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ความคิดเห็นที่ 75 |
Footnote: Evidence that FLAC is lossless.
Some people have expressed a preference for Wave format because they aren't convinced that any compression can really be lossless. (This belief is understandable, but very much incorrect.) To provide evidence that FLAC really is lossless, I ripped several varied tracks to Wave format, then converted the Wave files to FLAC format, then converted the FLAC files to Wave format, and compared the before and after Wave audio files.
For variety, the tracks came from four genres: metal, hip-hop, jungle, and a melodic vocal track which I can't fit into any convenient genre. The tracks are:
I, Zombie and More Human Than Human, both by White Zombie, from the album Astro-Creep 2000 (Songs of Love, Destruction and other Synthetic Delusions of the Electric Head).
Woo Hah!! Got You All In Check and Put Your Hands Where My Eyes Could See, both by Busta Rhymes (and first track featuring Rampage The Last Boy Scout) from the album Turn It Up! The Very Best.
Original Nuttah, by Shy FX & UK Apachi, from the album Jungle Mania 94.
Open Heart Zoo, by Martin Grech, from the album Open Heart Zoo.
For file comparison, I used an MD5 checksum utility called MD5summer by Luke Pascoe. This reads through all the bits of the file and uses them to calculate a fixed-length hash string. A good hash algorithm, such as MD5, should rarely-if-ever produce the same hash string for two different files of the same size. In fact, there are more than ten-to-the-power-thirty-eight possible MD5 hash string results, so the chances of any two files having the same MD5 hash result are pretty tiny. Changing even a single bit of the target file will change the MD5 checksum quite significantly. So the MD5 checksum is a fairly safe way of checking that two files are digitally the same, bit-for-bit.
To examine the result of lossless compression, each wave file was examined with MD5summer and the checksum noted in the table, in the "MD5 of original" column. Then the wave file was converted to FLAC format. Then the FLAC file was converted to a wave file with the same quality settings as the original. The restored wave file was then examined with MD5summer and the checksum noted in the table, in the "MD5 of restored" column. If the FLAC compression really is lossless, then the restored wave file will be exactly the same as the original wave file, and the checksums will match up exactly.
The results of the process were: (ดูรูปกระทู้ล่าง)
http://www.bobulous.org.uk/misc/audioFormats.html
การทดลอง แปลงไปแปลงกลับระหว่าง WAVกับ FLACครับ ถ้าทำ MD5 checksum ได้เท่ากัน แสดงว่าบิททุกบิทเท่ากัน แล้วคุณจะได้ยินเสียงแตกต่างได้อย่างไรล่ะ ในเมื่อ input bit ที่ส่งไป DACมันเท่าเทียมกัน หรือว่าเป็นเสียงผี!?!?!?!!? ยังงี้ต้องขอเชิญคุณ think4223 มาร่วมพิสูจน์ว่าผีมีจริงไหมซะแล้ว ^^
แก้ไขเมื่อ 20 พ.ค. 54 21:18:12
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แก้ไขเมื่อ 20 พ.ค. 54 21:14:54
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20 พ.ค. 54 21:12:34
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