Engineering Guides

The Snell & Wilcox Test Chart #2

Boundaries
Top and bottom boundaries of the active part are designated by four black triangular markers.

Interlace Check
This black box contains a narrow (horizontal cross-section 168 ns) oblique white line. The purpose of this test is to show the effect of lack of proper interlacing. It appears as a staircase instead of a smooth line when interlacing is incorrect.

Tartan Bars
These are 75% tartan colour bars. The purpose of this test pattern is to measure the chroma sharpness both horizontally and vertically. It shows comb type Y/C separator failure on sharp vertical transients. Below the tartan bars is a grey scale with 20 %, 40 %, 60 % and 80 % levels useful for non-linearity tests.

Circular Grey Scale
The grey scale circular pattern (grey cone) is useful for visual assessment of quantisation distortion, dither etc

Chart Format
This tells the viewer the chart format, e.g. 'PAL'.

Registration Check
This area contains a black box with a white cross (200 ns). It can be used for raster registration check, 2D aperture correction symmetry measurement etc.

Frequency/Vertical Response Check
This section contains slightly oblique bursts (almost horizontal) with frequencies 100, 200 and 300 tvl. They are useful for testing scan converters and vertical enhancers.

Horizontal and Diagonal Frequency Response Check
Bursts with frequencies correspondingly to 3.58 MHz verticals (NTSC SC), 300 tvl verticals, 300 tvl diagonals, 400 tvl diagonals, 400 tvl verticals, 4.43 MHz verticals (PAL SC). They are useful for quick check of frequency response, testing 2D aperture correction devices and evaluation of cross-colour effects with conventional and comb type decoders.

SECAM Bell Filter Check
This area contains a 4.286 MHz burst (SECAM bell filter centre frequency).

Frequency Response Wedge
This wedge covers the band from 1.5 MHz to 5.5 MHz

Radial Wedge
This radial wedge covers spatial frequencies up to 450 tvl. It shows decoding cross-effects and horizontal/vertical enhancement proportion.

Moving Zone Plate
This area is occupied by a static or moving circular grating (Fresnel zone plate) covering spatial frequencies range up to 429 tvl (5.5 MHz).The radius of this grating is 0.15 of picture height.The moving zone plate is especially useful for checking line or frame based comb decoders and scan converters performance.

Chroma Frequency Response

Y/C Timing Checks
This area contains blue/yellow and green/magenta bursts with frequencies 1.0, 0.5 and 1.5 MHz. They are useful for chroma resolution and Y/C timing tests.

PLUGE (Picture Line Up Generator)
The three boxes comprising the PLUGE (a BBC acronym for Picture LineUp Generating Equipment) are as follows:
- a small 93.75% white box on a 100% white background.
- a grey level (35%) box.
- a small dark grey (7.5%) box on a black background.

Pulse and Bar Tests
Three types of 2T (200ns) pulses are provided: white on black, black on white and white on dark grey (20%). This last pulse is useful for ringing and echo measurements if equipment under test includes black clipper concealing distortions below black level.

Chroma Non-Linearity Tests
This area contains a 3 level chroma staircase in the form of 33.3%, 66.6% and 100% magenta boxes. It can be used for chroma nonlinearity and differential phase measurements.

Large Area Chroma Tests
A 100% red box is provided for visual assessment of chroma noise, large area chroma flicker, and Hanover bars type distortions.