  We present a simple way of coding and compressing the data on board the
Planck instruments (HFI and LFI) to address the problem of the on board data
reduction. This is a critical issue in the Planck mission. The total
information that can be downloaded to Earth is severely limited by the
telemetry allocation. This limitation could reduce the amount of diagnostics
sent on the stability of the radiometers and, as a consequence, curb the final
sensitivity of the CMB anisotropy maps. Our proposal to address this problem
consists in taking differences of consecutive circles at a given sky pointing.
To a good approximation, these differences are independent of the external
signal, and are dominated by thermal (white) instrumental noise. Using
simulations and analytical predictions we show that high compression rates,
$c_r \simeq 10$, can be obtained with minor or zero loss of CMB sensitivity.
Possible effects of digital distortion are also analized. The proposed scheme
allows for flexibility to optimize the relation with other critical aspects of
the mission. Thus, this study constitutes an important step towards a more
realistic modeling of the final sensitivity of the CMB temperature anisotropy
maps.
