  Compressed sensing investigates the recovery of sparse signals from linear
measurements. But often, in a wide range of applications, one is given only the
absolute values (squared) of the linear measurements. Recovering such signals
(not necessarily sparse) is known as the phase retrieval problem. We consider
this problem in the case when the measurements are time-frequency shifts of a
suitably chosen generator, i.e. coming from a Gabor frame. We prove an easily
checkable injectivity condition for recovery of any signal from all $N^2$
time-frequency shifts, and for recovery of sparse signals, when only some of
those measurements are given.
