Poor audio quality on 10M FM receive

[w-u-2-o]

With 10M FM DX starting to show a lot more life, I was, for the first time for me, doing some 10M FM listening. Unfortunately, even large signals were received with quite a bit of nasty crackling sounds.

There are not a lot of adjustments available in FM mode. I tried a variety of settings in Settings > DSP > Options with no improvement.

Any 10M FM aficionados out there using Thetis successfully?

[ea3aqr]

Same issue here.

[Cosmo1stgen]

Scott,

Not exactly the same but I have been on thetis with a transverter down converted to 10m for years on fm repeaters. It worked fine. Haven’t tried it with the latest thetis release so maybe something has changed? I do remember something like this when I first tried protocol 2 years ago did a data base reset.

Ryan

[KA1GXR]

Try wider receive bandwidth filter. For US should be at least 5Khz for wide deviation .It may default to 2.5 khz wide .
Tom
KA1GXR

[w-u-2-o]

5KHz sounds better than 2.5KHz but it is still horrendously bad.

[W2PA]

Never tried it until today, Scott.

I'm now hearing the KQ2H repeater on 29.610 (somewhat local) at S8 with a bunch of EU stations using it. I'm not hearing the effect you describe - it sounds fairly good. Of course the various users have better or worse audio but the repeater's signal itself sounds clean.

Edited to add my Options settings:
Buffer size 64, Filter size 2048, Linear Phase, BH-7

[w-u-2-o]

Not at home right now but I will make a recording and post it.

[w9ac]

FM audio sounds good here. Of course, it's bandwidth-limited but I don't hear any obvious distortion, artifacts, nor data breakups. The signal source is an Icom 7610 in FM mode transmitting into a dummy load. Icom audio input source: USB 3.0 sound interface and a studio-grade mic.

Thetis FM DSP settings:

Buffer: 256; Filter Size: 1024; Filter Type: Linear Phase; Filter Window: BH-7

Thetis 2.9.0.6 FW: P2 v2.1.18

Paul, W9AC

Edit: The IC-7610's maximum FM deviation is 4.4 kHz and 3 kHz is the maximum FM modulated audio frequency. Carson's Rule for required FM channel bandwidth results in 7.4 kHz * 2 = 14.8 kHz of significant sideband spectrum.

Thetis' actual Rx bandwidth at the 5 kHz setting is 16 kHz, or 8 kHz either side of center. So, Thetis should be able to demodulate typical NBFM modulation without issue if the sending station's audio bandwidth and deviation is limited to the normal NBFM standard.

[w-u-2-o]

I've been doing some more listening. It seems that I need to see a minimum of 18dB (3 S-units) of SNR before I get full quieting. Anything less than that I get a lot of noise. Worse, the quality of the noise is extremely unpleasant.

I'll publish some recordings when I get time (after this next massive snow storm is cleaned up, perhaps Friday).

Also, I've noticed that none of my audio metering seems to work, from Mic all the way to COMP.

[w-u-2-o]

And it would appear that I've got no modulation on transmit. I'm using VAC. Does FM work with VAC?

[w9ac]

And it would appear that I've got no modulation on transmit. I'm using VAC. Does FM work with VAC?

FWIW - VAC in FM mode works here with the WDM driver, but not ASIO. However, I am using a modified ChannelMaster.dll file and have full FM audio control and metering with ASIO. I only engage VAC for digital modes with WDM.

The modified ChannelMaster file disables all front panel audio connectivity and digital audio is routed directly to a Presonus ASIO sound interface without engaging VAC.

Paul, W9AC

[w-u-2-o]

It should not require any software mod's.

I'm suspecting a bad database. I probably will not have time to investigate that until tomorrow.

[w-u-2-o]

I made some good progress today on my FM problems.

It was not a bad database.

My transmit problems were operator error.

However, on receive there is something weird about audio scaling inside Thetis when in FM mode. When using VAC as a virtual line output normally RX1 AF (or RX2 AF) should be set to 100. At this setting, when Thetis produces a maximal audio output (0dB) this is the level that is sent out of the VAC interface for processing by external audio applications. This works perfectly for SSB, CW, DSB, AM and SAM. In these modes, with AGC gain set to 10dB above the displayed average noise floor on the panadapter, NR OFF, one sees an audio output of about -15dB peak with just noise. With a signal in the passband, this will climb perhaps as high as -2dB peak (obviously there are some AGC effects in play).

When in FM mode, with RX1 AF set to 100, with just noise the audio output is pinned at 0dB and is badly clipped on negative peaks (but not positive peaks). Obviously any signals that appear in the passband cause the same sort of audio output. However, if I reduce RX1 AF to 30, then the audio level for noise drops to about -3dB peak, negative peaks are no longer clipped, and it sounds OK.

Some oscilloscope traces to follow when I get a chance.