  In this paper we demonstrate that a large, unexplored reservoir of
information about pulsar emission exists, that is directly linked to the
radiating particles and their radiation process: We present a study of flux
density measurements of individual pulses simultaneously observed at four
different frequencies. Correcting for effects caused by the interstellar
medium, we derive intrinsic flux density spectra of individual radio pulses
observed at several frequencies for the first time. Pulsar B0329+54 was
observed at 238, 626, 1412 and 4850 MHz, while observations of PSR B1133+16
were made at 341, 626, 1412 and 4850 MHz. We derive intrinsic pulse-to-pulse
modulation indices which show a minimum around 1 GHz. Correlations between the
flux densities of different frequency pairs worsen as the frequency separation
widens and also tend to be worse for outer profile components. The single pulse
spectra of PSR B0329+54 resemble the spectra of the integrated profile.
However, the spectral index distributions for the single pulses of PSR B1133+16
show significant deviations from a Gaussian. This asymmetry is caused by very
strong pulses with flux densities exceeding the mean value by more than a
factor of ten. These strong pulses occur preferentially at the trailing edge of
the leading component and appear to be broadband in most cases. Their
properties are similar to those of so-called giant pulses, suggesting that
these phenomena are related.
