Change in osmo-trx[master]: Transceiver: use proper factor for amplitude scaling

[fixeria]

fixeria has uploaded this change for review. ( https://gerrit.osmocom.org/c/osmo-trx/+/22456 )


Change subject: Transceiver: use proper factor for amplitude scaling
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Transceiver: use proper factor for amplitude scaling

In Transceiver::addRadioVector() we scale the I/Q samples by scaling
the output voltage of the DAC.   A relative factor/divisor/ration in
the voltage domain cannot be used 1:1 in the power domain.

There exist two similar formulas:

  a) X_dB = 10 * log10(X_lin / X_ref)
  b) Y_db = 20 * log10(Y_lin / Y_ref)

both of them are correct, and according to [1]:

  a) If you convert a quantity X that relates to power or energy,
     => the factor is 10.
  b) If you convert a quantity Y that relates to amplitude,
     => the factor is 20.

Therefore we should be using 20 instead of 10.  This change makes
osmo-trx apply per-lchan attenuation values correctly.  Otherwise
it would double the values indicated in TRXD messages.

[1] https://dspillustrations.com/pages/posts/misc/decibel-conversion-factor-10-or-factor-20.html

Change-Id: I98bc00bd25df4913d45e55eb008d715aca76fc7c
Related: SYS#4918
---
M Transceiver52M/Transceiver.cpp
1 file changed, 1 insertion(+), 1 deletion(-)



  git pull ssh://gerrit.osmocom.org:29418/osmo-trx refs/changes/56/22456/1

diff --git a/Transceiver52M/Transceiver.cpp b/Transceiver52M/Transceiver.cpp
index 3188c70..6de266c 100644
--- a/Transceiver52M/Transceiver.cpp
+++ b/Transceiver52M/Transceiver.cpp
@@ -396,7 +396,7 @@
   else
     burst = modulateBurst(bits, 8 + (wTime.TN() % 4 == 0), cfg->tx_sps);
 
-  scaleVector(*burst, txFullScale * pow(10, (double) -RSSI / 10));
+  scaleVector(*burst, txFullScale * pow(10, (double) -RSSI / 20));
 
   radio_burst = new radioVector(wTime, burst);
 

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Gerrit-Project: osmo-trx
Gerrit-Branch: master
Gerrit-Change-Id: I98bc00bd25df4913d45e55eb008d715aca76fc7c
Gerrit-Change-Number: 22456
Gerrit-PatchSet: 1
Gerrit-Owner: fixeria <vyanitskiy at sysmocom.de>
Gerrit-MessageType: newchange
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