[App_rpt-users] recordings

[Ken]

If the recordings sound muddy and Allison is OK, then I think it's obvious
where the problem lies.  Have a look at the frequency response of the
recording you made using Audacity's "plot spectrum."  If there is a lot of
energy below 300 Hz, that will be largely wasted energy, as most radios
filter out the low end because that is where the PL tones are. Yet when you
set the level of the recording (using "normalize") that energy will help
determine the gain that Audacity uses, so it will rob energy from the higher
frequencies that actually do pass through the repeater/radio.  So it will
sound extra-muddy when it goes through your repeater/radio.   You should run
a high pass filter on your recordings before normalizing, to fix this (see
the Effect menu).  A corner frequency of 100 Hz works well.  With a female
voice, you can use up to 300 Hz without losing any intelligibility. 

In a similar way, too much high frequency energy will mess up your levels
too.  Due to the 8 kHz sampling used internally, Asterisk will completely
eliminate any energy above 4 Khz  (actually it's about zero at 3.8).  If you
set your levels using a recording that was made at (for example) 44.1 KHz,
energy between 4 and 22 Khz will be part of the level setting, but will be
completely gone when it gets down-sampled  to 8 kHz. 

So, the minus 6 dB is correct. But that level-setting needs to be done to a
recording that matches the passband of what is going to be passed through
the radio.  Set the minus 6 AFTER fixing up the frequency response of your
recording.

In a related matter - there is one well-known bulletin famous for putting a
big peak in their recordings at about 3.5 kHz, so their audio will sound
sharper when passed through an SSB filter (I suppose).  This is annoying in
Asterisk, since this is right near the top of the frequency range and will
tend to be distorted if exaggerated.  I usually run a notch filter at 3900
Hz with a Q of 0.95 to provide a sudden fall-off in response, right at the
top, when I work with one of these bulletins.

Lastly - if Allison sounds muddy, that is another story. In that case, you
may have your transmit pre-emphasis configured wrong  - off when it should
be on (in usbradio.conf). 

73
Ken