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Diffstat (limited to 'doc/codec2.tex')
| -rw-r--r-- | doc/codec2.tex | 2 |
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diff --git a/doc/codec2.tex b/doc/codec2.tex index 73d2565..fe0939b 100644 --- a/doc/codec2.tex +++ b/doc/codec2.tex @@ -81,7 +81,7 @@ This production of this document was kindly supported by an ARDC grant \cite{ard A speech codec takes speech samples from an A/D converter (e.g. 16 bit samples at 8 kHz or 128 kbits/s) and compresses them down to a low bit rate that can be more easily sent over a narrow bandwidth channel (e.g. 700 bits/s for HF). Speech coding is the art of ``what can we throw away". We need to lower the bit rate of the speech while retaining speech you can understand, and making it sound as natural as possible. -As such low bit rates we use a speech production ``model". The input speech is anlaysed, and we extract model parameters, which are then sent over the channel. An example of a model based parameter is the pitch of the person speaking. We estimate the pitch of the speaker, quantise it to a 7 bit number, and send that over the channel every 20ms. +As such low bit rates we use a speech production ``model". The input speech is analysed, and we extract model parameters, which are then sent over the channel. An example of a model based parameter is the pitch of the person speaking. We estimate the pitch of the speaker, quantise it to a 7 bit number, and send that over the channel every 20ms. The model based approach used by Codec 2 allows high compression, with some trade offs such as noticeable artefacts in the decoded speech. Higher bit rate codecs (above 5000 bit/s), such as those use for mobile telephony or voice on the Internet, tend to pay more attention to preserving the speech waveform, or use a hybrid approach of waveform and model based techniques. They sound better but require a higher bit rate. |