Monday, November 17, 2008

ATSC peak to average - straight from the horse's mouth

These guys are in the business, they ought to know. This article in Broadcast Engineering talks about ATSC peak-to-average ratios. I was wrong to merely assume that the 3 dB ratio between average and the top symbol was all that was involved. There is overshoot all the way to 7 dB - 5 times the average power. However, look at Figure 3 in that article - the peaks are present only for very brief instances - the signal spends less than 2% of its time above 5 dB, which is triple the average. That does explain why the 10 watt picture of my 7025PA doesn't look as pretty is the 6 watt picture. At 6 watts, the peaks are entirely within 30 watts, which is almost entirely within the amp's linear range. However, at 10 watts, we're going to exceed the 30 watt number 2% of the time, meaning that those peaks almost certainly will be clipped, which is why the skirts start to grow.

This puts the impending arrival of the 200W amp in a different perspective. 200/3 = 66 watts, which is likely what I'll be able to get out of the thing and still be reasonably clean. It may have more headroom above 200W. If it could make it to 250W without clipping, then that would be 83 watts, but I still think I'd be happy with 66. That's 330 watts ERP.

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