An apparatus comprising: at least one processor ; and at least one memory including a computer program code , wherein the at least one memory and the computer program code are configured, with the at least one processor, to cause the apparatus to: acquire a heart activity measurement signal of a user, wherein the heart activity measurement signal characterizes electric activity of a heart measured by at least one electrode pair in contact with the user's skin, characterized in that the heart activity measurement signal is processed into a complex form comprising an in-phase component and a quadrature component; compute an auxiliary complex signal approximating a wavelet transform of the in-phase component and quadrature components of the heart activity signal; compute a phase of the heart activity measurement signal by computing an argument of the auxiliary complex signal; determine interference signal components in the acquired heart activity signal on the basis of the phase of the heart activity measurement signal; and eliminate the determined interference signal components from further analysis of the heart activity measurement signal. at least one processor ; and at least one memory including a computer program code , wherein the at least one memory and the computer program code are configured, with the at least one processor, to cause the apparatus to: acquire a heart activity measurement signal of a user, wherein the heart activity measurement signal characterizes electric activity of a heart measured by at least one electrode pair in contact with the user's skin, characterized in that the heart activity measurement signal is processed into a complex form comprising an in-phase component and a quadrature component; compute an auxiliary complex signal approximating a wavelet transform of the in-phase component and quadrature components of the heart activity signal; compute a phase of the heart activity measurement signal by computing an argument of the auxiliary complex signal; determine interference signal components in the acquired heart activity signal on the basis of the phase of the heart activity measurement signal; and eliminate the determined interference signal components from further analysis of the heart activity measurement signal. acquire a heart activity measurement signal of a user, wherein the heart activity measurement signal characterizes electric activity of a heart measured by at least one electrode pair in contact with the user's skin, characterized in that the heart activity measurement signal is processed into a complex form comprising an in-phase component and a quadrature component; compute an auxiliary complex signal approximating a wavelet transform of the in-phase component and quadrature components of the heart activity signal; compute a phase of the heart activity measurement signal by computing an argument of the auxiliary complex signal; determine interference signal components in the acquired heart activity signal on the basis of the phase of the heart activity measurement signal; and eliminate the determined interference signal components from further analysis of the heart activity measurement signal. <EOS>