  We calculate the spectral distortions from Hawking evaporation of primordial
black holes before the epoch of recombination, taking into account emission of
all standard model particles, including quark and gluons, and evolving the
resulting particle cascades in the expanding Universe. We show that the
constraints on the abundance of primordial black holes are stronger by more
than an order of magnitude compared to the previous calculations which take
only the primary photon emission into account. We also show that the shapes of
the spectral distortions is different from the $y$ or $i$-type distortions and
are sensitive to the mass of the primordial black holes. We also extend
previous constraints on the decay of long lived unstable particles before
recombination to additional decay channels. We show that for dark matter mass
$\lesssim$ 1 GeV, the spectral distortion shape is a function of the dark
matter mass as well as the decay channel to standard model particles. We also
provide new spectral distortion constraints on superconducting cosmic string
decay. We explicitly show that consideration of emitted photon spectrum from
string decay is not only important for the future experiments but also for
already available COBE-FIRAS data.
