Guilt and kilohertz and high definition

I am very, very sorry to those who have been visiting here the last ten days to find, well, nothing new. There is a reason for it.

For the past few weeks I’ve been executing subroutine [bulk_reviews]. This time around it’s 23 reviews for Australia’s Best Buys Home Theatre, which amounts to around 24,000 words. So I’m half way through that when my program goes recursive and re-executives subroutine [bulk_reviews]. This time it’s for a widescreen display comparison for ‘Livewire’ in The Age newspaper (it’ll appear on 30 October for those interested). Sixteen reviews. Ouch! Yesterday that level of the subroutine finally wound to an end, all 16 reviews submitted along with a short piece on alternative display technologies.

I pause here briefly to thank Samsung for promising two units for this review, then failing to deliver and, further, failing to return my increasingly desperate phonebank messages including one on deadline day. No doubt there was a good reason. I would like to thank LG for pitching in with an LCD TV, and Duratone Hi Fi for allowing me to do an in-store review of a Metz Spectral 82, at the last moment to replace the missing products.

Anyway, now I’m back on Best Buys, I at least have some time to breathe.

While doing all those display devices, I took the opportunity to check them to see whether they’d cope with various high definition signals from a DGTEC DG-5000i digital receiver. This has a convenient button on the remote to change the output signal, cycling between 576i (PAL), 576p, 720p and 1080i. Several of the devices would display all of these except for 720p. Which reminded me of something. 1080i is seen by some to be the most demanding signal standard, and it is because of the 1920 pixel horizontal resolution. But the horizontal scan frequency of both 576p and 720p is actually higher than 1080i, since the latter is interlaced and the others are progressive.

Here are the scan frequencies and total amount of uncompressed data required to be accepted by a display device. Note that these are for a PAL-based country like Australia which uses 50 hertz. For the US equivalents, multiply the scan frequencies and Megabits per second columns by 6/5.

Name Horizontal
Frequency (Hz)
Resolution (pixels)
Megabits per
second (approx)
576p 31,250 1,024 32
720p 37,500 1,280 48
1080i 28,125 1,920 54

Now even though 1080i clearly delivers more data than the other two, 576p and 720p are actually harder to handle for most display devices than 1080i. The reason is that the total amount of data per second is reflected in the bandwidth (1080i implies a bandwidth requirement of somewhat more than 100MHz!!!) But a bandwidth wider than the display device can use does not stop the display of the signal. It simply filters out the frequencies higher than its limits allow, shedding resolution.

For the picture to be displayed at all, though, the display device must be able to lock onto the horizontal scan frequency, otherwise it doesn’t know where to begin each scan line. That’s apparently why some of the devices (read the reviews if you want to know which ones) could display both 576p and 1080i, but not 720p. Somewhere between 31.25kHz and 37.5kHz their lock-on ability was exceeded.

But there’s an alternative explanation. These devices might, in fact, be capable of handling 720p at a 60 hertz frame rate (scan frequency: 45kHz). One of the devices was clearly designed for 60, not 50, hertz. The LG 15 inch LCD TV displayed 576p, but overscanned maybe 10% of the picture off the bottom of the screen. And it displayed 720p, but squeezed the picture into the left three-quarters of the screen. That suggests to me it was interpreting these 50 hertz signals as 60 hertz. It would not display 1080i/50 hertz at all.

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