There is no precisely defined specification for an MP3 encoder in the MPEG-1 standard. To generate MP3 files, encoders can use their own algorithm to remove the inaudible part of the audio file. However, the quality of the audio files will differ depending on the MP3 encoder used.
During encoding , time domain samples are converted into frequency domain samples and if there are transients in the sound, fewer samples (about 1/4th) will be taken to limit the transient noise. Encoders which produce good quality audio at higher sampling rates may not produce similar quality audio at lower sampling rates.
However, the decompressed output produced by MP3 decoders is usually exactly as per the specifications in the ISO standards. Depending on the MPEG version and layer, the MP3 file has a frame consisting of 384, 576 or 1152 samples, header and side information. The header and side information is used by the decoder to decode encoded data.
