Electronic Design

The Devil Is In The Design Details

In addressing some of the standard versus resolution issues resident with flat-panel HDTV design, Intersil first decided to upgrade the PLL that generates the pixel clock. HD resolution requires a PLL with low jitter. However, the range of resolutions is tough for analog PLLs to handle. This is because designers would have to optimize the loop filter across horizontal frequencies from 10 to 150 kHz. So, Intersil designed a digital PLL with a digital loop controller and a numerically controlled oscillator feeding into a digital phase controller that provides 64 precisely spaced phase choices.

Another interesting bit of design in Intersil's parts is the way the engineers dealt with offset. The usual approach is to cancel offsets with an offset digital-to-analog converter (DAC). But that means offset cancellation must be accomplished on each TV monitor or receiver, either during final test or by the user. And since offsets are temperature-sensitive, re-adjustments are necessary from time to time.

Intersil's approach takes advantage of the black reference set by the ?back porch? that follows the trailing edge of the video signal's HSYNC pulse and precedes the start of active video. Intersil's Automatic Black Level Compensation (ABLC) function monitors the value of the black level coming out of the analog-to-digital converter (ADC) during the back porch period and adjusts the DAC until the output code is zero.

Previous attempts to implement this function failed because the offset DAC resolution was too coarse. An offset DAC correction of only one step would cause the image to get visibly brighter or darker ? an unacceptable phenomenon called ?frame noise.?

Intersil solved this problem by increasing the offset DAC resolution to 10 bits. To control the 10-bit DAC, the ABLC function averages dozens of black-level samples of ADC data over hundreds of lines to generate an error signal with well over 10 bits of precision. The end result is a black level error of about ± 1/8 an LSB on average, or 1/2 of an LSB maximum.

To deal with false triggering from distorted sync signals from legacy sources, Intersil provides a programmable 3-bit threshold control for horizontal sync with levels from 0.4 to 3.2 V, in steps of 0.4 V, along with 300 mV of hysteresis for noise rejection and a digital de-glitcher that rejects glitches due to ringing. For the slower vertical sync, there's just the de-glitcher and 500 mV of hysteresis. (There are similar provisions for computer-output Sync-on-Green component video, albeit with slightly different parameters.)

The digital PLL nicely stabilizes the picture during head switching on old, stretched tapes and during fast-forward and rewind. For Macrovision, Intersil ORs sync with an internal mask signal, automatically adjusting the delay between the end of the mask and the valid horizontal sync transition.

Taking A Lesson From Delta-Sigma
For the ADCs in its HD video chips, Analog Devices created a technique called ?Noise Shaped Video? (NSV) that manipulates the noise spectrum of the ADC. NSV takes its lead from delta-sigma concepts without using quite so high a degree of oversampling. It's based on combining data converters with 12 bits of resolution (or more) with oversampling the signal's Nyquist frequency by at least four times.

(Well, not exactly Nyquist; I'm indebted to ADI's product line director for high-speed signal processing, Bill Bucklen, for an explanation of how television engineers approximated Nyquist when they first began digitizing video signals and how the sample rates for converters have evolved since then. Bucklen explained that around 1977, ADCs had more limited performance than they have today, and the initial ADC sampling rate for NTSC video was 10.7 Msamples/s, or three times the NTSC color subcarrier frequency. That provided a little wiggle room for designing the anti-aliasing filter, but not much. For PAL, the sample rate was three times 4.43, or 13.3 Msamples/s.)

This ?subcarrier locked? sampling had the advantage of locking any distortion artifacts (like linearity errors) to the color subcarrier and avoiding low-frequency ?beat? patterns. Later, NTSC and PAL sampling moved up to 14.3 and 17.7 Msamples/s. This eased the filtering problem, but it required 33% more memory. That was balanced by the fact that quadrature-sampling the color subcarrier made it simpler to demodulate the color components.

As the TV industry contemplated interconnecting boxes in the digital domain, it resolved the difference between the NTSC and PAL sampling frequencies by picking 13.5 Msamples/s as its ?Nyquist? rate. Sampling 525/60 video and 625/50 video at that rate yields roughly the same number of samples in an active video line, which simplifies standards conversion. And that explains why the converters in ADI products with NSV operate at 54 Msamples/s, which is four times what TV engineers consider ?Nyquist.?

Naturally, the high sampling rate reduces rolloff demands on the anti-aliasing filters, and thus complexity and cost. But as we learned from sigma-delta converter theory, oversampling also improves signal to noise ratio (SNR) because any noise (including quantization noise) present at the ADC's output is spread from dc to half the sampling frequency. Oversampling provides 1 full bit of improvement in SNR/ADC resolution for every quadrupling of the sampling frequency.

Analog Devices' NSV video chips combine this oversampling with what ADI calls dynamic element matching (DEM). DEM is an algorithm applied after quantization. It compensates for the natural mismatches in the ADC that lead to integral nonlinearity (INL) distortion. In addition, DEM reduces low-frequency artifacts resulting from settling-time violations in the ADC's switched-capacitor input circuit, moving them from inside the signal bandwidth to half the sampling frequency.

Intersil's AFE is the ISL98001, which supports HDTV resolution up to 1080p and computer monitor resolutions up to QXGA. Thousand-unit pricing is $10.06 each. Analog Devices also makes products for the AFE application, notably the AD9888 Graphics Digitizer. However, ADI's NSV products are the ADV7xxx decoder family and AD9889 High Definition Multimedia Interface (HDMI)/DVI Transmitter, which supports HDTV formats up to 1080i and 720p and graphic resolutions up to XGA. Thousand-unit pricing for the AD9889 is $4.73 each.

TAGS: Intersil
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