D-A conversion
Jun 1, 2009 12:00 PM, By Aldo Cugnini
Step one to digital is converting analog signals into digital equivalents.
For video signals, nonlinearity can be tolerated to some extent, but not so with audio. With audio signals, an oversampling converter can be used to both simplify the D-A conversion and to lower the quantization noise of the signal. This is because upconverting the signal to a very high sample rate will spread the quantization noise over the entire sampling spectrum; thus, the final lowpass filter will remove a great deal of the quantization noise. In addition, the D-A converter can be constructed using a pulse-width modulation scheme that will result in a very high linearity of the conversion. This is not practical for video circuits, however, due to the extremely high sampling rates that would be required.
In practice, the video D-A converter within a DTV converter or receiver is integrated within the DENC chip mentioned earlier. This device takes 8-bit multiplexed 4:2:2 YCbCr video at a 13.5MHz rate and converts this to Y/C video (for S-Video interface), or CVBS (composite video blanking signal), for composite or RF interface. To support low-end TV sets that are subject to cross-color artifacts, the luminance signal can be notch-filtered at 3.58MHz (NTSC) or 4.43MHz (PAL). A similar notch filter is also available to prevent problems with the sound carrier. Both of these filters, as well as the chroma interpolation filters (needed to convert 4:2:2 sampling to 4:4:4), are implemented digitally within the DENC. A D-A converter on the same chip (usually 10 bits to account for addition of the Y and C signals) then provides the final analog output.
D-A converter boxes are an important part of the digital transition strategy for broadcasters, enabling all consumers to afford keeping up with this technology easily. Within these units, D-A converter technology provides support for analog displays. However, as integrated TVs and digital interfaces such as HDMI become more commonplace, we may eventually arrive at the situation where analog video signals are no longer present in any of these devices!
Aldo Cugnini is a consultant in the digital television industry.
Send questions and comments to:
aldo.cugnini@penton.com
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