tempest has uploaded this change for review.

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trx: Fix tx/rx-sps 4 for USRP1 devices

The existing code sets the pingOffset variable to -7231(269 - 7500) with tx-sps = 4. The (apparent) correct value for pingOffset for tx/rx-sps 4 is 272.
While I'm not completely sure how this patch works to fix the increased sps value, my theory is that(as seen in the block comment on lines 72-77) OpenBTS, where this code is adapted from, only supported a split sps setup(tx-sps=4, rx_sps=1 for sps=4), which created a differing decimation rate between Tx and Rx on the board. In this case, apparently you needed to account for an advance in the timestamp for a correct output.
However, in the case tx_sps == rx_sps, the decimation/sample rates will be the same, and(I guess) this advance doesn't need to be accounted for. I also had to change a line in the initialization of the Rx end of the USRP device to allow it to have a correct decimation rate when the sps values are 4.
Also commented out the line that would create different pingOffset values for split sps values, since osmo-trx won't allow nonequal values for tx-sps and rx-sps. I left it visible in the file for reference however.

Tested & working on the URAN-1, a USRP1 clone designed for GSM.

Change-Id: I3b08c23a3e7d4a226cd23b9fe42558f596a63257
---
M Transceiver52M/device/usrp1/USRPDevice.cpp
1 file changed, 8 insertions(+), 7 deletions(-)

git pull ssh://gerrit.osmocom.org:29418/osmo-trx refs/changes/31/41231/1

diff --git a/Transceiver52M/device/usrp1/USRPDevice.cpp b/Transceiver52M/device/usrp1/USRPDevice.cpp
index 63a9bcc..21cbd7e 100644
--- a/Transceiver52M/device/usrp1/USRPDevice.cpp
+++ b/Transceiver52M/device/usrp1/USRPDevice.cpp
@@ -75,18 +75,19 @@
    * split sample rate Tx/Rx - 4/1 sps we need to need to
    * compensate for advance rather than delay.
    */
-  if (tx_sps == 1)
-    pingOffset = 272;
-  else if (tx_sps == 4)
-    pingOffset = 269 - 7500;
-  else
-    pingOffset = 0;
+  pingOffset = 272;
+
+  /* Split SPS (4/1) unsupported on USRP1
+   * if(tx_sps == 4 && rx_sps == 1)
+   *   pingOffset = 269 - 7500;
+   */
 
 #ifdef SWLOOPBACK
   samplePeriod = 1.0e6/actualSampleRate;
   loopbackBufferSize = 0;
   gettimeofday(&lastReadTime,NULL);
   firstRead = false;
+  pingOffset = 0;
 #endif
 }
 
@@ -101,7 +102,7 @@
   m_uRx.reset();
   try {
     m_uRx = usrp_standard_rx_sptr(usrp_standard_rx::make(
-                                        0, decimRate * tx_sps, 1, -1,
+                                        0, decimRate, 1, -1,
                                         usrp_standard_rx::FPGA_MODE_NORMAL,
                                         1024, 16 * 8, rbf));
     m_uRx->set_fpga_master_clock_freq(masterClockRate);

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