CS485 Sylabus

### Application of the Nyquist Rate: Digital Telephone

• Digital telephone

• 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

• Nyquist rate for "telephone" quality audio

• 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 used by telephone companies

• 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

• Encoding "telephone" quality audio:

 Each sample is encoded using signed binary value of 8 bits

• Data rate of "telephone" quality audio

• 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 ```

• Digital telephone

• 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

• A gadget from the past

• 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.