This avoids picking up the audio clock
with the RF ADC as spurs in the spectrum,
and requires a different PCM1802 mode config
than before for 512fs oversampling:
MODE0 = 1
MODE1 = 0
The resulting audio samplerate is 78125 Hz.
Currently the stream ID is always inserted,
so we have three reserved words.
Eventually if only one stream is present
this word is not needed, and the presence
of it should be signalled through the metadata.
After the recent change pipelining over three DMA transfers,
we obviously need two different idle line buffers, otherwise
the the metadata in the idle line currently in flight will
be overwritten in case we have two idle lines in a row.
When utilizing the full data rate, this cannot be observed,
as there won't be two idle lines in a row.
Previously when using an HSTX clock > sysclk/2,
there was a DMA underrun from time to time,
which limited the achievable data rate to
around 75 MByte/s. By using a third DMA channel
and employing some trickery to be still able to
use the DMA CRC sniffer, we now can achieve
128 MByte/s (or even more) by using sysclk/1
as HSTX clock.
The counter example has been updated to
generate those ~128 MByte/s.
