In article ,
"Tom Holden" wrote:

"Telamon" wrote in
message
Basically the radio has to adapt to the signal level you are tuned
to by changing its sensitivity.

... in order to avoid overload distortion at any stage in the
receiver by excessive signal and to even out the loudness between
weak and strong signals so that the listener does not have to adjust
the volume control.

For any kind of digital mode you want fast AGC ....

Telamon, I've read that elsewhere and it seems to be embedded in
specs but I don't understand why after having experimented with
modifying a DX-394A. My impression from the literature I have read is
that the 'ideal' receiver is supposed to have fast attack (under 10
milliseconds) and variable release (slow 1 second, medium ~ 100's
of milliseconds, fast ~ 10's of milliseconds).

1. A problem is if the attack is under 10 ms and the release is slow,
then a very short noise burst reduces the receiver gain for a long
time. Seems to me that the attack should be proportional to the
release to avoid that problem but that might not be ideal for SSB or
Morse. I can see that, for most data, a fast attack/release combo
would likely minimise dropouts. Maybe the ideal has independently
variable attack and release to accommodate all conditions.

2. Many DRM experimenters have found that the maximum DRM SNR is
achieved by turning AGC off but that may result in a higher long term
error rate due to signal fades and surges after the gain has been
set. The reasons why the Max SNR is reduced by AGC are not fully
understood - one, specific to the DX-394A and original DX-394 but
possibly a factor in other radios, is that a local oscillator was
pulled as the mixer gain varied, whether by manual RF Gain control or
by AGC. This results in a phase noise and, in extreme swings in gain,
in the loss of lock. There are other more subtle factors degrading
the Max DRM SNR that remain that I don't know about. (Note that the
DRM SNR has nothing to do with the perceived audio SNR but is a
simple representation of the quality of the transmission channel
calculated from the scattering of the transmission coding
constellation.)

Maybe I should not have commented on the digital mode since it has been
years that I have used a decoder on short wave but what I recommended is
what I recall working best for me most of the time. I was using an R8
with a decoder and looking at the R8B manual the AGC spec is:
Mode Attack release
Slow 1 ms 2 sec
Fast 1 ms 300 ms

Assuming the R8 and R8B AGC specs are the same the Fast AGC was the
right choice for most situations.

Although the R8 and R8B have a filter for RTTY I also used the CW filter
for the small shift 450Hz signals. Then there were some modes where I
had to use the LSB filter so 500Hz, 1.8 KHz and 2.3 KHz. Generally you
used the smallest filter for the shift you were trying to get as the
smaller filter lowers the noise floor.

I have no experience with DRM other than the noise it generates on the
bands but since it is a digital mode I would think Fast AGC would the
best choice. If your radio is programmable then I would set the fastest
settings for attack, hang and release. Here you don't actually care what
the received digital signal sounds like and you want the AGC on to
prevent an kind of overloading together with as rapid response as
possible to signal increases or decreases. This would be the best case
for getting the most bits in the stream on a continuous basis. The sound
you do care about is after the digital processing and then conversion to
audio. One problem DRM has on the receiver end is the high noise floor
due to the very wide bandwidth needed. This is yet another maddening
tradeoff when one looks at the entire system of transmitter, ionosphere
and receiver. Increased intermodulation products, high noise floor and
probably other things I have not thought about.

I generally stay away from turning the AGC off. The only reason to do
that is if the signal you are trying to get is very weak and noise is
desensitizing the radio though the AGC action.

--
Telamon
Ventura, California