VSB generation; VSB for TV; SSB applications

Generation of VSB AM

Vestigial Sideband (VSB) modulation transmits one sideband almost fully and only a small part (vestige) of the other sideband. It is a compromise between DSB (easy to generate, but wide) and SSB (narrow, but hard to filter).

 m(t) --> Product   --> DSB-SC --> VSB filter --> s_VSB(t)
          modulator               H(f)
             ^
             |
         Ac cos wc t

1. A product (balanced) modulator gives DSB-SC: $A_cm(t)\cos\omega_ct$.
2. A VSB shaping filter $H(f)$ passes the wanted sideband fully and a vestige of the other, with a gradual roll-off around $f_c$.
3. For distortion-free coherent detection, the filter must satisfy

$$H(f + f_c) + H(f - f_c) = \text{constant}, \quad |f| \le W$$

i.e. the filter response is odd-symmetric about $f_c$ (it passes through half its full value at $f_c$), so the part of the wanted sideband lost near $f_c$ is exactly made up by the vestige of the other sideband.

Spectrum

       |H(f)|
   1 -              ____________________
                   /                    |
  0.5 - - - - - - *                     |
                 /|                     |
   0 -----------/-+---+-----------------+----> f
           fc-fv  fc  fc+fv            fc+W
          <--roll-off-->
     vestige of LSB    full upper sideband

Bandwidth $B_T = W + f_v$, where $f_v$ is the vestige width.

Why VSB for TV

• The video signal has a very large bandwidth (about 4.2–5 MHz) and significant low-frequency and DC content. SSB filters cannot cut sharply at $f_c$ without removing these, while DSB would need about 10 MHz.
• VSB saves almost half the bandwidth: in CCIR-B, video uses a 5 MHz upper sideband plus a 0.75 MHz vestige, so the channel fits into 7 MHz.
• With a carrier added, VSB+C can be detected with a simple envelope detector in TV receivers.
• Practical filters with a gradual slope are easy to build.

Applications of SSB

• HF point-to-point and ship-to-shore radio telephony.
• Amateur (ham) radio voice communication.
• Military and aviation HF communication.
• Frequency division multiplexing in analog telephone carrier systems.
• Police and mobile radio, where power and bandwidth are limited.