Does editing a lossy format without converting to another format file hurt quality?

Question

I've heard that converting a file from one lossy format to another one further reduces quality. That makes sense! But what happens if I edit a lossy file and export using the same file format, does the compression further hurt quality?

For example, if I open an MP3 into a program like Audacity, edit it a bit, and then export it out as an MP3 again, will that incur further quality losses during compression? Or does the compression algorithm affect the file in the same lossy way that it did when it was first compressed, thus keeping quality more or less the same? How would this (MP3->MP3) compare to exporting as a different lossy format (like MP3->Ogg/Vorbis, for example)?

Answer 1

Yes, editing an MP3 file and saving it again in a lossy format will reduce the quality.

On opening the MP3 file, the editor first transforms it into a stream of uncompressed audio. The editing then takes place on a raw data. It doesn't cause a loss in quality itself, but saving the file again in a lossy format does, ad with the read-edit-write procedure, you are effectively recompressing the file.

Basically it doesn't matter if you choose the destination format to be the same as the original or a different one, as long as the compression takes place, you will lose some information (compared to the file you opened for editing).

Saving in a format with a higher bitrate might however preserve the quality as explained here. The exact values depend not only on the parameters of the destination format, but also on the audio data itself and the transformation (editing) you applied, so it's difficult to compare the methods.