Short note: Line coding principles in PCM

Line coding is the conversion of the binary PCM bit stream into an electrical waveform (pulse pattern) suitable for transmission over the cable and through regenerative repeaters. A plain on–off signal has DC, loses timing on long runs and gives no error check, so a line code is always used.

Requirements of a good line code

• No DC component: lines are coupled through transformers and capacitors (and carry DC power for repeaters), so the signal must have no DC and little low-frequency energy.
• Timing (clock) content: regenerative repeaters extract the clock from the transitions of the received signal; there must be enough transitions whatever the data.
• Small bandwidth: the spectrum should be narrow to reduce attenuation and crosstalk on the cable.
• Error monitoring: the code should have redundancy so that errors can be detected in service (e.g. bipolar violations).
• Transparency: any data pattern, including long runs of 0s or 1s, must pass without loss of timing.
• Simple, low-cost coding and decoding.

Common line codes

 data:     1   0   1   1   0   0   0   0
          +---+   +-------+
 NRZ-L    |   |   |       |
 (unipol) +   +---+       +---------------  0

          +-+     +-+ +-+
 RZ       | |     | | | |
 (unipol) + +-----+ +-+ +-----------------  0

          +---+       +---+
 AMI      |   |       |   |
       ---+   +---+   +   +---+------------ 0
                  |   |   (no pulses
                  +---+    for 0s)

1. Unipolar NRZ: 1 = +V, 0 = 0 for the full bit. Simple, but has large DC and no timing in long runs. Not used on lines.
2. Polar NRZ: 1 = +V, 0 = −V. DC is zero only for balanced data; poor timing.
3. Unipolar RZ: pulse for half a bit for 1s. Has DC, double bandwidth, timing lost on 0-runs.
4. Manchester (biphase): a transition in the middle of every bit. Excellent timing and no DC, but needs twice the bandwidth; used in Ethernet, not on PCM trunks.
5. Bipolar AMI: 0 = no pulse, 1s alternately + and −. No DC, energy concentrated around $f_b/2$, and bipolar violations reveal errors. Its weakness: long runs of zeros give no pulses, so repeaters lose timing.
6. HDB3: AMI in which each string of four zeros is replaced by 000V or B00V (chosen so violations alternate). At most three zeros in a row. Standard for E1/E2/E3.
7. B8ZS: AMI in which eight zeros are replaced by 000VB0VB. Standard for T1; B3ZS/B6ZS for higher T levels.
8. CMI, 4B3T, 2B1Q: used on some interfaces and ISDN lines.

Example HDB3 coding (first pulse +):

Choice

Thus PCM lines use bipolar codes with zero substitution (HDB3, B8ZS), which keep AMI's low bandwidth and zero DC while guaranteeing clock content.