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+README for the SFN patch
+========================
+
+Concept
+-------
+
+Each modulator receives ETI frames with absolute timestamps. These timestamps
+are composed of two parts:
+- The TIST field as defined in the ETI standard, giving an offset after the
+ pulse per second signal;
+- A time information transmitted using the MNSC, representing the precise time
+ when the frame must be transmitted, with one-second resolution.
+
+When CRC-DABMUX is called with -s, the TIST is defined in each frame. The
+Time is always encoded in the MNSC.
+
+In a SFN setup, it is not possible to rely on a modulator to slow down the
+Ensemble multiplexer. CRC-DABMUX takes an additional parameter -r to output one
+ETI frame every 24ms. This output parameter only adds a simul:// output.
+
+The ETI stream must then be made available to several transmission sites. For
+this, the eti_tcp server can be used.
+The eti_tcp server listens on the port specified on the command line.
+An example invocation is:
+./mux-throttled.sh -s -m 1 | ./eti_tcp/eti_tcp.py 54001
+
+Each modulator then receives the ETI stream through a TCP connection. Each frame
+contains the complete timestamp, to which an per-modulator offset is added.
+The sum is then given to the USRP. The offset can be specified in two ways:
+
+using -o delay:
+Adds a constant delay specified on the command line (see 1st example).
+
+using -O delayfile
+Adds the delay specified in the given file. The file must contains a single
+line containing a floating point number. This file is read each 50 frames, and
+the modulator delay is updated.
+
+In both cases, the units are seconds.
+
+Example modulator invocation (for a B100 USRP supporting 2048Msps and with a
+fixed delay of 2.3 seconds)
+nc localhost 54001 | ./moduhd-2048.sh -o 2.3
+
+Example modulator invocation (for an USRP requiring resampling and with a delay
+specified in a separate file "moddelay")
+nc localhost 54001 | ./moduhd-resample.sh -O moddelay -G 5
+
+It is also possible to set daugherboard gain using -G. Furthermore, the transmission
+frequency is set on the command line, as seen in the examples in moduhd-XXX.sh.
+
+CRC-DABMOD uses the UHD library to output modulated samples to the USRP device.
+When started, it defines the USRP time using the local time and the PPS signal.
+It is therefore important to synchronise computer time using NTPd.
+
+When a frame arrives with a timestamp that is in the past, the frame is dropped.
+If the timestamp is too far in the future, the output module waits a short
+delay.
+
+Synchonisation can be verified by using an oscilloscope and a receiver. It is
+very easy to see if the null symbols align.
+
+
+Hardware requirements
+---------------------
+
+The following hardware is required to test the SFN patch to the CRC mmbTools:
+- Two USRPs ;
+- One or two computers with the mmbTools installed ;
+- A network connection between the two computers ;
+- A 10MHz refclk source ;
+- A 1PPS source synchronised to the 10MHz ;
+- An oscilloscope to check synchronisation.
+
+The lab setup used in the experiments :
+http://pic.mpb.li/IMAG0411.jpg
+
+In this example, one USRP B100 and one USRP2 are used. Modulation is done on two
+different computers, but it also works with a single computer. In that case, the
+device string for the -u option for CRC-DABMOD must specify the device. (e.g.
+-u "type=b100", -u "type=usrp2" or -u "serial=ABC123")
+
+The HP33120A generator at the bottom is used to generate the 10MHz REFCLK (sine,
+2Vpp). It's sync output is used as 10MHz IN for the Agilent 33250A generator.
+The Agilent is set to a square wave, 1Hz. Its sync output is then wired to the
+PPS input of the USRPs.
+
+List of changes made
+--------------------
+To CRC-DABMUX:
+- Throttled output option instanciates a simul output
+- Set TIST and use MNSC to transmit time
+- Command line arguments handling
+
+Files:
+Modified Eti.h and DabMux.cpp
+
+To CRC-DABMOD:
+- Added multithreaded UHD Output module
+- Added normalisation for UHD (we must divide output samples by
+ 32768.0)
+- Command line arguments handling
+- Changed the flowgraph to be able to chose between outputs
+- Added decoding of time in MNSC
+- Added delay control for the modulator using -o and -O
+- Initially set the timezone to UTC, because the MNSC is in UTC
+
+Files:
+Added OutputUHDThreaded.{cpp,h}, TimestampDecoder.{cpp,h}; Modified DabMod.cpp,
+DabModulator.h, Eti.h, EtiReader.{cpp,h}; Changed some .h files to include
+<stdio.h> because of compilation issues; Corrected OutputMemory char* name().
+
+
+Future elements to be changed
+-----------------------------
+- Communicate delay and buffer information between multiplexer and modulators,
+ to have a better control about delays.
+- Handle UHD asynchronous messages in another thread, and use this information
+ for monitoring
+
+Repository:
+http://hg.mpb.li/mmbtools-sfn/
+
+2012-07-11
+Matthias P. Braendli
+matthias@mpb.li
+