V.FC: technical details¶

V.FC's specification was never published. This page records what D-Modem's version (dsp/vfc_data.cpp) is based on. For an overview, see V.FC.

Where it comes from¶

The Courier V.Everything's DSP (a TMS320C5x) runs V.FC from overlay 7 of its firmware image IDSDL302 (flash offset 0x3c2f0, loaded at program 0xb000). The courier-emu project (docs/vfc.md) found the overlay, the symbol rate table, the point table and the two trellis codes. courier-emu has no licence for most of its files, so D-Modem takes its findings as facts and copies none of its code.

For this work the overlay was run in a small C5x simulator of D-Modem's own, with known bits in place of the scrambler's output, and a model of the mapper was written until its points matched the firmware's: 80 symbols at each of the 35 rates below with the 32 state code, and several with the 8 state one. dsp/test/vfc_test.cpp checks D-Modem's encoder against 48 of the Courier's points at five of them. All addresses below are program addresses in that overlay or in the resident code under it.

Rates¶

The rate index r (0 to 6) gives (r + 6) x 2400 bit/s, 14400 to 28800 (0xb77c reads it from the rate word). The symbol rates are V.34's six. Above the speeds in the table in V.FC a frame's points would leave the point table, so those are taken to be the limits.

Frames¶

The data goes in frames of 8 symbols. A frame carries b = ceil(8R/S) bits for rate R at symbol rate S. Where 8R/S is not a whole number, some frames carry one bit less: in each cycle of 5 frames (7 at 2800 baud) the long ones come first, then the short ones, and the very first frame of the call is a short one (0xc88b, the pattern at 0x3a2).

From b, with q = floor((b - 32) / 8) and k = b - 32 - 8q:

The 16 + k shell mapping bits of a frame pick the 8 symbols' rings with V.34's shell mapping (9.3/V.34), M rings with M the smallest for which M^8 >= 2^(16 + k) (4 to 8). Which symbols carry 3 bits and which 2 is a 16-bit pattern by k (0xcb66: aaaa aaab aaaf aabf aaff abff afff bfff, 2 bits a symbol from the low end); a short frame takes one bit from the last symbol.

The bits are taken in order, least significant first: for each symbol its 2 + q bits (the low 2 the quadrant change), then its shell mapping bits, which go into the shell mapper's number at the next higher bits.

Points and trellis codes¶

For each symbol:

1. The quadrant z = (z + the 2 bits) mod 4.
2. The point index is the uncoded bits plus ring << q.
3. The subset c = 0 5 6 1 2 7 4 3[s0 + 2z] (0xcb2e), where s0 is the trellis encoder state's low bit.
4. The point is entry 2 x index + (c >> 2) of a quarter-plane table (0xaecf in the resident code, vfc_points in D-Modem), turned clockwise by c & 3 quarter turns. The table's points lie on the lattice of x + y odd, which is V.34's lattice turned by 45 degrees; courier-emu found that its energy shells hold the same points as V.34's, in another order.
5. The encoder's next state: the bits of state | z << 5 each pick a tap word (seven taps), the taps are XORed into B, and the state is (B ^ ((B >> 5) & (B >> 7) & 3)) & 0x1f. One set of taps (0xcb5f) is V.32's 8 state code, the other (0xcb58) a 32 state one. The encoder starts at state 0 and z at 0.

The scrambler is V.32's (1 + x^-18 + x^-23 or 1 + x^-5 + x^-23), which the Courier runs on bytes; D-Modem runs V.34's bit by bit, which has not been checked against the Courier's byte version.

The Courier also has a precoder (0xc9c5) and a non-linear encoder (0xc9f8) like V.34's, with one more term in the latter's series; D-Modem uses neither.

D-Modem's start-up¶

With +MS=VFC the V.34 pump runs V.34's start-up and sets bit 34 of its MP (reserved in V.34). When both MPs have it, both directions use V.FC's data mode at the fastest rate up to V.34's choice that V.FC has at that symbol rate, with the 32 state code when the MP asks for 32 or 64 V.34 states and the 8 state code for 16. If there is no such rate, V.34's data mode runs.

The Courier's own start-up¶

What is known so far from the overlay (scripts of signals at 2400 baud, run by 0xc529):

• The answerer (entry 0xb000, script 0xc560) sends 16 symbols of silence, 256 of alternating points (like V.32's AC), 16 of the other pair (CA), 256 two-phase and 1024 four-phase training symbols, 16 more, then repeats a rate word until it hears the caller.
• The caller (entry 0xb028) stays silent until its receiver has chosen one of five scripts.
• The training uses V.32's 4800 bit/s points and its scrambler.
• The DSP reports rate choices from the signal to noise ratio to the 80186 supervisor (call 0x83c8), which takes part in the negotiation.

The line probing, the symbol rate change and the rate negotiation have not been read.

Not done¶

• V.FC's own start-up, and so a call to a real V.FC modem.
• The rates below 14400.
• Precoding and the non-linear encoder.
• Rate renegotiation in V.FC's terms (D-Modem's is V.34's, then V.FC's data mode again).