The soundcard is 24 bit. Like every other 24 bit soundcard, its dynamic range is less than 24 bit, but it works at 24 bit. Thermal properties of basic electronic components prevent converters getting very near 24 bit without cryogenic cooling.
This cannot be reproduced at will. So far, no cassette has made a distorted recording twice. It might go away completely if I only recorded at 16 bit but I've been using the soundcard at both 16 and 24 bit (32 bit floating point is the file format) at will, for years with no problem. If the problem resides in the soundcard, and not in the computer or WinXP, it is a problem that just developed.
Yes 16 bit is plenty for the material. It is probably plenty for the processing too but the relative amount of quantization error is markedly smaller for each step when in floating point format, so I choose to use it.
As I said in the beginning, this is speech only, although I have no reason to believe that the same thing could not have happened had I been recording music. In this material, breaks between words, and pauses between phrases, sentences, etc., are the norm. The strong high frequencies are only, and always, there during words.
Could it be some really atrocious and unwanted attempt at sample rate conversion somewhere? e.g. a driver dropping or duplicating samples? or performing nearest neighbour interpolation or something?You can always find a bug on a specific combination of hardware, software and driver versions that no one put together before.Cheers,David.