Cutting the Confusing Chatter about VoIP CODECs

The animated explanation can be found at:
http://www.techtionary.com/fastcasts/codec-mos.swf

A CODEC-COder-DECoder (also known as an encoder-decoder and COmpression-DECompression system when used in video systems) is a computer chip (semiconductor) processing system.  Source codecs are designed specifically for speech, whereas Waveform codecs work well with any type of sound.  That is, depending on the audio or voice application would drive the selection of the Source or Waveform CODEC. Compression algorithms operate by sampling voice and quantizing the analog sound into digital values.  G.711 is based on traditional Nyquist-Shannon sampling theorem that the sampling frequency rate must be at least twice as high as the highest input frequency for the result to closely resemble the original signal.  A 4,000 Hz-Hertz voice pattern would be sampled at a rate of 8,000 BPS-Bits Per Second.  

A G.711 encoded audio stream is 64/56/48 KBPS-Kilo Bits Per Second. Each 13/14 bit sample of the original signal (voice-audio) is encoded into an 8 Byte/Octet.  In G.711, there are two forms of companding (COMpression-EXPANDing) standards: µ-Law (µ from the Greek mu or M or Modulation) and the A-Law.  G.711 µ-Law compresses frames of 14-bit linear (additive) PCM-Pulse Code Modulation samples into frames of 8-bit logarithmic (multiplicative) PCM code words.  G.711 A-Law compresses 13-bit linear (additive) PCM samples into 8-bit logarithmic (multiplicative) PCM code words. 13 to 8 bits is A-Law transformation used in Europe, 14 to 8 bits is µ-Law transformation used in North America and Japan.  For example, natural steps increase in an additive fashion (linear scale) or in a multiplicative fashion (logarithmic scale).  Shown here is G.729 coding at 8,000, not 64,000 samples per second.  Transcoding is also related to the concept of Tandem Encoding or Tandem Compression.  Tandem Encoding is traditional the concept of the transfer of TDM traffic between different telephone carriers via tandem Class 4/5 switches as they process telephone calls.  Tandem Encoding is also the process of interconnecting same or different company packet voice.  That is, different CODECs (e.g. G.711/729) may be used at different locations within the same company and more likely between different companies.  MOS-Mean Opinion Scores reduces rapidly with each time a voice conversation is processed by a CODEC.  Whether you call it Transcoding or Tandem Encoding is a network element to be standardized when ever possible.

So What

Here's the "so what" or "why should I care about this."  For example, different CODEC sampling rates may start synchronized but shortly become un-synchronized which can cause encoding problems and voice to jitter.  To measure and manage jitter RTP-Real Time Protocol uses the time-stamp function in the protocol to assess jitter based on the delay between arrival (interarrival) times of each packet.  Changing the number of bits sampled and quantized can dramatically impact the voice quality.  However, LAN-Local Are Network and WAN-Wide Area Network bandwidth limitations may have an equal or greater impact. Echo can also occur as a result of Asynchronous Transcoding. Transcoding is the process of conversion between circuit-switched (PSTN-Public Switched Telephone Network) and packet-switched networks such as Frame Relay, Internet, and ATM-Asynchronous Transfer Mode. However, Asynchronous Transcoding is to be avoided.  According to Intel, "The term "asynchronous transcoding" refers to a situation when, for example, one endpoint is talking G.711 to another endpoint talking G.723 (two different encodings)."

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