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CD Writing Query

Discussion in 'Software Development' started by scrollisalegend, Nov 11, 2001.

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  1. scrollisalegend

    scrollisalegend Thread Starter

    Joined:
    Feb 20, 2000
    Messages:
    48
    Hello there,

    I was just wondering if anyone out there knows if it is possible to read off one disk whilst writing this data to another drive (ie CD/R). What I'm getting at is - can a Cd be read directly and written to another disc in a different drive, without actually having to rip the Cd first (ie saving info on hard drive) and then writing this to a rightable cd. See im keen on making a small visual basic app to do just this, but not sure if i have to get the app to save the files first and then write the cd. If this is possible would anyone know any threads or anything were'd I'd find out more about it.

    Thankyou heaps for your time and help!!!!

    Scroll :)
     
  2. plejon

    plejon

    Joined:
    Jul 26, 2001
    Messages:
    671
    What you describe is a copy on the fly.

    It is certainly possible, CD creation programs such as Nero (www.nero.com) support the options, but also warn you for the pitfalls. See this extract from the Nero online help below.


    Nero versions 3.0 and higher are capable of burning “on the fly" copies using a CD-ROM drive as input device. You don't need several hundred megabytes of free volume space on your hard disk any longer and will be able to copy CDs much faster. That's the good news.
    The dark side of the moon.....
    Being able to copy CDs faster without needing lots of free volume space on your hard disk sounds like very good news. But unfortunately things are not that easy. There are several things to consider before copying “on the fly":

    1. You need a CD-ROM drive capable of reading input data quickly enough to burn a CD. This means if you want to burn with 2x then your CD-ROM drive must be able to read faster than 2x (better 4x or more) to prevent a “buffer underrun". To make sure this won't happen is not so simple. One problem is that lots of CD-ROM drives (even brand new models among them!) read audio data considerably slower than normal data (mode 1 or mode 2) For example there are some 24x CD-ROM drives which read audio slower than 3x.

    Even more problems cause CD-ROMs which can't read audio at all or can only read audio slower than 1x (even brand new models!). Of course this makes “on the fly" copies of audio or mixed mode CDs impossible because every CD recorder needs input data at at least 1x (depending upon the selected write speed).

    2. If a read error occurs on the source CD (for example if there is a scratch on the CD), then the read error may not be corrected by reading a sector several times because there's no time to repeat the read process because every delay could cause a buffer underrun on the CD recorder of course trashing the unfinished copy. All that Nero can do in this case is to show a warning and write garbage data or just zeros to keep the recorder satisfied. The result will be a copy of the CD with some unreadable files which are better than a trashed CD but still pretty bad. That means that “on the fly" copies are very sensitive concerning read errors.

    3. Many CD-ROM drives can't read any session information. All they can tell is how many tracks of which type were found on the CD. This means there may be a loss of session information.
    4. Audio tracks may contain audio index positions. This information is available while reading the track that contains the index position. Unfortunately this is too late for “on the fly" copies because recorders want to know about audio index positions before starting to write the track. In the end this means that any audio index positions will be lost if copying “on the fly".

    5. The last bad news concerns the quality of digitally read audio data. To explain why it's necessary to discuss some technical details: While copying an audio track “on the fly" Nero reads some audio data and then writes it on CD. This action is repeated as often as necessary to copy the track. The result is a very long repeated read/write/read/write... command sequence. Concurrently, the source and the destination CD keep spinning at independent speeds. The CD-ROM drive reads data from CD and stores it inside it's cache memory. Nero will then receive it's input data from the CD drive's cache. If Nero reads digital audio data the procedure is just the same as for data. The trouble is that the source CD drive normally should read faster than the recorder would write. So the CD-ROM drive fills it's internal cache with “read ahead" digital audio data to be able to respond quickly to read commands. Nero reads slower than the CD-ROM drive because it reads at the slower recorder's speed. So at some point the CD-ROM drive's cache is full (a so called “buffer overflow") and some “old data" (from the CD drive's point of view) must be thrown away to be able to keep on reading ahead. But this “old data" is from slower Nero's point of view “current data" and is urgently needed to be sent to the CD recorder. What the drive needs to do now is to seek to an earlier position of the audio track. Unfortunately, audio tracks consist of sectors which don't contain any direct time information (data tracks do contain time information!). Therefore, what the CD-ROM drive does is to estimate a “rough guess" of where the required audio data might be found on the CD. It then starts to read again at this “fuzzy" position and fills it's cache with data to send it to Nero. So the audio data Nero gets is not necessary exactly what Nero wanted to read, but may contain data from another (very close but not “correct") position of the CD. Nero then uses this data to write it to the recorder believing the data is correct. The result is a copied track with some “clicks" or “scratches". This unwanted effect is called “jitter". The amount of “jitter" depends highly upon the model of your CD-ROM drives. There are drives which read audio perfectly and other produce such a huge amount of “jitter" that they are almost impossible to use as input device for audio CD copies. To find out to which category your CD-ROM drive belongs all you got to do is to save an audio track from your CD-ROM drive with Nero's “Save track" menu command and then play the wave file. If it sounds alright, then you've got a “good" CD-ROM drive. If it sounds bad, then it's better not to use this drive as input for “on the fly" copies of audio or mixed mode CDs. By the way: for “image CD copies" Nero can do “jitter correction" to avoid scratches and clicks. While copying “on the fly" there is no time to do that.
     
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