Soundcard Levels for Digital Modes - The Bad Waterfall

The image above is a too common sight on the waterfall of digital mode operations. This article aims to help promote the proper configuration for having a clean signal on the waterfall to avoid interference to other users and also to vastly increase the effectiveness of your signal.

What you see in the image is a PSK-31 operator who is over-driving their signal to their rig. This is what it looks like when your audio drive level is causing ALC to engage.


  • Rig _always_ set to max RF power output, whatever it is capable of (SSB mode)

  • Adjust sound device drive levels to control desired rig output power level

  • Use the OS sound device drive levels first to adjust, followed by rig menu controls and then digi mode software controls to fine tune.

  • Do NOT set the soundcard device as the default sound device for the operating system.

  • ZERO ALC is the right answer, there is nothing to be gained with ALC deflection on digital signals. You should easily reach the rig's rated output power without any ALC action if you follow these simple steps.

The causes

  • Digi mode software or software packet modems have the audio output level too hot

  • The hardware interface or PC soundcard is set to too high a drive level

  • The rig itself if it has a built in soundcard has the menu levels too hot for the inbound audio level.

  • The rig itself if the mic input is used has the mic gain too high for the incoming audio level

When dealing with audio chains, the path the audio takes from the point of origination to each progressive step from one stage of processing through to the final output has to be managed carefully to stay within the dynamic range of each stage. If you exceed the range, you introduce distortion or compression and will have unwanted results.

Think of operating voice in SSB mode. You set your rig for 100W output, adjust your mic gain and away you go. If you whisper into the mic, you will output very low power. If you get close to the mic or raise your voice, the output power will go up. This is what we are going to do here, just using the audio levels driven by the OUTPUT of your digi mode software.

You will be able to run 1W through max power with ZERO ALC if you follow the instructions here just by controlling your audio drive level into the rig.

The fix is pretty straightforward and requires thinking in a logical step by step from the origin of the signal source through to the transmitted signal.

The Solution

  • Start with your rig, the transmitter. Put your rig in full output power, 100W or in some cases 150 or 200W output. The RF power output of the rig should be on its maximum output power, period. (For SSB in USB mode)

  • Find where your digital mode software sets its drive levels. In FLdigi it's the number at the far bottom right corner next to the AFC button, left of the diamond (RX level indicator). Set that number to 0, which is maximum drive. In JS8Call it is the lever on the far right side in the lower pane, slide it all the way up.

  • Make sure the sound device that is being used, could be a sound card in a PC, could be a sound card built into your rig, could be an external sound card like a Signalink is set up the following way.

    • It should NOT be the default device for the operating system or alarms, bells, blips and beeps will all wind up going out over the air. Just make sure your operating system sees the device.

    • You get to the devices in Windows by going to Control Panel -> Hardware and Sound -> Manage Audio Devices. This is a good time to click the Playback tab and Properties and rename the Playback device to "Rig Drive Level" and the Recording tab device to "Waterfall Level". You will need to reconfigure digital software to select those new device names if you make this change.

    • Find and select the device in your digital mode software configuration, I prefer the open source fldigi software.

    • Set the operating system default levels for the devices, input (Playback device, drive level) and output (Recording device and waterfall level) to 0% to start

    • With the Signalink USB, set TX and RX knobs to 12 o'clock on the box as a starting point. If you can't get levels high or low enough in the next steps, use those knobs. Once set up you can run with the knobs alone if you like. Also be sure your Delay setting on the Signalink USB is set to very short, I may have run mine at the shortest stop on the pot but I forget now. You don't really want VOX delay for digital modes, it's more for if you are feeding voice audio in to hang TX between words so it doesn't chatter. You don't want it chattering and you don't want it hanging too long after TX ends.

  • Using a dummy load or a tuned antenna (this is going to max the output of your rig), fire up the test tones or "tune" button in your digital mode software. You should now see a tone going through your rig and generating an output signal level.

