Good point Amir.
Amir's point is a generalised point about digital audio & jitter but it doesn't set out to explain:
- how a USB cable causes jitter
- how a USB cable that connects a USB to SPDIF converter (signal still digital) might effect the final audio signal
Don't mean to complicate your shootout process but I've been using Acoustic Revive USB cables for several months now and cannot say enough good things about them. They have 2 models both of which go to pretty extensive lengths to separate power and signal, the SP version has 2 USB connectors at the output side. Anyway, if you can add them to your shootout it would be interesting. I also think very highly of the Nordost Blue Heaven USB which sounds dramatically different than the Acoustic Revive but still very very good. Thanks for posting the thread, look forward to your findings.
Listening Room: McIntosh C46, MEN220, MCD500, MR78-Modafferi modified, MPI4, MC602 (2), Pass Labs XVR1 (three-way), tri-amped Infinity IRS Series V, TailTwister T2X rotator, AtlasSound FMA Rack, dedicated electrical sub-panel, NO TV!
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I'm sure that our resident experts will correct any/all of the above if I'm deluded!
Amir will answer for himself but I'll add a couple of comments:
1. A USB cable with improper impedance, poor connectors, and/or limited bandwidth can add jitter to the signal, and deterministic jitter which I think is more important than random.
2. IME the biggest problem with USB cables in general (assuming asynchronous operation) is that they provide a noise conduction path from PC to audio through the ground, and in some cases the power wire (if used). The ground noise can be coupled into the audio signal path. Galvanic isolation can help a lot.
"After silence, that which best expresses the inexpressible, is music" - Aldous Huxley
Even if USB is used, the target clock needs to track its rate. The host is in charge and how fast it sends out the samples must be the rate at which the DAC follows.
The larger problem with USB is not that but the buffering segments (I wrote the previous response because of the generic comment you made that digital data cannot vary analog output of a DAC). The USB transport will send a block of data, and then go fetch the next. That causes a glitch during that period. A great case study of it is here: http://www.eetimes.com/design/audio-...he-and-triumph
As you see there, the DAC was designed without regard to jitter induced by USB rebuffering. The result was this:
"“At such a time it is human nature to want various people to see (hear) the result, so we demonstrated it to all of those purported to be 'Golden Ears.' The audio signal came through the PCM1716, a DAC with an industry-wide reputation, and the PLL as the PLL1700, which has excellent C/N performance.” When the guys in charge listened to the prototype I saw dubious faces and was asked a variety of questions such as "Is the source coming from the PC corrupted?" In the end I was told to measure the audio performance. When I announced the results in a subsequent meeting I was told the distortion was an order of magnitude too high; the THD+N was 0.03%.”
“I went into this thinking "Since we are processing digital signals, we can expect good sound as a matter of course, and from here on we are dealing with digital!" So this experience was a real shock.”
Upon Raising the FFT Resolution . . . A 100 Hz Monster Appeared!
Next, in order to investigate the skirt around the fundamental, I decided to increase the FFT resolution to a higher setting than I usually use. Naturally it took longer to make the measurement. After a wait time that would best be measured in a fractions of an hour, I was amazed at the FFT analyzer's output graph. The measured FFT is shown in Figure 9:
“Even for a sample rate of 44.1 kHz, the USB isochronous mode packets have a period of 1 ms (1 kHz). In order to distribute 44.1 kHz across 1 ms intervals, one 45-sample packet is sent for every nine 44-sample packets. The tracking pulse (as we will call it here) for every 45 sample packet occurs once every 10 packets, or with a frequency of 100 Hz. Since the PLL loop filter, a so-called low pass filter, has its corner in the tens of kHz range, this 100 Hz tracking pulse goes right on through and shows up on the PLL's VCO control voltage. It appears as frequency jitter.
From the graph it is seen that the PLL frequency fluctuates impulsively right at 10 ms intervals. As a test I changed the sampling frequency to 48 kHz and measured the same 1 kHz signal.”"
So as we see, sudden timing changes from the source providing data to the DAC caused distortion to travel to the DAC output and not be filtered out by the usual circuit (PLL). He redesigned the PLL and the jitter was filtered out. The point then is that just because digital is digital, it does not mean that you get the same analog output regardless of how the system works upstream. There is a level of complexity below that which needs to be looked at.
As noted, the picture is more complex in that you are also electrically coupling a noisy computer to a DAC. To my knowledge no one has tried to characterize any of this as variations are infinite.
Founder, Madrona Digital Audio, Video, Home Automation
Contributing Editor, Widescreen Review Magazine