x_lk
100+ Head-Fier
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I was wondering what kind of USB cables people are using. A search on 'USB cable' led me to this thread. 'USB cable matters' - I keep hearing this statement these days and am always amazed by the sensitiveness a pair of human ears can reach. To me, this is by far the most ridicules statement I've seen on this forum. I don't want to be rude, but please, stop passing wrong knowledge onto other people ...
Let me make my point first. USB cable DOES NOT matter, well, at least for audio applications. USB transport is jitter-immune.
Why? I'll try my best to explain. Although USB is a serial bus(using NRZI encoding - a NRZI encoded SPDIF would be cool
), the USB communication is actually data packets based. There are three basic packet types: handshake(who said USB audio is one way communication?), token(address) and data(payload with CRC). A simplified picture of USB communication is like this: host sends token+data, device accepts payload and responds with handshake ACK, or rejects payload with handshake NAK. In later case, host sends the token+data again. So in USB world, there is absolutely no wrong or partial data packets being passed to application layer. A data packet either arrives immediately or eventually arrives after retransmissions. A data packet can carry a payload up to 1K bytes, that is 8192bits, or 512 16bit samples. Use CD audio as an example, a USB data packet can hold a little less than 6ms audio stream (two channels). Let's calculate how many packets can be exchanged on USB in this 6ms. Although USB 2.0 supports up to 480Mbps, most devices typically work at an average 24Mbps with a peak transfer rate up to 80-160Mbps. To be conservative, we use 24Mbps for our calculation. 24Mbps/1000*6=144K bits, that is to say, in 6ms, USB can transfer 144K bits, or 9K 16bit samples, which are 18x of the bandwidth required by CD audio. See my point here? If there ever is a wrong packet being detected, it can take up to 17 retries to get it right without interrupting the playback of an audio stream (un-buffered, worst case). I would say any decent 'certified' USB cable should have a BER(bit error rate) far lower than the required 17/18, otherwise, how they got certified at first place? Now the job of USB cable is done, you can see no jitter being introduced at all. The jitter actually kicks in at the following stage when a USB receiver chip converts the received payload into bit stream(serialization/re-clocking). I won't go there since I'm not an expert.
OK, people may ask, why are there super expensive USB cables out there? Well, every product has its own market. But using these high-class cables for USB audio is definitely an overkill, IMO. I can see their usage, when the application is bandwidth sensitive, or simply requires the maximum throughput out of USB transport. Mass-storage and video are two obvious example I can think of.
Let me summarize:
1. Lossless transmission is the beauty of digitalization(SPDIF is rough and bad, shame on SONY and Philips).
2. Any USB 2.0 certified cable is good cable(for audio use).
3. There is no sonic difference between a $10 USB cable and a $300 USB cable.
4. Jitter comes from USB receiver chip and beyond.
Edit: A cable matters only if the USB receiver is bus powered, which is a bad design to start with anyways.
lk
Let me make my point first. USB cable DOES NOT matter, well, at least for audio applications. USB transport is jitter-immune.
Why? I'll try my best to explain. Although USB is a serial bus(using NRZI encoding - a NRZI encoded SPDIF would be cool
OK, people may ask, why are there super expensive USB cables out there? Well, every product has its own market. But using these high-class cables for USB audio is definitely an overkill, IMO. I can see their usage, when the application is bandwidth sensitive, or simply requires the maximum throughput out of USB transport. Mass-storage and video are two obvious example I can think of.
Let me summarize:
1. Lossless transmission is the beauty of digitalization(SPDIF is rough and bad, shame on SONY and Philips).
2. Any USB 2.0 certified cable is good cable(for audio use).
3. There is no sonic difference between a $10 USB cable and a $300 USB cable.
4. Jitter comes from USB receiver chip and beyond.
Edit: A cable matters only if the USB receiver is bus powered, which is a bad design to start with anyways.
lk