[Elecraft] K3S USB audio questions

[Richard Lamont]

Hi,

FWIW I've done this. I've increased the gain by 12 dB, i.e. two bits.

Incidentally, the scaling on CONFIG:LIN OUT is linear - i.e. the output
voltage is proportional to the number. So for +12 dB w.r.t. the default
I've increased it from 10 to 40.

Richard Lamont G4DYA

On 06/11/16 16:48, ab2tc wrote:
> Hi,
> 
> You make a good point for some digital modes. But you should be able to make
> up for most or all of these 3 bits by increasing the LINE OUT as it must
> surely be before the codec ADC but probably not before the K3's DSP DAC.
> It's a shame that they have to do it this way (back to back DAC/ADC, but
> everybody else is doing the same). But by adjusting LINE OUT, the USB audio
> interface should be no worse than the audio LINE OUT considering the
> nonlinearity of the transformer in that path.
> 
> AB2TC - Knut
> 
> 
> 
> Richard Lamont wrote
>> On 05/11/16 18:13, ab2tc wrote:
>> [
>>>>> 4. When listening to SSB audio at S9+, with slow AGC, the output level
>>>>> via this interface seems to be -22 dBFS true peak. Why is it throwing
>>>>> away three bits of resolution? Or is this amount of headroom
>>>>> necessary/useful?
>>
>>> The audio level seen by, say a recording program on the PC depends on the
>>> LINE OUT setting on the K3. I left that as the default, 10, and used the
>>> Windows mixer to adjust the level. My audio is now reading  approximately
>>> -6dB below clipping which is probably a little high, but I have set my
>>> AGC
>>> SLP to the flattest possible setting (highest number). If you prefer a
>>> considerable slope on the AGC curve above threshold, you will need a
>>> lower
>>> LINE OUT setting. Remember that even when throwing away 3 bits, you still
>>> have far more resolution than you can use with an HF receiver.
>>
>> Consider JT65+JT9. There might be 30 or more signals within a 4 kHz
>> band, with widely varying levels. If they're all at the same level, then
>> each signal will be about 30 dB below the sum of the signals. If the
>> signal you want is 30 dB weaker than the others, then it will be 60 dB
>> below the sum.
>>
>> The data sheet shows the ADC has an SNR of 89 dB and THD+N of 0.01%
>> (i.e. -80 dB).
>>
>> If the sum of the signals is peaking to -22 dBFS true peak, than that
>> weak JT65/JT9 signal is going to be at -82 dBFS - close to the ADC noise
>> floor. Maybe getting at least some of those three bits back could make
>> quite a difference.
>>
>>
>> -- 
>> Richard Lamont G4DYA
>> <
> 
>> richard at .me
> 
>> >
>> <snip>
> 
> 
> 
> 
> 
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