Saturday, November 17, 2012

NTSC and PAL

The other day, I captured the composite video signal coming out from an infotainment device. The device had a setting which allows you to select encoding for the output signal. By setting the device to output a particular image on the composite video RCA connector (hence the waveform shown below are of unmodulated baseband video signals) and switching the device between NTSC and PAL, I was able to capture and analyse various aspects of the two standards.

Test Pattern 

Here is the test pattern I used. I loaded the JPEG onto a USB flash drive and connected it to the infotainment device. The device was put in image playback mode and the selected file was rendered on the LCD display as well as output on the composite video out.

The image is composed of exactly 640 x 480 pixels which corresponds to an aspect ratio of 4:3.

Test Pattern


A short table of differences between the two standards

NTSC PAL
Stands for National Television System Committee Phase Alternating Line
Major Contries using it North America, Most of South America, Japan etc.
(mostly regions where AC mains is 60Hz)
India, Australia, South Asia, Eastern parts of Africa etc.
(mostly regions where AC mains is 50Hz)
Wikipedia Link http://en.wikipedia.org/wiki/NTSC http://en.wikipedia.org/wiki/PAL
frames per second (fps) 60i or 30p (59.94i incase of colour) 50i or 25p (p stands for progressive scan,
i stand for interlaced)
Tint control NTSC receivers have a tint control to perform colour correction manually. If this is not adjusted correctly, the colours may be faulty.  The PAL standard automatically cancels hue errors by phase reversal, so a tint control is unnecessary.
Scan lines 525 625
Horizontal Frequency 15.734 kHz 15.625 kHz
Aspect Ratio 4:3
Colour Subcarrier roughly (2x227+1)x15750/2 = 3.58 MHz
exactly (2x227+1)x15734.26/2 = 3.579545 MHz 
roughly (2x283+1)x15625/2 = 4.43 MHz
exactly 4.43361875 MHz
Video Bandwidth 4.2 MHz 5.0 MHz
Sound Carrier 4.5 MHz 5.5 MHz
Bandwidth 6 MHz 7 or 8 MHz


Waveforms


So here is the comparison of various sections of the signal. Practical measurement differ slightly from the ideal figures presented in the table above. The signals captured were pure baseband video signals only. No modulated audio signals were present.


 Many Lines

Single Line

Color Band signal

Color Burst, White band and Yellow band

Color Burst

Color subcarrier frequency measurement

Horizontal Frequency Measurement
 Phase change between green and magenta bands

Vertical Blanking

Spectrum Comparison

References

  1. Maxim APP 734 Video Basics - A nice short summary of all related concepts
  2. Chapter 08 of the book Video Demystified: A Handbook for the Digital Engineer by Keith Jack (PDF here)
  3. PAL vs NTSC - An article clarifying the relationship between pixel resolution and video encoding format (NTSC/PAL) with respect to DVDs.
  4. 16x9 Enhancement - An article elucidating aspect ratios with respect to videos.
  5. Vertical Blanking Interval of 625-Line Standard (PAL Colour) (PDF Diagram)
  6. World Analogue Television Standards and Waveforms 

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