Fine business Jeroen,
Great set up and experiment!
I've been experimenting with direct FM modulation of my FT817 using the
"data port" connector on the back. This works well, and I can pick up
the 2400A signal on a SDR OK. I'm using an external DAC (actually a
stm32f4 discovery) with response down to DC (not a sound card) to
modulate the FT817.
There's a cmd line utility (fsk_mod_ext_vco.c) I wrote to support the tx
side. By using an external SDR for Rx we can (hopefully) get a 10dB
lower MDS than FM or otehr DV systems.
Cheers,
David
On 02/05/18 07:30, Jeroen Vreeken wrote:
Hi all,
A small update: the dra818 module is a bit dissapointing. The worst
problem: it takes almost a second before it modulates after switching to TX.
So I went for a different approach:
A while back I modified my FT817 a bit to make the 455kHz IF available
on the back. This is done by using the connections available for the
optional filter. This filter is used both for receiving and for
transmitting.
http://dmlinking.net/ft817_24khz/IMG_20161219_212016small.jpg
For receiving I use a 455kHz filter and a simple one transistor
amplifier and feed the result directly to the atsame70.
The CPU samples as 96kHz resulting in a 25kHz digital IF due to aliasing.
Transmission is done similarly by outputting a signal at 25kHz on de DAC
and mixing it (with a 4053) with a 480kHz square wave signal generated
by a timer on the CPU. After filtering it with a 455kHz filter it is fed
back to the FT817.
A seconds switch of the 4053 is used to switch between the rx and tx path.
http://dmlinking.net/ft817_24khz/455kHz_trx_small.jpg
Right now the following works:
-FM demodulation of voice (filtered and with deemphasis)
-pure FM demodulation (also capable of receiving DC)
-FM modulator (both voice and sine test signals)
-FreeDV 2400A transmission (not sure if it works due to lack of a 2400a
capable receiver)
-FreeDV 2400B transmission
The local repeater PI2EHV successfully received the 2400B signal and
decoded it just fine.
Recording of the received audio at the repeater can be found here:
http://dmlinking.net/audio/2018/05/pi2ehv.ampr.org.201805012000.wav
(The signal was fine, the audio going into the encoder was terrible,
just a simple electret mic connected to the atsame70 cpu)
The first results are encouraging and it looks like this is an easy way
to get on air with the 2400 modes on any band the FT817 supports. But
there is still a lot of debugging to do. For some reason encoding 2400B
stops after about a minute of transmitting...
73,
Jeroen PE1RXQ
On 01/09/2018 03:32 PM, Jeroen Vreeken wrote:
Hi,
For a while now our local repeater PI2EHV has been capable of
receiving both analog FM and FreeDV mode 2400B.
However the amount of digital users is low (not a big surprise).
Inspired by the SM1000 I have been thinking about a project to make
using it easier: A small handheld capable of mode 2400B.
Nothing really fancy, but it should be able to show of some features
(like FPRS position reports interleaved in an audio stream) while
making communication in the field as simple as possible. And of course
it should be an open design. Since the inspiration came from the
SM1000 I would like to call it the 'HT1000'.
The basic design would consist of:
- DRA818 FM transceiver module
- Small GPS receiver
- LiPo battery
- Atmel ATSAME70 cpu
- Speaker & mic
- Display and controls
- Glue circuitry to let it work together
- 3D printed housing
As of last night the first step is working: an ATSAME70 running
freedv_rx() and freedv_tx() between two AD converters and two DA
converters. Next step will be connect a DRA818 module so I can
actually test on air.
73,
Jeroen PE1RXQ
------------------------------------------------------------------------------
Check out the vibrant tech community on one of the world's most
engaging tech sites, Slashdot.org! http://sdm.link/slashdot
_______________________________________________
Freetel-codec2 mailing list
Freetel-codec2@lists.sourceforge.net
https://lists.sourceforge.net/lists/listinfo/freetel-codec2
------------------------------------------------------------------------------
Check out the vibrant tech community on one of the world's most
engaging tech sites, Slashdot.org! http://sdm.link/slashdot
_______________________________________________
Freetel-codec2 mailing list
Freetel-codec2@lists.sourceforge.net
https://lists.sourceforge.net/lists/listinfo/freetel-codec2
------------------------------------------------------------------------------
Check out the vibrant tech community on one of the world's most
engaging tech sites, Slashdot.org! http://sdm.link/slashdot
_______________________________________________
Freetel-codec2 mailing list
Freetel-codec2@lists.sourceforge.net
https://lists.sourceforge.net/lists/listinfo/freetel-codec2
