                           Auto-adaptative equalizer

Linear and synchronous equalizer.
If needed, downstream part can be recursive.

Clock must be centered, by adding to upstream modules filters (two ray model
included) the opposite of their group delay at central frequency.

The following circuits are simulated, in turn:
- two ray model, if corresponding module is present,
- an automatic gain control, based on the knowledge of modulated signal mean
  level before equalization,
- a carrier recovery, by multiplication and division by 2, 4, ... of the
  signal phase.

Then, the resultant periodic temporal signal is continuously equalized
until convergence (the user specifies a convergence time, integer multiple
of the frame period Tm).



At every symbol,
- the nearer nominal constellation symbol is determined,
- for each modulation axis, the rms error (difference between A or B signal
  level and nominal signal level) is computed,
- every coefficient i of the two delay lines sets is incremented
  proportionnally to error at time t and to signal sign at time t - iT.
Incrementation of central coefficient leads to an automatic and continuous
gain and phase control of equalized signal.

If a two rays model is present upstream, the relative amplitude Ro of ray
#2 may be progressively increased to measure equalizer "hold" performance.
In the same way, sampling instant To may be varied.
If error is too high, Ro can be decreased: in this case, before computation,
coefficients recover the values saved before the last incrementation.

In any case, choose the higher possible value of Ns and, consequently, of N,
to avoid a blockage of convergence for some parameters values.
If needed, try the "RAND" (random) option of "Digital modulator" module.

Options:
- Error may also be computed from the real symbol value (applied to digital
  modulator). In this case, generally, convergence is faster.
- Only extremum symbols in AM, and symbols situated on the external square
  of QAM constellation, may be selected fort incrementation of coefficients;
  doing so, convergence is sometimes faster.
- Errors may be replaced by their sign.
- The incrementation step Eps is adjustable:
    - a too small value slows convergence down,
    - a too high or too small value may cause equalizer to diverge.

- It is possible to compute the maximal selectivity -20 log(1-Ro), so called
  signature, in acquisition mode or hold mode.
  In both cases, a maximal rms error ERQM, and a maximal selectivity RoLogMax
  must be specified
  - In acquisition mode, initial selectivity applied to equalizer is
    incremented until equalizer is unable to reach ERQM, all coefficients
    being put to zero before incrementation.
    Incrementation is done by dichotomy, accuracy of result being set by dRo.
    If result is 31.5 dB for dRo = 1 dB, that means that ERQM is not reached
    for 30 dB, and reached for 31 dB.
  - In Hold mode, same process, but coefficients keep the values of the
    last incrementation.

- If the canal terminates by a "Loop" module varying the notch frequency of
  a "Two rays model" module, a full signature contour is obtained.
  In this case, specify in "Two rays model" a sufficiently small selectivity
  in order that convergence be warranted for every notch frequency.

