IQ vs 4FSK

[KevinC]

In the latest beta SDS update it lets you display whether the radio is using IQ or 4FSK to decode a P25 signal. I’m guessing 4FSK=C4FM and IQ=CQPSK? I wonder how it determines which one to use? I know Icom professional radios used to pick automatically and that was a disaster.

Maybe they could give us the option to pick which digital modulator (in MSI speak) to use in a future update. Or just make all P25 use IQ???

 

[werinshades]

In the latest beta SDS update it lets you display whether the radio is using IQ or 4FSK to decode a P25 signal. I’m guessing 4FSK=C4FM and IQ=CQPSK? I wonder how it determines which one to use? I know Icom professional radios used to pick automatically and that was a disaster.

Maybe they could give us the option to pick which digital modulator (in MSI speak) to use in a future update. Or just make all P25 use IQ???

I've been experimenting with a P25 P2 system, scanning 2 sites. On one site (P25-simulcast) IQ displayed, talkgroups all showing IQ. On another site (P25 non-simulcast), 4FSK displayed, and talk groups all showing IQ while transmitting on this site. This is the system that has occasional garble, and in order to clear it, turn the selector knob and the scanner recycles through the system with perfect decode. If I selected IQ in theory, would the system ignore the stand alone site?

 

[KevinC]

If I selected IQ in theory, would the system ignore the stand alone site?

If "IQ" is CQPSK then no as it is backwards with C4FM (assuming this is 4FSK). But C4FM is NOT compatible with CQPSK (it would still work sort of/sometimes at best).

 

[prcguy]

IQ is “In phase” and “Quadrature” information which contains amplitude and phase information, usually demodulated and has nothing to do with modulation type. 4FSK, C4FM, QPSK, CQPSK, etc, are all modulation types.

 

[KevinC]

IQ is “In phase” and “Quadrature” information which contains amplitude and phase information, usually demodulated and has nothing to do with modulation type. 4FSK, C4FM, QPSK, CQPSK, etc, are all modulation types.

Ok.

 

[werinshades]

IQ is “In phase” and “Quadrature” information which contains amplitude and phase information, usually demodulated and has nothing to do with modulation type. 4FSK, C4FM, QPSK, CQPSK, etc, are all modulation types.

No idea what this means but it sounds cool. Why the interest and display option added for modulation type in regards to reception if one doesn't have anything to with the other?

 

[prcguy]

Sounds like a radio is giving you the option to use either an IQ or 4FSK demodulator for specifically demodulating P25 format, which might yield different results. Here is a snip from Wikipedia about P25 modulation types.

“Phase 1 radio systems operate in 12.5 kHz digital mode using a single user per channel access method. Phase 1 radios use Continuous 4 level FM (C4FM) modulation—a special type of 4FSK modulation[12]—for digital transmissions at 4,800 baud and 2 bits per symbol, yielding 9,600 bits per second total channel throughput. Of this 9,600, 4,400 is voice data generated by the IMBE codec, 2,800 is forward error correction, and 2,400 is signalling and other control functions. Receivers designed for the C4FM standard can also demodulate the "Compatible quadrature phase shift keying" (CQPSK) standard, as the parameters of the CQPSK signal were chosen to yield the same signal deviation at symbol time as C4FM. Phase 1 uses the IMBE voice codec.”

 

[KevinC]

Sounds like a radio is giving you the option to use either an IQ or 4FSK demodulator for specifically demodulating P25 format, which might yield different results. Here is a snip from Wikipedia about P25 modulation types.

“Phase 1 radio systems operate in 12.5 kHz digital mode using a single user per channel access method. Phase 1 radios use Continuous 4 level FM (C4FM) modulation—a special type of 4FSK modulation[12]—for digital transmissions at 4,800 baud and 2 bits per symbol, yielding 9,600 bits per second total channel throughput. Of this 9,600, 4,400 is voice data generated by the IMBE codec, 2,800 is forward error correction, and 2,400 is signalling and other control functions. Receivers designed for the C4FM standard can also demodulate the "Compatible quadrature phase shift keying" (CQPSK) standard, as the parameters of the CQPSK signal were chosen to yield the same signal deviation at symbol time as C4FM. Phase 1 uses the IMBE voice codec.”

It doesn't give the option, it's auto-magic.