1. An encoder comprising a low-frequency portion encoder for encoding a low-frequency portion of an information signal in units of frames of the information signal; a localizer for localizing transients within the information signal; an associator for, as a function of the localization, associating a respective reconstruction mode from among at least two possible reconstruction modes with the frames of the information signal, and, for frames which have associated therewith a first one of the plurality of reconstruction modes, associating a respective absence indication with these frames; a generator for generating a representation of a spectral envelope of a high-frequency portion of the information signal in a temporal grid which depends on reconstruction modes associated with the frames, such that frames which have the first one of the plurality of possible reconstruction modes associated therewith, the frame boundaries of these frames coincide with grid boundaries of the grid; and a combiner for combining the encoded low-frequency portion, the representation of the spectral envelope and information on the associated reconstruction modes and the transient absence indication into an encoded information signal, the generator and the combiner being formed to introduce, for a frame having the first reconstruction mode associated with it, either no or one or several spectral envelope value(s) describing the spectral envelope with a respective frequency resolution, as part of the representation of the spectral envelope, into the encoded information signal for the first, in terms of time, grid area of this frame as a function of the transient absence indication, wherein at least one of the low-frequency portion encoder, the localizer, the associator, the generator and the combiner comprises a hardware implementation.