Presbyterian College > Academic Server > Jon Bell > Digital TV > DVD Recorder Quality Modes


Comparing Different Quality Modes
on a DVD Recorder

Introduction

Old-fashioned video cassette recorders (VCRs) allow you to record in different quality modes (SP, LP, etc.). They run the video tape through the recorder at different speeds, giving higher quality with faster speeds, in exchange for shorter recording times on a given length of tape.

Similarly, modern DVD recorders allow you to record in different quality modes, by changing the amount of compression of the digital video data. Less compression produces a better quality image, but the resulting data files are bigger. A DVD has a fixed capacity in terms of file size (about 4.2 gigabytes), so there is a tradeoff between image quality and the length (in time) of the video that can fit on it.

My Panasonic DMR-EH75V DVD recorder has four fixed recording quality modes, corresponding to four different (time) capacities per DVD:

Mode Disc Capacity
(hr:min)
XP 1:00
SP 2:00
LP 4:00
EP 8:00

It also has a FR (flexible recording) mode, in which the user specifies the desired capacity of a DVD, in the range 1:00 to 8:00 hours, down to the nearest minute, and the recorder adjusts the compression accordingly.

On this recorder, if the disc capacity is set to more than 4:00 hours, the image is recorded with pixel dimensions of 352x240 (instead of 720x480 as for the higher-quality modes), which has a dramatic effect on the image quality when viewed on a fixed-size TV screen. In effect, the image has to be scaled back up to 720x480 when it is played back, which cannot recover the detail that was lost in the original down-scaling.

It's obviously useful to have some idea of the image quality of different recording modes, so I generated some sample images.

Procedure

I recorded a high-definition over-the-air broadcast on a Sony DHG-HDD500 high-definition hard-disk based digital video recorder.

I dubbed the recording to the hard disk of a Panasonic DMR-EH75V DVD recorder via an S-video connection, repeating it several times using different recording modes. I used all four of the fixed modes (XP, SP, LP and EP), and I used FR mode three times with DVD-capacity settings interpolated between the fixed modes: 1:30, 3:00 and 6:00 hours.

I copied the samples to a DVD in high-speed mode, which simply copies the data files, leaving the video at the same quality as on the hard disk.

I read the DVD on a Macintosh computer and used MPEG Streamclip software to extract frames from the same point in each sample, to TIFF files in a 16:9 aspect ratio. The original images on the DVD have rectangular (not square) pixels, so MPEG Streamclip has to re-scale them to get the proper aspect ratio on a computer screen. Modes up to 4 hours produced 852x480 pixel images; the 6- and 8-hour modes produced 426x240 pixel images.

I used Photoshop CS2 to re-size the 6- and 8-hour mode images from 426x240 to 852x480 pixels, to match the others; then converted the images from TIFF to JPEG (quality level 8).

Results

I repeated the procedure above for two different frames, taken from an October 2007 baseball World Series game broadcast on Fox. Follow a link below to see a page with all the images for one frame:

I don't think these static images tell the complete story. When watching these recordings on my 32" LCD TV, I can see slight differences between the various modes, up to LP, especially when there's a lot of motion. Baseball is rather static overall, so it's not as sensitive to motion artifacts.


This page was last updated on 27 October 2007.


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