Do I lose quality when I move a media file from one place to another?

Does moving video and picture files back and forth from one external hard drive to another deplete the image quality?


An interesting question was left on the Ask A Question page by Ronnie regarding the possibility of losing image quality when moving files from one place to another. I guess the source of this question comes from the experience you get when you copy an audio tape from one cassette to another, say if you’re making a copy of a ColdPlay tape. The target cassette has the same music, but the copying process introduces “noise” which reduces the quality of the music. If you copy it again and again, you’ll notice a progressive deterioration in the overall sound. Another example of this deterioration can be seen when you photocopy a leaflet about diet pills for example, then photocopy the photocopy etc. After a number of generations you can see the gradual deterioration in the quality of your image.

Luckily, when we move a file from one disk to another, we don’t experience this same symptom. Coping a file from one place to another, for example, from a hard disk to a floppy disk, from a removable hard disk to a CD, the resulting file remains 100% intact. This is definitely a good thing. Media files would lose quality otherwise, but even worse, data files and executables can be completely destroyed if even one bit of the data has been inadvertently changed.

So, what’s the difference between the two processes? Well, in the latter case, moving a file from one place to another is a purely digital process. Files on a disk are stored as a digital sequence of 1s and 0s and this sequence can be replicated perfectly. The former case however, involves converting or translating information from an “analogue” format to a “digital” format and back to “analogue”. The problem here is that the conversion process is approximate and can never be 100%. Even if the process is “practically” perfect, the target can never be exactly the same as the source, and the more generations we make of the copy, the more the differences become obvious.

Okay, that wasn’t a very technical explanation, but my aim here is to convey what is happening in layman terms and to assure Ronnie that it doesn’t matter if you copy a file once, a hundred times or a million times; as long as there’s no fault in the media you are copying to, your file will never lose quality. (If you want something a bit more technical, there’s a great write up on analogue to digital conversion on Wikipedia)

2 thoughts on “Do I lose quality when I move a media file from one place to another?”

  1. One other thing to consider is whether you are indeed simply copying the file, or whether there is some conversion (from one digital format to another) going on in the process. For example, if when you copy a piece of music or video to your portable media player, it is _possible_ that the file format of the original requires your PC to convert it into a new version. This conversion will usually result in some loss of fidelity or raw data.
    One thing I remember finding a friend do back in the late 90s was re-ripping his 128Kbit MP3s to 320kbit… he was convinced that this would result in better quality sounds. If he had ripped the ORIGINAL MP3 @ 320kbit per second, from a CD or other HQ source, he would have been correct. However, when he ripped it to just 128kbit, the information lost, was just that… lost. No amount of fiddling (for all intents and purposes) with the bitrate after this step in the process would have helped.

    Anyway, good answer, and for most situations, it hits the nail on the head. An easy way to tell if your media is being converted, rather than directly copied to your portable media player, is the amount of time taken for the transfer to complete – compare this to copying the file to a thumbdrive or other external (non-hard magnetic drive)device. If the PMP xfer time is a lot higher, it might be worth checking what exactly is going on 🙂

  2. @Chris: Thanks for the comment. You’re right of course. If there is some conversion process taking place, there is room for the same “noise” mentioned above caused by approximation in the conversion process.

    However, as Ronnie’s question was about moving media files from one disk to another, I suspect he’s not actually doing any conversion.

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