Awesome work David and all who are spearheading this. It takes a tremendous
amount of energy fine tuning this kind of thing, and major kudos to all
involved!
________________________________
From: David Rowe <[email protected]>
To: [email protected]
Sent: Thursday, 12 April 2012, 15:41
Subject: [Freetel-codec2] modem update
Hello,
I have been continuing my side project of building a modem to support
Codec 2 testing over HF channels. I have been nicely supported by some
HF modem guys. We also have some guys working on a cross platform GUI
to put the modem and codec together into a usable Digital Voice
application.
I have improved the frequency offset estimation, implemented freq
offset tracking, and worked out how to handle differences in sample
clocks between the tx and rx sound cards of up to +/- 2000 ppm. I am
now testing the entire modem using a HF channel simulator.
Have attached some screen dumps from my simulation. Its running at a
SNR of 5dB, 100Hz frequency offset, and a sample clock offset of
1000ppm, with the CCIR "moderate" HF model.
fig1 - spectrogram (waterfall). You can see that the channel is messed
up in some way all of the time, e.g. at least one tone has some
impairment on it. HF is tough!
fig2 - shows the timing and freq offset estimation. It "lets go" in the
fades but can recover after a loss of signal. This is what I was aiming
for. fig2_awgn is the same parameters but just a AWGN channel - no HF
model (like a VHF channel I guess). The ramp in the timing is due to the
sample clock errors between tx and rx.
fig 4 - shows various sync and BER measures, top is the received 10101
BPSK pilot, middle is BER of some test data, bottom shows when the test
data frames were received (about 50% of the time in this simulation).
Working on this modem has been a good way to get my head around modems
for HF channels, and got me thinking about how to optimise the combined
modem/codec system.
Cheers,
David
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Boundary is the first to Know...and Tell You.
Monitor Your Applications in Ultra-Fine Resolution. Try it FREE!
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