Low Frequency Audio Noise

Ed, K0KC

I recently built a cable so that I could use my Flex-6700 on the digital modes. I used the Tigertronics SignalLink USB sound card which normally is connected to my IC-7700. My normal mode of operation is JT65 and JT9 with Joe Taylor's WSJT-X software. Joe's software works great with my Flex-3000, my Flex-5000, and my IC-7700. With the Flex-6700, however, I see some very strong audio noise/spurs at the low end of the waterfall as seen in the screen shot below (click on image to enlarge): This noise is omnipresent when the 6700 is on a dummy load or if the receive frequency is changed, so it apparently is coming from the audio chain. I have been able to work a number of JT65 and JT9 stations as long as I stay away from the low end. SSDR 0.12.17 running under Win 7/32, DIGIU mode.

George KF2T

Same issue here. Had thought it was to do with the Signalink. Haven't tested further, since I've been hanging out up "north" with higher audio frequencies.

Ed, K0KC

George, The SignalLink did not display this problem with my IC-7700 to the best of my recollection. If I have some time this week, I will change the internal jumpers and cables back to what is required for the IC-7700 and check things out again.

Carl/K5HK

Ed, Wonder if adjusting the lower frequency response in the RX equilizer setting might not elimiate this. It appears to be an area which isn't really optium for digital communications anyway? 73, Carl/ K5HK

Ed, K0KC

Carl, Apparently the equalizer (on receive) is not functional on the Line 1 output from the 15 pin ACC connector. I could not see any effect on the noise as I adjusted the sliders. You are correct in that there is not much JT65 or JT9 activity in that region, but the large noise signal does affect my level and waterfall adjustments...I would like to "zap" it if possible. The problem will be moot whenever the software implements DAX.

Tim - W4TME

Ed, You have the RX filter low cut on Slice A set to 0 Hz. There is no useable audio information that low. I suspect that the 7700 had an audio high pass filter down around 100 Hz or so to remove the very low frequency noise that the sound card is not filtering out properly. Set the low cut RX filter setting on Slice A to 100 or 150 Hz.

Ed, K0KC

Tim, I set the filter to various values up to 200 Hz and saw no difference on the waterfall. I also see nothing on the panadapter at the low end which could be causing this noise... which is why I suspect that it is being generated somewhere in the audio chain.

Keith - AE5XN

Ed/George, Interesting - I'm not seeing this at all with my Signalink USB hooked up to my 6700's ACC HD15 connector while using DM780. My SSDR filter is set to 0Hz to 3000Hz in the screen capture below. Out of curiosity, if you try to listen to the USB Audio Codec Microphone audio input of the Signalink through your speakers or headphones in Windows, can you hear any low-frequency noise showing up on the waterfall (providing your speakers can reproduce <100Hz)? Flex 6700, GPSDO, SSDR V0.12.17.18545, Radio HW V0.12.2.2 Keith - AE5XN

Ed, K0KC

Keith, I will do some more testing tomorrow, but I noted that your setup is different from mine in the Transmit Control Panel. First of all, you have MIC selected and I have LINE selected. Next, you have COMP on and I have it off. Thirdly, you have not selected ACC while I have. Lastly, you have 20 dB selected and that is not available to me since I did not select MIC. I am not sure if any of this makes any difference as some of these controls have not yet been implemented properly; perhaps Tim can comment. I was using WSJT-X in my screen shot and you are using DM780 in your screen shot. I fired-up DM780 here and saw some "stuff" near zero audio frequency, but it may have been legit signals. My audio levels in DM780 were not optimized, so I will have to experiment with the levels some more before I draw any conclusions.

Ed, K0KC

I also forgot to mention that I am using two panadapters (split mode) and you are using one...this should not make any difference, but we are working with beta software now, so who knows?

George KF2T

For what it's worth - a third picture with a third perspective. You can pick out the noise below about 50 Hz (also visible is a JT65 signal). Filter on SSDR set to 0-3300 Hz. True, there isn't anything useful to do with those few Hz, but...

Keith - AE5XN

Ed, George, I'll try duplicating Ed's settings tonight to see if I can see anything with DM780, and then also look with WSJT-X. My thought here is that some of the high-frequency aliasing that's currently in the audio chain may be getting folded back to low frequency in the WSJT-X waterfall, and it isn't happening (or reduced) in DM780, due to differences in their waterfall algorithms. I'm just wondering if the codec fixes in the next SmartSDR preview will solve this issue. Keith - AE5XN

Keith - AE5XN

Ed, George, I'm still not seeing it on my end, either in WSJT-X or DM780. Not sure why? Keith - AE5XN

John Stuart KM6QX

Keith, Looks like you are probably correct about the aliasing noise folding back into the very low audio spectrum. Below is my 6700 receiving a 4,100 Hz 'test tone' that also pops up at 100 Hz, below the audio filter bandpass (you can see the bandpass skirts). As the test tone was moved towards 4,000 Hz, the aliasing(?) peak moved towards 0 Hz. So any data tones inside the audio bandpass near 4,000 Hz will produce images down in the 100 Hz and lower range. John Stuart, KM6QX

Keith - AE5XN

John, Thanks! I figured that might be what's going on based on my past experience in characterizing class D audio amplifiers and codecs at TI. Keith - AE5XN

Ed, K0KC

This problem still exists for me in 0.13.10...might even be a bit worse.

John Stuart KM6QX

Ed, I just check for low frequency audio aliasing and couldn't find any here. If I moved the low end audi filter up to about 400 Hz, I found some 60Hz pulse noise (and numerous odd harmonics) but was way down below the (no antenna) noise level (see 1st graplh below). Then (this is wierd) every 1 or 2 minutes, a 20 Hz peak + harmonics rise out of the noise for about 10 seconds (2nd graph). But these are really low amplitude (inaudible) audio noise, but obviously my Delta-44 sound card 'hears all'. John Stuart, KM6QX PS: I love the ability to edit my posts (but I still need a spell checker).