- entry 2009/01/31

Something lurking in my PVR

All of the pictures you may see are from ITV1's repeat of "Dancing On Ice" shown this afternoon, it was what was on as I was writing this.

If you select Display Ratio, the following will appear:

Under the SysSetup menu are a number of options:

If you select PlayMode Setting, you can choose:

In the SysSetup menu, the Clock Setting is as expected.

The Display Setting menu leads to this:

The System option gives the firmware details and reports on the USB device's free space.

The Record Setup is as expected (PAL/NTSC, resolution...).

Perhaps the most interesting option, if your PVR seems to be acting a little oddly, is Master reset:

Resolution tests

All of the formats are 24bpp (16m colour). The correct aspect ratio of an NTSC picture is 1.47:1 (720×480) and for a PAL picture it is 1.25 (720×576); so all of the intermediate sizes will be a compromise. To be honest, I am not sure if this is a particularly big issue - I record most stuff nowadays at 640×480 and my computer/DVD player stretches this to fill the screen. Well, actually I record most stuff anamorphic so the computer/DVD will letterbox it to fill the screen... You can see what I mean at the end.

• 176×144, 15fps [aspect 1.22:1]
  
  
  This is intended for mobile phones, for the later generation that can cope with MP4 video. I don't think mine can, it works with .3GP video at a lowish quality and disgustingly poor audio (about 8000Hz sampling rate, mono). But, then, my phone is old in comparison with today's gadgets.
  One of the problems that you may encounter is that the scaling is harsh. If we zoom up the source we can see that it is quite lacking in quality:
  
  To give you an idea of what I mean, this is how it could look if resized with anti-aliasing:
  
  The PVR probably does not anti-alias as it is an operation that is fairly processor-intensive (even using a DSP) to manage at 30fps on full-frame input. It is quicker by far to simply pluck every fourth pixel (or however the PVR does it) and use that; though it does show in the results.
  Given this, unless you wish to directly record/view something on your mobile, it might be preferable to record at a higher resolution, then scale it down yourself using something such as, say, the Lanczos3 filter.
   
  
• 320×240, 30fps [aspect 1.33:1]
  
  
  This is possibly the smallest size likely to be useful. If the PVR could output DivX .AVIs, I would record some things in this format - like episodes of Party Of Five, stuff that I could drop onto my Zen and delete after watching.
  The video scaling is still a little harsh, however the increased resolution does make things readable - though overlaid subtitles may still present a problem if they are small. It may look a bit teeny-tiny on your computer screen, especially if you are trying to read this on a big high-resolution LCD jobbie, however on a two-and-a-half inch media device, 320×240 can be surprisingly good, though I suspect a lot of that is down to the Zen's display quality.
   
  
• 480×360, 30fps [aspect 1.33:1]
  
  
  I can only imagine this odd size is tailored to some specific version of iPod that can't manage VGA (below) but is larger than QVGA (above)...
   
  
• 640×480, 30fps [aspect 1.33:1]
  
  
  This, VGA size, seems to me to be a perfectly acceptable compromise between image quality and data rate. The latter is an important factor for me as I am not using zippy equipment to process video. To put this into context, a friend of mine said it is possible to rip a DVD to his media player in around 25 minutes. For me, the same conversion would be something in the order of 12-16 hours (depending on codec options/quality). Besides, in reality, you are not likely to notice 60 lines less horizontal resolution. Hell, you lose more than that vertically as a factor of the device not working to full-frame PAL!
   
  
• 720×480, 30fps [aspect 1.47:1]
  
  
  And finally, NTSC resolution, the highest quality the device can record.

It may appear that each size is better than the previous as it is getting bigger. It is a fact that any inspecific size recording will be stretched to the full frame size for playback. My computer records at 352×288 (CIF), which, when scaled up, is on par with a videotaped recording. So what I shall do is present my computer's output, and the three most useful of the PVR resolutions, all scaled to the dimensions of a PAL frame.

• Benchmark [edited full-frame capture]
  
  
  This was edited back in January to remove the news banner and bordering... which I have added back in for the purposes of this demonstration. I saved the picture because she's cute and one day I hope to be able to figure out what her name is from the caption.
  Essentially, this is what a full-resolution PAL frame should look like.
   
  
• 352×288 [CIF, my computer's capture card]
  
  
  This is how my PC's capture card's recordings look when played back (i.e. from DVD-R). The resolution isn't great, but you can see what I mean about it being about the same as videotape. I can't complain, I mean, what did I expect a 450MHz CPU to manage to achieve?!?
  Actually, if you look, the picture is quite sharp so the perceived quality is more than larger PVR sizes.
   
  
• 320×240 [QVGA, PVR's smallest useful]
  
  
  Just by way of comparison, the Zen-size as it would be seen on a TV. At the moment I make my recordings at 352×288 (on the PC) or 640×480 (on the PVR) and then scale down to Zen resolution; so for any given Zen media file, I will have a better quality version around; though as I have said, if the PVR could output DivX/XviD directly, I would look to recording 'disposable' items directly in this size so no transcoding would be required.
   
  
• 640×480 [VGA, useful PVR size]
  
  
  This is the size that I use nowadays. If you compare with the example below, you will see what I mean that there is little perceptual difference between this and the full frame recording. You will still achieve about 2½ hours/Gb; the actual size depends a lot on the 'complexity' of the source material. For example, "Your Country Needs You" (BBC1) includes a lot of lighting effects and movement so an hour of this will run to approx. 550Mb. On the other hand, "Party Of Five" is more static so an hour of this is only 380Mb.
  It does, however, take my computer slightly less time to process, the first pass running in at ~33fps instead of ~28. That means an hour will complete in 55 minutes, instead of 64. Only nine minutes? Now if you are processing about eight hours worth of video in one go, that's a noticable difference.
   
  
• 720×480 [NTSC, PVR's largest size]
  
  
  Full-frame, the best the PVR can manage. Might be a consideration if I had NTSC input, but as I don't and some degree of scaling will be necessary in any case, I might as well use the VGA size and shave off some time in the transcoding.
  Move your mouse pointer over the picture to swap in the 640×480 version, and move your mouse off of the picture to return to the 720×480 version. Can you see a difference?

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