Sunday, 21 February 2010

Sound Card Trouble

In the last few days I'd been thinking that maybe there was something wrong with my transmitted signal from home on packet, as no stations were receiving me. I had a listen to my own signal and the deviation didn't seem too high but there was a lot of hum in the background (not a harmonic of 50Hz, some other frequencies).
Changing the transmitter power made no difference so it wasn't RF getting back into the audio leads. The hum went away when I unplugged the cable between the Edirol UA-25's output jack and the radio's data socket. I tried the onboard sound and the noise also went away. So the hum was coming from the UA-25 but never used to be there.
When I moved cables around at the back of the PC, the noise went away. In the end I found it was a cable between the Record output of my audio amplifier which went to a 3.5mm stereo jack plug. This had been unplugged from the back of the PC but left nearby. At some point, the tip of the jack plug had touched the metal case of the PC.
Somehow this had created a loop which made noise from inside the PC appear on the outputs of the USB sound device. For now I'm sticking with the internal sound for radio stuff because it's good enough.
The output volume of the internal sound had to be lowered to 10% to stop the IC-7000 being overdriven through its data port. The data port is far more sensitive than a 'line' level signal which you would get from a mixer or other equipment. It also doesn't work when keying the rig by a CI-V command. When the IC-7000 gets the transmit command by CI-V, it expects you to be talking into the microphone. That's a pity because it would have meant one less serial port being used.
Things I don't like about the IC-7000's serial port:
1)Distorted on narrow FM - see the picture below. This is FM noise (no signal present) from the data port on the 7000 (filter setting 2). There is a sharp bandpass filter between 300 and 3000Hz but something is distorting the audio after this.


Below is the same noise but taken from the headphone socket:
There is less noise above 3KHz and the noise sounds more erm...noisy, the distortion on the data port made it more crackly sounding.

2)No squelch - the audio is taken from before the squelch control so is useless for recording programs which use VOX. There is a squelch output so you could build a circuit for muting the audio.
3)No operation by remote control - must use a serial/parallel/VOX interface to key the rig for data. The rig's own VOX doesn't work on the data port either

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