SM1000 Part 10 – First Over the Air Tests

Last night I visited Matt, VK5ZM, with a SM1000 Beta. He configured the SM1000 to interface to his IC706 using the CN12 connector patch socket. This allowed an RJ45 cable from the SM1000 to connect to the RJ45 mic/audio connector on the IC706. A few level adjustments and he was using FreeDV to talk to Andy, VK5AKH on 40m! It all went very smoothly.

Here is Matt in action, using the SM1000 in the hand-held mic configuration. He also tried a Heilsound headset, which also worked well.

Here is the audio received by Andy:

I was monitoring Matt’s FreeDV signal using FT817 and FreeDV on a laptop. I did notice a slope to the spectrum, and a cross shape to the scatter diagram (screenshot below). This suggests some of the carriers are at a lower level than others. This is bad news as these carriers will hit the noise floor sooner, causing bit errors and poor speech quality. I really need to look into this carefully, testing a selection of radios with a carefully crafted test signal to measure the tx transmit spectrum slope and flatness. This may explain why people running Flex Radios and SDRs get consistently good performance with FreeDV.

SM1000 Progress Update

Since the last post Rick has been doing some fine work on the enclosure. My beta PCB works well so we are happy with the electronics and firmware. Once we have checked the next revision of the enclosure we plan to start manufacturing the Beta units after the Chinese New Year holiday. I’ll start taking pre-orders shortly.

6 thoughts on “SM1000 Part 10 – First Over the Air Tests”

  1. Fantastic guys! The audio sounds really natural, and the smartmic surprised me as I’d imagined something the size of a packet of 20 cigarettes bur it appears to be more like a match box.

    When’s the Chinese New Year? I’m waiting to pre-order!!

    Stu

  2. We perhaps should have noted in this article that;

    – Radio was a bog standard IC706Mk2G (generic shack-in-a-box)
    – Voice compressor was disabled
    – All DSP processing, including noise blanker disabled
    – Level into mic was set just *under* where the ALC would begin to reduce gain (still in linear region)
    – Radio was making 90W PEP with an average power of 17W

    I know from a technical perspective that the audio path from either the microphone connector or data port is the same. I’ve already explored, measured and plotted these responses previously for other reasons.

    What we are seeing in the received spectrogram is the shape of the crystal filter within the IC706 (and the FT817 for that matter). Its the combined result. The small standard Japanese 455kHz crystal filters used in these radios are not that great, they are borderline useful even for voice. Plots of the modulator in the IC706mk2G show amplitude variations of up to 4dB in the pass band with phase variations at the edge of the xtal filter of more than 100 degrees.

    This radio is definitely amateur grade, meaning it’s linearity and performance is mediocre at best. The IC706mk2G was great in its day, but its age and performance are now certainly showing.

    For FreeDV to succeed it needs to be able to cope with these “mediocre” modulators and receivers. This does mean that the demodulator and perhaps modulator need to be able to account for the “flat-ness” of the crystal filters especially on transmit, something which David is now thinking about.

    However make no mistake it worked, right out of the box and the SM1000 successfully sent and received the waveforms and a QSO was made. So there is no doubt that the first test was a resounding success, meaning David can push on bringing the SM1000 into the world.

    I for one will be watching David’s blog for the first signs of pre-orders. I’ll certainly be in there from day one looking for DX contacts, these are exciting times.

    73, Matthew VK5ZM

  3. Matthew,

    Just checking – the issue David saw would have happened if you used the 1200 pin on the TNC interface instead of the 9600 pin, because it has pre-emphasis. And it’s not clear what you connected to from what David wrote. On a rig with a 9600 connection, like the 706, that’s definitely the interface to use.

  4. I am extremely excited to see how the unit will perform on my VX1700. The transceiver has a 9600 baud port and has very little audio processing compared to many amateur rigs. I am hopeful that it will provide a very clean a useful platform for the SM1000.

  5. been waiting for this for quite some time now, finally time to give digital voice a shot :)
    so where is the pre-order page Hihi.

    Regin

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