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)."
ABOUT
TECHtionary.com produces dealer and customer training programs,
online presentations including iPod and PC formats sales
brochures, virtual installation manuals and animated online
presentations. The company has more than 2,850+ free online
presentations on data communications,, internet, wireless,
VoIP-Voice over Internet Protocol, PBX Systems, central office
switching, protocols, telephony, telecommunications, networking,
routing, IPTV, WiMax,
power systems, broadband, WiFi-wireless fidelity and other related
technologies available at
http://www.techtionary.com. TECHtionary also produces VoIP
Dealer Training. Some of the key highlights are: Building a VoIP
Business, Selling & Marketing VoIP, Customer and End
User Training, VoIP Technology, Network Design, Provisioning,
Customer Service, Dealer Portal, and Enhanced & Professional
Services. VoIP training is also available as a one-day
introduction to a five-day indepth course and can be customized
and delivered via
web seminar or online tutorial series. More details on VoIP Dealer
training are available by calling Tom Cross at 303-594-1694 or
cross@gocross.com Thomas Cross is a magazine columnist with
many key technology publications and a member of the Technical
Board of
Advisors for the VoIP-Security Alliance.