Camrad62:
Ни чего.
Ясно, полное невежество налицо.
Vitek11:
вы много процетировали, черт возьми, не поленились. Только где кординальная разница в передаче для принтера и для камеры или ЦАП-а. Вы не разобрались в прочитанном, совсем.
Не, в случае USB Audio он прав.
5.6.5 Isochronous Transfer Data Sequences
Isochronous transfers do not support data retransmission in response to errors on the bus. A receiver can
determine that a transmission error occurred. The low-level USB protocol does not allow handshakes to be
returned to the transmitter of an isochronous pipe. Normally, handshakes would be returned to tell the
transmitter whether a packet was successfully received or not. For isochronous transfers, timeliness is more
important than correctness/retransmission, and, given the low error rates expected on the bus, the protocol is
optimized by assuming transfers normally succeed. Isochronous receivers can determine whether they
missed data during a (micro)frame. Also, a receiver can determine how much data was lost. Section 5.12
describes these USB mechanisms in more detail.
An endpoint for isochronous transfers never halts because there is no handshake to report a halt condition.
Errors are reported as status associated with the IRP for an isochronous transfer, but the isochronous pipe is
not halted in an error case. If an error is detected, the host continues to process the data associated with the
next (micro)frame of the transfer. Only limited error detection is possible because the protocol for
isochronous transactions does not allow per-transaction handshakes.
(с) Universal Serial Bus Specification Revision 2.0
Но! В случае асинхронного подтипа изохронной передачи данных по интерфейсу USB,
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 are not synchronized to a start of frame (SOF) or to any other clock in the USB domain.
3.11.4.1 SINGLE DIRECTION SINK ENDPOINT
A typical example for this scenario is a USB speaker, implementing a sink endpoint that receives streaming audio data from the Host.
Asynchronous
o Host: The Host driver needs to be able to handle an explicit feedback endpoint. From the feedback data, the Host then decides how many samples to send over the data streaming endpoint in subsequent Bus Intervals.
o Device: The Device has its own local, free-running audio sample clock, which determines how many samples are consumed by the Device each Bus Interval. The Device shall implement an explicit feedback endpoint as well as the necessary logic to provide the correct feedback values to send over said endpoint back to the Host. The advantage of this mode of operation is that it is fairly easy to generate a robust, stable, jitter-free, high-quality audio sample clock (derived from a crystal-based master clock, for example).
(c) UNIVERSAL SERIAL BUS DEVICE CLASS DEFINITION FOR AUDIO DEVICES, Revision 3.0