Leaving aside the subject of drift in free-running VFOs (pre PLL  & TCOs ) one regular feature of old CW transmitters, particularly where they were simply high-power oscillators, was the chirp you got as the frequency dipped on key-down whilst the HT re-stabilized.

In an effort to simulate this an audio keyer is being prototyped to bring back those old days.   The video shows the first hack ( 20 minutes today ) .  The sound is terrible as I have not added low pass filtering to the square wave output and it is playing on a piezo sounder.   It is also dual-tone (selectable by design).

The next step is to change the chirp algorithm to natural log as this better mimics the capacitive recovery effect on old HT keyed circuits.

73  G0FOZ

Update – some drift on this one, a different chirp and amplifier stage was added (3rd-order RC LPF too ).