[Elecraft] [K3] Rx/Tx equalization

[Jim Brown]

On Mon, 25 Aug 2008 07:04:21 -0700, Mike Scott wrote:

>Jim (K9YC), what part of the 50 Hz spectrum will get by a 2.7 KHz roofing
>filter at any equalization setting? At least on my system I could put 50 Hz
>at +16 dB gain and 50 Hz will still be down 20 dB. The DSP skirts put it
>lower. I ask this question because I have been staring at my actual roofing
>filter plots.

>So I don't quite understand where you are coming from. Perhaps the devil is
>in the details and breath-pop energy which would be cut off by the roofing
>filter/DSP combination is still affecting the transmit-audio gain in
>deleterious ways. Maybe it is just my lack of understanding of how the K3
>works. In this area it is a bit of a black box to me, the price we pay for
>not soldering.

Hi Mike,

You're looking at the total signal path from mic in to RF out, but you're not 
considering what happens in the audio blocks that precede the transmitter. 
Good audio processing for communications uses careful equalization to shape 
the audio response for maximum use of the transmitter bandwidth, as well as 
compression and peak limiting to bring softer parts of the transmitted speech 
up to nearly full modulation level. There's also VOX, that looks at the 
transmit audio and turns the transmitter on when it sees audio. 

The compressor, limiter, and VOX all look at transmit audio to decide how to 
do their thing. If your microphone is producing lots of output on low 
frequency energy, it will cause the compressor and/or limiter to turn down the 
gain, reducing your modulation. That LF energy can also trigger the VOX 
prematurely. 

Yes, the transmit crystal filter imposes some hard limits on the transmitted 
bandwidth, but all of that is FAR up the signal chain from the audio 
processing noted above. The audio chain can suffer from these problems, and it 
can also produce distortion if over driven. Another point -- proximity effect 
is a LARGE effect when it is present, easily 15-20 dB of LF boost. It's only 
present in directional mics, but most pro mics suffer from it. If you're using 
one of those mics, it's easy for breath pops and other LF noise to be 10-20 dB 
louder than your voice. So your audio gain may be just fine for your 
transmitted speech, but pops and LF noise may be distortiing in the audio 
chain. 

The value of equalization and dynamics processing for ham radio are discussed 
in an appendix of http://audiosystemsgroup.com/RFI-Ham.pdf. Good audio 
processing -- equalization and compression/limiting -- can increase your 
effective talk power by 6-10 dB! That's equivalent to adding a 400W - 1,000W 
amp to your K3. Broadcasters learned this MANY years ago -- all of them are 
carefully equalized and processed. 

73,

Jim Brown K9YC