  • Using the operating system sound card device levels, while sending a tune, go to the Playback device tab in control panel, select levels and adjust that level while watching the rig meter. You want to adjust that device level until you see the max power out of the rig that you intend to ever use for digi work, but NO ALC indication at all on the rig meter. A 100W rig should show 100W or just shy of it with no ALC. Leave that device alone once this is set. I recommend only running 50W at the most and not for long.

  • Using the digital mode software, find the setting to control the audio output drive level in the software you are using, and while transmitting, lower that output level until you see your transmitter output level drop to the desired level for operation. In fldigi this is the box with the number in it at the far lower right corner called the TX level attenuator with -30db the lowest it will go.

    • Typically due to the high duty cycle of digital modes and their very high sensitivity, a 100W rig on SSB mode (digital modes are almost entirely done in USB SSB mode) should have the audio input level dropped to show 25-50W output power at most or you risk overheating your rig.

    • If you followed this properly you will see maximum rig output power possible with ZERO ALC deflection. Even a very old rig like a TS-140 or TS-680 with little user controls will behave this way.

    • If you are seeing any ALC operation at all, check that your rig's RF output power is set to its maximum.

  • If you followed the above and cannot get the output audio drive level low enough, do the following.

    • Check that your rig doesn't have menu setting for input/output sound levels if it has a sound card built in. (My Kenwood TS-590SG menu 71 needs to be at 1, it's extremely hot on input level). You may need to lower the input levels for that. If you don't have enough waterfall output, adjust the output drive levels.

If you follow this carefully, you will have a whistle clean output signal (unless you have other issues like RF in the shack or buzz on your interface cables from ground issues). This is super critical because it will make your signal far more effective for receiving stations to decode it and it will stop interference to adjacent signals.

Odds and Ends and Waterfall/RX level set

If your rig has a built-in sound card as my TS-590SG and the IC-9700 I have do, there is another variable. They have input and output drive levels specific to their USB interfaces within their configuration menu systems.

With a built in sound card in your rig, you will need to balance the rig device controls in the rig's menu system.

For your INPUT levels, so the audio coming out FROM the rig to the software, you similarly need to adjust those levels so you aren't over driving the software you are using. For the waterfall level, the RX levels, you want to adjust the device in the Recording tab of the sound devices control panel.

Each software has different indicators, but with fldigi there is a VU meter in the bottom center bar . If your level is too low (diamond reads black) your software may not decode the incoming transmissions. If it is too high (diamond reading yellow or red) it is clipping and decode will not work well either. Take your RX level with RF gain wide open and AGC off, tuned to a passband with no signals and bring it up so the VU meter shows -50 to -40 range.

JS8Call as another example, is very sensitive and it's CAT (not to be confused with rig control) indicator should be about mid-way, 40-60db and not hotter. JS8Call on my setup requires lower input levels for audio coming from my rigs so I do a quick adjustment to tweak it.

This Also Affects FM

With FM a similar concept of how to configure drive levels also exists and it's a bit trickier to solve for. This applies not just to sound card based modems but also with packet hardware TNCs.

With FM you want to adjust your output drive level just shy of where the clipper/limiter for FM deviation cuts in or it too will distort your signals much like ALC action does on SSB. The quick and dirty way is to transmit into a dummy load and monitor on another radio while sending test tones. You want to gradually lower the audio level until you can audibly hear the volume drop. Clean packets are effective packets.

For AFSK modes over FM for NBEMS, also you want to center the signals at 1500 to avoid either end of the frequency response. Older or more poorly designed FM rigs can have quite a bit of attenuation on either end of the waterfall. Try to keep your signals and width of mode as centered as possible and IMO a bit narrower than where you see the waterfall drop off sharply.

Also, I always recommend running with open squelch if on simplex, as you can easily decode signals that would be well below the squelch threshold. Play with MT-63 on open squelch for simplex, it is pretty remarkable.

The image below is a Thor 16 signal on 80m propagated over a path of a few hundred miles that is greatly magnified.

It was a 200W signal driven by a linear amplifier and was 20 over S9 to my station at the time of testing. It is fairly clean as a result of following all the guidelines to configure it.