Half Rate (HR or GSM-HR or GSM 06.20) is a speech coding system for GSM, developed in the early 1990s.
Since the codec, operating at 5.6 kbit/s, requires half the bandwidth of the Full Rate codec, network capacity for voice traffic is doubled, at the expense of audio quality.
[1] Previous specification was in ETSI ETS 300 581–2, which first edition was published in December 1995.
[2] For some Nokia phones one can configure the use of this codec:
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