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Back Next Contents Digital Video for Multimedia: Considerations for Capture, Use and Delivery

Section 3: Digital Video on Trial

During our investigations into digital video for multimedia applications a series of experiments were performed to investigate the various parameters involved with video capture such as, choice of codec, frame rates, key frames, compression quality and size of video window. The results, a series of digital video clips, can be accessed and downloaded from the following World Wide Web site: http://www/.ets.bris.ac.uk. Further information expanding on the issues already discussed in this report is also available, as are links to other sites with relevant information.

Method:

A video sequence containing several scenes with differing content (scenes with little change, rapidly changing scenes, scenes with multiple content and special effects, etc) was captured 'raw' and using real-time compression at various frames rates and window sizes, and compressed using a variety of different quality settings and codecs. The sequence was captured from a non-degradable video source, an analogue write once laserdisc, to eliminate tape degradation effects and maintain quality. All sequences were captured on a PC. Two cards were used in the study:

Results:

Table 1: Compression times and file sizes for various quality settings and codecs using MS VidEdit

Initial file: 30seconds of Intel Raw video, 25frames per second, 320 x 240, original file size 70.4MB. All files are at 24 bit colour except MSVideo 1 as this only supports 16 bit colour.
Output settingsCodecconversion time (minutes)size (MB)output file
100% quality key frame every frameMS Video 19:0070.1msvid_1.avi
Indeo 3.2 10:45 13ir32_1.avi
Indeo Interactive 17:00 12.4 ir4_1.avi
Cinepak 48:30 18.7 cnpk_1.avi
0% quality key frame every 10 framesMS Video 12:500.61msvid_2.avi
Indeo 3.214:152.65ir32_2.avi
Indeo Interactive43:451.79ir4_2.avi
Cinepak33:1018cnpk_2.avi
Codec defaults for quality and key frame rateMS Video 1 3:405.17msvid_3.avi
Indeo 3.214:052.89ir32_3.avi
Indeo Interactive46:554.95ir4_3.avi
Cinepak 34:5018cnpk_3.avi

Comparing the conversions for MS VidEdit and Adobe Premiere there was no noticeable difference in either the time taken or the final file size. Adobe Premiere does afford the opportunity to output to the Apple Quicktime format, corresponding times and file sizes are given in the table below.

Table 2: Compression times and file sizes for various codecs at the highest quality setting using Adobe Premiere

All starting with the same initial file, 30seconds of Intel Raw video, 25fps capture, 320 x 240, original file size 70.4MB

Output settingsCodecconversion time (minutes)size (MB)output file
quality 5 key frame every frameMS Video 16:3562.8video_1q.mov
Indeo 3.211:4012.6ir32_1q.mov
Cinepak52:0518.2cnpk_1q.mov

Note: At the time of writing the Quicktime format did not support the Indeo Interactive codec. Quicktime provides a quality setting range of 0-5.

Table 3: A comparison of file sizes versus frame size.

All files captured to RAM as Indeo Raw (via the Videoblaster RT 300) and recompressed at 25 fps, highest quality, key every frame using MS VidEdit
Frame sizecodecfile size (bytes)
160x120Indeo 3.24420298
Indeo Interactive4420814
192x144Indeo 3.26181010
Indeo Interactive6516170
288x216Indeo 3.211681090
Indeo Interactive11399920
320x240Indeo 3.213280700
Indeo Interactive12658408
640x480*Indeo 3.21884426
Indeo Interactive3956890

*Note: The low file sizes here are due to the computer and card being inable to capture at 640 x 480, captures were therefore carried out manually (i.e. frame by frame) and represent sequences of 100 frames (4 seconds).

Table 4: A comparison of file sizes versus frame rate

All files captured to RAM as Indeo Raw (via the Videoblaster RT 300) and recompressed at 320 x 240, highest quality, key every frame, using MS VidEdit
Frame ratecodecfile size (bytes)
25Indeo 3.213280700
Indeo Interactive12658408
15Indeo 3.27782486
Indeo Interactive7440252
12.5Indeo 3.26597990
Indeo Interactive6331812

Table 5: A comparison of file sizes versus codec using the Video NT card.

Initial file: Approximately 17.5 seconds of Vitec 422 video (Vitec's form of raw compression), 25 frames per second, 192 x 144, original file size 25 MB, recompressed at highest quality, key frame every frame, using Vitec editing software
codecfile size (bytes)output file
Cinepak3792598cnpk.avi
MS Video 113669916msvid.avi
Intel Indeo 3.23264318ir32.avi
Intel Interactive3441432ir4.avi
Vitec DCT4484472vitec2.avi

Note: Vitec's own DCT (Discrete Cosine Transform) codec only gave us control over some of its settings others were greyed out.

Table 6: MPEG files.

Comparison of file size and quality settings of MPEG files created from either Indeo Raw at two different frame sizes, or compressed AVI files, using MPEG compression software (MPEG maker) supplied with the Video NT card.
Frame sizeInitial codecfile size (bytes)output fileMPEG compressor settings
192x144 file from table 3Intel Raw611884hi_cmprs.mpgdefault high compression
Intel Raw1713813quality.mpgdefault high quality compression
Intel Raw2474927quick.mpgdefault fast compression
320x240 original file from table 1Intel Raw1223695hi_cmprs.mpgdefault high compression
Intel Raw3250012quality.mpgdefault high quality compression
Intel Raw4823189quick.mpgdefault fast compression
Recompressed using files from table 1Cinepak4370476ex_cnpk.mpgdefault high quality compression
MS Video4245941ex_msvid.mpgdefault high quality compression
Intel Indeo 3.23812733ex_ir32.mpgdefault high quality compression
Intel Interactive4073239ex_ir4.mpgdefault high quality compression

One sequence was captured directly to MPEG using the Vitec Video NT card using the 'high quality' setting but with the stuttering playback, general poor quality and time constraints we did not consider worthwhile continuing with this approach. The file is vitec.mpg

Initial Discussions:

The above tables reveal some interesting facts about the nature of the various codecs listed above. The nature of the relationship between quality level, compression and file size varies from one codec to the next. The balance of tradeoffs is dependent upon the video material (content and quality) and its purpose. However, assessment of the various codecs is a subjective process and the above results should be considered while viewing the files. Refer to the following World Wide Web site for further comparisons and information and to download and view the video files: http://www.ets.bris.ac.uk
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