- Telephone quality audio:
- Human ear can hear frequencies:
20 - 20,000 Hz....
But:
- You do not need the full range to understand what someone is saying...
- Understandble conversions
can be carried using the following
range of frequencies:
- 300Hz - 3400 Hz
- Human ear can hear frequencies:
20 - 20,000 Hz....
- Maximum frequency
of "telephone" quality audio:
- 3400 Hz !!!
- Nyquist sampling rate
for "telephone" quality
audio:
Nyquist Sampling Rate (telephone quality) = 2 × MaxFrequency = 2 × 3400 Hz = 6800 Hz i.e.: Minimum sampling rate = 6800 times per second.
- Sampling rate of
(digital) telephone:
- Today's (digital) telephone
samples the voice at:
- 8000 Hz
I.e.: 8000 samples in 1 sec or 1 sample in 1/8000 sec
- 8000 Hz
- Today's (digital) telephone
samples the voice at:
- Encoding
"telephone" quality audio:
- Each sample is encoded using signed binary value of 8 bits
- Data rate
of telephone quality audio:
Sampling Rate = 8000 Hz ====> 8000 samples in 1 sec 1 sample outputs 8 bits ====> 8000 × 8 bits = 64000 bits in 1 sec
Data rate of telephone quality audio = 64,000 bps
- Fact:
- The telephone company reserves a 64,000 bps communication channel for each phone conversation
However:
- The telephone company "steals"
one bit from
every byte (8 bits)
for
administration purposes
- E.g., send the caller ID to the phone
- Effective
data rate of
a telephone channel:
- effectively,
a telephone channel transmits:
- 7/8 × 64,000 bps = 56,000 bps
of real audio data
- effectively,
a telephone channel transmits:
- The equipment that
allows you to transmit
data over a
telephone connection
is:
- a modem (modulator/de-modulator)
-
Due to the channel capacity
of a telephone connection
(see above !!!),
the modem speed
is limitted to 56 kbps !!!
Modem is a thing of the past because telecom companies now sells Data Services to their customers.