BT815A Ver la hoja de datos (PDF) - Unspecified

Número de pieza

componentes Descripción

Fabricante

BT815A

VideoStream™ Decoders

Unspecified 

Bt819A/7A/5A

FUNCTIONAL DESCRIPTION

UltraLock

UltraLock

The Challenge

The line length (the interval between the midpoints of succeeding horizontal sync

pulses) of analog video sources is not constant. For a stable source such as studio

quality source or test signal generators, this variation is very small: ±2 ns. Howev-

er, for an unstable source such as a VCR, laser disk player, or TV tuner, line length

variation is as much as a few microseconds.

Digital display systems require a fixed number of pixels per line despite these

variations. The Bt819A employs a technique known as UltraLock to implement

locking to the horizontal sync and the subcarrier of the incoming analog video sig-

nal by generating the required number of pixels per line.

Operation Principles

of UltraLock

UltraLock is based on sampling using a fixed-frequency stable clock. Since the

video line length will vary, the number of samples generated using a fixed-frequen-

cy sample clock will also vary from line to line. If the number of generated samples

per line is always greater than the number of samples per line required by the par-

ticular video format, the number of acquired samples can be reduced to fit the re-

quired number of pixels per line.

The Bt819A requires an 8*Fsc (28.64 MHz for NTSC and 35.47 MHz for PAL)

crystal or oscillator input signal source. The 8*Fsc clock signal, or CLKx2, is di-

vided down to CLKx1 internally (14.32 MHz for NTSC and 17.73 MHz for PAL).

Both CLKx2 and CLKx1 are made available to the system. UltraLock operates at

CLKx1 although the input waveform is sampled at CLKx2 then low pass filtered

and decimated to CLKx1 sample rate.

At a 4*Fsc (CLKx1) sample rate there are 910 pixels for NTSC and 1,135 pixels

for PAL within a nominal line time interval (63.5 µs for NTSC and 64 µs for PAL).

For square pixel NTSC and PAL formats there should only be 780 and 944 pixels

per video line, respectively. This is because the square pixel clock rates are slower

than a 4*Fsc clock rate, i.e., 12.27 MHz for NTSC and 14.75 MHz for PAL.

UltraLock accommodates line length variations from nominal in the incoming

video by always acquiring more samples, at an effective 4*Fsc rate, than are re-

quired by the particular video format and outputting the correct number of pixels

per line. UltraLock then interpolates the required number of pixels in a way that

maintains the stability of the original image despite variation in the line length of

the incoming analog waveform.

The example illustrated in Figure 5 shows three successive lines of video being

decoded for square pixel NTSC output. The first line is shorter than the nominal

NTSC line time interval of 63.5 µs. On this first line, a line time of 63.2 µs sampled

at 4*Fsc (14.32 MHz) generates only 905 pixels. The second line matches the

nominal line time of 63.5 µs and provides the expected 910 pixels. Finally, the

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