kvik skrev:Lavricables snakker om RFI, EMI, lavkapasitans mm - hvad er relevansen for en digitalkabel? (Også set i lyset af msp’s erfaring et par indlæg herover)
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i Mojo-trĂĄden pĂĄ HeadFi (post #3)
Influences *upon* RF noise and *of* RF noiseÂ
Quote: musicheaven
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Digital transmission is based on SPDIF standard which transmits data and clock information as an encoded signal usually using PCM, that information is decoded on the Mojo into data and clock signal so it's important that the encoded information be jittered free and not degraded over short distance.
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The USB transmission on the other end is a device to device transmission mechanism using an encoding scheme and handshaking mechanism, it is usually stream based so more tolerant to poorer wire as frames are transmitted and decoded from the source to the target device. The target device will reconstruct the data and clock signal from the frame and then feed it to the DAC to be analog reconstructed and eventually band pass filtered to remove any residual high and low frequency signals out of the audio band.I still think you need to keep the USB cable short but it is more tolerant of longer lengths up to a limit.
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To make a story short, the short USB cable is fine but an analog cable used as a digital one is just a bad idea. Again, that's just my opinion.Quote: Rob WattsJust to clarify:
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1. SPDIF decoding is all digital within the FPGA. The FPGA uses a digital phase lock loop (DPLL) and a tiny buffer. This re-clocks the data and eliminates the incoming jitter from the source. This system took 6 years to perfect, and means that the sound quality defects from source jitter is eliminated. How do I know that? Measurements - 2 uS of jitter has no affect whatsoever on measurements (and I can resolve noise floor at -180dB with my APX555) and sound quality tests against RAM buffer systems revealed no significant difference. You can (almost) use a piece of damp string and the source jitter will be eliminated.
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2. USB is isochronous asynchronous. This means that the FPGA supplies the timing to the source, and incoming USB data is re clocked from the low jitter master clock. So again source jitter is eliminated.
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So does this mean that any digital cable will do?
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Sadly no. Mojo is a DAC, that means its an analogue component, and all analogue components are sensitive to RF noise and signal correlated in-band noise, so the RF character of the electrical cables can have an influence. What happens is random RF noise gets into the analogue electronics, creating intermodulation distortion with the wanted audio signal. The result of this is noise floor modulation. Now the brain is incredibly sensitive to noise floor modulation, and perceives this has a hardness to the sound - easily confused as better detail resolution as it sounds brighter. Reduce RF noise, and it will sound darker and smoother. The second source is distorted in band noise, and this mixes with the wanted signal (crosstalk source) and subtly alters the levels of small signals - this in turn degrades the perception of sound stage depth. This is another source of error for which the brain is astonishingly sensitive too. The distorted in band noise comes from the DAP, phone or PC internal electronics processing the digital data, with the maximum noise coming as the signal crosses through zero - all digital data going from all zeroes to all ones. Fortunately mobile electronics are power frugal and create less RF and signal correlated noise than PC's. Note that optical connection does not have any of these problems, and is my preferred connection.Â
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Does this mean that high end cables are better? Sadly not necessarily. What one needs is good RF characteristics, and some expensive cables are RF poor. Also note that if it sounds brighter its worse, as noise floor modulation is spicing up the sound (its the MSG of sound). So be careful when listening and if its brighter its superficially more impressive but in the long term musically worse. At the end of the day, its musicality only that counts, not how impressive it sounds.        Â
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Rob
Quote: Rob WattsÂ
The reasons why sources and digital interconnects sound different are well understood - see some of my posts. In a nutshell it is not jitter (all my DACs are completely immune to source jitter) but down to RF noise and distorted currents from the source flowing into the DAC's ground plane. The RF noise inter-modulates with the analogue electronics, creating random noise as a by product, which creates noise floor modulation, and that makes it sound brighter or harder. The correlated or distorted currents very subtly add or subtract to small signals, thus changing the fundamental linearity, which in turn mucks up depth perception.
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But I also agree in that lots of people hear changes that are not there - I for one have never heard any difference with optical cables (assuming all are bit perfect) with my DAC's, but lots of folks claim big differences. Placebo, or listening with your wallet, plays a part here. Then there are cases of people preferring more distortion... Listening tests must be done in a very controlled and careful fashion, particularly if you are trying to design and develop things.
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Rob
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Rob's take on whether cables can affect SQÂ
Quote: HiFlight
At the risk of being flamed, I don't see how the composition of the USB cable wire can add warmth to digital data from whatever device is being used as a transportQuote: Rob WattsI understand those concerns too - after all the data is the same. But there are solid scientific reasons why they can make a difference.
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In the 1980's, people started talking about mains cables making a difference to the sound quality - and I didn't believe it either - particularly as my pre-amp had 300 dB of PSU rejection in the power supply. But I did a listening test, and yes I could hear a difference. Frankly I still could not believe the evidence of my own ears, so did a blind listening test with my girl friend. She reported exactly the same observation - mains cables did make a difference to SQ.
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To cut a long story short, I proved the problem was down to RF noise. RF noise inter-modulates with the wanted audio signal within the analogue electronics, and if the RF noise is random, then the distortion is random too and you get a increase in noise floor with signal. This increase in noise floor is noise floor modulation, and the brain is very sensitive to it; you can perceive tiny amounts of noise floor modulation as a brightening or hardening of the sound. By tiny I mean the noise floor modulation needs to be well below -200 dB, so the brain is very sensitive to it. With the right test equipment, you (APX5555 is only test equipment that has no innate noise floor modulation) can easily measure the effect.
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The RF characteristics of the cable can change the RF noise that gets injected into Mojo's ground plane, and this is the mechanism for changes in smoothness. You may say why can't you make it insensitive to it; well I go to silly lengths to RF filter and decouple, and use dual solid ground planes on the PCB, but you can't remove the problem. For Dave, Hugo TT and 2 Qute I have galvanic isolation, and this eliminates the problem (along with other SQ problems such as sound-stage depth). But I can't do this with portable devices, as it draws power from the 'phone. That said it's less of an issue with portable electronics as they are less power hungry and create less noise.
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So what are the best USB cables? Firstly, be careful. A lot of audiophile USB cables actually increase RF noise and make it sound brighter, and superficially impressive - but this is just distortion brightening things up. Go for USB cables that have ferrites in the cable is a good idea - it may also solve any RF issues from the mobile that you may have too.
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Rob