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how to backup pc on usbThis article is the Internet to find the author: Laser mouse \u0026 nbsp;

In January 2009, the United States CES, Ayat (Ayre) released a model for the QB-9 decoder, in the HiFi sector caused a sensation. The $ 2500 small box is not as good as the general decoder, there are optical fiber, coaxial and other input interface, QB-9 only a USB input port, it is clear that it is dedicated to the computer. In the QB-9 before the emergence of a variety of USB decoder on the market, why is the reason why QB-9 appeared to lead the world's serious concern? Because the QB-9 uses a technology called USB asynchronous, completely solve the PC-HiFi in a headache Jitter problem.

Figure 1: Yi Ya QB-9 decoder, the advent of aroused widespread concern

Before we introduce USB asynchronous, we must first remember that the jitter that really affects the sound is the clock jitter of the DAC chip, not the other (such as the jitter given to the decoder data). How to minimize the jitter of the DAC chip clock is our ultimate task. Numerous experiments show that, whether it is a few hundred dollars DVD, low-end CD player, computer track player or hundreds of thousands of top turntable, through the SPDIF digital output data are exactly the same, the probability of error is small, do not consider. But we all know that the low-end turntable sound is much worse than the high-end turntable, the reason is that the turntable digital signal Jitter led to the decoder clock Jitter, low-end turntable digital output Jitter large, high-end turntable Jitter small, there is a good difference between sound quality.

An article on my translation: Jitter and Digital Interface, which mentions that the clock signal is transmitted over long distances will introduce additional jitter, which means that the decoder's clock is best not to Through the external device input, in this case, we can easily think of the most perfect solution is: a very accurate clock on the DAC chip edge, and the clock as the main system of the main clock, that clock does not need to The clock is synchronized and the other clock is kept in sync usb with pc with it.

According to this requirement, we can easily think of the perfect CD machine architecture, in fact, there are many CD machine is done in accordance with this framework:

Figure 2: the perfect structure of the CD machine, the Jitter impact to a minimum  a clock close to the DAC chip, and as the entire CD player's main clock

This clock also instructs the servo circuit to read data from the CD. When the servo circuit finds the FIFO underrun, it speeds up the spindle motor speed and speeds up the data. When the servo circuit finds FIFO overload, it slows down the spindle motor speed , The data readout slows down to keep the FIFO half full.

Figure 3: the perfect structure of the CD machine into a turntable and decoding, the clock should be in the decoder side, turntable by the clock control

As the servo system to read the disk when the action will produce electromagnetic interference, harmful to sound quality, for which we can put the servo circuit, FIFO, movement part and DAC, clock separately, respectively, installed in a separate chassis, which became the turntable And the decoder, to minimize the servo on the decoding part of the interference. From the above diagram we can see that the perfect architecture of the CD machine is divided into turntables and decoders, the clock should be placed on the decoder side, so you need two data lines (or a two-way data lines) were transmitted data and clock, Can still maintain a perfect architecture. Although the long distance transmission, the clock sent to the turntable and the data sent to the decoder Jitter, but these Jitter will not affect the sound, because we are most concerned about the decoder clock is local, no transmission, Jitter minimum. Unfortunately, the original development of SPDIF data transmission standards, Jitter on the sound effects and sources can not be fully studied, the results SPDIF defined as a one-way data transfer standard, the clock can only be placed on the side of the turntable, so that the decoder must Face the Jitter problem that could have been without worrying.

Figure 4: SPDIF clock on the side of the clock, making the decoder is inevitably affected by the Jitter

In some high-End turntables and decoders, there is a dedicated clock interface, the turntable can receive the decoder clock. Enable the clock interface, the user can clearly feel the sound quality upgrade. Unfortunately, due to lack of standards, this clock interface is not strong, usually require dial and decoding supporting the use of its limitations.

Summary: A perfect turntable, the decoder solution should also meet the following conditions:

1. Clock on the decoder side, and as the system master clock, do not need to keep pace with others

2. Two-way data transfer between the decoder and the turntable: The clock signal is passed from the decoder to the turntable, and the data is passed from the turntable to the decoder.

