[arm-allstar] ARM-allstar Digest, Vol 82, Issue 22

[Patrick Perdue]

Yeah, I'm aware of the Nyquist frequency and all that. Before I started 
using Allstar, I would sometimes control nets on DMR with a DVStick and 
my own processing, using a multi-band dynamics processor in what 
basically amounted to a de-esser in reverse to dynamically increase the 
energy in that range. People couldn't understand why I sounded so much 
better than the typical DMR.

Relating this back to HamVoIP, I thought the LPF was a bit below 
Nyquist, maybe around 3.5 kHz or so, which is why I was confused that 
frequencies in the range of 3.8 were still passing.

Now that I've read more on that filter's implementation, it's less 
surprising to me. I thought it was more of a cut-off, which isn't the case.


On 3/19/2021 3:48 PM, "Ken via ARM-allstar" wrote:
>>> I remember, some time ago, you mentioned that simpleusb has filters to
> prevent anti-aliasing. Does this only apply to TX audio?
>
> The data on the receive side cannot include any audio components above 4
> kHz, so no similar anti-alias filters are needed.  The USB fob D/A converter
> is driven at 48 kHz, so the filtering you need is up at that frequency range
> and hardware-based (after the D/A converter).
>
> On the transmit side I find it helpful to limit the energy starting at 3 kHz
> because there are  just not very many bits/sec available for digitizing that
> energy in that range and the result is in increasing distortion products
> (not including aliasing).
>
> This is at odds with audio from YSF whose vocoder does some magic to pump
> through a lot of energy in that audio frequency range .... (vocoder vs
> codec).
>
> Ken/KE2N
>
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