The Encoding Bitrate
The "bitrate" is a measure of how much material must be downloaded per second in order to allow a lecture to play continuously. A lecture recorded at less than 50,000 bits per second (50 kbps) is considered to be within telephone modem speeds. Anything much larger than that becomes the domain of "broadband."
In either circumstance, the Player will download sufficient material so that once play begins, it will not need to pause during the talk to download more.
The amount of material that must be downloaded before play can begin is determined by the lecture's average encoded bitrate and your connection speed.
The effect is like a bucket that is being fed by a faucet, but one with a hole in it. If your connection bandwidth (the rate of data feed) is much greater than that of the lecture's encoded bitrate (the rate of consumption), play will begin almost immediately. In this circumstance, we can be assured that the bucket will fill with data faster than the player can consume it. Only the image and sound files for the first slide will need be downloaded before play can begin.
But, if your connection speed is much less than the encoding rate of the lecture, we will have to wait for the bucket to fill to the point that we can be assured that we will not run out of data before the lecture ends. Only then can we unplug the hole in the bottom of the bucket and begin to let the data flow. In this situation, the initial download time may be significant, perhaps half as long as the lecture itself, but the lecture will eventually begin, and once it does, it will play to the end without stopping.
On the other hand, if your telephone modem connection speed is approximately equal to the lecture's encoded bitrate, a number of slides will need to be downloaded before play can safely begin. This may require one, two or three minutes.
Only one "quality" level is recorded in QCShow lectures. Everyone sees and hears the same high quality image and sound presentation regardless of their download bandwidth. The only difference between bandwidths will be the time required before play can begin.
The design of QCShow is based on the premise that the lecturer will show approximately one slide per minute. If that proves to be true, the average talk will require an encoded bitrate of approximately 30 to 35 kbps, divided in this manner:
If the lecturer shows his slides at an average rate greater than one slide per minute, the encoding bitrate for the lecture concomitantly rises. Two slides per minute generally result in an average encoding rate of 45 to 50 kbps; three slides per minute are in the range 50 to 60 kbps.
- The audio is recorded at a constant 18.5 kbps, using an Ogg Vorbis compression.
- The "bitrate" of the image files are highly dependent on the complexity of the images, but in general require an additional 3 to 13 kbps bandwidth, multiplied by how many slides per minute are shown.
- An additional 8 kbps is added to these rates to account for transmission overheads. The encoded bitrate for the lecture is the sum of these three values.