[aprssig] Use of 9k6 port for 1k2 packet

[Greg D]

Hi folks,

I expect this is a simple R + C circuit, but have no idea what values to
use.  Any suggestions?

What I'm looking to do is to use the 9k6 port on my mobile rig (an
Alinco DR-610) for regular 1k2 packet radio.  The advantages of the 9k6
port is that the level is unaffected by the rig's volume control, and it
has a separate connection at the back of the rig for both the audio and
PTT, leaving the radio's voice function intact.  I'd be using one side
of the radio for voice and the other for packet, not at the same time,
of course.  I did a quick "smoke test", and at least the receive side
seems to work.  I need to wire up a PTT circuit (simple 1-transistor
buffer) for the transmit side, but that will need to wait for a few
minutes of spare time.  The TNC is a Raspberry Pi 3B with a USB sound
dongle and Direwolf.  PTT is via a GPIO pin, through the buffer.

The usual problem with using a regular audio connection for 9k6 packet
is the audio response shaping (pre / post emphasis), which kills the
higher speed operation.  I want to do the reverse.  How do I properly
"shape" the 9k6 port's audio stream for reliable 1k2 packet operation? 
Do both transmit and receive each need shaping, or (given the success
with receive) just transmit?  Or, neither, and just wire the ports
directly to the USB dongle and call it close enough?

Thanks,

Greg  KO6TH