USB audio is complicated, and we humans don’t like that. So we start simplifying things, even vendors do that. Unfortunately, simplification can lead to wrong information. Like as with USB async audio, the DAC “asks” for data (some vendor’s flyer, long time ago). With USB, there is one master, the host. Only firewire was able to reverse the roles. Or what about digital is digital, so cables don’t matter. The data packets send with USB audio are fire-and-forget. Bad cable means packet loss. But since it’s digital, a good cable is good enough. Better “copper” does not improve things.
Since USB audio is fire-and-forget (USB isochronous mode) and you want your DAC to have it’s own local clock, you have to tell the sending party to hurry-up or back-off. USB uses a separate channel (endpoint) for that, and that’s called feedback. As part of being complicated, this feedback can be implicit or explicit (WTF?). As described here the explicit method of feedback is the DAC telling the sending party (host) how fast it receives the data. Using this number, the host adjusts its rate of data it’s sending. The USB audio packet-rate is fixed, so to adjust the data rate, the number of audio frames in each packet is adjusted. Pretty simple and this feedback method is used most.
So what’s the implicit method then? The implicit feedback method is mostly used by audio interfaces that have input channels, so for recording. The timing of this recording channel is then used to adjust the sending channel speed. This assumes the audio interface uses the same clock for both. There is no dedicated channel (endpoint) for feedback, hence implicit. The logic to adjust/align sending speed is much different and as a result, the linux audio system does not always work well with these kind of interfaces. I could not get my MOTU M2 to work with Debian on a beaglebone black. On an Intel mini-pc, it worked perfect. These type of audio interfaces are designed for low and perferably fixed latency, here the implicit feedback method makes sense.
Note that both feedback methods support asynchronous clock on the DAC/Audio interface.