The paper presents modifications of the sine-approximation method (SAM) specified in ISO 16063-11:1998, focusing on two of the various well-established versions: (i) SAM version using heterodyne quadrature signals (HQS) and (ii) SAM version using time interval analysis (TIA), and their implementation in the national medium-frequency vibration standard of NIM, China. For the HQS version, modified techniques and procedures have been introduced applicable to suppress disturbing effects from frequency instability of the carrier signal and from displacement drift. For the TIA version, a new algorithm has been developed for which the frequency demodulation is based on the estimation of time intervals between neighbouring peaks and valleys of the laser Doppler interferometer instead of successive positive or negative zero crossings. A spline approximation has been implemented allowing relatively slow sampling rates (e.g. 5 MS s⁻¹) to be applied. The low requirements on hardware techniques allow primary vibration calibrations with high accuracy to be performed efficiently and cost effectively. Some results of investigations of the modified versions of SAM are demonstrated. The results achieved by these modified SAM versions are compared with each other and with PTB results measured within the complementary key comparison CCAUV.V-K1.1 piloted by PTB from 2006 to 2007.