[opendtv] Re: Europe now debates i vs p

  • From: Craig Birkmaier <craig@xxxxxxxxx>
  • To: opendtv@xxxxxxxxxxxxx
  • Date: Tue, 14 Sep 2004 08:40:38 -0400

At 10:32 AM -0500 9/13/04, Doug McDonald wrote:
>Are you sure that is true for 720p cameras? If is it, then there
>is room for quite a bit ...  a very lot indeed ... of additional
>detail in 720p pictures.

I'm not absolutely certain, but I'm fairly sure I'm correct.

The camera that is most likely the one you were watching is the 
Thomson/Grass Valley LDK 6000 MKII, which currently dominates 
installations in HD remote broadcast truck, due to it's ability to 
switch between 720P and 1080i modes. The spec sheet for this camera 
does not provide any details about the frequency response. There is 
an interesting spec: 55% MTF for 720P at 27 MHz.

There are several issues here.

First is the ability of any given sensor to acquire high frequency 
details. Obviously, a 2 Mpixel sensor can capture higher frequency 
detail than a 1 Mpixel sensor. But the difference may not be easy to 
perceive because of the MTF issue. A slightly lower resolution 
picture may look sharper if the MTF of the higher resolution camera 
is lower. In essence, the high frequency details lack contrast when 
the MTF of the camera is reduced.

Mark Schubin may correct me if I am wrong, but the 55% MTF at 27 MHz 
suggests that a fine black detail at 27MHz will be captured as a 
midtone grey. The lack of contrast will make it difficult to perceive 
the detail. For example a B& W herringbone pattern may turn into a 
solid grey pattern.

In order to capture the high frequency details with GOOD MTF, you 
need to oversample, which is another benefit of the Thomson LDK 6000 
(although most of the oversampling is in the vertical, not the 
horizontal). For a 720P capture the LDK 6000 does some horizontal 
oversampling - 2048 samples are resampled to 1280. This is not quite 
the 2:1 oversampling that would be desirable, but it is better than a 
1920 x 1080 sensor being used for 1920 x 1080 with NO oversampling.

Another issue is the overall response of the production chain - what 
happens as the HD signals move through the production switcher, 
signal distribution, and finally MPEG encoding.

Bottom line, you are correct. There is still plenty of headroom for 

>Actually, if I had not been careful to avoid it, it could
>also have been real content from computer generated
>stuff. The first day our local NBC station did HD, they
>had a computer generated bug that had very high frequency
>content indeed.

Yes, it is easier to generate high frequency details in graphics than 
it is to capture equivalent levels of detail with a camera. But there 
are some trade-offs here too. The higher the frequency of the graphic 
edges, the greater the problems with MPEG compression, which tries to 
quantize away the high frequency coefficients. It is better to 
roll-off the response on the graphics a bit to help the downstream 

Remember, hardly anyone has a display capable of resolving these high 
frequency details, so why stress the system? A spectrum analyzer lets 
you "see" stuff that is not perceptible on the display.

>If indeed there is nothing real above 25 MHZ, then there is
>no need for 720p horizontal resolution. For a couple of
>weeks before yesterday Fox broadcast 720p football signals which
>were camera generated at 720p, cut down to professional grade
>digital, not MPEG, 480i, and then upconverted to 720p. Those
>signals did indeed cut off at about 20-22 MHZ. And they looked
>good ... real real good indeed ... but not the wow factor of the
>real 720P.

720P (and all the other formats for that matter) are just containers 
for samples. How good those samples are is the real question. The 
same could be said for NTSC: it took about four decades to max out 
the NTSC container.

The ability to fill the spectra for any HD format will improve over 
time, as we are able to deal with the harsh realities of high 
resolution image capture. We need to oversample images to fill the 
spectra; unfortunately, this means that we need to divide the 
available photons among even more sensor sites. Its all about the 
signal to noise ratio and sensitivity. The solution is likely to be 
found in a new generation of sensors that do not have the physical 
limitations of the venerable old CCD. keep an eye on CMOS sensors.

>If they are not getting any real signal above 25 MHz, they
>need to get better cameras, considering the alias point is 37 MHZ.

All things in good time.

For now, I believe that 720P is the HD sweet spot.

- It has more than adequate resolution for virtually ALL consumer 
display applications. And it is now possible to build affordable 1 
Mpixel LCD, DLP and LCoS display chips.

- It is possible to do some oversampling during 720P acquisition, 
which improves the MTF.

- And 720P is a far better optimized for MPEG-2 compression that 1080i.

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