Low quality audio material, due to either poor encoding or poor quality originals, is no rarity. One of the most common symptoms of such audio is missing high frequency range (because of e.g. an inappropriately chosen low pass filter applied before encoding). The two classes of tools that exist to deal with the problem -- equalizers and harmonic exciters -- prove largely ineffective: equalizers because they work by amplifying frequency ranges, and there is not much they can do if there is nothing to amplify; harmonic exciters because they are very constrained as to what frequencies and with what amplitudes they can regenerate. The latter, often recommended for our case, are usually described as adding "subtle brightness" to the signal. Unfortunately that is all they can do -- and it is clear that when we are dealing with hardcore low pass filtered signal, there are no "subtleties" involved -- we need an appropriately big hammer for the job. This problem was first encountered by me a few years ago when I was trying to remaster some hard to find tracks from 96 kbps MP3 sources. Later, it came to restoring some poorly encoded movie sound tracks. Any tools that I could obtain (even commercial ones, let alone freely available) proved to be inadequate for the task, which led to the development of this tool.
BandR is a band replication tool for remastering audio with missing higher frequency range. Its core principle of operation is adequately illustrated in the graphic above: it takes a lower part of the frequency spectrum and copies it (with some minor amplitude and phase adjustments) to replace the otherwise missing upper part. There are some nasty problems to deal with along the way, but overall, the algorithm is pretty simple, and did a good job for me. From what I understand, a similar technique (SBR, or spectral band replication) is used by some more recent low bit rate audio encoding schemes, but I was unable to find any existing tools that did it for plain low passed WAVE files.
This is a command line tool. The program accepts and outputs uncompressed WAVE files, in either integer or floating point format. For more info, the command line syntax (if you can call it that), and the user guide, refer to the README file. Good luck in your remastering endeavors.
BandR v1.2 (550K). Download BandR version 1.2 (last updated 4/13/2010).