Publishing a 40-Minute Lecture on a 1 Mbps Audience and a 20 GB Plan
A 40-minute physics lecture was recorded at 1280 x 720 pixels, 25 fps, 24-bit colour, with stereo audio at 44.1 kHz/16-bit. The school wants it on its website for students who mostly use 3G phones on roughly 1 Mbps connections and pay for their own data. A second copy is needed for the 8 GB pen drives given to students with no internet. The editing software reports the master as 'uncompressed AVI'. One student argues that uploading the master is fine 'because MP4 is compressed anyway'; another points out that the CD copy made earlier was only 700 MB and 'looked fine on the phone'. The teacher wants on-screen equations to stay readable on a 5-inch phone at arm's length. There is no video server, only ordinary shared hosting with a 20 GB monthly traffic limit, and about 300 students are expected to download the lecture.
1. Estimate the size of the uncompressed master and use the estimate to settle the argument about uploading it.
Read the worked answer
Video: one frame is 1280 x 720 x 3 bytes = 2,764,800 bytes, about 2.64 MB. At 25 fps that is about 66 MB per second. Over 40 minutes (2,400 s): 66 x 2,400 = roughly 158,000 MB, i.e. about 158 GB. Audio: 44,100 samples/s x 2 bytes x 2 channels = 176,400 B/s, about 176 kB/s; over 2,400 s that is about 423 MB. Total: roughly 158 GB.
That settles it. The hosting plan allows 20 GB per month, so a single upload attempt is eight times the monthly allowance before anyone downloads it.
The 'MP4 is already compressed' argument confuses container with codec: an AVI container can hold raw frames, and this master does. The earlier 700 MB CD was re-encoded and downscaled, so it was never the master; that its picture looked fine on a 5-inch screen proves only that a small display hides compression artefacts. Quality has to be judged by whether the equations can be read, not by file size or by a thumbnail.
2. Propose encoding and delivery settings for the web version and the pen-drive version, with supporting numbers.
Read the worked answer
Web version. Split the lecture at slide changes into chapters of about 5 minutes (300 s) so a student on a weak link can finish one part. Encode H.264 at 854 x 480, 15 fps (lecture slides, not sport), video bitrate 250-300 kbps; audio mono AAC 48-64 kbps at 22.05 kHz, since one speaker does not need stereo and mono halves the audio bits. Total about 320 kbps, roughly 40 kB/s, so one chapter is 40 x 300 = about 12 MB and the whole lecture about 96-110 MB. On a 1 Mbps link (about 125 kB/s) the stream uses roughly 30% of the pipe: playback starts within a few seconds and a full chapter downloads in under two minutes. Readability check: 854 x 480 at 300 kbps softens small type, so the teacher must use large dark-on-light on-screen text and re-check one chapter on a real 5-inch phone before the rest is encoded. Pen-drive version. Space is not scarce on 8 GB, so keep quality: 1280 x 720, 25 fps, 1.2-1.5 Mbps video plus 96 kbps stereo, about 1.6 Mbps = 200 kB/s, giving 200 x 2,400 = about 480 MB per lecture. Twenty lectures still fit on one drive.
3. The plan allows 20 GB of traffic per month and 300 students are expected to download the web lecture. Check the arithmetic and propose a workable publishing plan.
Read the worked answer
At roughly 110 MB per full lecture, 300 downloads = 300 x 110 MB = 33,000 MB, about 33 GB, which is 65% over the 20 GB monthly allowance, and that is before ordinary site traffic. Left alone, the site will be suspended mid-month, and it will be the students who are cut off.
Options, in order of effort:
- Cut the bytes. Drop video to 200 kbps and audio to 40 kbps (about 30 kB/s): a 12 MB chapter becomes about 9 MB and the lecture about 80 MB, so 300 downloads = 24 GB. Still too close to the limit.
- Serve only what is watched. Chapter-level downloads mean most students take two or three chapters rather than all eight; at an average of three chapters (36 MB) the total falls to about 11 GB.
- Move the heavy file off the school host. Publish the video to a video platform with the school account and embed it, keeping the site's HTML and slides on the shared host so the 20 GB covers only pages, images and the PDF slide handouts (about 2 MB each).
- Use offline distribution for the rest. Keep the pen-drive copy as the primary route for students with no data, and publish a small audio-only version (64 kbps mono, about 19 MB per lecture) for those on very expensive data.
Whichever mix is chosen, monitor the host's traffic report weekly rather than discovering the limit at suspension.