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Hello,
This is my first post. I am new to hi fi and my interest comes from both my love for good quality music and profession.
I have recently setup a yamaha receiver with two Polk speakers for 2 channel music listening but I am far from satisfied with the results.
Since I intend to use digital stored music most of the time (mp3s or streaming) I started reading about digital music reproduction.
One thing that was obvious from the beginning is that DAC design and quality is important.
However, since I am coming from an engineering background, never understood about the importance of good cables for transmitting digital signal.
I did some reading and I realized that the issue is not with the digital signal transfer but rather with the jitter.
I am copying some pieces from http://www.positive-feedback.com/Issue43/jitter.htm
This explains the difference between transferring files and music:
The audio data transfer must include both 1) accurate data and 2) accurate timing, whereas non-real-time transfers only require accurate data.
This explains the impact of cables in jitter:
Cables don't actively add jitter to the signal, however they can slow the signal transitions or "edges". When the edges are slowed, the receiver or buffer at the cable destination is less likely to detect the transition at the correct time with certainty, which results in jitter.
So here is my question. Why do we need to transfer digital signal in real time? This signal is fed to the DAC. Why doesn't the DAC buffer the signal so that the usb is used just for file transfer and then use a high quality internal connection to transfer the file in real time to the DAC?
According to this, this is already happening in CDs:
2. Servo-system/rotational jitter
This is the system in the CD player/Transport that determines the spin-rate of the CD. It is an electromechanical system. Even though most modern CD players have buffering of the data to create some tolerance to this jitter, there is usually a PLL (Phase-locked-loop) involved, which is usually still somewhat susceptible to jitter. For newer players that completely buffer the data at high-speed from a CDROM reader to a memory buffer, this jitter is not an issue.
Georgios
Follow Ups:
Buffering an entire file requires a lot of memory, and if its buffered, you will likely lose the ability to pause, fast-forward and reverse etc..We get the same benefit with the Async USB protocol, by buffering a smaller amount and controlling the flow from the source (pull) based on a local clock. A backchannel is used to regulate the flow of data, preventing memory buffer overflow and allowing the local clock to spool the data out.
With Async, the jitter in the signal on the cable is not important. Jitter at the receiver is still impacted by the USB cable however. This occurs due to common-mode noise on the USB cable. USB filters and galvanic isolation of grounds minimizes this noise.
It is alsopossible to get errors on the USB cable, depending on the USB receiver and termination. A better quality cable can reduce losses and improve bandwidth, reducing errors.
Edits: 01/29/15
Yes, I have noticed that most modern DACs have asynchronous mode, meaning they don't depend on the digital signal for clock.
So the only issue would be errors? I am not sure how common it is to have decoding errors. I have transfered many files and never had a crc failure yet.
Anyway when I get a DAC with usb I will test and see if there is any difference! At the moment my receiver accepts only hdmi.
Do you think using a dedicated DAC and then connecting the DAC output to my received would improve sound q?
Any DACs that I can get on Frys?
Thanks
George
Georgios
The issues are errors and common-mode noise.
A good DAC will improve most systems. Frys? I doubt it.
Even with buffers, there is LIM effects.
See:
How digital audio interconnects can affect the sound:
http://www.AudioAsylum.com/audio/digital/messages/55974.html
Latest post with a lot of cited URLs:
http://www.audioasylum.com/audio/tweaks/messages/133408.html
ALSO:
http://www.audioasylum.com/audio/cables/messages/99487.html
for a thread on jitter that gets into the nitty gritty.
Hope these help you to understand that jitter can not be truly eliminated due to all the non-perfect components and sub-systems in a CD/DVD player, etc. Even computer based hard-drive digital playback systems have similar issues, along with the added burden of dealing with the computer based power supplies and all of their issues.
Jon Risch
The author of the article posts here so he might respond. My tests of USB cables goes back about five years. I bought two identical cables. I had one cryogenically treated at Jena Labs. I could hear a definite improvement through the cryoed cable. more recent improvements have been due to the same considerations given to analog low level signal cables plus isolation of the DC voltage wires.
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