[arm-allstar] real world audio values

[David McGough]

Hi Chris,

The txdsplvl setting is a reduction in TX audio level. There is no 
absolute value, since the absolute setting comes from the CM1xyz mixer 
level setting; this being adjusted using the txmixaset and txmixbset 
parameters.

Setting the output levels of CM1xyz chips is a little convoluted!  
Without going into the gory details, the reason for this is that different
chip versions require radically different settings and output levels. For
example, the original CM108 chips had 152 mixer level settings (from 0% to
100% output).  Later chips (like the "B" versions) have only 37 mixer
levels from 0 to max....37 steps is very course; big output changes
between steps!

After becoming frustrated when testing many "clone" CM1xyz adapters, the
txdsplvl setting evolved. The basics are that a txdsplvl setting of 999 is
a unity-gain output level---no change due to this parameter.  As you step 
down from 999 to 0, there is a linear reduction in audio voltage.  
Converting this to dB would be:   dB(W) = 20 * log (V1 / V2), where V1 
goes from 0 to 999 and V2 = 999.  Here are the levels.  Note that the 
reduction starts out gradually and increases exponentially, as expected:

txdsplvl -------- dB
--------------------
999		  0 (unity--no level change)
951		 -1
905		 -2
861		 -3
819		 -4
779		 -5
741		 -6
705		 -7
670		 -8
638		 -9
606		-10
577		-11
549		-12
522		-13
497		-14
472		-15
449		-16
427		-17
407		-18
387		-19
368		-20
350		-21
333		-22
317		-23
301		-24
286		-25

223		-30
173		-35
135		-40
105		-45
82		-50
64		-55
50		-60
39		-65
30		-70

----------------------------------------------------------------------------

The procedure to set the output level is to start with txdsplvl == 999.  
Set the mixer (txmixaset/txmixbset) values so that the output is slightly
high.  Then, don't change the mixer further and fine tune by reducing the
output level, setting the txdsplvl below 999.

73, David K4FXC


On Sat, 27 May 2023, Chris via ARM-allstar wrote:

> Hi.
> 
>  
> 
> RX, TX, and DSP range from 0 to 999 in SimpleUSB. Does anyone know how much
> change equals a dB? Just trying to convert these to values that make sense
> to me.
> 
>  
> 
> Thanks
> 
>  
> 
> Chris
> 
>  
> 
>  
> 
> 73,
> 
> Chris
> 
>  
> 
> VE3RWJ on qrz.com <https://www.qrz.com/db/ve3rwj> 
> 
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> 
>  
> 
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