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How Many CDs Can You Burn at Once?
kfs27 asks: "In an attempt to help a professor of mine record and duplicate his lectures. I have been asked to put together a CD duplicating box. Commercial products seem to be very expensive and I figured a PC with some SCSI160 Cards (HW or SW Raid maybe), SCSI Burners and a 15K RPM drive (size not an issue) could do the job for cheaper. But the question is, how many CDs can you burn at once of 30 minutes, mono audio. 10 at a time would be excellent I think. More of course better. Cost is not a huge issue, as long as it's less than Commercial Duplicators, it's more of an experiment, but must be stable and easy to operate (I'd be willing to script up a frontend)."
With the prices of RAM being as low as they are, you might want to consider building a ram disk to store the data that's going to get burned. That way you don't have to worry about the speed of the hard drives, the ram will always be faster.
We have one box here with 4 SCSI burners in it with a 700 meg ram drive. Everything works wonderfully in it.
Why CDs? 30 minutes of mono audio, encoded in 32Kb/s MP3, is (30 * 60 * 32 / 8) = 7200 KB (with the last "/ 8" to get us to kiloBYTES instead of kiloBITS). There are MUCH better codecs then MP3 for this at this bitrate, I just use MP3 as a convenient and easily obtained example. Record the lecture, convert to (lossily-compressed-audio-format-of-your-choice), and load it on the web.
At the end of the semester, give each student ONE CD with the entire course on it!
Nowadays, if your student can use a CD, they can play an MP3. And even a 7MB download is doable over a modem connection. (And you might cut that down to perhaps 1 or 2 MB or less by using a codec designed to do voice-only, but you'll probably have to pay for it.)
First, I have to say that I agree with the comments already posted suggesting that you just compress the audio and make it available for download. It's much more efficient. Especially when semester end comes and students want to review data from several lectures, not swapping disks frequently will be more convenient.
I'd mod them up if I didn't have to say that RAM disks are a bad idea. If you simply add the RAM to the system, then the OS can cache the data in the most efficient manner possible. As long as you have the RAM to cache the image, the OS shouldn't be reading it from the drive constantly. Using a RAM disk is actually harmful to system performance, because the OS may not be able to cache disk sectors that are frequently needed. Statically allocating the RAM only works if you have more information about disk use than your OS, which you almost certainly do not.
First of all, if these MUST be in standard CD-audio format, then the answer to the question about how many disks you can burn of 30 minutes of audio in a given time can be calculated by dividing 30 minutes by the speed of the reader (say 15x), and then adding a minute or two for lead-in lead-out, toc, loading, etc. In this case, a 15x drive should be able to burn a 30 min CD in about 3-4 mins. A single drive should be able to turn out around 15-20 an hour.
The poster did say he wanted to do this on the cheap. The bandwidth bottleneck in a PC environment will most likely be the PCI bus. Even with two IDE drives on an IDE chain, you should be able to keep up with the burning at 15x (150MB/min per drive). If I was going to do this on the cheap, I'd get me a used Pentium-II/Celeron class machine, or possibly a higher end pentium machine, get 4 IDE chains in it, and load it up with 6 CDR drives. Total cost should be under $1000, assuming you use linux or freebsd or similar. ($600 for drives, $50 for controller card, $350 for used machine). You may need to add a little for memory expansion, as I think the idea of a Ramdisk (300Megish) would be good, but memory is cheap (512 total MB should be sufficient). If you need more drives, add another machine. If you find that the machine can't keep up with this many, drop one or two and put them on a second machine.
If these are for delivery to students which aren't at the lecture, or for review, perhaps the best thing would be to not focus on bulk duplication, but instead to figure out an on-demand system. What I mean is that if a student WANTS the lecture, then they can visit a computer at a specific location, select the date of the lecture, insert media and wait 5 mins for it to spit it out. That would be *Really* cheap (linux box w/CDR and suitably sized hard drive).