2400 bit/s modem sound (V.22bis)

The V.22 bis handshake

V.22 bis splits the line into two bands: the caller transmits on a 1200 Hz carrier and the answerer on 2400 Hz, both at 600 symbols per second. After the V.25 answer tone and 75 ms of silence, the answerer sends unscrambled binary ones at 1200 bit/s. With V.22 DPSK coding that is a steady stream of 270° phase steps, a tone slightly off 2400 Hz.

The caller listens for 155 ms, waits a further 456 ms and sends S1: 100 ms of the unscrambled double dibit 00/11. Hearing S1 is how each side learns the other can do 2400 bit/s. Both then send scrambled ones at 1200 bit/s and, 600 ms later, at 2400 bit/s with 16-point QAM (§6.3.1.1).

In the UK, Germany, France and Australia the answerer also sends an 1800 Hz guard tone, a national option in §2.1. Switch the country to hear it.

Play Unscrambled ones Play S1 from the caller

Approximate: which point inside each quadrant carries which pair of bits (Figure 2/V.22 bis).