It appears that the 'hetro' function (utility or standalone) has problems in Csound 5 that it didn't have in 4.

The backstory: I was playing with a complex sound (Big Ben's bong, actually...) and I thought hetro would be a good way of getting at its components. I normally stick with Csound 4 [for reasons I think I've explained before] so that was what I used to create a 'het' file and play it back with 'adsyn'. The result, when I used about 40 harmonics, was very gratifying -- sounding very close to the original 'bong' -- but I was curious to know if using Csound 5 would give a different result.

Indeed it does... and not in a good way! The playback was a good quarter-tone higher! It doesn't matter which machine/OS or Csound version I play the het file back on (though I find the gain has changed by a factor of 32768 in 5! (:-/), it sounds the same.

To make sure it wasn't some artefact of the source audio, I created a WAV file of a simple 'A' note (440+880 Hz sine waves) and ran it through the same process. Here's the original file: http://goodeveca.net/hetro/A440_880.wav

and here is a recording of the outputs generated by the various 'het' files: http://goodeveca.net/hetro/A_hetro.wav

In order the generating files were:
1. Csound 4 default het output (no options).
2. Csound 5 default.
3. Csound 5 with '-f440' (!!)
4. Csound 4 with '-f440'

Number 3 is particularly out of line -- nearly 4 semitones sharp! In fact, if you look at the het file, the base frequency it homes in on is (exactly) 550 Hz!

If you want to look at all the files individually, they are available at http://goodeveca.net/hetro/ including a zip of everything if it's more convenient.

I don't know anything about heterodyne filter analysis, and I can't find any info about it on the web [maybe I'll eventually get motivated to dig Moorer's article out of the campus library] so I'm not competent to look at the problem. I hope somebody else can.