Compact Disc Data Transfer RatesAndrew DavidsonPhilips Interactive Media of AmericaThis note is an attempt to provide a derivation of the standard CD-ROM datarate of 150 kb per second. In this note, "kb" means kilobyte, "mb" meansmegabyte, and "Mb" means megabit. A byte is 8 bits. A kilobyte is 1024 bytes(not 1000 bytes!). I claim that a megabyte is only 1000 kilobytes, not 1024,but there is some debate about that. I don't have a definitive answer.It all starts with CD-DA (digital audio used for music CDs). CD-DA is sampledat 44.1 khz. This means that there are 44,100 samples (or digital values) ofaudio per second. In PCM audio, which is the standard for CD-DA, samples are16 bits per channel. So for stereo, that means 32 bits per sample or 4 bytes.Multiplying by 44,100 gives 176,400 bytes per second, or ~172 kb, rememberingthat a kilobyte is 1024 bytes, not 1000 bytes: 44100 * 4 / 1024 = 172.26 kbSo if CDs can transfer at 172 kb per second, where does the 150 kb figure comefrom? We seem to have lost 22 kb per second.Digital audio data is a continuous stream of bits on disc - there are nosectors or files, only a table of contents tucked away on the disc that tellsyour CD player where each track begins in that stream. For CD-ROM, weobviously need some method of putting data into discrete chunks, or sectors.The Yellow Book (the spec for CD-ROM) defines this method.Sectors on CD-ROM discs are 2352 bytes each and are transferred at the rate of75 per second. So: 2352 * 75 = 176,400and that number should be familiar from above: ~172 kb.CD-DA data is very tolerant of errors in the data stream. Even if you "lose"4 bytes during a transfer, that is only 1/44,100-th of a second of audio andthe human ear (brain, actually) hardly notices the loss. But if we lose 4bytes of computer data randomly while using our CD-ROM disc, someone is likelyto get upset. So, each CD-ROM sector reserves 304 bytes of those 2352 forerror detection and error correction codes. These codes guarantee thattransfer errors can always be corrected, protecting the integrity of the data.(Of course, it can't necessarily recover from peanut butter or scratches onthe disc.) So now we have fewer bytes in each sector actually available foruser data: 2352 - 304 = 2048 bytes per sector, and 2048 * 75 sectors per second = 153,600 bytes per second 153600 / 1024 = 150.0 kb per secondTherewith, the mythical 150 kb per second.Newer double speed CD-ROM drives have to transfer at this rate to supportCD-DA, but can also go faster (150 sectors per second) for computer data toachieve the 300 kb per second rate.One also hears transfer rates quoted in terms of megabits per second (Mb).Remembering that a byte is 8 bits, we can recast the 150 kb figure in termsof Mb: 150 * 8 * 1024 = 1,228,800 bits per second 1228800 / 1024 / 1000 = 1.2 megabits per secondSo 150 kb per second equals 1.2 Mb per second. You will often hear datatransfer rates for compressed digital video (like MPEG) referred to in Mbper second. 1.2Mb per second is the nominal rate for optical disc-basedtransmission of MPEG compressed video. In this case, we can sacrifice theerror correction data (since errors in real-time audio and video are alsoless critical) and use the remaining bandwidth for MPEG compressed audio.The total is then back to approximately 1.5 megabits per second, whichis compatible with CD-DA transfer rates.
