When the Droitwich transmitter is behaving itself the 1kHz pips look to be quite good on 198kHz here. I'm really just trying to describe the
difference in harmonic distortion levels (of the time pips) seen on the analyser data between times when the transmitter has the sibilance issue and when it is working normally. When it is working normally the pips look fairly decent to me and I get 0.7% THD on the demodulated pips. I guess it's up to the individual to decide if this is a fairly acceptable sine wave or not. When the sibilance issue was present the THD shot up to 10%. That difference is all I'm really trying to describe
Obviously, there's not much point in the BBC trying for Hifi levels of distortion for a LW AM transmitter but I can't see any significant odd order harmonics (the components of a square wave) when I look on the analyser when the transmitter is behaving itself without the sibilance issue.
I'm only a few miles from the transmitter and I'm doing all my tests and demodulation in the digital domain using data from the spectrum analyser and I'm doing it in daylight hours to minimise propagation effects.
Obviously, it would be risky to try and assess the transmitter quality (wrt the time pips) with a low cost domestic LW AM receiver as the rapidly rising pip tones would contribute to (AF) AGC induced distortion, distortion in the IF amplifier and detector and maybe even distortion in the AF stages.