15年前，音響迷都說CD player比USB DAC好，如今誰這麼說？
會不會覺得羞恥？
如果說"眼前的CD player能做到比USB DAC好"，在15年前，可能就無疑問。
經驗，不是真理。
證明一件事正確，可以用邏輯推論，如數學那般，也可以用窮舉法，但無法窮舉的事物太多了，只能靠邏輯推論，前提是，你得明白其原理，不懂原理，妄想用窮舉法證明？別鬧了⋯
某DAC外加個clock後，聲音比較好，這表示？
外面那個clock比裡面的clock好，沒錯吧？
試問，為何裡面的不做好，偏要再外加？難道是有空間限制嗎？還是有啥其他限制？
知道是啥嗎？說來聽聽？要不要先去問清楚？
外面那個clock是44.1KHz？96KHz？都不是？那怎麼由那個頻率轉成44.1KHz和96KHz？需要一組電路是吧？這組電路若是隨便做，例如用爛零件、爛電源，結果會好嗎？不可能吧？
那麼，該DAC是砸了不少錢在接收外面的clock再轉成44.1, 96KHz的電路囉？
這錢為何不砸在裡面的clock？
據說live演出時A4 440Hz會調成442Hz，你聽得出來嗎？典型的石英震盪子的頻率誤差是25ppm，也就是44100Hz可能會是44101.1025Hz，你聽得出來嗎？
頻率精準不重要，也就是每秒幾次不重要，夠準了；癥結點在每次的誤差，也就jitter。
再說一次，經驗，不是真理。
3.10 Clock Domains
A Clock Domain is defined as a zone within which all sampling clocks are derived from the same master
clock. Therefore, within the same Clock Domain, all sampling clocks are synchronous and their timing
relationship is constant. However, the sampling clocks can be at different sampling frequencies. The master
clock can be generated in many different ways. An internal crystal could be the master clock, the USB start
of frame (SOF) could be used or even an externally supplied clock could serve as a master clock.
In general, multiple different Clock Domains can exist within the same audio function.
3.11 Audio Synchronization Types
Each isochronous audio endpoint used in an AudioStreaming interface belongs to a synchronization type as
defined in Section 5 of the USB Specification. The following sections briefly describe the possible
synchronization types.
3.11.1 Asynchronous
Asynchronous isochronous audio endpoints produce or consume data at a rate that is locked either to a
clock external to the USB or to a free-running internal clock. These endpoints cannot be synchronized to a
start of frame (SOF) or to any other clock in the USB domain.
3.11.2 Synchronous
The clock system of synchronous isochronous audio endpoints can be controlled externally through SOF
synchronization. Such an endpoint must lock its sample clock to the 1ms SOF tick. Optionally, a high-
speed endpoint could lock its clock to the 125 µs SOF that occurs at the beginning of every microframe to
improve accuracy.
3.11.3 Adaptive
Adaptive isochronous audio endpoints are able to source or sink data at any rate within their operating
range. This implies that these endpoints must run an internal process that allows them to match their natural
data rate to the data rate that is imposed at their interface.
來個簡單問題，I2S bus，若有啥雜訊或是阻抗偏移之類的導致某個時刻，波形扭曲的太嚴重，導致0誤判成1或反之，那，你會聽到啥聲音？
同樣的問題，若是USB線呢？
若是這波形扭曲出現在I2S的word clock/LR clock/frame clock = 取樣頻率勒？你會聽到啥聲音？
對Async USB DAC而言，USB bus上的D+/D-訊號jitter，與DAC IC的clock jitter，有啥關聯？
USB端收到的資料只要正確，根本不用管其jitter，資料正確，就證明其jitter夠低。而資料正確這點，是由傳輸的packet內的檢查碼驗證；一旦有錯誤，就會要求重發一份；連續一直錯，來不及補上，資料就會跟不上DAC IC的節奏，也就會斷音；很短的斷音，聽來就像是雜音，啪一聲。反過來想，沒聽到雜音或斷音，就表示資料正確，一切OK，完全不用管其jitter。
別傻傻的，人家說DAC的jitter很重要，就四處亂瞎猜，這裡也要低jitter，那裡也要低jitter，這裡也會影響jitter，那裡也會影響jitter⋯連電路圖都看不懂⋯還掰的頭頭是道⋯根本捕風捉影無限滑坡啊⋯2024年了，還在學上古神農氏嚐百草⋯你是有幾條命⋯幾毛錢可以浪費？
不過，有些電腦的USB介面不見得能用這麼長的線，請洽貓窩試用吧。
https://youtu.be/Wsh6i8SNi-c?si=98ZraF6xZ74jEB5s