3. Transmission interface must be the industry standard, with interchangeability, that is, different manufacturers of the turntable and decoder can be used with free

4. The transmission interface has enough transmission bandwidth to meet the transmission of audio data stream

Conditions 1 and 2 are not difficult to meet, most of the data interface standards support two-way data transmission, but the conditions are difficult to meet the 3,4, especially the conditions 3. Until the USB and WindowsXP \\ MAC OS / X and other operating systems appear to meet the above conditions became a reality.

USB specification fully takes into account the needs of multimedia audio and video applications, providing the perfect support, here only with the relevant data transmission part of this article for some introduction.

First introduce the relevant noun definition:

Host: a USB environment can only have a host, usually a PC, the data can only be transmitted between the host and the USB device, the device needs to forward data through the host.

Endpoint: The data transmission with the host is done through the endpoint, a device may have a lot of endpoints, for different functions of data transmission.

Pipeline: The connection of the endpoint to the host is called a pipe and can be imagined as a data line.

Frame: USB specifies to send a data frame every 1 millisecond, a frame can have multiple packets. The data frame starts with the SOF flag and ends with the EOF flag.

USB support data bi-directional transmission, for audio applications, specifically the audio device class definition specification, the highest version is USB Audio2.0, multi-channel high-definition audio specifications, currently only OS / Xlion operating system support, the market to see USB decoders that support high-definition audio must be installed with special drivers for use under Windows. And earlier versions of USBAudio 1.0 get Winodw XP \\ Vista \\ 7, MACOS \\ X and other mainstream operating system system support, in accordance with the norms designed decoder can be plug and play, without the need to install a dedicated driver, the maximum support 24/96 Specifications Two-channel audio.

Audio and video data and other real-time requirements continue to send data according to a fixed bandwidth, USB and audio applications related to the type of data transmission is called isochronous transmission, the English IsochronousDataTransfer, there are simultaneous translation of the translation, but easy to be mentioned later In the clock synchronization mode, 'synchronous' confusion, so this article translated as isochronous transmission.

USB Audio 1.0 is USB1.1 framework of the specification, the maximum transmission bandwidth is limited to 12Mbps (1.5M bytes / second) within. USB specifies that a data frame is sent every millisecond so that the data that can be transmitted per frame does not exceed 1500 bytes. At the same time USB specification, isochronous transmission mode, a valid data sent (that is, excluding the protocol overhead) does not exceed 1023 bytes, the following table is a different length of effective data when the bandwidth usage, you can see in a send Valid data in the 64,128,256 bytes, each frame can transmit 1280 bytes (1.28 megabytes per second), so USB Audio 1.0 supports up to two-channel 24bit96KHz audio, to meet the needs of the vast majority of occasions.

Table 1: USB 1.1 isochronous transmission mode data transmission efficiency table

In the USB isochronous mode, three endpoints of the clock synchronization type are supported:

 adaptive endpoint: adaptive endpoint of the strongest adaptability, can be said to be submissive, according to the other side of the request to adjust their clock beat, to achieve the purpose of synchronization.

The adaptive sender adjusts its own clock according to the transmission rate of the receiving end. Therefore, a feedback pipeline needs to be recommended between the receiving end and the transmitting end, and the data transmission rate of the receiving end is notified to the transmitting end.

The adaptive sender can communicate with all types of receivers. For an adaptive receiver, there is no need to establish a feedback pipeline with the sender, which can obtain the transmission rate information directly from the data stream.

 Synchronous endpoint: The characteristics of a synchronous endpoint are that its clock is controlled by the USB frame start flag (SOF). There are two ways, one is to use the PLL with its own clock and SOF synchronization, which is nothing with the adaptation; the other is to control the transmission of SOF, making the amount of data transmitted per frame to maintain consistency. Since only the host can control the SOF transmission, it is only for the external device that can not change the SOF to keep the data volume between the SOFs by throwing out the excess data or inserting the interpolation.

 asynchronous endpoint: asynchronous endpoint clock will not be synchronized with the outside world, for the asynchronous receiver, the sender must establish a feedback pipeline, the data transmission rate information to tell the sender. Since this feedback can be sent at any time, the sender at any time according to the feedback information to adjust the transmission rate, so you can eliminate the impact of clock frequency drift.

