On 2015-03-22 9:25 PM, Don Wilhelm wrote:
> Even though the 13kHz filter theoretically has zero offset, you may
> find a better balance for the bass response by experimenting a bit
> with the offset value for that filter.
The 13 KHz filter has more than adequate bandwidth for Sync (USB/LSB)
AM. Even if it is off by as much as 2 KHz the bandwidth is enough to
pass the full 4.5 KHz recovered bandwidth. The upper/lower edge is
determined by the DSP characteristic, not the crystal filter bandwidth.
Looking at an audio spectrum display with "hold" it's fairly clear to
see there is roughly a 50 Hz difference in the lower frequency limit
between "LSB" and "USB" in Sync detection while by ear there is a
qualitative difference ("more open") high end in "USB". There is no
perceptible difference when moving the FM filter offset from limit
to limit. The difference persists even when forcing use of the 2.8
KHz crystal filter. It's purely an issue in the DSP function.
73,
... Joe, W4TV
On 2015-03-22 9:25 PM, Don Wilhelm wrote:
Grant,
Synchronous AM reception is rather like SSB, so the edges of the filter
are significant rather than the center. For non-synch AM, the center is
significant as you state.
The filter slope on the low frequency side will not be the same as on
the high frequency side.
73,
Don W3FPR
On 3/22/2015 4:50 PM, Grant Youngman wrote:
That could be. But the DSP bandwidth (10 kHz or 5 kHz in sync mode) is
sitting in the middle of a 13 kHz filter. There's a lot of room for
slop before the filter edges should come into play ...
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