Question on digital data transmission for flex-6500

Noel

Hello,

I'm trying to get a better understanding on how digital modes are transmitted from the Flex-6500. With voice, because it's already audio, I'm assuming that it just gets modulated with SSB and is then sent out. However, when dealing with the digital modes, is the I/Q data sent in a similar way by first converting the in-phase and quadrature streams into audio and then performing SSB modulation?

Thanks,
Noel 

Erika - KØDD

Jim Gilliam

KD0RC

The various digital modes are really analog signals when they hit the air. If you think about good ol rtty, there are two tones that represent a binary state, zero or one. Combinations of zeros and ones create the characters of a message. More modern modes like FT8 have more tones and different timing. Those tones can indeed be sent as audio to an SSB transmitter. Your 6500 set up in DIGIU, for example, is doing just that. Because you are feeding audio from a program like WSJT, it is just audio, not quadrature components. If an SDR radio was generating the modulation directly, it would likely be a different story. Did that help? 73, Len, KD0RC

Noel

Dale - AE6DS

Noel

Hi Dale,

Thank you for the response. So just to make sure I'm understanding it, this means that the DigiU modes in the Flex-6500 are strictly generating audio (i.e. there is no initial generation of I/Q samples and then conversion to audio)? Does the same follow for the analog modes supported on the Flex-6500?

Thanks,
Noel 

Noel

Dale - AE6DS

The Flex modes set the modulation (FM/USB/LSB/etc). The DIGU mode is USB modulation without additional audio processing. For transmitting, the audio is generated by the digi program (WSJT-X, FLDigi, etc). The Flex DAX system is doing audio transport. It replaces what would be a sound card and audio cables with most rigs.

KD0RC

Correct!

KD0RC

Noel, I think your confusion is around the word "digital". We call them digital modes, but the RF signal is a set of tones. In the simple rtty case, you can have a transmitter send mark and space tones by shifting the freq of the radio, or you can send two different audio tones to a SSB transmitter. While generated differently, the signal at the receiver is identical. Since the Flex is not generating any digital modes on its own, we rely on third party programs to generate the tones. They will work on any SSB transmitter as they are just audio. On receive, programs like CW Skimmer use I/Q inputs so that they can look at a band of frequencies. This lets you simultaneously decode many signals. This use of quadrature signals is not needed for generation of the signal for transmission as you are only creating one output at a time. Make sense? 73, Len, KD0RC

Ted VE3TRQ

KD0RC

Hi Ted, if the Flex were coded to have the digital modes natively, creating an audio stream would not be necessary. The Flex team decided not to clutter the radio with all of these modes, relying on third parties and a robust audio exchange (DAX) to accomplish digital communications. I was kind of hoping for the more popular modes to be implemented natively, but I certainly understand Flex's reasoning. Now that I have owned my 6400 for a few months, I don't miss having the additional modes implemented natively vs via third party apps. 73, Len, KD0RC

Noel

Dale - AE6DS

The software uses those I and Q streams to generate audio. That audio is what gets shipped to the Flex for transmission. Radios are big gearboxes that convert between EM signals in the MHz range and audio vibrations in the kHz range. If you want to transmit something with your Flex, it needs to start as audio. You can generate that audio with a tone (cw), your voice (phone), or software (digi).

KD0RC

Hi Noel, The Flex 6000 series radios and SmartSDR have not implemented other modulation schemes besides the usual, SSB, CW, AM etc.that you see when you drop down the mode window.  QPSK and BPSK signals, like PSK31, are generated in whatever program you are using (e.g. FLDigi), then passed to the transmitter (analog or SDR) as an audio stream. The I and Q streams used to generate the QPSK signal are summed after a 90 degree phase shift so you lose the identity of I and Q, and you are left with a plain old audio feed to the transmitter. If it didn't work this way, analog rigs would not be able to transmit on the digital modes.

There is a waveform manager in SmartSDR that I believe was intended to allow other modulation schemes, but I don't know if there are any waveforms currently available.

73,
Len

David Decoons, wo2x

DIGIU removes compression and equalization from the transmitted audio. This keeps the TX audio flat across the TX bandwidth. Dave wo2x