S/PDIF or USB? (4)
Improving on PC side 3
Efforts to improve in this domain can be concentrated on several main problems.
Jitter
As we have just seen, the jitter performance of the USB port is relatively similar to the jitter performance of the S/PDIF output of the soundcard hosted in the given PC. So, if we were using a different PC, would this performance also be different? Can some PC's perform better than is just shown?
The previous graph shows the performance of the USB input of The Model. It was made using one PC both as a USB source and as a host for measuring A/D front end. This was done intentionally, to lower possible common mode and ground loop artifacts caused by having two PCs in the chain. However, to find the answer on this last question, I used another PC to source the USB signal to the same USB input of The Model. This second PC again runs under Windows XP and uses Foobar2000 player. The same first PC is again used for measurement. Jitter related artifacts (red curve) dropped visibly. In fact, in most of the audio band they got down to the noise floor of the whole set up, which in turn expectedly, as a consequence of use of two PCs, increased (black curve - measurement taken in the absence of the audio signal).
[click to see a higher res graph]
One will normally want to investigate deeper within PC architecture on exact causes of these differences, and consequently on possibilities to further improve on this performance, but it shell remain out of the scope of this article.
Noise
One will normally also want to further investigate possibilities to lower the noise floor, searching for its causes. Going deeper in this direction is again also out of the scope of this article but following graph may be indicative. I firstly turned the source PC, i.e. its operating system, off, but it didn't lower the noise floor really importantly (blue curve). Then I turned the main PC mains switch off, and it again helped only slightly, in the 15-16kHz area (black curve). Only completely unplugging the mains cable from the wall yielded the noise floor down to the limit of the measuring set up (red curve). Please note that this wasn't the ground loop, since only the measurement PC has been using the connection to the safety earth.
[click to see a higher res graph]
Further investigation would tell whether some part of supply worked even when the switch was turned off, or this was solely a common mode noise, but this should be enough to make importance of noise issues quite obvious. In fact this doesn't look like anything new but, unfortunately, despite all the PC tweaks proposed these days, the problems in this area are still more or less totally intact.
PC noise related issues however are not even remotely limited to the noise shown in the graph above, i.e. to the noise within the audio band. They are probably not even primarily related to the noise within the audio band. In fact, most of the audio gear has relatively reliable power supply rejection within the audio band. Consequently, many advanced modern PCI audio cards, which are both supplied by and located in the PC, achieve vanishingly low noise level within the audio band. (Actually the soundcard used for these measurements is exactly such a card and its noise floor is well shown by the last curve of the last graph). It is however also a high frequency noise that disturbs our audio circuits the same, and many times it is the actual reason for their disappointing subjective performance.
According to the feedback I've had from Audial DACs users, in some cases decoupling for noise appeared more important than jitter performance. In other words, the external USB to S/PDIF converter that was feeding the S/PDIF input of the DAC, soundwise worked better than a direct connection with the USB input of the DAC, with USB decoding hardware being more or less equal. (As opposed to their USB inputs, S/PDIF inputs of Audial DACs provide galvanic (transformer) decoupling.4) This may be further emphasized by known susceptibility of TDA1541A to the high frequency noise.
The problem, as I understand it, and putting aside ground loop problems which would have to be addressed additionally, is made by two major groups of contributors:
1. Supply noise
PCs almost without exception use switching mode power supplies, including battery supplied laptops, since these also employ DC-DC converters. Use of such supplies means major savings in big industry manufacturing. On the other hand, savings per one item doesn't mean much in the world of high quality audio. One may talk about the improved noise performance of the recent SMPS generation, however there is no guarantee with regard to the actual performance of the units available. Obviously, this suggests a move to a quality linear supply. Additionally it would solve the problem of noise caused by PC supply fan (both the mechanical one, as the electrical one dumped back into the supply line). Current lack of adequate commercial products however requires some kind of DIY solution.
2. Noise generators like ICs and components with mechanically rotating parts (fans, HD)
These parts dump the switching currents back into the ground, thus disturbing the system ground (“ground bounce”). As the USB galvanically couples the PC ground to the USB peripherals, the DAC’s ground is directly violated. This is apparently harder to solve. Use of USB decoupling devices is suggested, however the prices of currently available device significantly add to the overall system price, and reported results are not consistent. Solving the problem right at the start is recommended anyhow. The usual suggestions for using the most powerful PC machines may be wrong in this regard, since the noise generated by them is normally higher in frequency.
A lot of it, however, depends on the implementation. There is no doubt that the computer hardware designers do a lot of very good work in this regard, however the usual PC environment is still too noisy even for basic audio standards. Unfortunately, at this point there is quite a gap that severely limits a possible move forward: PC designers are usually not audio oriented, and audio guys are normally total strangers in PC architecture - and there is practically no communication between these two worlds.
Ground loops and cables
Use of two or more galvanically coupled devices where each is using own connection to the safety earth is a recipe for ground loops. Use of switching supplies makes the use of safety earth a must (otherwise the ground may shift notably above the earth voltage, sometimes even 100 or more Volts). Optical isolators are sometimes recommended to break this loop, however one has to be cautious with them. They are often not good jitter performers and usual USB devices still use USB stream to retrieve the sampling clock.
USB cables have to be as short as possible.
Long cables won't only increase the ground loops, but will also increase a current induced common mode noise and bring 1kHz USB packet frequency into the audio signal. Please see the third and fourth graph in this post that show the difference between 1m and 3m of the same cable.
Spread spectrum clock
This technique is used in some PCs to make them easier to pass the FCC regulative. The graphs published here are made using PCs not employing spread spectrum clock. The second PC however had the option to turn spread spectrum on in the BIOS setting. So I tried turning it on to see the possible consequences. This may come as a surprise but use of the spread spectrum clock didn't change anything about USB jitter performance of this PC.
Software
Software was among more discussed USB PC audio issues. Like many others, I also found software to contribute to the final sonic results but, as opposed to what you can read at some places, it has nothing to do with jitter. I went through many programs, including both those claimed to improve on the jitter and those without any ambitions in this direction (WinAmp, Foobar2000, XXHighEnd, and a couple of other more or less known players), as well as through a couple of output drivers (kernel mixer, a few ASIO drivers), and found no trace of any relation between the jitter performance and software used. In other words, unless one really screws up something on his own to make it perform unnecessarily badly, software doesn't change a squat about jitter. I have to repeat that all these tests were performed on Windows XP, and speaking strictly this statement would apply to the application layer and not to the operating system. However, there are no indications of things being different when it comes to the operating system. And Windows XP is what the vast majority of consumers use these days anyway.
Of course, the question is, if it is not a jitter, what is it then? Unfortunately, up to now to my knowledge there has been no serious research in this domain, and I honestly think the things here are at least if not more hopeless than those related to the hardware. Software for now thus remains practically exclusively listening experiments area.
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3 - Many texts have been written on this topic last year or so. Most of them were unfortunately useless and even misleading.
4 (added in 2011) - This applies to the Audial DACs available back in 2008, The Model and AYA II. Later Audial DACs with USB inputs, D-09 and Model S USB, do isolate USB.