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by lxgr 820 days ago
No, unlike G.711, G.722 is lossily compressed using psychoacoustic concepts, and modems wouldn’t know what to do with the extra acoustic frequencies anyway.

G.722 might sound better to humans, but G.711 is definitely better for modems since it’s effectively just uncompressed PCM (disregarding a bit of dynamic range compression which modems are generally fine with).
2 comments
throwaway67743 820 days ago
It is compressed, yes, but at higher bitrates it is actually usable for modems/faxes, you're limited to low baud rates anyway due to jitter and sample intervals. But really the benefit is 16k sampling rate instead of 8, I'm not saying it's great but it's the best we've got.

It would be possible to mask this with a relatively simple FXS device to hide all of this and pretend it's a modem while packetising the actual data, but I guess the demand is almost zero so why bother.

I've been looking for a solution for many years to retain dialin services without having racks full of modems and trunks that are rapidly going out of fashion and at some point will not be available except via IP (and now we have the same problem at both ends), but they just don't exist.

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lxgr 820 days ago
> really the benefit is 16k sampling rate instead of 8

Regular modems can't make use of that, since the actual phone lines they were designed for (whether analog or digital via G.711) didn't support more than 300-3400 kHz anyway, so anything beyond 8 kHz sampling (corresponding to a Nyquist frequency of 4 kHz) is wasted on them.

Maybe you could go crazy with a custom modem that knows how to exploit the additional high-frequency components of G.722 while dealing with the lossy/psychoacoustic compression across all bands, but I doubt it would yield any improvement over G.711:

> It is compressed, yes, but at higher bitrates it is actually usable for modems/faxes

No, they're both exactly 64 kbps (after compression/"compression"), so you wouldn't be able to fit any more signal in it from an information theoretical point of view.

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ssl-3 820 days ago
Have we forgotten about T.38[1]? It is designed just to let analog fax machines/modems send documents over IP.

1: https://en.wikipedia.org/wiki/T.38

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throwaway67743 819 days ago
But only fax, and it's basically a software version of what I suggested above. It isn't actually a modem of any kind, it's basically the equivalent of sending a fax via sip messages. The closest we have is iaxmodem which does actually work for low baud faxes, but it's no DSP emulation for higher speeds, even if the line was good enough.
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