TT 3285 and USB audio class (technical introduction is boring !)

I remember that more than two years ago, the rhythm decided to develop the USB audio product. Meeting with R & D departmentUSBDuring the core solution, the engineer asked me whether you want to use the solution quickly or a complete solution, and then explained the One-afternoon USB audio solution to me! Finally, I will say how long it will take us to get the most complete response for 10 months. Result .......... More than two years later, I finally got it!
I learned what I understood that afternoon and what I learned later.MaterialsTo share with you! The content is boring! If you are interested, let's take a look!

Let's talk about the USB transmission mode.
The USB transmission modes include bulk transaction, control transaction, interrupt transaction, and isochronous transaction. Bulk transaction and isochronous transaction can be used as the transmission mode of the product audio stream.

Bulk transaction: the main application is in the case of a large amount of data transmission and acceptance, and there is no bandwidth requirement, that is, as long as Accurate transfer, no time required . Features: ensure transmission. Mobile hard disks and scanners belong to this type Device, This transmission mode only considers the data accuracy, not the real-time performance. . In this status, the transmission mode is extremely vulnerable. Others USB connection device interference, when using a USB flash drive to transmit data, its Unstable transmission rate This is the embodiment of this problem! In addition, when this method is used for audio transmission, a large buffer is required to control the audio stream. Although it can obtain better jitter, it is easy to produce latency. Burst sound And phase difference.

Isochronous transaction, that is, USB audio class, is used Stream Data Transmission with strict time and strong fault tolerance , Or used for requirements Real-time applications with constant data transfer rates . For example, the network phone number for instant calls. Features: ensure synchronization of transmission. Guaranteed Fixed transmission bandwidth!

In the isochronous transaction mode, the data is divided into three modes: SYN (synchronous) Ada (Adaptive) asy (asynchronous.
In isochronous transaction SYN synchronous mode: low-end USBSound CardWidely used . This method is available Stable bandwidth, but the clock is dominated by computer clock , But because The USB clock provided by the computer and the clock required for audio file playback sometimes have a base difference , Will generate Impact Sound QualityJitter , So Applicable onlyHeadset or simple voice equipment, But itsUseSystemUnderlying audioDriverStrong support for USB drive mode .
In isochronous transaction Ada adaptive mode is based on itselfChipData transmission method for secondary processing and transmission The product itself does not conform to the clock of the computer's internal data, After receiving the signal, refresh the clock in buffer mode based on the internal phase-locked loop of the chip, and completely clear the jitter of the USB part. . However, this requires the chip itself to have the digital processing capability to handle the clock and then send it to the backend. Decoding Chip. This design method has low requirements on system capabilities and related connection circuits. Because the underlying audio driver of the system can be used, the driver mode has strong support.

Isochronous
The asy asynchronous mode in transaction is the audio transmission mode controlled by the driver. The independent clock used by the driver according to the requirements of the chip and the audio resources is implemented by the chip's own lock loop to direct the computer to the USB clock to provide signals, theoretically, this method can be used to control the jitter of USB most effectively, but this method requires high system performance and related connection circuit processing. Because this mode cannot use the underlying driver of the system, the driver mode has weak support capabilities and needs to be independently developed by the vendor for different operating systems.

After learning the above information, let's take a look at our 3 series chips 3185 and 3285. Work Mode!
Although USB audio is not supported
Class is a simple way to design the transmission of USB audio data streams, but all the responsibility for system stability is lost to the customer. Continue to study USB audio
Class, let us find a solution to completely solve all the problems of the customer? Finally, I chose the latter. After more than two years of cooperation, I finally found a solution for accurately transmitting audio data at a stable bandwidth!

Tempotec 3285 is used Ada adaptive and asy asynchronous modes in isochronous transaction mode . Taking tempotec 3285's built-in tempotec proprietary high-precision digital PLL Technology (t pll) as the core, it can work in combination with the two modes.

Tempotec 3285 is smarter than other so-called asynchronous mode chips. First, tempotec 3285 uses isochronous
The transaction mode is not the bulk transaction mode of other vendors. When the device is plugged in, tempotec
3285 will automatically notify the system that we are isochronous
In this case, the system provides a constant high bandwidth for the transmission of audio streams. This completely avoids the problem of Phase Difference and delay in bulk transaction mode.

In Ada (Adaptive Mode), the chip receives The Internal Computing Capability of the chip is used to re-organize the data link based on the clock requirements of the backend decoder chip through the (t pll) Digital Phase-Locked Loop Technology, in this way, the jitter generated during transmission between the computer and the USB sound card is cleared.

In asy (asynchronous mode), you only need to press the asynchronous transmission mode button without connecting to the computer. When the system identifies the driver and our new hardware ID, the product automatically enters
In asy asynchronous mode in isochronous transaction mode, when obtaining stable bandwidth, the driver
The Digital Phase-lock loop (PLL) technology directs the computer to transmit audio data according to the requirements of the chip through the exclusive driver, thus completely avoiding jitter.