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發表於 2020-8-5 10:43:20
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本帖最後由 HungLiu@FB 於 2020-8-5 10:45 編輯
我曾在國外論壇上很詳細的解釋過PC的jitter來源，看過後就能更清楚明白什麼是jitter
還是那句老話，PC本身巨大的多工缺陷以及他所用的USB，不斷地投資改善，根本就是試圖純化大染缸的水而已，還不如簡單輕鬆其實又省錢的手段--避開USB，例如CD轉盤，或是Raspberry Pi
Let me explain why the sound quality of raspberry pi or the computer is very poor, and USB has undergone a conversion to generate more jitter so that USB is more worse. For the digital part, if you want to improve the sound quality, you only have to reduce jitter.
Although the computer CPU will be intermittent in the delivery of packets, and let the packet too slow delivery, but the order of the packet is not chaos. Of course, it may be because the computer clock is not accurate, resulting in too slow or too fast delivery packet. But if the packet is too slow to be passed immediately to the DAC, the sound will be strange, so the packet may be too slow or too fast must be resolved first.
The working principle of the USB chip is basically not much different from CPU. Therefore, as long as the music signal passes through the USB chip, jitter will be greatly increased, which seriously deteriorates the sound quality. The USB isolator or filter actually only makes the USB chip work more stable, reduce jitter, but The USB isolator or filter still can not completely solve the jitter generated by the normal operation of the USB chip, which is caused by the USB, CPU innate design.
The order of the packets will not be destroyed so we have a chances to solve jitter from computer if the reclock board has enough buffer to make the data can be stored more , for example 44khz music signal will transmit delay at least 1 second, and it be solved too slow or too fast delivery packet. That's why when using Ian' FIFO, when you press to play music, you will always hear the sound after one second. It's to solve the tolerances between clocks. It uses the better MCK to continue to pass the data orderly read from the buffer.
Relatively, if the buffer is not big enough, no way to solve too slow or too fast delivery packet , for example less than 1 second delay, and it is difficult using the better MCK to pass the information more correctly.
FIFO itself maybe not necessary, but the main useful because his buffer is big enough and send the packet back to the DAC using a better quality clock chip. The whole process is called reclock.
Based on our understanding of jitter, it is easier to understand why CD is still the best music storage medium, not a hard drive.
A WAV file is made of, for example, 100 data units. When the WAV file is stored on CD, the 100 data units are stored continuously so that the time difference between each data unit is relatively small when the read head reads the data from CD, and there is almost no jiiter.
The same WAV file is stored in the hard driver, the 100 data units may be scattered on the driver, even if the hard drive has been defrag, so that the head read WAV file from the hard driver, the time difference between each data unit is relatively bigger and bigger jitter.
Tape and CD are the same to store continuously the data units, but CD has many holes on the surface of the CD to make the reflected light easily deflected, resulting in jitter. Also, the tape is fixed speed, unlike the CD is not fixed speed. Tape is the best storage media for a music file.
Flash memory is the worst one, because it uses of the same the hard driver with the decentralized storage means and when read WAVE file from flash memory, there will be a large number of transistors are turned on/off, and each switch speed is not the same, this is very easy to produce jitter. Flash memory will have a bunch of high-frequency noise.
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