Delayed QSK

In most cases your own CW can be heard via the websdr. Audio latency is about 400msec +/-50msec. Incidentally it can go up for a short period of time. Muting audio while transmitting is then necessary, because it is delayed and disturbs sending CW.

Keep in mind that your own CW also controls the AGC in the websdr. The present AGC in the websdr is very fast and recovers fast. That supports QSK.

The more predictable and constant the latency, the better the timing of your own delayed received CW is known and the better QSK (muting the audio) is possible.


Software semi break-in QSK

See SSB operation.


PTT line for semi break-in

If using semi break-in the PTT line can directly be used to mute the audio. The delay has to be set to the maximum delay in the websdr e.g. 450msec.


Hardware for almost full break-in QSK

In order to mute your own CW signal the transmitted CW keying has to be delayed.

Your own CW keying can be derived from your external CW keyer, your transceiver or from the transmitted RF. Detecting the RF to the antenna also supports SSB.

A digital circuit is needed to delay the keying. The delayed keying is then used to mute the audio from the websdr.

An Arduino Uno can be used very well. The delay can be set by a potentiometer and the dots/dashes can be extended in time to handle variations on the latency. So adding pre and post extra mute time.

By adding a detection of CW in the audio of the websdr the Arduino can also be programmed to control the needed delay automatically. The detection of CW can be implemented by an envelope detection or by e.g. the NE567 tone decoder/phase-lock loop.


AGC behavior and CW

The present AGC in the WebSDR is not optimized for CW. For strong CW signals with a high signal to noise ratio, the start of a dot of dash can overshoot in amplitude. This sounds like clicks on the start of a dot or dash.

An APF post-filter is able to minimize this.

Audio peak filter (APF) for CW

This is a very useful option for CW. In combination with the 500Hz bandwidth in the WebSDR. The WebSDR’s fast AGC is controlled on the 500Hz bandwidth. This reduces the contribution of the CW signal to the AGC. Very weak signals will almost have no effect on the AGC. Just as if the AGC knee level (RF gain) has been increased.

When very weak signals increase further in strength the output level after the APF will increase also. Until they are strong enough to reduce the AGC gain. Then the output level will stop to increase. This is a smooth transition.



Schematic of an APF filter.



The center frequency range can be increased by using a double 10k potmeter. The resistor R4 is to be replaced by the second potmeter and a series 8.2k resistor.

Changing the center frequency doesn’t change the bandwidth.

High Q 18.2nF capacitors is a must! Simple ceramic capacitors will result in deviating center frequencies and higher bandwidths.