I read in one of the comments in the FreeDV blog-
someone was wondering why FreeDV did better than expected amongst local
noise.
The main reason is that many noises are single carriers. As long as the
level of the interferer does not cause too much AGC action (which would
reduce the SNR of the desired signal), the OFDM modem will survive it.
In the case of 700D with 14 carriers, , if one of the data carriers gets
wiped out by the strong single carrier interferer, this simply makes the
error correction (FEC) work harder. Ultimately, the cost is not free,
the cost will be some reduction in overall sensitivity, because some of
the error correction effort has been utilized .
* In other cases, it might simply be the interference landed outside the
1kHz width of the 700D, and while you could hear it, it did not fall
within the passband of the modem.
To reduce the potential for signals to trigger AGC action, it is
beneficial to use a IF bandwidth only as wide as the signal plus some drift.
On my IC7300, I can dial up 100 Hz steps from 1 KHz to 3.6kHz. I set it
at about 1.2kHz for 700D, giving room for drift.
and thus I can reduce the likelihood of strong interferers from getting
inside my AGC passband
-glen
_______________________________________________
Freetel-codec2 mailing list
[email protected]
https://lists.sourceforge.net/lists/listinfo/freetel-codec2
