Low bit-rate frequency extension coding

There are now a number of applications, most notably streamed Internet audio, which require audio and speech to be encoded at a low bit rate, typically 16 kbit/sec or below. To achieve an acceptable quality, the original signal is normally low-pass filtered to somewhere between 4 and 5.5 kHz before encoding. Rather than discard the upper frequency band completely, we propose encoding just the noise component of it using about 500 bits/sec. This greatly enhances contemporary music and close-microphone speech, but has little effect on classical music. The process can be used to enhance any audio or speech codec, knowing only its encoding/decoding delay.