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Independent Sideband, the forgotten mode

Discussion in 'Discussions, Opinions & Editorials' started by KL7AJ, Nov 13, 2008.

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  1. KL7AJ

    KL7AJ Ham Member QRZ Page

    Independent Sideband has great possibliities for amateur radio. Back in the 1970s there was some flirtation with the technique (see May, 1977 QST) but it never really caught on. The main application was with could send your photos on upper sideband and audio on lower sideband. It was actually quite a lot of work to set this up...but with new DSP techniques it should be a snap.

    But I see something even cooler about ISB. With a very well aligned system, you could run FULL DUPLEX SSB on HF. (Maritime radio has used full duplex H.F. for years, but with widely separated channels. With a good ISB system, ou could talk and listen at the same time on the same "carrier" direction on usb and the other on lsb. With a high isolation hybrid or circulator, you could even use the same antenna. I think this is something worth experimenting with.

  2. K4KYV

    K4KYV Subscriber QRZ Page

    Armed Forces Radio and VOA used to run feeder links to their overseas stations using HF ISB. They could be heard on odd frequencies outside the regular shortwave broadcast bands. You could hear one program on USB and another on LSB. This was before the days of long distance transmission by satellite.
  3. KL7AJ

    KL7AJ Ham Member QRZ Page

    MOTHER is the necessity of invention. :)

  4. KE4YGS

    KE4YGS Ham Member QRZ Page

    Maybe it isn't necessary but I can envision the need for a timing chain somewhere for both sets to lock onto for really accurate signal processing. Maybe transmitted by the TX terminal and the Rx terminal locks its PLL, ref osc or its LO to it and vice versa so there is no freq disparity. Maybe I'm a victim of the modern age. I seem to remember some Navy testing circa. 1968-9 where they tried USB for voice and LSB for TTY. I think the intent was the same as a primitive orderwire on Satellite Links. Raven Electronics was working with the Navy on it. Was a dismal failure as I remember. I can think of several ways to make it work but they all involve dollars in fairly significant quantity compared to the cost of the radio. Voice of course, as in full duplex voice would be much less touchy.

    Last edited: Nov 13, 2008
  5. KL7AJ

    KL7AJ Ham Member QRZ Page

    In an ISB transmitter, you use the same generator...they WILL be locked.

  6. K7JEM

    K7JEM Ham Member QRZ Page

    I don't think that would work. You still have a strong transmitted signal very near the RX signal. This would cause extreme overload to the RX side. A hybrid or circulator just isn't going to be enough. Even opposite sideband suppression would have to be immense, generally it is only about 50dB. Even 100dB wouldn't be enough.

  7. K8ERV

    K8ERV Ham Member QRZ Page

    I was associated with a 10kw Collins xmitter on Eniwetok. The two sidebands carried separate (multiplexed) RTTY channels. Why not?

    TOM K8ERV Montrose Colo
  8. KB2VXA

    KB2VXA Ham Member QRZ Page

    Hmmm, sounds like the Kahn mono compatible AM stereo system. I know where there's an exciter siting in a rack just waiting to be re tuned for 160M and fed into a 1KW transmitter already there but alas, that'll never happen.
  9. N2RJ

    N2RJ Ham Member QRZ Page

    I think it could be done with modern digital signal processing.
  10. KL7AJ

    KL7AJ Ham Member QRZ Page

    Funny you should mention that. I talked to Leonard Kahn once, about 20 years ago. He was a cranky eccentric back then, and I guess he's still going strong. He made some fine equipment though. We had a Symmetra-Peak on our FM station for did exactly what it was supposed to.

  11. W5HTW

    W5HTW Ham Member QRZ Page

    Want to experiment with it? Easy! Put two identical SSB transceivers side by side. Separate antennas. 100 watts output. Same carrier frequency, say 7250.000 khz. Put one on USB and one on LSB. You now have ISB.

    Ok, transmit on the USB rig. How much can you hear on the LSB rig at the same time?

    I spent a lot of time with ISB with the government. MUX, (AFSK multiplex) Not much use for it in amateur radio, though your suggestion of voice on one sideband and sstv on the other might be workable. Possibly a problem with bandwidth restrictions.

