Zero Beat again

I'd like to consdier the zero beat on the direct conversion receiver again.

Suppose that the CW singal is received on 7010KHz. The local oscillator generates 7009KHz. The difference between the received signal and the local sinigal is 1KHz and you can hear it at audio frequency at 1KHz.
The frequency of the local oscillator is increasing to 7010KHz and the difference is getting smaller. The audio frequency is getting lower. When the received signal and the local oscillator is the same, there is no audio singal that is Zero Beat.

At each side of zero beat, you can hear the audio signal as 1KHz. In this example, you can hear 1KHz audio signals on either 7009KHz or 7010KHz. The receiving characterisitics called "double-sideband effect". It means that you can hear the audio tone for both side of the frequency.

This characteristic can be used to advantage. It gives you a choice of 'two spots' that you can choose for the reception of a CW signal - you can tune your oscillator to above or below the incoming signal and still get a suitable tone for reception of the wanted signal. One spot may have an interference advantage over the other by changing the pitch of an adjacent and unwanted signal whilst keeping the pitch of the wanted signal the same.

If the transmitter frequency is negatively offset (TX frequency is 600-700 Hz below the RX frequency), one needs to tune on the “high side” of zero beat in order to have the transmit frequency fall within the other station's receiver pass band.