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+On the Usage of ODR-DabMod for Synchronous Transmissions
+========================================================
+
+Summary
+-------
+ODR-DabMux and ODR-DabMod offer support for timestamped transmission
+when the UHD output is used. This README explains how this functionality
+works, and how to set it up.
+
+This feature is a prerequisite for the creation of a single-frequency
+network.
+
+
+Concept
+-------
+The goal of this functionality is to synchronise the transmission for
+several transmitters. This has been tested with the USRP B100, B200 and the
+USRP2, that both have the necessary REFCLK and 1PPS inputs. Both are
+required to synchronise two USRPs:
+- The REFCLK is used in the USRP for timekeeping. If we want two
+  USRPs to stay synchronised, they both must have a precise 10MHz
+  source at the REFCLK, otherwise their internal clocks will drift
+  off.
+- The 1PPS signal is used to set the time inside the USRPs. The rising
+  edge of the 1PPS signal has happen synchronously for all transmitters.
+  Usually, GPS is used to drive this 1PPS.
+
+For such a system, there will be one multiplexer, which will send the ETI
+stream to several modulators. The ETI stream, in this case, is transported
+over the ZMQ interconnection.
+
+Each modulator receives ETI frames that contain absolute timestamps, defining
+the exact point in time when the frame has to be transmitted. These in-band
+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 ODR-DabMux is configured accordingly, the TIST is defined in each frame.
+The time is always encoded in the MNSC.
+
+When the ETI stream is sent to several modulators using non-blocking I/O, it
+is not possible to rely on a modulator to back-pressure the Ensemble multiplexer.
+It is therefore necessary to throttle multiplexer output.
+
+Each modulator then receives the ETI stream through a ZMQ 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 the
+configuration file of ODR-DabMod, and can be modified on the fly using
+the remote control interface.
+
+ODR-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 NTP.
+
+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. Then tune the receiver to the
+ensemble, and alternatively lower the tx gain of each modulator to see if the
+receiver is still able to receive the ensemble without hiccup.
+
+Time and frequency references
+-----------------------------
+In addition to the 10MHz refclk and 1PPS inputs on the USRP, some USRPs also
+support an integrated GPSDO. For the B200, there are two GPSDOs modules that
+can be used: The Ettus GPSDO (Jackson Labs Firefly), and the u-blox LEA-M8F on
+the [Opendigitalradio board](http://www.opendigitalradio.org/lea-m8f-gpsdo).
+
+To use the LEA-M8F, some modifications in the UHD library are necessary, because
+the module outputs a 30.72MHz refclk instead of a 10MHz. The changes are
+available in the [ODR repository of UHD](https://github.com/Opendigitalradio/uhd),
+with branch names called *lea-m8f-UHDVERSION*.
+
+When using the integrated GPSDO, ODR-DabMod will also monitor if the GPS
+reception is ok, and if the time reference is usable.
+
+Hardware requirements
+---------------------
+The following hardware is required to build a SFN with the ODR-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.
+
+It is possible to use signal generators as REFCLK source and 1PPS, if there is
+no GPS-disciplined oscillator available. It is necessary to synchronise the
+1PPS source to the 10MHz source.
+
+
+###########
+june 2012, initial version, Matthias P. Braendli
+feb 2014, renamed crc-dabXYZ to odr-dabXYZ, mpb
+sep 2015, overhaul, talk about GPSDOs, mpb
+