CDP是否會讀錯數位資料? (讀書心得) - 喇叭
By Zenobia
at 2019-11-22T17:33
at 2019-11-22T17:33
Table of Contents
寫在前言的前面:
本人非CDP或音頻專業,所以此篇為讀書心得,有錯請見諒。
所以相同資料讀完後,版上專業先進有更正確的意見,請不吝指教!
(真)前言:
有關CD player 「數位」資料讀取是否有錯誤的問題,一直找不到確切的講法。
主要是因為技術細節太多,唯有研發CDP servo晶片的專業人士才能夠真正的就
實務面回答。
不過就我個人而言,數位的資料傳輸,是必定可以比對確認資料的正確性,只是
要怎樣去比對而己。上次是USB線材的資料傳輸,這應該算是有定論。但是CDP在
網路上卻找不到結論,讓我深感遺憾。
不過真的在網路上找到了相關的研究資料,所以提供給大家參考。
一、 懶人包結論
甲、 CPD是會”讀錯”資料的,正常情況下約1兆bit會出現1個bit的資料
無法被正確傳到後面。(讀錯:讀到錯誤資料但是無法用糾錯機制救回)
乙、 1兆bit的錯誤發生率,等於你聽完整的160片CD,才會發生1個bit的錯
誤。(PS:完整 = 74分鐘)
丙、 如果發生1 bit的錯誤,CDP的主控晶片會用內插法去修正資料,就
實際聽感上來說是幾乎無差的(許多文件上所述,就相信是飛利浦和
SONY的實驗結果吧)。如果晶片沒有去修正資料,會發生clipping或
是氣泡爆的聲音。(聲音檔請參考adamptt網友提供的案例)
二、 資料來源
特別超級感謝中興大學幾位教授的貢獻。
http://audi.nchu.edu.tw/~imtech/
http://audi.nchu.edu.tw/~imtech/course/odsguideline.htm
會認為此份資料值得分享,主要是因為畢境是教授做學術性的研究,應該
具有相當的可靠性。(而且這份資料還公開+拿教育部補助)
PS:建議資料要存起來,很怕網站關了。
有關CDP、CDDA與糾錯的原因,可以參考第1章~第3章。
即使不是工科專業的網友也可以讀讀,因為前面的結論非常清楚簡單,不
用去管後面的一堆數學公式與圖形。(事實上,我也沒有看後面那堆數學)
基本資料(中文):
https://zh.wikipedia.org/wiki/CDDA
https://www.techbang.com/posts/9827
英文:
https://www.cdrfaq.org/faq02.html#S2-17
三、 資料計算
甲、CDP 讀取錯誤率
第二章第36頁有寫到,CD讀取資料時約有10萬~100萬的資料錯誤率,
靠著CIRS糾錯回來後,錯誤率降到100億~1000億。
第三章第二頁:
「With proper design, CD and DVD can be reduced to an error rate
of 10^(-12), that is, less than one uncorrectable error in 10^(12)
bits」
中譯:在正確設計使用下,約每1,000,000,000,000 bits (1兆 bits) 會出
1 bit 的錯誤。
以上二段資料,我選擇採用第三章第二頁的資料。因為第二章為中間講
述糾錯機制CIRC,而第三章是就整個系統的糾錯而言。
乙、CD片數換算
每秒鐘聲音資料為:44100(Hz) * 16bit * 2(兩聲道) =1,411,200 bits。
一張CD聲音bit總數:1411200 * 60(秒/分) * 74(分/片) = 6,265,728,000 bits
1000000000000 / 6265728000 = 159.6 = 160 片CD。
四、結論
對,100萬的CDP"可能"真的有效果,"可能"可以把160片CD會錯1個音的機率降
到161片 ~ 16000片吧!
至於是 161 還是 16100,就請廠商提供數據。
~~~~
後記
教授的資料,往往都是在正常一般的情況下。
在音響的世界,「低音震動」其實是會影響所有的電子元件。CDP
讀取頭的高速移動和震動,可以想見必定會受影響。
就我個人所知,如果是百萬級的低音喇叭發功時,那個皮膚都會有
感受,CDP就更不用說了。
BUT,但是,影響到底多少?這個舉證責任,應該是在CDP廠商了。
--
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