    Last edited: Nov 14, 2008
  12. K7KBN

    K7KBN Ham Member QRZ Page

    We used to come up with KMI in Oakland CA from my ship (USN) when we were at anchor in places like Hong Kong. We used ISB with a WRT-2 transmitter/R-390 receiver and full duplex. The antennas were maybe 700 feet apart, if that. At 1KW from the WRT-2, there was not even a hint of cross-sideband interference. Talked home quite a few times.
  13. KI4NGN

    KI4NGN Ham Member QRZ Page

    I agree with some here that it wouldn't work.

    It's one thing to discuss transmitting one piece of information in one sideband and another piece of information in the other sideband simultaneously, but it's altogether very different to discuss transmitting on one sideband while simultaneously receiving on the other.

    No digital processing is going to solve the problem of having the extremely strong transmit signal as little as 600Hz away from the receive side. That signal could be millions of times stronger than any signal coming in the receive side, and it would have to be filtered out at the beginning of the front end or else the receiver would be worthless. There would also have to be a huge degree of RF isolation between the transmitter and receiver within the rig itself.

    The last poster discussed doing it with transmit and receive antennas 700 feet apart. If it was done, then I suppose that distance reduced by magnitudes the level of the transmitted signal on the receiver, but how many hams can have antennas 700 feet apart?

  14. AB0WR

    AB0WR Ham Member QRZ Page

    Are you *sure* you were operating full duplex with both transmitters on the same frequency, one in USB and one in LSB?

    I've done full duplex with separate xmtr/rcvr setups but not on the same frequency.

    Think about 2m FM repeaters and the efforts required to allow full duplex operation and then try to duplicate it on HF. Very, very difficult -- even for DSP.

    tim ab0wr
  15. W5HTW

    W5HTW Ham Member QRZ Page

    Not only will you have a transmitter signal extremely close in frequency to the receive signal, it will be extremely close in proximity as well, like a couple of inches. Inside the same box on the table! Imagine Field Day with an HF transceiver on say 14,210 and another, on the same table, on 14212. Even on widely separated antennas, it would be amazing if either station could hear much of anything.

    I really don't want to discouorage experimentation. I have worked extensively with ISB operations.

    I would imagine the R390A and the WRT-2 were on widely separated frequencies, receiving on one, transmitting on a very different one. That is indeed full duplex.

    Exactly what overseas telco's did back in the late 50s and through the 1960s. , using full duplex SSB. Here's how. They put up one transmitter with two (or more, but usually two) sidebands being transmitted to the same geographic
    area. For example, from a transmitter in Calif, to a receiver site in
    Hawaii. That would mean two one-sided, signals TO Hawaii. To get the
    other side of the conversations, a similar transmitter in Hawaii would
    be beamed to Calif, only at a greatly removed frequency. For example,
    the one from Calif to Hawaii might be on 5500 khz and the one from Hawaii to Calif on 5850 khz. Or further apart. You can, of course, do
    the same with SSB. Or AM. Transmit TO the distant point on one
    frequency and FROM the distant point on a greatly removed frequency.
    Exactly what we do with repeaters!

    ISB can be used, as you say, to transmit video on one sideband and audio
    on another. From the same transmitter.

    In commercial use, ISB uses a pilot carrier. But whether it has such a
    carrier or not, there IS one present, just suppressed, and at a couple of inches away inside that transceiver box, it is going to be a very strong signal. That is the center frequency. Even at a few milliwats of suppressed carrier (55 db down) the adjacent opposite sideband is not usable for reception.

    I worked with four channel MUX in ISB operations. The exciter had two audio
    inputs, one for each sideband, upper and lower. But each sideband's
    audio input is fed by extremely sharp filters, that will put two audio
    signals into that input. Example. The carrier frequency might be
    5500.000. On USB two different audio signals are applied. The first
    ranges from 400 to 570 Hertz, for 170 hertz shift, and the second from
    700 to 770 Hertz, for 170 Hertz shift. Each is independent information,
    so is in effect, two intelligent signals. We duplicate that on the
    lower sideband. That allows for AFSK signals on the same transmitter,
    to be received by four different stations at the distant end, perhaps in
    Bolivia, Argentia, Brazil and Columbia, for example. Each station tunes
    to his desired signal.

