[vorbis] Downsampling

[ÍôÇç´¨]

It is commonly said here that if I want to make AM radio-quality
stuff at very low bitrates, a good way is to downsample.
I downsampled a song to 11025Hz mono and encoded with -q 0,
the result is about 18kbps and is at least radio quality.
The downsampler I used is from Edinburgh speech tools, named
ch_wave.  `sox' performs terribly, so I didn't use it.

However, I heard some unpleasant sound at relatively high
frequencies in the *downsampled WAV file*, but not in the
encoded OGG file.  So I assume I need some even better
downsamplers.  Is there any on Linux?

P.S. Why not put such downsampling things in the encoder,
activated when the quality setting is really low?  Or better,
save bits by some way cleverer than downsampling?

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