On Thu, Oct 17, 2024 at 12:27:30AM +0200, yoann.le.bor...@free.fr wrote: > Regarding your approach of white box reverse engineering, have you had a > look at Jatin Chowdhury's work on the subject? > (https://github.com/jatinchowdhury18/AnalogTapeModel). > He went quite far simulating the full chain and his plugin has acquired > quite a reputation.
Yes, I know of his work. But reading the paper, I don't think his physical model is correct. Unless I'm missing something, the algorithm he uses amounts to - upsampling, giving S(t) - adding the bias signal B(t) - evaluating the hysteresis loop on S(t) + B(t), giving M(t) - downsampling, which filters out the HF bias. But that NOT how things work in reality. Imagine following a small fragment of tape (corresponding to one sample in the upsampled domain), as it moves past the recording head and away for it. The magnetic field of the record head extends well beyond the head gap, decreasing in strength as you move away from it. So the small tape fragment will go through to many cycles of the bias + signal waveform with decreasing amplitude. The resulting magnetisation is not just the 'average' of M(t). For an example, see <http://kokkinizita.linuxaudio.org/linuxaudio/downloads/magnetisation.png> X-axis is time in microseconds, bias frequency is 100 kHz, bias amplitude is 1.0, signal (assumed constant during the short time shown) = 0.5. The green trace shows the magnetic field as seen by a small tape fragment as it moves past the recording head. The red trace is the resulting magnetisation, taking the hysteresis effects into account. The constant value it converges to is the signal that remains on the tape. This is what I simulate, and it is way more complex than what is described in Chowdhury's paper - that would just be the average value of the red trace while it is still at full amplitude. If the signal is HF, it can't be assumed constant during the time the field decay takes, it may well go through a complete cycle or more. This is what causes the 'self erasure' at higher frequencies which Chowdhury doesn't take into account at all (AFAICS). Ciao, -- FA _______________________________________________ Linux-audio-dev mailing list -- linux-audio-dev@lists.linuxaudio.org To unsubscribe send an email to linux-audio-dev-le...@lists.linuxaudio.org