    The carrier is transmitted, but 20 db below PEP power output in the
    sidebands. The carrier is used as a phase lock signal, for distant
    receivers to lock on. Then they use a "sideband slicer" which allows them to select which of the four AFSK signals they will receive. This is only useful when you have two or more stations in the same general direction from the transmitter.

    It is very old technology, and probably led to the invention of PSK31.
    This was AFSK mutliplex (MUX) operations in the mid 1960s, and it
    followed the earlier FSK 850 Hertz RTTY. It was, actually, sound card
    operation before sound cards were invented!

    Now, as to voice operation, that, too, is possible, but only two
    channel in analog operation, due to the bandwidth required for intelligible voice. Theoretically, 300 to 3000 hertz below and 300 to 3000 hertz above the carrier. Those same dual channel exciters (TMC SB1E models) could do
    voice just as easily and often did. Reception at the distant end was
    simply a matter of selection USB or LSB. In these operations, no pilot
    carrier was necessary, but was usually used to provide long term stability.
    Again, it was 20 db below PEP ouput.

    The application in amateur radio would be pretty limited, about what
    you say, SSTV on one sideband, voice on the other. Or some sort of data
    on the other. In that case, it is at the receiving end where the difficulty lies. The fellow there must be using either two separate receivers, or a DSB receiver with two channel outputs. This makes it very difficult to apply to amateur radio.

    In another application, some microwave links used the same technique,
    but with up to 48 total channels, 24 of them narrow shift below the
    carrier freq and 24 of them narrow shift above the carrier freq. In
    some of these systems, the total shift was only 85 hertz (cycles back
    then!!) between Mark and Space. Most of our HF stuff was 170 Hertz, but
    we could run two ISB channels at 850 hertz, one upper and one lower.
    And we did a little of that.

    Later, more sidebands were created with the use of digitized voice.
    Certain crypto equipment could run as many as 16 channels, one of them
    digital voice, into a single SSB exciter. And, if needed, a second
    crypto unit could do the same on the opposite sideband! That meant two
    secure voice conversations, totally unrelated to each other, and 32
    channels of data, all independent, at the same time. On HF. Neat.

  16. K7FE

    K7FE QRZ Lifetime Member #1 Life Member Platinum Subscriber QRZ Page

    Tom, My father helped blow that place up.

    I foresee bandwidth complaints, but hey, look at HI FI SSB.

    Terry, K7FE
  17. KL7AJ

    KL7AJ Ham Member QRZ Page

    ISB is authorized for amateur use by the FCC. This was brought up in the may 77 QST article.

  18. AF6LJ

    AF6LJ Premium Subscriber QRZ Page

    That would be totally Kool.
  19. WA4ILH

    WA4ILH Ham Member QRZ Page

    Full duplex?, yes. same frequency?, hardly. for example, just below the 20 meter band, KMI transmitted to ships on channel 1229 or 13,161 Khz and listened on 12,314 Khz. (a "split" of nearly 850 Khz) On the 8 Mhz band, KMI transmitted to ships on channel 804 or 8,728 Khz and listened on 8,204 Khz. They also operated in the 2,4,6,16,22 and possibly the 26 Mhz band.
    Tom WA4ILH

    BTW, The R-390A was one fine receiver. Sure was a lot better than it's "replacement" the R1051.
    Last edited: Nov 14, 2008
  20. AI3V

    AI3V Ham Member QRZ Page

    Naval Communication Station, Diego Garcia,BIOT (British Indian Ocean Territory) checking in! (1980-1981)

    Back on VQ9 the transmitters and receivers were seperated by about ten miles, So there was not much crosstalk between them. That seperation was nothing on a HF path, compare that to a uhf repeater, where 10 miles would be terrible from a systems point.

    I was in for the R1051, we had a couple 390's for messing around, but didn't use them for NAVY traffic, the 1051 had the advantage of being synthesised, so the radiomen could just dial in the frequency to the 1 khz and go (5 knobs!) , and that revolving band turret in the front end was fun to watch go round!

    Note the ISB position all the way CW on the mode switch on the attached thumbnail.
    Also note the two meters,And two headphone jacks, one for lsb, one for usb, or both for isb.

    Here is a website to restore your 1051

    Now That's a bandswitch!:eek:

    Last edited: Nov 14, 2008
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