From the above we can see that a decoder if it is adaptive or synchronous, and its clock requires synchronization with the outside world, easy to introduce Jitter, which does not meet our requirements, only asynchronous endpoint clock has nothing to do with the outside world, By the sender according to the feedback rate it sends to send data to keep the synchronization. So if the decoder is an asynchronous endpoint, it is exactly what we have mentioned before the four, together with the computer to form the perfect turntable, decoder architecture.

USB asynchronous use of two pipes to achieve two-way data transmission: a data pipeline from the computer to send data to the decoder, another feedback pipe to the decoder to the computer to report their own data transfer rate. This is different from the way the advanced turntable + decoder uses the clock interface to transfer the clock directly, but the effect is the same.

Figure 5: USB asynchronous decoder architecture

In the USB asynchronous transfer process, the decoder side of the microprocessor based on the local clock frequency and USB SOF interval to calculate their own data rate, and through the feedback pipeline to the computer to report how much data per frame, the computer is in accordance with this feedback Information, how much data do you want me to give you how much data, not more will not be less. The microprocessor simultaneously fetches data from the FIFO according to the tempo of the local clock for DAC decoding.

USB asynchronous architecture in full compliance with the perfect turntable + decoding requirements:

1. Clock on the decoder side, is the system master clock, do not need to keep pace with others

2. Decoder and computer through the USB two-way transmission of data, the decoder tells the computer how much data I need, the computer is required to send data

3. USB interface is the industry standard, different manufacturers of the decoder can be connected to a different computer

4.USB interface has sufficient transmission bandwidth, to meet the 24/96 specifications two-channel audio transmission, the future can support higher-specification multi-channel audio transmission

USB1.1 was developed in 1998 as early as possible, but since the operating system only supported the adaptive decoder, until the recent Windows XPSP3 appears to support asynchronous decoder. So a long time on the market USB sound card, the decoder has been working in the adaptive mode, the interface chip to Ti PCM27XX majority. This type of chip only supports

Adaptive mode, the maximum sampling rate is only 48KHz, the output clock Jitter up to 2-3ns, if the direct drive DAC chip, it sounds very obvious digital sound. Enthusiasts generally believe that USB sound quality is derived from this. In order to solve the Jitter problem, you can add a side of the PCM27XX ASRC chip to eliminate the vast majority of Jitter, using this program has a domestic ARCEMDAC, the United States Benchmark DAC1 and other decoders.

2002 appeared more advanced audio dedicated USB interface control chip, such as TAS1020B, supports all USB data transfer mode and synchronization type, and built-in audio Codec. However, this chip built-in official firmware can only support adaptive mode, if it is working in asynchronous mode, you must write their own procedures, making programming and debugging becomes extremely difficult, the developer must work on the bottom of the USB, WindowsUSBAudio The working principle of the kernel driver and the working principle of the TAS1020B chip have a deep understanding to complete this work. Coupled with the operating system has not been able to support USB asynchronous, so many of the decoder although using this chip, but only to use it to support 24/96 features, still work in adaptive mode, the output clock Jitter up to 2ns or more The For example, Benchmark DAC1, USB interface is used TAS1020B, work in the adaptive mode, had to add an ASRC to suppress Jitter.

Over the past few years many people have seen the potential of USB asynchronous, try to write asynchronous programs for the TAS1020B, but have been failed, only one master was successful. He rewrote the firmware of the TAS1020B, joined the USB asynchronous support, and the software license to the Arts and DCS, so the market will have the beginning of this article Yi Ya QB-9 decoder. XMOS has introduced a USBAudio 2.0 development kit that supports asynchronous mode, and the asynchronous decoder that conforms to the USB Audio 2.0 specification is used without exception.

USB asynchronous is a technology that is not too new, but the official application of only two years, with its excellent performance and interoperability, more and more in the decoder has been applied. Now understand the technology is not too many people, most enthusiasts still think that USB sound quality is not good, PC-HiFi not. I believe there will be more and more decoders use this excellent technology, so that PC completely replace the CD become a reality.

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