• Codec deals with digital signals • Modem deals with analog signals Digital Data, Digital Signals Encoding schemes • Simple scheme ○ Bit 1 = high ○ Bit 0 = 0 low • Other schemes use pulses • NRZ-L ○ Simple scheme ○ 0 is high, and stays high until we get a 1 ○ 1 is low, and stays low until a zero ○ Used in USB • NRZ-I ○ Switches on 1s • Bipolar AMI ○ Returns to zero ○ Switches on 1s § Always alternates, will never have 2 spikes in a row § This is the in-built error detection • Pseudoternary ○ Opposite of BAMI • Manchester ○ Used in ethernet • HDB3 ○ Count bipolar pulses from the beginning of the last substitution (not the beginning of the entire bit stream) ○ A bipolar pulse is when you go from high to low Why different schemes? • BAMI can detect errors by checking for two negatives in a row • Some encoding schemes use more bandwidth than others ○ BAMI needs 3 levels, which is more than the others • NR2L may cause synchronization issues if a long series of 1s is sent ○ The receiver may have difficulty determining how many ones, if the clock isn't perfectly synchronized with the transmitter ○ B8ZS handles this error by doing what AMI does for 1s, but having variations for long strings of zeroes Digital Data, Analog Signals • Modulation of analogy signals for digital data ○ 3 vars to change: Amplitude, frequency, phase ○ 3 modulation techniques, ASK, BFSK, BPSK, for each var • ASK ○ We identify 1s by checking for higher amplitude ○ Not efficient, suitable for optical fibre, RFID • BFSK ○ We identify 1s by checking for higher frequency ○ Can have multiple levels by sending in multiple different frequencies • BPSK ○ We identify 1s by changing the phase • QAM ○ Combination of PSK and ASK § Different phases and amplitudes ○ QAM 256 means 256 levels Analog Data, Analog Signals • We often want to change the frequency of data during transmission ○ Typical human voice ranges from 0 - 4khz ○ Sent via 900khz microphone • AM radio: amplitude modulation • FM radio: frequency modulation • 93.5Mhz is the carrier signal
