Regarding the encoder name: I went for " hmp3 MPEG Layer III audio encoder [...] Utilizing the Helix MP3 encoder" as "Helix" and "Xing" are most likely still trademarked somewhere, so cannot be used as name for this particular encoder.
As for the "TX" advanced mystery options: The value specified for the TX option ends up in the aptly named variable "test1" of the struct "BA_CONTROL" (bit allocation control). Unlike the also present "test2" and "test3", "test1" actually appears to "do" something, in the method "CBitAllo3::startup_adjustNT1B":
void
CBitAllo3::startup_adjustNT1B ( )
{
int ch, i, a, na;
int ab, nab;
int d, dmax;
int f;
static int sthres[21] = {
0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0,
100, 100, 100, 200, 300, 300, 300,
};
//#define STHRES 300
//#define STHRES 0
if ( ba_control.test1 == 0 )
return;
f = ba_control.test1;
for ( ch = 0; ch < nchan; ch++ )
{
na = 1;
a = 0;
nab = 1;
ab = 0;
for ( i = 0; i < nsf[ch]; i++ )
{
//if( snr[ch][i] > STHRES ) {
if ( snr[ch][i] > sthres[i] )
{
a += NT[ch][i];
na++;
ab += nBand_l[i] * NT[ch][i];
nab += nBand_l[i];
}
}
a = a / na;
ab = ab / nab;
if ( na < 5 )
continue;
for ( i = 0; i < nsf[ch]; i++ )
{
//if( snr[ch][i] > STHRES ) {
if ( snr[ch][i] > sthres[i] )
{
dmax = HX_MAX ( snr[ch][i] - 400, 0 );
//d = (ab - NT[ch][i]) >> 1;
d = ( f * ( ab - NT[ch][i] ) ) >> 4;
d = HX_MIN ( d, dmax );
NT[ch][i] = NT[ch][i] + d;
}
}
}
}
The value "test1" provides a value for the variable "f", which then further down the road is used as a multiplier to compute an adjusted noise threshold (?) for 21 scalefactor bands. As is a common theme with that code base, I find it hard to actually determine what is happening there (non-descriptive variable names, magic numbers), but "test1" defaults to 6.
Why 6? Because Neal said so! ("set -1 for default, test reserved 6 or 8 seems best, Neal says 6" and "default 6 re. Neal").
Topic: Resurrecting/Preserving the Helix MP3 encoder (Read 46755 times)
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