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C4FM Cross banding....hmmmmmmm Surely

Discussion in 'VHF/UHF - 50Mhz and Beyond' started by W4EGE, Apr 15, 2018.

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  1. W4EGE

    W4EGE Ham Member QRZ Page

    I’m not sure if this has been talked to death yet or not, but at least with respect to the ftm400 is has not.

    I would like to pass C4FM DN through something (likely an ftm400) from an HT to a node. On paper this should work. Transmit VHF DN from an FT2D to VHF on a FTM400 and then out UHF (analog) to a node. Once that UHF RF is received by another C4FM radio/node it should be intact right? RF is RF...how I understand this is just a a matter of what the endpoints are using to decode the C4FM signal. It Should not matter what the machines are doing in the middle with that DN RF...on paper, it should pass through should it not?

    In support of that idea...I can hear C4FM encoding on my Ft60...but of course it can’t decode it. If that signal is heard by any other C4FM radio it would know what to do with that RF as long as it was in DN mode.

    So...what am I missing? Why wouldn’t a FTM400xdr in analog cross repeat mode pass a C4FM transmission?
     
  2. K7JEM

    K7JEM Ham Member QRZ Page

    It might work, but probably not. A C4FM signal will get distorted when passing through an analog repeater. Some of the low frequencies needed for proper operation will be cut off by the filtering in the repeater. If you could just take discriminator audio from the RX and apply it to the analog modulator, at the right level and with minimal filtering, it might work.

    Digital signals like this have to have a very flat response from nearly DC to around 3000 Hz. I think the analog connection in the middle will destroy the needed bandwidth.
     
    W4EGE likes this.
  3. W4EGE

    W4EGE Ham Member QRZ Page

    Here is where the different levels of license are apparent. Thank you for your thoughts.
    I had not thought about the filtering aspect. That being said I had another thought.
    If regular fm analog modulation is 25khz wide...it would seem that C4FM at 12.5khz would sit nicely in the center of the transmitting/receiving frequencies. I would think NEGATING the need to filter anything since C4FM is more compressed.

    Shouldn’t a 12.khz wide carrier frequency pass through something intended to be 25khz wide (analog)?

    I guess I’m missing something still...I’ll call it technician ignorance!
     
  4. KV6O

    KV6O Ham Member QRZ Page

    The 25kHz wide is the conventional channel spacing used by hams for a 5kHz deviated signal. It's not 25kHz wide. It more like 16kHz for a 3kHz audio signal.

    https://en.wikipedia.org/wiki/Frequency_modulation
    https://en.wikipedia.org/wiki/Carson_bandwidth_rule

    An FM analog repeater is made to pass a limited bandwidth signal, and does so by demodulating the received signal, and re-modulating it for transmission, sometimes with different characteristics - flatter or altered frequency response, added or removed PL tones, etc. This arrangement will not pass the full 12.5kHz (or so) of a C4FM signal.

    A non-inverting linear transponder would "pass" the signal, but even then there will be degradation, which is why a proper digital repeater demodulates/decodes the received signal and regenerates it for transmission.
     
    W4EGE likes this.
  5. W4EGE

    W4EGE Ham Member QRZ Page

    Thank you for that!

    I talked to Yaesu USA Today and asked this same question to them. The response back was a bit of a surprise. It was simply a matter of the FTM400 only having a single digital vocoder on VFOA only. The technician stated that it’s possible that there could be a future FTM400 that will repeat C4FM...this was not the first time they had heard this question.
     

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