What is digital voice?
Digital voice is a voice in a digital form. Why would we convert
analog voice to digital voice? As analog signal travels down the wire,
it loses its strength and attenuates. To boost the signal strength and
keep the volume at a predefined level, devices that amplify the analog
signals are placed every mile. The amplifier device, which is called a
repeater, will amplify all parts of the signal including the noise that
is picked up along the way. In order to avoid this problem we would want
to convert the analog voice signals to digital form. (Carr and Snyder p.
171) By transmitting analog voice signal as digital signal, the signal
is regenerated and amplified leaving the noise behind and a message is
communicated clearer at the destination.
How do we convert analog voice into digital form?
Pulse-code modulation (PCM), differential pulse-code modulation (DPCM),
and adaptive differential pulse-code also called adaptive delta
pulse-code (ADPCM) are techniques used for storing analog voice/audio
data in a digital format.
What are PCM, DPCM, and ADPCM?
Pulse-code modulation (PCM): Modulation in which a signal is sampled,
and the magnitude (with respect to a fixed reference) of each sample is
quantized and digitized for transmission over a common transmission
medium. (1)
Differential pulse-code modulation (DPCM): Pulse-code modulation in
which an analog signal is sampled and the difference between the actual
value of each sample and its predicted value, derived from the previous
sample or samples, is quantized and converted, by encoding, to a digital
signal. (1)
Adaptive differential pulse-code modulation (ADPCM): Differential
pulse-code modulation (DPCM) in which the prediction algorithm is
adjusted in accordance with specific characteristics of the input
signal. (1)
What is the process involved in converting analog voice into digital
form?
To convert the analog voice to digital form, PCM, samples the level
of a voice conversation 8000 times per second using a scale of 256
integer points, converts each integer measurement into an 8-bit code and
transmits the resultant information at a speed of 64,000 bps. The
receiving unit reverses the process and a voice conversation is
regenerated. (Carr and Snyder p. 172)
A second technique of converting the analog voice to digital form is
ADPCM. ADPCM stores the value differences between two adjacent PCM
samples and makes some assumptions that allow data reduction. (2) ADPCM
allows the use of a 4-bit code to represent the difference value and
reduces the signal requirement to 32,000 bps. (Carr and Snyder p. 173)
Almost all long distance carriers use analog-to-digital and
digital-to-analog conversion at this point and move analog telephone
conversations as digital data for clear and noise-free conversations.
(Carr and Snyder p.173)
Reference:
Houston H. Carr, and Charles A. Snyder. "The Management of
Telecommunication." McGral-Hill Companies, 1997.
Web Page Reference:
http://www-library.itsi.disa.mil/org/fed_std/html/dir-011/_1622.htm
http://support.microsoft.com/support/kb/articles/Q89/8/79.asp
