As an alternative to Fourier Transformations, one can also use classic audio filters applying high pass and low pass filters, or a band pass filter (or similar EQ settings with the aim of boosting the signal around 2KHz, and silencing the signal below about 1KHz and above 4KHz.<p>Caveat: I love doing this in a musical context for a specific part (e.g. vocals or drums or guitar) amongst many, and so I tend to fine-tune the exact frequencies in the context of the other parts so they evoke the memory and emotion of a telephone line more than the exact specifications of a POTS[0] line.<p>[0] POTS = plain old telephone service or plain ordinary telephone service
AMR is a very "cheap" codec to run, if you want something to sound like it's been over the GSM phone system you could just run it through AMR and back again. Possible patent encumberance but implementations are available in ffmpeg.
The "too good" part is due to missing compander - very similar or identical to ADPCM compander for better analog lines. ADPCM itself in form of G.711 is still used in older digital phone exchanges.
Coming soon, AI prediction of what untelephonic voices would have sounded like! Starting with a detelephonised <i>One Night in Bangkok.</i><p><a href="https://www.youtube.com/watch?v=rgc_LRjlbTU" rel="nofollow">https://www.youtube.com/watch?v=rgc_LRjlbTU</a>
The phone system has been mostly digital "forever." Pretty much by the 1990s, even if you had an analog copper phone line, it was digital by the time it made it to the switchboard.
Every now and then I hear someone call a radio station using a landline phone -in good repair-. The difference in quality (despite the LL bandwidth filtering) compared to mobile-phone callers is often unmistakable. For one thing, the mobile artifactual garbling and missing audio segments stand out.<p>So it's helpful to stipulate what kind of 'phone audio' you're trying to mangle into 'fidelity'.
Eventide’s classic DSP4000b has algorithms for phone call sonic simulation. The patches are highly modular (and thus can be inspected for analysis - at least down to the blocks used to construct the patch) and it would be interesting to see how that old school hardware DSP approach compares to the author’s design and implementation.
I remember landlines sounding quite different from the author. To me they sound much better than cell phones. It sounds as if the other person is actually on the other end of the line. Cell phones sound very artificial with compression artifacts and such.