V.wb: wideband on a bare pair¶

A phone line only carries sound up to about 3.4 kHz, which is why dial-up modems top out around 56 kbit/s. A bare pair of copper wires (one you own between two buildings, an old alarm circuit, a dry pair rented from a phone company) has no such limit. V.wb is D-Modem's own mode for such a pair. It uses frequencies up to 36 kHz and reaches about 149000 bit/s each way.

It needs D-Modem at both ends, and hardware that samples the line at 48 kHz or more: the pico-serial WB stick (see Softmodem hardware), or any sound codec that fast. It is not ADSL and does not connect to a phone company's equipment.

For the signal, the start-up and the measurements, see the technical details.

Using it¶

At 48 kHz and above, V.wb is the only mode, and the line behaves like a leased line: no dial tone, no ringing.

AT&L1S0=1        at the answering end
AT&L1            at the calling end, then ATD

If the connection drops, the modems reconnect by themselves. Error correction and compression run on top, as on any other call.

How it works¶

The signal is split into many narrow tones, 187.5 Hz apart (the same idea as ADSL and Wi-Fi). At the start the modems measure how clean each tone is, then put more bits on clean tones and fewer on noisy ones. One direction uses the lower part of the band and the other the upper part.

Speeds in simulation¶

These were measured on a simulated pair with noise, interference and a clock offset between the two ends, with no bit errors. A short cut of the line costs one retrain, and error correction resends what was lost. One end uses about 1.5% of one processor core.

Still to do¶

• Moving bits between tones during a call as the noise changes.
• Trellis coding, for a little more speed.
• A test on a real pair with real WB sticks.