiple carriers within a 25kHz emission mask. Analog development was 6:1. In digital they had it 8:1. They put all their eggs into the 800 MHz basket. When Nextel bought up almost all of the 800 MHz spectrum there was no market and they went bankrupt.

 

[kb5udf]

Vietnam era single frequency analog repeater, closest thing I could find, have a look at this.

S.F.R. - Single Frequency Repeater
https://apps.dtic.mil/dtic/tr/fulltext/u2/666781.pdf

 

[RFI-EMI-GUY]

Vietnam era single frequency analog repeater, closest thing I could find, have a look at this.

S.F.R. - Single Frequency Repeater
https://apps.dtic.mil/dtic/tr/fulltext/u2/666781.pdf

I don't think there is any TDMA demonstrated in that experiment.

There was supposedly a single frequency repeater for mines that used bi directional amplifiers and pin diodes to steer the signal to go either direction quasi simultaneously on a leaky cable. The switching frequency was well beyond the modulation frequency of standard analog FM LMR radios so users would not hear the wideband generated. It would not meet FCC spurious limits if used above ground.

Sent from my SM-T350 using Tapatalk

 

[RFI-EMI-GUY]

Do you ever get a nutty idea in your head and you can't quite shake it?

Back when I was first getting into DMR, I got to reading up on TDMA. One oversimplified explanation of DMR in particular was that it took 60ms of audio and "compressed" it so it could be transmitted in 30ms. And it left me wondering if TDMA really had to be digital.

Has analog TMDA ever been attempted (or deployed)? It seems that, in theory, you could buffer 60ms (or whatever) of audio, speed it up 2x, and transmit it in a 30ms block, with the receiver then listening to one "slot" or the other and playing it back at half the transmitted speed. I suspect the technology to do this reliably probably emerged at about the same time as usable digital voice codecs, since I can't imagine that trying to buffer 60ms of audio and then speed it up 2x near-instantaneously would be remotely practical with tape. But I've had a hard time trying to Google "analog tdma," since the matches are all comparing the two things. So I'm curious if there were some attempts to do TDMA with two analog voice signals.

To be clear, I'm not remotely trying to argue that there should be a two-slot analog TDMA system, or that it would confer any advantage. I just wonder if it's a thing that has been tried before.

What you describe could be done, though the complexity of doing it today would be same as using a vocoder to digitize voice.

FHSS frequency hopped spread spectrum can be done with normal analog FM. There are many examples from cordless phones to Have Quick military radios. Bendix King made LMR radios with this capability for the feds, I have toyed with the possibility of applying FHSS to CB radios.

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[kb5udf]

I agree, I don't think the experiments I cited demonstrated actual TDMA, just was closest to an actual implementation of analog TDMA I could find.

Currently, though I am lusting over the idea of toying with single frequency TDMA-DMR repeater, as there are showing up online for about $500.

 

[n1das]

I agree, I don't think the experiments I cited demonstrated actual TDMA, just was closest to an actual implementation of analog TDMA I could find.

Currently, though I am lusting over the idea of toying with single frequency TDMA-DMR repeater, as there are showing up online for about $500.

The Hytera PD982 DMR portable can function as a TDMA single frequency repeater (SFR). The feature is an upcharge from the bare bones PD982 model.

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[kf8yk]

Bell Labs came up with TASI:

Time-assignment speech interpolation - Wikipedia

While not quite the same as TDMA, it is an analog example of time slicing of a communications path.

 

[fog]

Huh, thanks all! There's some really interesting stuff here. I never realized that FHSS was also used with analog FM.

The DMR SFR stuff is interesting, but the analog FM single-frequency repeater boggles my mind. I can't wrap my head around how the transmit wouldn't overpower the receive signal, short of using highly-directional antennas, which the linked stuff doesn't seem to describe as necessary.

I've learned a lot of neat stuff in this thread!

 

[RFI-EMI-GUY]

Huh, thanks all! There's some really interesting stuff here. I never realized that FHSS was also used with analog FM.

The DMR SFR stuff is interesting, but the analog FM single-frequency repeater boggles my mind. I can't wrap my head around how the transmit wouldn't overpower the receive signal, short of using highly-directional antennas, which the linked stuff doesn't seem to describe as necessary.

I've learned a lot of neat stuff in this thread!

A guy I worked with had a Panasonic FHSS cordless phone that failed. The PA in the base unit went out. I went to the FCC website and found the schematic and discovered that the phone used a fixed IF around 460 Mhz that was modulated by FM and then was upconverted to 2.4 GHz FHSS via a mixer. This means that anyone with a UHF scanner in close proximity coukd clearly hear what was being said. I showed the schematic to him and suggested he buy a real DECT digital phone.

I don't think that any of those single frequency repeater ideas have panned out as being practical for the reasons you mentioned.

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[fwradio]

As far as DMR SFR, I have used it and it does work really well. Hytera also now has the MD-782i full duplex radio that has the SFR option available for it. A little pricey, probably about the same as a PD982i with SFR, but it works really well and full mobile power. On VHF they have worked out really well.