Re: the DSA parameter
I think that in the study of transients it is appropriate to develop new analysis methods in the time domain.
Problem with studying transients: the duration is, by definition, short. So the amount of information that can be acquired is small (few samples) and noise is high, because averaging cannot be used.
There's something I wanted to do: make a DAC output short pulses, and measure either the peak amplitude or the energy of the pulses. Varying the digital pulse amplitude, how accurately does the DAC reproduce the pulse? This would apply to 1-bit DACs mostly, because they only have a limited number of output bit slots to actually encode a short pulse.
I never got around to doing it, but since SigmaDelta DACs can be emulated digitally (output stage not included), if you got the SigmaDelta algorithm, you could do it digitally, fast, on a large number of pulses, and plot the probability density of the error.
If someone has a software DSD encoder, please share, I'd like to run a file through it, lowpass, and substract from the original.
Back to transients. These signals tend to have a wide spectrum, making FFT useless. So I've used my approach of high amplitude LF sine + low amplitude HF sine to measure self-heating induced bias current drift in an amplifier. As I explained above, the amplitude of the HF sine on the output gives the derivative of the transfer function directly (incremental gain) on the operating point set by the LF sine. It is a time domain measurement. As the output stage heats up, its transfer function changes. So the measured transfer function is different on each cycle of the LF sine. Plotting it as a gm wing diagram, the changes are exactly as expected.
Likewise, consider a Lin/Blameless amplifier. Its open loop gain depends on VAS transistor hFe which depends on its temperature, so it depends on what signal was played in the previous milliseconds. Open loop gain at one frequency can be measured via variations of output impedance, because that depends on the amount of feedback available. So it should be possible to inject an AC HF current into the output and measure its amplitude continuously, giving a real-time measurement of output impedance and therefore open loop gain at that frequency. Then, make the amp output a large voltage transient, and check how open loop gain changes and then settles back to normal.
This could be done with a soundcard (digital AM detection, max frequency 48kHz) or with an AM receiver, which would allow a much higher frequency like 1MHz.
G. Perrot did a similar test a long time ago, using DC. Basically examining the settling of the amplifier back to 0V after a sine burst.