It has been discovered that the DSP ROM in the Calypso GSM baseband processor
implements one nifty feature which is not used at all in standard phone or modem
operation, but which can be used for all kinds of interesting hacks: the traffic
channel (TCH) bits coming out of the GSM 05.03 channel decoder in the downlink
direction (to be fed to one of several speech decoders) can be read out of the
DSP's API RAM in real time, and in the uplink direction the user can feed her
own bits to the input of the GSM 05.03 channel encoder, effectively suppressing
the output of the internal vocoder.

I (Mother Mychaela) previously played with this functionality back in 2016, and
it's been mostly shelved since then.  This functionality is now getting renewed
attention in late 2022: now that we have a proper set of codec libraries (the
present package) and a proper understanding of Rx DTX handling requirements, we
can take another shot at decoding TCH downlink captures taken from Calypso GSM
MS.

For reasons of project resource planning, the work to study Calypso DSP downlink
bits in EFR mode is currently postponed until after we build our long-overdue FC
Venus board, but in the case of FR1 codec, we have a TCH downlink recording that
was captured back in 2016, and by analyzing this recording, we can make the
following observations:

* DSP status word 0 contains various flags; we already knew some of them, but
  now we got additional knowledge:

  bit 2 is BFI
  bits 4:3 give a 2-bit SID flag per GSM 06.31 section 6.1.1

  Unfortunately I couldn't find TAF anywhere - but oh well.

* DSP status word 1 appears to be garbage, DSP status word 2 appears to be some
  kind of error count - no new knowledge here.

* By heeding the newly-understood BFI flag in DSP status word 0, we can do a
  much cleaner decoding of TCH downlink capture than we did back in 2016.

The new gsmfr-cvt-dlcap utility in the present package replaces the old
fc-tch2fr utility from FC host tools - the latter should now be considered a
bogon.  The output from gsmfr-cvt-dlcap is in our gsmx format, containing
Themyscira BFI markers in those frame positions where Calypso DSP indicated BFI
on TCH downlink, and it needs to be fed to gsmfr-decode from the present
package; our FR1 Rx DTX preprocessor will take care of SIDs and BFIs, and the
complete decoding chain should then closely replicate what must be happening
inside the original DSP.