Chroma Systems TX

1
Chroma Systems (TX)

• ENCODING Why encoding? RGB alone was not
  compatible with black and white receivers. Each
  of RGB contains only part of picture.
• Remedy? Encoding- arranging colour video signals
  in specific proportions to provide all video
  information.
• 2 separate signals 1.) C or chrominance ( I and
  Q) and 2.) Y or luminance.
• ADDING COLOURS combining primary colours
  produces wide range of mixtures. ex. GBcyan

2
Chroma Systems contd. (TX)

• 3 MAIN CHARATERISTICS 1.) Hue - Tint, different
  hues for different wavelengths. Phase angle of C
  signal. 2.) Saturation - Degree to which colour
  is saturated. Amplitude of C signal. 3.)
  Luminance - amount of light intensity perceived
  by the eye. Lighter areas have more luminance
  than darker areas. Y level indicates how colour
  will appear in black and white.
• SIGNALS 1.) Chrominance A.) I signal - B-Y from
  matrix B.) Q signal R-Y from matrix. 90 degree
  phase difference with I signal.

3
CHROMA SYSTEMS contd. (TX)

• 2. Luminance - provides relative brightness for
  the C signal. Y 0.30R 0.59G 0.11B
• Note chroma subcarrier is PHASE modulated by I
  and Q which in turn modulates the main picture
  carrier.

4
Chroma Systems (RX)

• How does the chroma RX synchronize to the
  transmitted signal? Samples colour burst signal
  from TX for FREQUENCY and PHASE.
• SIGNIFICANT BLOCKS OF THE CHROMA RX
• Comb filter - separates the video (Y) signal and
  colour signal. Older systems use narrowband
  filter. Comb filter constructed of a glass cavity
  with a transducer and speaker at opposite ends.
  SAW in nature.
• BPA - 4 tuned narrowband I.F. stages for
  sideband amplification.

5
Chroma Systems (RX) contd.

• Why do we require chroma subcarrier reinsertion?
• Burst Gate - detects the presence of a colour
  burst and enables or disables BPA by way of the
  COLOUR KILLER.
• ACC - automatic chroma control. Acts like AGC for
  R.F. and I.F.

6
Chroma Demodulator System

• Colur burst is at 0 degrees reference
• I is at 57 degrees and Q is at 147 degrees
• Where is R-Y and B-Y relative to each other?
• Are they in quadrature in the colour phase
  diagram?
• How is green produced? A two transistor system
  combines (R-Y) and (B-Y).
• Demodulators include I, Q, (R-Y)(B-Y),
  (R-Y)(G-Y),(R-Y)(G-Y)(B-Y), X-Z and RGB.
• Vectors lengths represent the magnitude of the C
  signal.